The influence of the relative timing of arterial and subarachnoid space pulse waves on spinal perivascular cerebrospinal fluid flow as a possible factor in syrinx development

Characterization of pulsations in the brain and cerebrospinal fluid using ultra-high field magnetic resonance imaging

The development of innovative non-invasive neuroimaging methods and biomarkers is critical for studying brain disease. Imaging of cerebrospinal fluid (CSF) pulsatility may inform the brain fluid dynamics involved in clearance of cerebral metabolic waste. In this work, we developed a methodology to characterize the frequency and spatial localization of whole brain CSF pulsations in humans. Using 7 Tesla (T) human magnetic resonance imaging (MRI) and ultrafast echo-planar imaging (EPI), in-vivo images were obtained to capture pulsations of the CSF signal. Physiological data were simultaneously collected and compared with the 7 T MR data. The primary components of signal pulsations were identified using spectral analysis, with the most evident frequency bands identified around 0.3, 1.2, and 2.4 Hz. These pulsations were mapped spatially and temporally onto the MR image domain and temporally onto the physiological measures of electrocardiogram and respiration. We identified peaks in CSF pulsations that were distinct from peaks in grey matter and white matter regions. This methodology may provide novel in vivo biomarkers of disrupted brain fluid dynamics.

Quantitative Magnetic Resonance Cerebral Spinal Fluid Flow Properties and Executive Function Cognitive Outcomes in Congenital Heart Disease

Cerebrospinal fluid (CSF) circulation has recently been shown to be important in nutrient distribution, waste removal, and neurogenesis. Increased CSF volumes are frequently observed in congenital heart disease (CHD) and are associated with neurodevelopmental deficits. This suggests prolonged perturbation to the CSF system and possible interference to its homeostatic function, which may contribute to the neurodevelopmental deficits in CHD. CSF flow has yet to be studied in CHD patients, but the pulsatile flow of CSF throughout the brain is driven mainly by cardiopulmonary circulation. Given the underlying heart defects in CHD, the cardiopulmonary circulatory mechanisms in CHD might be impaired with resultant perturbation on the CSF circulation. In this study, we determine whether CSF flow, using MRI measurements of static and dynamic pulsatile flow, is abnormal in youths with CHD compared to healthy controls in relation to executive cognitive function. CSF flow measurements were obtained on a total of 58 child and young adult participants (CHD=20, healthy controls = 38). The CSF flow was measured across the lumen of the Aqueduct of Sylvius using cardiac-gated phase-contrast MRI at 3.0T. Static pulsatility was characterized as anterograde and retrograde peak velocities, mean velocity, velocity variance measurements, and dynamic pulsatility calculated as each participant's CSF flow deviation from the study cohort's consensus flow measured with root mean squared deviation (RMSD) were obtained. The participants had neurocognitive assessments for executive function with focus on inhibition, cognitive flexibility, and working memory domains. The CHD group demonstrated greater dynamic pulsatility (higher overall flow RMSD over the entire CSF flow cycle) compared to controls (p=0.0353), with no difference detected in static pulsatility measures. However, lower static CSF flow pulsatility (anterograde peak velocity: p=0.0323) and lower dynamic CSF flow pulsatility (RMSD: p=0.0181) predicted poor inhibitory executive function outcome. Taken together, while the whole CHD group exhibited higher dynamic CSF flow pulsatility compared to controls, the subset of CHD subjects with relatively reduced static and dynamic CSF flow pulsatility had the worst executive functioning, specifically the inhibition domain. These findings suggest that altered CSF flow pulsatility may be central to not only brain compensatory mechanisms but can also drive cognitive impairment in CHD. Further studies are needed to investigate possible mechanistic etiologies of aberrant CSF pulsatility (i.e. primary cardiac hemodynamic disturbances, intrinsic brain vascular stiffness, altered visco-elastic properties of tissue, or glial-lymphatic disturbances), which can result in acquired small vessel brain injury (including microbleeds and white matter hyperintensities).

An analytic model for the flow induced in syringomyelia cavities

A simple two-dimensional fluid-structure-interaction problem, involving viscous oscillatory flow in a channel separated by an elastic membrane from a fluid-filled slender cavity, is analyzed to shed light on the flow dynamics pertaining to syringomyelia, a neurological disorder characterized by the appearance of a large tubular cavity (syrinx) within the spinal cord. The focus is on configurations in which the velocity induced in the cavity, representing the syrinx, is comparable to that found in the channel, representing the subarachnoid space surrounding the spinal cord, both flows being coupled through a linear elastic equation describing the membrane deformation. An asymptotic analysis for small stroke lengths leads to closed-form expressions for the leading-order oscillatory flow, and also for the stationary flow associated with the first-order corrections, the latter involving a steady distribution of transmembrane pressure. The magnitude of the induced flow is found to depend strongly on the frequency, with the result that for channel flow rates of non-sinusoidal waveform, as those found in the spinal canal, higher harmonics can dominate the sloshing motion in the cavity, in agreement with previous in vivo observations. Under some conditions, the cycle-averaged transmembrane pressure, also showing a marked dependence on the frequency, changes sign on increasing the cavity transverse dimension (i.e. orthogonal to the cord axis), underscoring the importance of cavity size in connection with the underlying hydrodynamics. The analytic results presented here can be instrumental in guiding future numerical investigations, needed to clarify the pathogenesis of syringomyelia cavities.

Characterization of oscillations in the brain and cerebrospinal fluid using ultra-high field magnetic resonance imaging

Development of innovative non-invasive neuroimaging methods and biomarkers are critical for studying brain disease. In this work, we have developed a methodology to characterize the frequency responses and spatial localization of oscillations and movements of cerebrospinal fluid (CSF) flow in the human brain. Using 7 Tesla human MRI and ultrafast echo-planar imaging (EPI), in-vivo images were obtained to capture CSF oscillations and movements. Physiological data was simultaneously collected and correlated with the 7T MR data. The primary components of CSF oscillations were identified using spectral analysis (with frequency bands identified around 0.3Hz, 1.2Hz and 2.4Hz) and were mapped spatially and temporally onto the MR image domain and temporally onto the physiological domain. The developed methodology shows a good consistency and repeatability (standard deviation of 0.052 and 0.078 for 0.3Hz and 1.2Hz bands respectively) in-vivo for potential brain dynamics and CSF flow and clearance studies.

Cerebrospinal fluid solute transport associated with sensorimotor brain activity in rodents

Cerebrospinal fluid (CSF) is crucial for maintaining neuronal homeostasis, providing nutrition, and removing metabolic waste from the brain. However, the relationship between neuronal activity and CSF solute transport remains poorly understood. To investigate the effect of regional neuronal activity on CSF solute transport, Sprague-Dawley rats (all male, n = 30) under anesthesia received an intracisternal injection of a fluorescent tracer (Texas Red ovalbumin) and were subjected to unilateral electrical stimulation of a forelimb. Two groups (n = 10 each) underwent two different types of stimulation protocols for 90 min, one including intermittent 7.5-s resting periods and the other without rest. The control group was not stimulated. Compared to the control, the stimulation without resting periods led to increased transport across most of the cortical areas, including the ventricles. The group that received intermittent stimulation showed an elevated level of solute uptake in limited areas, i.e., near/within the ventricles and on the ventral brain surface. Interhemispheric differences in CSF solute transport were also found in the cortical regions that overlap with the forelimb sensorimotor area. These findings suggest that neuronal activity may trigger local and brain-wide increases in CSF solute transport, contributing to waste clearance.

Pulsatile cerebral paraarterial flow by peristalsis, pressure and directional resistance

BACKGROUND

A glymphatic system has been proposed that comprises flow that enters along cerebral paraarterial channels between the artery wall and the surrounding glial layer, continues through the parenchyma, and then exits along similar paravenous channels. The mechanism driving flow through this system is unclear. The pulsatile (oscillatory plus mean) flow measured in the space surrounding the middle cerebral artery (MCA) suggests that peristalsis created by intravascular blood pressure pulses is a candidate for the paraarterial flow in the subarachnoid spaces. However, peristalsis is ineffective in driving significant mean flow when the amplitude of channel wall motion is small, as has been observed in the MCA artery wall. In this paper, peristalsis in combination with two additional mechanisms, a longitudinal pressure gradient and directional flow resistance, is evaluated to match the measured MCA paraarterial oscillatory and mean flows.

METHODS

Two analytical models are used that simplify the paraarterial branched network to a long continuous channel with a traveling wave in order to maximize the potential effect of peristalsis on the mean flow. The two models have parallel-plate and annulus geometries, respectively, with and without an added longitudinal pressure gradient. The effect of directional flow resistors was also evaluated for the parallel-plate geometry.

RESULTS

For these models, the measured amplitude of arterial wall motion is too large to cause the small measured amplitude of oscillatory velocity, indicating that the outer wall must also move. At a combined motion matching the measured oscillatory velocity, peristalsis is incapable of driving enough mean flow. Directional flow resistance elements augment the mean flow, but not enough to provide a match. With a steady longitudinal pressure gradient, both oscillatory and mean flows can be matched to the measurements.

CONCLUSIONS

These results suggest that peristalsis drives the oscillatory flow in the subarachnoid paraarterial space, but is incapable of driving the mean flow. The effect of directional flow resistors is insufficient to produce a match, but a small longitudinal pressure gradient is capable of creating the mean flow. Additional experiments are needed to confirm whether the outer wall also moves, as well as to validate the pressure gradient.

Syringomyelia Associated with Chiari 1 Malformation in Adults: Positive Outcome Predictors after Posterior Fossa Decompression with Duraplasty

BACKGROUND

Syringomyelia (Syr) in patients with Chiari 1 malformation (CM1) may be attributable to abnormal dynamics of cerebrospinal fluid (CSF) in the upper cervical segment; fourth ventricle enlargement has been reported in association with a worse clinical and radiological presentation, independently of the posterior fossa volume. In this study, we analyzed presurgery hydrodynamic markers to evaluate if their changes could be associated with clinical and radiological improvement after posterior fossa decompression and duraplasty (PFDD). As a primary endpoint, we aimed to correlate improvement in the fourth ventricle area with positive clinical outcomes.

METHODS

In total, in this study, we enrolled 36 consecutive adults with Syr and CM1 who were followed by a multidisciplinary team. All the patients were prospectively evaluated with clinical scales and neuroimaging, including CSF flow, the fourth ventricle area, and the Vaquero Index by using a phase-contrast MRI before (T0) and after surgical treatment (T1-Tlast, with a range of 12-108 months). The CSF flow at the craniocervical junction (CCJ), the fourth ventricle area, and the Vaquero Index changes were statistically analyzed and compared to the clinical and quality of life improvement after surgery. The good outcome prediction ability of presurgical radiological variables was tested.

RESULTS

Surgery was associated with positive clinical and radiological outcomes in more than 90% of cases. The fourth ventricle area significantly reduced after surgery (T0-Tlast, p = 0.0093), but no significant associations with clinical improvement were found. The presurgical presence of CSF flow at the CCJ was able to predict a good outcome (AUC = 0.68, 95% CI 0.50-0.87 and LH+ = 2.1, IC 95% 1.16-3.07) and was also significantly associated with post-surgical pain relief (rho = 0.61 and p = 0.0144).

CONCLUSIONS

Presurgery CSF flow at the CCJ is proposed as a radiological marker with the ability to predict a positive outcome after PFDD in adults with syringomyelia and CM1. Measurements of the fourth ventricle area could be useful additional information for evaluating surgical long-term follow-up; further experience on larger cohorts is required to better define the prognostic yield of this radiological parameter.

Post-traumatic syringomyelia resolution following surgical treatment: the moniliform syrinx with a better prognosis

BACKGROUND

Post-traumatic syringomyelia (PTS) presented as a serious delayed complication after spinal cord injury (SCI). In our preliminary pathological investigation of PTS in an animal model, the endogenous repair was activated during the early stage of the central canal expansion. We thought about whether there might be an "early syringomyelia state" with a better outcome.

OBJECTIVE

This study aimed to further understand the pathophysiological basis of PTS's occurrence, development, and outcome.

MATERIALS AND METHODS

A cross-sectional observational study from a single-center syringomyelia database prospectively maintained at China International Neuroscience Institute (CHINA-INI). A consecutive series of 28 PTS patients at our institution for surgical treatment met the inclusion criteria of this study. Their clinical and imaging data in a long-term follow-up were reviewed retrospectively. We compared the surgical outcome between moniliform and distended syringomyelia based on high‑resolution MRI and syringomyelia-related symptoms. American Spinal Injury Association (ASIA) impairment scale (AIS) grade to assess their neurological status.

RESULTS

Through a series of phenotypic comparisons, we found that moniliform-like syrinx belongs to a special morphological state with a shorter natural history. The patients in the moniliform group had a better surgical outcome compared with those in the distended group (P = 0.028): more obvious symptom improvement as shown in Kaplan-Meier analysis (P = 0.033, Chi square = 4.523) and a higher syringomyelia resolution rate (P = 0.024).

CONCLUSION

We consider the delayed post-traumatic syringomyelia with moniliform type with a better surgical outcome and emphasize the importance of timely intervention to restore cerebrospinal fluid circulation.

Preoperative assessment of dominant occipital sinus in patients with Chiari malformation type I: anatomical variations and implications for preventing potentially life-threatening surgical complications

OBJECTIVE

The surgical treatment of Chiari malformation type I (CM-I) frequently involves dural incision at the posterior cranial fossa. In cases of persistent patent occipital sinus (OS), the sinus is usually obliterated and divided. However, there are some patients whose OS is prominent and requires crucial modification of the operative planning to avoid potentially life-threatening massive hemorrhage and disturbance of cerebral venous circulation. In the present study, the anatomical variations of the dominant OS in patients with CM-I were analyzed and the authors attempted to develop treatment recommendations for patients with CM-I with dominant OS.

METHODS

The study included 213 patients with CM-I who underwent MR venography (MRV) prior to surgical treatment. OS dominance was assessed using 2D time-of-flight MRV or 3D phase-contrast MRV. Particular attention was paid to the pattern of venous outflow channels. The characteristics of the patients with dominant OS and the surgical outcomes were retrospectively reviewed.

RESULTS

Dominant OS was identified in 7 patients (3.3%). The age in those with dominant OS was significantly younger than in those without (p = 0.0202). The incidence of concurrent scoliosis in the patients with dominant OS was significantly higher than in those without (p = 0.0366). All the dominant OSs were found to be of the oblique type. Unilateral oblique OS (OOS) with normal ipsilateral transverse sinus (TS) and hypoplastic contralateral TS was found in 2 patients (0.9%). The authors found 1 patient each (0.5%) who had unilateral OOS with hypoplastic ipsilateral TS and normal contralateral TS, unilateral OOS with bilateral hypoplastic TSs, and bilateral OOSs with bilateral normal TSs. Bilateral OOSs with bilateral hypoplastic TSs were found in 2 patients (0.9%). All these patients had syringomyelia. Instead of performing Y-shaped dural incision and duraplasty, surgical procedures were modified depending on the types of the OOSs to preserve their venous drainage routes. Although massive bleeding from the dominant OS during dural incision occurred in 1 patient, none suffered neurological deterioration. The syrinx volume decreased in all but 1 of the patients postoperatively.

CONCLUSIONS

Assessment of the venous drainage pattern using MRV is indispensable for safe surgical treatment in patients with CM-I. The surgical procedure should be modified based on the type of dominant OS to minimize the surgical risks.

Chiari Malformation Type 1 in Adults

The term Chiari malformation refers to a heterogeneous group of anatomical abnormalities at the craniovertebral junction. Chiari malformation type 1 (CM1) refers to the abnormal protrusion of cerebellar tonsils through the foramen magnum and is by far the commonest type. Its prevalence is estimated approximately 1%; it is more common in women and is associated with syringomyelia in 25-70% of cases. The prevalent pathophysiological theory proposes a morphological mismatch between a small posterior cranial fossa and a normally developed hindbrain that results in ectopia of the tonsils.In most people, CM1 is asymptomatic and diagnosed incidentally. In symptomatic cases, headache is the cardinal symptom. The typical headache is induced by Valsalva-like maneuvers. Many of the other symptoms are nonspecific, and in the absence of syringomyelia, the natural history is benign. Syringomyelia manifests with spinal cord dysfunction of varying severity. The approach to patients with CM1 should be multidisciplinary, and the first step in the management is phenotyping the symptoms, because they may be due to other pathologies, like a primary headache syndrome. Magnetic resonance imaging, which shows cerebellar tonsillar decent 5 mm or more below the foramen magnum, is the gold standard investigative modality. The diagnostic workup may include dynamic imaging of the craniocervical junction and intracranial pressure monitoring.The management of CM1 is variable and sometimes controversial. Surgery is usually reserved for patients with disabling headaches or neurological deficits from the syrinx. Surgical decompression of the craniocervical junction is the most widely used procedure. Several surgical techniques have been proposed, but there is no consensus on the best treatment strategy, mainly due to lack of high-quality evidence. The management of the condition during pregnancy, restriction to lifestyle related to athletic activities, and the coexistence of hypermobility require special considerations.

Spontaneous resolution of syringomyelia following pregnancy and parturition in a patient with type I chiari malformation: A case and systematic review

BACKGROUND

Spontaneous resolution of syringomyelia has rarely been reported in the literature. Rarer still are cases wherein this process is associated with pregnancy and parturition. We review theories on syringomyelia development and spontaneous resolution to better understand the role pregnancy and parturition may play in both processes.

METHODS

We present a 30-year-old female with MRI-confirmed spontaneous syrinx regression following caesarean delivery of a full-term pregnancy. We additionally review the literature to identify previously reported cases of spontaneous syrinx regression both independent of and associated with pregnancy.

RESULTS

Including the present case, 39 cases describing spontaneous regression of syringomyelia have been reported in the literature, of which only four are associated with pregnancy and parturition. 75% of all reported cases were associated with type I Chiari malformation, though several disorders of the craniocervical junction and spinal canal were implicated. Complete syrinx regression was achieved in 33.3% of cases and 5% of cases described recurrence of syringomyelia following the spontaneous resolution.

CONCLUSION

Syringomyelia likely develops due to disturbance of the physiologic flow of cerebrospinal fluid around the craniocervical junction and the obex. Several mechanisms including fissuring of the spinal cord parenchyma and reduction of subarachnoid scarring are likely involved in this process. In the setting of pregnancy, additional mechanisms surrounding the increased intraabdominal forces imparted by a growing fetus, Valsalva-like strain experienced during labor, and hemodynamic changes that occur to accommodate gestation are likely implicated. Nevertheless, patients should continue to be monitored periodically for syrinx recurrence.

Surgical Management of Chiari Malformation Type I Associated with Syringomyelia: Outcome of Surgeries Based on the New Classification and Study of Cerebrospinal Fluid Dynamics

Introduction: The mainstay of treatment of syringomyelia associated with Chiari malformation type I (CM-I) is the management of CM-I to normalize the cerebrospinal fluid (CSF) flow at the foramen magnum. CM-I is classified into three independent types. Surgical treatment was selected based on the mechanism of hindbrain ptosis in each CM-I type. Materials and Methods: Foramen magnum decompression (FMD: 213 cases), expansive suboccipital cranioplasty (ESCP: 87 cases), and craniocervical fixation (CCF: 30 cases) were performed. CSF flow dynamics were assessed pre- and post-surgery using cine phase contrast magnetic resonance imaging. During surgery, CSF flow dynamics were examined using color Doppler ultrasonography (CDU). Results: ESCP and FMD demonstrated high rates of improvement in neurological symptoms and signs (82.7%), whereas CCF demonstrated a high rate of improvement in neurological symptoms (89%). The pre-operative maximum flow velocity (cm/s) was significantly lower in patients than in controls and increased post-operatively. During surgery, CDU indicated that the volume of the major cistern was 8 mL, and the maximum flow velocity was >3 mL/s. Conclusions: An appropriate surgical treatment should be selected for CM-I to correct hindbrain ptosis. In addition, it is necessary to confirm the normalization of CSF flow at the foramen of Magendie.

Changes in intrathoracic pressure, not arterial pulsations, exert the greatest effect on tracer influx in the spinal cord

BACKGROUND

Cerebrospinal fluid (CSF) circulation in the brain has garnered considerable attention in recent times. In contrast, there have been fewer studies focused on the spine, despite the expected importance of CSF circulation in disorders specific to the spine, including syringomyelia. The driving forces that regulate spinal CSF flow are not well defined and are likely to be different to the brain given the anatomical differences and proximity to the heart and lungs. The aims of this study were to determine the effects of heart rate, blood pressure and respiration on the distribution of CSF tracers in the spinal subarachnoid space, as well as into the spinal cord interstitium.

METHODS

In Sprague Dawley rats, physiological parameters were manipulated such that the effects of spontaneous breathing (generating alternating positive and negative intrathoracic pressures), mechanical ventilation (positive intrathoracic pressure only), tachy/bradycardia, as well as hyper/hypotension were separately studied. To investigate spinal CSF hydrodynamics, in vivo near-infrared imaging of intracisternally infused indocyanine green was performed. CSF tracer transport was further characterised with in vivo two-photon intravital imaging. Tracer influx at a microscopic level was quantitatively characterised by ex vivo epifluorescence imaging of fluorescent ovalbumin.

RESULTS

Compared to mechanically ventilated controls, spontaneous breathing animals had significantly greater movement of tracer in the subarachnoid space. There was also greater influx into the spinal cord interstitium. Hypertension and tachycardia had no significant effect on spinal subarachnoid spinal CSF tracer flux and exerted less effect than respiration on tracer influx into the spinal cord.

CONCLUSIONS

Intrathoracic pressure changes that occur over the respiratory cycle, particularly decreased intrathoracic pressures generated during inspiration, have a profound effect on tracer movement after injection into spinal CSF and increase cord parenchymal tracer influx. Arterial pulsations likely drive fluid transport from perivascular spaces into the surrounding interstitium, but their overall impact is less than that of the respiratory cycle on net tracer influx.

Tachycardia and hypertension enhance tracer efflux from the spinal cord

Background

Disruption of cerebrospinal fluid (CSF)/interstitial fluid (ISF) exchange in the spinal cord is likely to contribute to central nervous system (CNS) diseases that involve abnormal fluid accumulation, including spinal cord oedema and syringomyelia. However, the physiological factors that govern fluid transport in the spinal cord are poorly understood. The aims of this study were to determine the effects of cardiac pulsations and respiration on tracer signal increase, indicative of molecular movement following infusion into the spinal cord grey or white matter.

Methods

In Sprague Dawley rats, physiological parameters were manipulated such that the effects of spontaneous breathing (generating alternating positive and negative intrathoracic pressures), mechanical ventilation (positive intrathoracic pressure only), tachycardia (heart atrial pacing), as well as hypertension (pharmacologically induced) were separately studied. Since fluid outflow from the spinal cord cannot be directly measured, we assessed the molecular movement of fluorescent ovalbumin (AFO-647), visualised by an increase in tracer signal, following injection into the cervicothoracic spinal grey or white matter.

Results

Tachycardia and hypertension increased AFO-647 tracer efflux, while the concomitant negative and positive intrathoracic pressures generated during spontaneous breathing did not when compared to the positive-pressure ventilated controls. Following AFO-647 tracer injection into the spinal grey matter, increasing blood pressure and heart rate resulted in increased tracer movement away from the injection site compared to the hypotensive, bradycardic animals (hypertension: p = 0.05, tachycardia: p < 0.0001). Similarly, hypertension and tachycardia produced greater movement of AFO-647 tracer longitudinally along the spinal cord following injection into the spinal white matter (p < 0.0001 and p = 0.002, respectively). Tracer efflux was strongly associated with all blood vessel types.

Conclusions

Arterial pulsations have profound effects on spinal cord interstitial fluid homeostasis, generating greater tracer efflux than intrathoracic pressure changes that occur over the respiratory cycle, demonstrated by increased craniocaudal CSF tracer movement in the spinal cord parenchyma.

Slosh Simulation in a Computer Model of Canine Syringomyelia

The exact pathogenesis of syringomyelia is unknown. Epidural venous distention during raised intrathoracic pressure (Valsalva) may cause impulsive movement of fluid ("slosh") within the syrinx. Such a slosh mechanism is a proposed cause of syrinx dissection into spinal cord parenchyma resulting in craniocaudal propagation of the cavity. We sought to test the "slosh" hypothesis by epidural excitation of CSF pulse in a computer model of canine syringomyelia. Our previously developed canine syringomyelia computer model was modified to include an epidural pressure pulse. Simulations were run for: cord free of cavities; cord with small syringes at different locations; and cord with a syrinx that was progressively expanding caudally. If small syringes are present, there are peaks of stress at those locations. This effect is most pronounced at the locations at which syringes initially form. When a syrinx is expanding caudally, the peak stress is typically at the caudal end of the syrinx. However, when the syrinx reaches the lumbar region; the stress becomes moderate. The findings support the "slosh" hypothesis, suggesting that small cervical syringes may propagate caudally. However, when the syrinx is large, there is less focal stress, which may explain why a syrinx can rapidly expand but then remain unchanged in shape over years.

Brain solute transport is more rapid in periarterial than perivenous spaces

Fluid flow in perivascular spaces is recognized as a key component underlying brain transport and clearance. An important open question is how and to what extent differences in vessel type or geometry affect perivascular fluid flow and transport. Using computational modelling in both idealized and image-based geometries, we study and compare fluid flow and solute transport in pial (surface) periarterial and perivenous spaces. Our findings demonstrate that differences in geometry between arterial and venous pial perivascular spaces (PVSs) lead to higher net CSF flow, more rapid tracer transport and earlier arrival times of injected tracers in periarterial spaces compared to perivenous spaces. These findings can explain the experimentally observed rapid appearance of tracers around arteries, and the delayed appearance around veins without the need of a circulation through the parenchyma, but rather by direct transport along the PVSs.

Phase offset between arterial pulsations and subarachnoid space pressure fluctuations are unlikely to drive periarterial cerebrospinal fluid flow

Circulation of fluid through the central nervous system maintains fluid homeostasis and is involved in solute clearance. The glymphatic system is hypothesised to facilitate waste clearance in the brain, with inflow via periarterial spaces, bulk flow through the parenchyma, and outflow via perivenous spaces. The driving force for this mechanism is unknown. Previous modelling in the spinal cord suggests that timing offsets between arterial and subarachnoid space pressure pulses can enable net inflow in perivascular spaces (PVS). This study adapted the spinal pulse offset mechanism to the brain and simulated movement of tracer particles used in experiments. Both bulk flow and diffusive movement of tracer were simulated. Intracranial pressure pulses were applied to one end of a 300-μm-long perivascular space combined with a moving arterial wall simulating arterial pulsations. The simulations indicate the pulse offset mechanism can enable net inflow via PVS; however, it is unknown whether the temporal offset required is physiologically realistic. Increasing the positive component of the ICP (intracranial pressure) pulse increased net flow. Tracer particles driven by bulk flow reached the outlet of the PVS with a net speed of ~ 16 μm/s when the permeability was two orders of magnitude higher than values in the literature. These particles were unable to penetrate into the parenchyma in the absence of diffusion. Dispersion dominated tracer movement in the parenchyma. Further research is required to reconcile discrepancies between these results, and both experimental and computational studies.

The mechanisms behind perivascular fluid flow

Flow of cerebrospinal fluid (CSF) in perivascular spaces (PVS) is one of the key concepts involved in theories concerning clearance from the brain. Experimental studies have demonstrated both net and oscillatory movement of microspheres in PVS (Mestre et al. (2018), Bedussi et al. (2018)). The oscillatory particle movement has a clear cardiac component, while the mechanisms involved in net movement remain disputed. Using computational fluid dynamics, we computed the CSF velocity and pressure in a PVS surrounding a cerebral artery subject to different forces, representing arterial wall expansion, systemic CSF pressure changes and rigid motions of the artery. The arterial wall expansion generated velocity amplitudes of 60-260 μm/s, which is in the upper range of previously observed values. In the absence of a static pressure gradient, predicted net flow velocities were small (<0.5 μm/s), though reaching up to 7 μm/s for non-physiological PVS lengths. In realistic geometries, a static systemic pressure increase of physiologically plausible magnitude was sufficient to induce net flow velocities of 20-30 μm/s. Moreover, rigid motions of the artery added to the complexity of flow patterns in the PVS. Our study demonstrates that the combination of arterial wall expansion, rigid motions and a static CSF pressure gradient generates net and oscillatory PVS flow, quantitatively comparable with experimental findings. The static CSF pressure gradient required for net flow is small, suggesting that its origin is yet to be determined.

Correlating Tissue Mechanics and Spinal Cord Injury: Patient-Specific Finite Element Models of Unilateral Cervical Contusion Spinal Cord Injury in Non-Human Primates

Non-human primate (NHP) models are the closest approximation of human spinal cord injury (SCI) available for pre-clinical trials. The NHP models, however, include broader morphological variability that can confound experimental outcomes. We developed subject-specific finite element (FE) models to quantify the relationship between impact mechanics and SCI, including the correlations between FE outcomes and tissue damage. Subject-specific models of cervical unilateral contusion SCI were generated from pre-injury MRIs of six NHPs. Stress and strain outcomes were compared with lesion histology using logit analysis. A parallel generic model was constructed to compare the outcomes of subject-specific and generic models. The FE outcomes were correlated more strongly with gray matter damage (0.29 < R² < 0.76) than white matter (0.18 < R² < 0.58). Maximum/minimum principal strain, Von-Mises and Tresca stresses showed the strongest correlations (0.31 < R² < 0.76) with tissue damage in the gray matter while minimum principal strain, Von-Mises stress, and Tresca stress best predicted white matter damage (0.23 < R² < 0.58). Tissue damage thresholds varied for each subject. The generic FE model captured the impact biomechanics in two of the four models; however, the correlations between FE outcomes and tissue damage were weaker than the subject-specific models (gray matter [0.25 < R² < 0.69] and white matter [R² < 0.06] except for one subject [0.26 < R² < 0.48]). The FE mechanical outputs correlated with tissue damage in spinal cord white and gray matters, and the subject-specific models accurately mimicked the biomechanics of NHP cervical contusion impacts.

Continuous Hypodynamic Change of Cerebrospinal Fluid Flow as A Potential Factor Working for Experimental Scoliotic Formation

Scoliosis is often associated with syringomyelia (SM). As an important role in SM formation, the influence from abnormal cerebrospinal fluid (CSF) flow is still unclear to scoliosis. The aim of this experimental work is to explore the connection between CSF flow and scoliosis through imaging and histological analysis on the basis of a kaolin-induced scoliotic rabbit model. For imaging observation, in 40 kaolin-induced rabbits by C7 spinal cord injection, through pre- and postoperative MRI and radiography, CSF flow and scoliosis formation were detected at consecutive phases. According to the final formation of scoliosis until postoperative week 12, the kaolin-induced rabbits were divided into 2 groups. Through comparing the 2 groups, the relationship between the changes of CSF flow velocity and scoliosis formation were reviewed and analyzed. For histological observation, another 20 kaolin-induced rabbits were used for consecutive histological observations of spinal cord at postoperative 3-day, 2-week, 4-week and 6-week. After kaolin-induction, abnormal spinal coronal curve was observed from postoperative week 6 in the 37 survived rabbits. At postoperative week 12, scoliosis formation was detected in 73.0% kaolin-induced rabbits and the mean Cobb angle was 27.4°. From the comparison between scoliotic and non-scoliotic groups, the difference of the velocities of CSF flow was more obviously from postoperative week 4 to 12, especially after week 6. In the scoliotic group, the peak velocity of CSF flow was diseased gradually following scoliosis formation after induction. Moreover, the decrease of the peak velocities of CSF flow from preoperation to postoperative 12 weeks (ΔVmax), including up-flow (ΔVUmax) and down-flow (ΔVDmax), were positively correlated to the final scoliotic Cobb angle (P < 0.01). Through histological observation at different phases, the distinctive pathological changes of the spinal cord included early inflammatory reaction, adhesion and blockage in the subarachnoid space and the central canal, perivascular space enlargement, central canal expansion, which suggested the CSF flow being blocked by multiple ways after kaolin-induction. In conclusion, experimental scoliosis can be successfully induced by intraspinal kaolin injection. In this model, continuous hypodynamic change of CSF flow was correlated to the formation of scoliosis, which could be an important factor of scoliotic pathogenesis being explored furtherly.

Abnormalities in spinal cord ultrastructure in a rat model of post-traumatic syringomyelia

BACKGROUND

Syringomyelia is a serious complication of spinal cord trauma, occurring in approximately 28% of spinal cord injuries. Treatment options are limited and often produce unsatisfactory results. Post-traumatic syringomyelia (PTS) is presumably related to abnormalities of cerebrospinal fluid (CSF) and interstitial fluid hydrodynamics, but the exact mechanisms are unknown.

METHODS

Transmission electron microscopy (TEM) was used to investigate in detail the interfaces between fluid and tissue in the spinal cords of healthy Sprague-Dawley rats (n = 3) and in a rat model of PTS (n = 3). PTS was induced by computer-controlled impact (75 kDyn) to the spinal cord between C6 and C8, followed by a subarachnoid injection of kaolin to produce focal arachnoiditis. Control animals received a laminectomy only to C6 and C7 vertebrae. Animals were sacrificed 12 weeks post-surgery, and spinal cords were prepared for TEM. Ultra-thin spinal cord sections at the level of the injury were counterstained for structural anatomy.

RESULTS

Spinal cords from animals with PTS displayed several abnormalities including enlarged perivascular spaces, extracellular edema, cell death and loss of tissue integrity. Additionally, alterations to endothelial tight junctions and an abundance of pinocytotic vesicles, in tissue adjacent to syrinx, suggested perturbations to blood-spinal cord barrier (BSCB) function.

CONCLUSIONS

These findings support the hypothesis that perivascular spaces are important pathways for CSF flow into and out of the spinal cord, but also suggest that fluid may enter the cord through vesicular transport and an altered BSCB.

Origin of Syrinx Fluid in Syringomyelia: A Physiological Study

BACKGROUND

The origin of syrinx fluid is controversial.

OBJECTIVE

To elucidate the mechanisms of syringomyelia associated with cerebrospinal fluid pathway obstruction and with intramedullary tumors, contrast transport from the spinal subarachnoid space (SAS) to syrinx was evaluated in syringomyelia patients.

METHODS

We prospectively studied patients with syringomyelia: 22 with Chiari I malformation and 16 with SAS obstruction-related syringomyelia before and 1 wk after surgery, and 9 with tumor-related syringomyelia before surgery only. Computed tomography-myelography quantified dye transport into the syrinx before and 0.5, 2, 4, 6, 8, 10, and 22 h after contrast injection by measuring contrast density in Hounsfield units (HU).

RESULTS

Before surgery, more contrast passed into the syrinx in Chiari I malformation-related syringomyelia and spinal obstruction-related syringomyelia than in tumor-related syringomyelia, as measured by (1) maximum syrinx HU, (2) area under the syrinx concentration-time curve (HU AUC), (3) ratio of syrinx HU to subarachnoid cerebrospinal fluid (CSF; SAS) HU, and (4) AUC syrinx/AUC SAS. More contrast (AUC) accumulated in the syrinx and subarachnoid space before than after surgery.

CONCLUSION

Transparenchymal bulk flow of CSF from the subarachnoid space to syrinx occurs in Chiari I malformation-related syringomyelia and spinal obstruction-related syringomyelia. Before surgery, more subarachnoid contrast entered syringes associated with CSF pathway obstruction than with tumor, consistent with syrinx fluid originating from the subarachnoid space in Chiari I malformation and spinal obstruction-related syringomyelia and not from the subarachnoid space in tumor-related syringomyelia. Decompressive surgery opened subarachnoid CSF pathways and reduced contrast entry into syringes associated with CSF pathway obstruction.

Chiari malformation and syringomyelia

Chiari malformation was first described over a century ago, and consists of posterior fossa anomalies that generally share the feature of cerebellar tonsillar descent through the foramen magnum. Our understanding of this disorder was initially based on autopsy studies, and has been greatly enhanced by the advent of MRI. The surgical management of Chiari anomalies has also evolved in a parallel fashion. Although the exact surgical technique varies among individual surgeons, the goals of surgery remain constant and consist of relieving brainstem compression and cranial nerve distortion, restoring the normal flow of CSF across the foramen magnum, and reducing the size of any associated syrinx cavity. Syrinx cavities are most commonly associated with Chiari anomalies, yet primary spinal syringomyelia (PSS) can be caused by traumatic, infectious, degenerative, and other etiologies that cause at least a partial CSF flow obstruction in the spinal subarachnoid space. As with syringomyelia associated with Chiari anomalies, the main goal of PSS surgery is to reestablish CSF flow across the area of obstruction. In addition to MRI, myelography with CT can be very helpful in the evaluation and management of these patients by identifying focal regions of CSF obstruction that may be amenable to surgical intervention. Future directions for the treatment of Chiari anomalies and syringomyelia include the application of advanced imaging techniques, more widespread use of genetic evaluation, large-scale outcome studies, and the further refinement of surgical technique.

Dispersion in porous media in oscillatory flow between flat plates: applications to intrathecal, periarterial and paraarterial solute transport in the central nervous system

Background

As an alternative to advection, solute transport by shear-augmented dispersion within oscillatory cerebrospinal fluid flow was investigated in small channels representing the basement membranes located between cerebral arterial smooth muscle cells, the paraarterial space surrounding the vessel wall and in large channels modeling the spinal subarachnoid space (SSS).

Methods

Geometries were modeled as two-dimensional. Fully developed flows in the channels were modeled by the Darcy–Brinkman momentum equation and dispersion by the passive transport equation. Scaling of the enhancement of axial dispersion relative to molecular diffusion was developed for regimes of flow including quasi-steady, porous and unsteady, and for regimes of dispersion including diffusive and unsteady.

Results

Maximum enhancement occurs when the characteristic time for lateral dispersion is matched to the cycle period. The Darcy–Brinkman model represents the porous media as a continuous flow resistance, and also imposes no-slip boundary conditions at the walls of the channel. Consequently, predicted dispersion is always reduced relative to that of a channel without porous media, except when the flow and dispersion are both unsteady.

Discussion/conclusions

In the basement membranes, flow and dispersion are both quasi-steady and enhancement of dispersion is small even if lateral dispersion is reduced by the porous media to achieve maximum enhancement. In the paraarterial space, maximum enhancement R_max = 73,200 has the potential to be significant. In the SSS, the dispersion is unsteady and the flow is in the transition zone between porous and unsteady. Enhancement is 5.8 times that of molecular diffusion, and grows to a maximum of 1.6E+6 when lateral dispersion is increased. The maximum enhancement produces rostral transport time in agreement with experiments.

Electronic supplementary material

The online version of this article (10.1186/s12987-019-0132-y) contains supplementary material, which is available to authorized users.

Effect of extradural constriction on CSF flow in rat spinal cord

Background

Fluid homeostasis in the central nervous system (CNS) is essential for normal neurological function. Cerebrospinal fluid (CSF) in the subarachnoid space and interstitial fluid circulation in the CNS parenchyma clears metabolites and neurotransmitters and removes pathogens and excess proteins. A thorough understanding of the normal physiology is required in order to understand CNS fluid disorders, including post-traumatic syringomyelia. The aim of this project was to compare fluid transport, using quantitative imaging of tracers, in the spinal cord from animals with normal and obstructed spinal subarachnoid spaces.

Methods

A modified extradural constriction model was used to obstruct CSF flow in the subarachnoid space at the cervicothoracic junction (C7–T1) in Sprague–Dawley rats. Alexa-Fluor 647 Ovalbumin conjugate was injected into the cisterna magna at either 1 or 6 weeks post–surgery. Macroscopic and microscopic fluorescent imaging were performed in animals sacrificed at 10 or 20 min post–injection. Tracer fluorescence intensity was compared at cervical and thoracic spinal cord levels between control and constriction animals at each post-surgery and post-injection time point. The distribution of tracer around arterioles, venules and capillaries was also compared.

Results

Macroscopically, the fluorescence intensity of CSF tracer was significantly greater in spinal cords from animals with a constricted subarachnoid space compared to controls, except at 1 week post-surgery and 10 min post-injection. CSF tracer fluorescence intensity from microscopic images was significantly higher in the white matter of constriction animals 1 week post surgery and 10 min post-injection. At 6 weeks post–constriction surgery, fluorescence intensity in both gray and white matter was significantly increased in animals sacrificed 10 min post-injection. At 20 min post-injection this difference was significant only in the white matter and was less prominent. CSF tracer was found predominantly in the perivascular spaces of arterioles and venules, as well as the basement membrane of capillaries, highlighting the importance of perivascular pathways in the transport of fluid and solutes in the spinal cord.

Conclusions

The presence of a subarachnoid space obstruction may lead to an increase in fluid flow within the spinal cord tissue, presenting as increased flow in the perivascular spaces of arterioles and venules, and the basement membranes of capillaries. Increased fluid retention in the spinal cord in the presence of an obstructed subarachnoid space may be a critical step in the development of post-traumatic syringomyelia.

Electronic supplementary material

The online version of this article (10.1186/s12987-019-0127-8) contains supplementary material, which is available to authorized users.

Characterization of intrathecal cerebrospinal fluid geometry and dynamics in cynomolgus monkeys (macaca fascicularis) by magnetic resonance imaging

Recent advancements have been made toward understanding the diagnostic and therapeutic potential of cerebrospinal fluid (CSF) and related hydrodynamics. Increased understanding of CSF dynamics may lead to improved detection of central nervous system (CNS) diseases and optimized delivery of CSF based CNS therapeutics, with many proposed therapeutics hoping to successfully treat or cure debilitating neurological conditions. Before significant strides can be made toward the research and development of interventions designed for human use, additional research must be carried out with representative subjects such as non-human primates (NHP). This study presents a geometric and hydrodynamic characterization of CSF in eight cynomolgus monkeys (Macaca fascicularis) at baseline and two-week follow-up.

Results showed that CSF flow along the entire spine was laminar with a Reynolds number ranging up to 80 and average Womersley number ranging from 4.1–7.7. Maximum CSF flow rate occurred ~25 mm caudal to the foramen magnum. Peak CSF flow rate ranged from 0.3–0.6 ml/s at the C3-C4 level. Geometric analysis indicated that average intrathecal CSF volume below the foramen magnum was 7.4 ml. The average surface area of the spinal cord and dura was 44.7 and 66.7 cm² respectively. Subarachnoid space cross-sectional area and hydraulic diameter ranged from 7–75 mm² and 2–3.7 mm, respectively. Stroke volume had the greatest value of 0.14 ml at an axial location corresponding to C3-C4.

Gadolinium and Multiple Sclerosis: Vessels, Barriers of the Brain, and Glymphatics

The pathogenesis of multiple sclerosis is characterized by a cascade of pathobiologic events, ranging from focal lymphocytic infiltration and microglia activation to demyelination and axonal degeneration. MS has several of the hallmarks of an inflammatory autoimmune disorder, including breakdown of the BBB. Gadolinium-enhanced MR imaging is currently the reference standard to detect active inflammatory lesions in MS. Knowledge of the patterns and mechanisms of contrast enhancement is vital to limit the radiologic differential diagnosis in the staging and evaluation of MS lesion activity. The aim of this review was the following: 1) to outline the pathophysiology of the effect of lymphocyte-driven inflammation in MS, 2) to describe the effects of gadolinium on the BBB and glymphatic system, and 3) to describe gadolinium enhancement patterns and artifacts that can mimic lesions in MS.

A Computational Model for the Dynamics of Cerebrospinal Fluid in the Spinal Subarachnoid Space

Global models for the dynamics of coupled fluid compartments of the central nervous system (CNS) require simplified representations of the individual components which are both accurate and computationally efficient. This paper presents a one-dimensional model for computing the flow of cerebrospinal fluid (CSF) within the spinal subarachnoid space (SSAS) under the simplifying assumption that it consists of two coaxial tubes representing the spinal cord and the dura. A rigorous analysis of the first-order nonlinear system demonstrates that the system is elliptic-hyperbolic, and hence ill-posed, for some values of parameters, being hyperbolic otherwise. In addition, the system cannot be written in conservation-law form, and thus, an appropriate numerical approach is required, namely the path conservative approach. The designed computational algorithm is shown to be second-order accurate in both space and time, capable of handling strongly nonlinear discontinuities, and a method of coupling it with an unsteady inflow condition is presented. Such an approach is sufficiently rapid to be integrated into a global, closed-loop model for computing the dynamics of coupled fluid compartments of the CNS.

Pulsatile flow drivers in brain parenchyma and perivascular spaces: a resistance network model study

Background

In animal models, dissolved compounds in the subarachnoid space and parenchyma have been found to preferentially transport through the cortex perivascular spaces (PVS) but the transport phenomena involved are unclear.

Methods

In this study two hydraulic network models were used to predict fluid motion produced by blood vessel pulsations and estimate the contribution made to solute transport in PVS and parenchyma. The effect of varying pulse amplitude and timing, PVS dimensions, and tissue hydraulic conductivity on fluid motion was investigated.

Results

Periodic vessel pulses resulted in oscillatory fluid motion in PVS and parenchyma but no net flow over time. For baseline parameters, PVS and parenchyma peak fluid velocity was on the order of 10 μm/s and 1 nm/s, with corresponding Peclet numbers below 10³ and 10⁻¹ respectively. Peak fluid velocity in the PVS and parenchyma tended to increase with increasing pulse amplitude and vessel size, and exhibited asymptotic relationships with hydraulic conductivity.

Conclusions

Solute transport in parenchyma was predicted to be diffusion dominated, with a negligible contribution from convection. In the PVS, dispersion due to oscillating flow likely plays a significant role in PVS rapid transport observed in previous in vivo experiments. This dispersive effect could be more significant than convective solute transport from net flow that may exist in PVS and should be studied further.

Computer simulation of syringomyelia in dogs

Background

Syringomyelia is a pathological condition in which fluid-filled cavities (syringes) form and expand in the spinal cord. Syringomyelia is often linked with obstruction of the craniocervical junction and a Chiari malformation, which is similar in both humans and animals. Some brachycephalic toy breed dogs such as Cavalier King Charles Spaniels (CKCS) are particularly predisposed. The exact mechanism of the formation of syringomyelia is undetermined and consequently with the lack of clinical explanation, engineers and mathematicians have resorted to computer models to identify possible physical mechanisms that can lead to syringes. We developed a computer model of the spinal cavity of a CKCS suffering from a large syrinx. The model was excited at the cranial end to simulate the movement of the cerebrospinal fluid (CSF) and the spinal cord due to the shift of blood volume in the cranium related to the cardiac cycle. To simulate the normal condition, the movement was prescribed to the CSF. To simulate the pathological condition, the movement of CSF was blocked.

Results

For normal conditions the pressure in the SAS was approximately 400 Pa and the same applied to all stress components in the spinal cord. The stress was uniformly distributed along the length of the spinal cord. When the blockage between the cranial and spinal CSF spaces forced the cord to move with the cardiac cycle, shear and axial normal stresses in the cord increased significantly. The sites where the elevated stress was most pronounced coincided with the axial locations where the syringes typically form, but they were at the perimeter rather than in the central portion of the cord. This elevated stress originated from the bending of the cord at the locations where its curvature was high.

Conclusions

The results suggest that it is possible that repetitive stressing of the spinal cord caused by its exaggerated movement could be a cause for the formation of initial syringes. Further consideration of factors such as cord tethering and the difference in mechanical properties of white and grey matter is needed to fully explore this possibility.

Electronic supplementary material

The online version of this article (10.1186/s12917-018-1410-7) contains supplementary material, which is available to authorized users.

A Poroelastic Fluid/Structure-Interaction Model of Cerebrospinal Fluid Dynamics in the Cord With Syringomyelia and Adjacent Subarachnoid-Space Stenosis

An existing axisymmetric fluid/structure-interaction (FSI) model of the spinal cord, pia mater, subarachnoid space, and dura mater in the presence of syringomyelia and subarachnoid-space stenosis was modified to include porous solids. This allowed investigation of a hypothesis for syrinx fluid ingress from cerebrospinal fluid (CSF). Gross model deformation was unchanged by the addition of porosity, but pressure oscillated more in the syrinx and the subarachnoid space below the stenosis. The poroelastic model still exhibited elevated mean pressure in the subarachnoid space below the stenosis and in the syrinx. With realistic cord permeability, there was slight oscillatory shunt flow bypassing the stenosis via the porous tissue over the syrinx. Weak steady streaming flow occurred in a circuit involving craniocaudal flow through the stenosis and back via the syrinx. Mean syrinx volume was scarcely altered when the adjacent stenosis bisected the syrinx, but increased slightly when the syrinx was predominantly located caudal to the stenosis. The fluid content of the tissues over the syrinx oscillated, absorbing most of the radial flow seeping from the subarachnoid space so that it did not reach the syrinx. To a lesser extent, this cyclic swelling in a boundary layer of cord tissue just below the pia occurred all along the cord, representing a mechanism for exchange of interstitial fluid (ISF) and cerebrospinal fluid which could explain recent tracer findings without invoking perivascular conduits. The model demonstrates that syrinx volume increase is possible when there is subarachnoid-space stenosis and the cord and pia are permeable.

The 3D characteristics of post-traumatic syringomyelia in a rat model: a propagation-based synchrotron radiation microtomography study

Many published literature sources have described the histopathological characteristics of post-traumatic syringomyelia (PTS). However, three-dimensional (3D) visualization studies of PTS have been limited due to the lack of reliable 3D imaging techniques. In this study, the imaging efficiency of propagation-based synchrotron radiation microtomography (PB-SRµCT) was determined to detect the 3D morphology of the cavity and surrounding microvasculature network in a rat model of PTS. The rat model of PTS was established using the infinite horizon impactor to produce spinal cord injury (SCI), followed by a subarachnoid injection of kaolin to produce arachnoiditis. PB-SRµCT imaging and histological examination, as well as fluorescence staining, were conducted on the animals at the tenth week after SCI. The 3D morphology of the cystic cavity was vividly visualized using PB-SRµCT imaging. The quantitative parameters analyzed by PB-SRµCT, including the lesion and spared spinal cord tissue area, the minimum and maximum diameters in the cystic cavity, and cavity volume, were largely consistent with the results of the histological assessment. Moreover, the 3D morphology of the cavity and surrounding angioarchitecture could be simultaneously detected on the PB-SRµCT images. This study demonstrated that high-resolution PB-SRµCT could be used for the 3D visualization of trauma-induced spinal cord cavities and provides valuable quantitative data for cavity characterization. PB-SRµCT could be used as a reliable imaging technique and offers a novel platform for tracking cavity formation and morphological changes in an experimental animal model of PTS.

Direct numerical simulation of transitional hydrodynamics of the cerebrospinal fluid in Chiari I malformation: The role of cranio-vertebral junction

Obstruction to the cerebrospinal fluid (CSF) outflow caused by the herniation of cerebellar tonsils as a result of Chiari malformation type I leads to altered CSF hydrodynamics. This contribution explores the minutest characteristics of the CSF hydrodynamics in cervical subarachnoid space (SAS) of a healthy subject and 2 Chiari patients by performing highly resolved direct numerical simulation. The lattice Boltzmann method is used for the simulations because of its scalability on modern supercomputers that allow us to simulate up to approximately 10⁹ cells while resolving the Kolmogorov microscales. The results depict that whereas the complex CSF flow remains largely laminar in the SAS of a healthy subject, constriction of the cranio-vertebral junction in Chiari I patients causes manifold fluctuations in the hydrodynamics of the CSF. These fluctuations resemble a flow that is in a transitional regime rather than laminar or fully developed turbulence. The fluctuations confine near the cranio-vertebral junction and are triggered due to the tonsillar herniation, which perturbs the flow as a result of altered anatomy of the SAS.

Characteristics of CSF Velocity-Time Profile in Posttraumatic Syringomyelia

BACKGROUND AND PURPOSE

The development of syringomyelia has been associated with changes in CSF flow dynamics in the spinal subarachnoid space. However, differences in CSF flow velocity between patients with posttraumatic syringomyelia and healthy participants remains unclear. The aim of this work was to define differences in CSF flow above and below a syrinx in participants with posttraumatic syringomyelia and compare the CSF flow with that in healthy controls.

MATERIALS AND METHODS

Six participants with posttraumatic syringomyelia were recruited for this study. Phase-contrast MR imaging was used to measure CSF flow velocity at the base of the skull and above and below the syrinx. Velocity magnitudes and temporal features of the CSF velocity profile were compared with those in healthy controls.

RESULTS

CSF flow velocity in the spinal subarachnoid space of participants with syringomyelia was similar at different locations despite differences in syrinx size and locations. Peak cranial and caudal velocities above and below the syrinx were not significantly different (peak cranial velocity, P = .9; peak caudal velocity, P = 1.0), but the peak velocities were significantly lower (P < .001, P = .007) in the participants with syringomyelia compared with matched controls. Most notably, the duration of caudal flow was significantly shorter (P = .003) in the participants with syringomyelia.

CONCLUSIONS

CSF flow within the posttraumatic syringomyelia group was relatively uniform along the spinal canal, but there are differences in the timing of CSF flow compared with that in matched healthy controls. This finding supports the hypothesis that syrinx development may be associated with temporal changes in spinal CSF flow.

Fluorescence imaging of lymphatic outflow of cerebrospinal fluid in mice

Cerebrospinal fluid (CSF) is known to be reabsorbed by the lymphatic vessels and drain into the lymph nodes (LNs) through peripheral lymphatic vessels. In the peripheral lymphatics, the contractile pumping action of lymphangions mediates lymph drainage; yet it is unknown whether lymphatic vessels draining cranial and spinal CSF show similar function. Herein, we used non-invasive near-infrared fluorescence imaging (NIRFI) to image (i) indocyanine green (ICG) distribution along the neuraxis and (ii) routes of ICG-laden CSF outflow into the lymphatics following intrathecal lumbar administration. We demonstrate lymphatic contractile function in peripheral lymphatics draining from the nasal lymphatics to the mandibular LNs. In addition, we observed afferent sciatic lymphatic vessels, which also show contractile activity and transport spinal CSF into the sciatic LNs. This drainage pattern was also visualized by NIRFI following intrathecal thoracic injection. In situ intravital imaging following intrathecal lumbar injection of blue dye shows similar distributions to that seen in vivo with ICG. NIRFI could be used as a tool to probe CSF pathology including neurological disorders by imaging CSF outflow dynamics to lymphatics.

Aquaporins in the Spinal Cord

Aquaporins (AQPs) are water channel proteins robustly expressed in the central nervous system (CNS). A number of previous studies described the cellular expression sites and investigated their major roles and function in the brain and spinal cord. Among thirteen different mammalian AQPs, AQP1 and AQP4 have been mainly studied in the CNS and evidence has been presented that they play important roles in the pathogenesis of CNS injury, edema and multiple diseases such as multiple sclerosis, neuromyelitis optica spectrum disorders, amyotrophic lateral sclerosis, glioblastoma multiforme, Alzheimer's disease and Parkinson's disease. The objective of this review is to highlight the current knowledge about AQPs in the spinal cord and their proposed roles in pathophysiology and pathogenesis related to spinal cord lesions and injury.

Computational Investigation of Cerebrospinal Fluid Dynamics in the Posterior Cranial Fossa and Cervical Subarachnoid Space in Patients with Chiari I Malformation

Purpose

Previous computational fluid dynamics (CFD) studies have demonstrated that the Chiari malformation is associated with abnormal cerebrospinal fluid (CSF) flow in the cervical part of the subarachnoid space (SAS), but the flow in the SAS of the posterior cranial fossa has received little attention. This study extends previous modelling efforts by including the cerebellomedullary cistern, pontine cistern, and 4th ventricle in addition to the cervical subarachnoid space.

Methods

The study included one healthy control, Con1, and two patients with Chiari I malformation, P1 and P2. Meshes were constructed by segmenting images obtained from T2-weighted turbo spin-echo sequences. CFD simulations were performed with a previously verified and validated code. Patient-specific flow conditions in the aqueduct and the cervical SAS were used. Two patients with the Chiari malformation and one control were modelled.

Results

The results demonstrated increased maximal flow velocities in the Chiari patients, ranging from factor 5 in P1 to 14.8 in P2, when compared to Con1 at the level of Foramen Magnum (FM). Maximal velocities in the cervical SAS varied by a factor 2.3, while the maximal flow in the aqueduct varied by a factor 3.5. The pressure drop from the pontine cistern to the cervical SAS was similar in Con1 and P1, but a factor two higher in P2. The pressure drop between the aqueduct and the cervical SAS varied by a factor 9.4 where P1 was the one with the lowest pressure jump and P2 and Con1 differed only by a factor 1.6.

Conclusion

This pilot study demonstrates that including the posterior cranial fossa is feasible and suggests that previously found flow differences between Chiari I patients and healthy individuals in the cervical SAS may be present also in the SAS of the posterior cranial fossa.

Research into the Physiology of Cerebrospinal Fluid Reaches a New Horizon: Intimate Exchange between Cerebrospinal Fluid and Interstitial Fluid May Contribute to Maintenance of Homeostasis in the Central Nervous System

Cerebrospinal fluid (CSF) plays an essential role in maintaining the homeostasis of the central nervous system. The functions of CSF include: (1) buoyancy of the brain, spinal cord, and nerves; (2) volume adjustment in the cranial cavity; (3) nutrient transport; (4) protein or peptide transport; (5) brain volume regulation through osmoregulation; (6) buffering effect against external forces; (7) signal transduction; (8) drug transport; (9) immune system control; (10) elimination of metabolites and unnecessary substances; and finally (11) cooling of heat generated by neural activity. For CSF to fully mediate these functions, fluid-like movement in the ventricles and subarachnoid space is necessary. Furthermore, the relationship between the behaviors of CSF and interstitial fluid in the brain and spinal cord is important. In this review, we will present classical studies on CSF circulation from its discovery over 2,000 years ago, and will subsequently introduce functions that were recently discovered such as CSF production and absorption, water molecule movement in the interstitial space, exchange between interstitial fluid and CSF, and drainage of CSF and interstitial fluid into both the venous and the lymphatic systems. Finally, we will summarize future challenges in research. This review includes articles published up to February 2016.

Novel method for dynamic control of intracranial pressure

OBJECT Intracranial pressure (ICP) pulsations are generally considered a passive result of the pulsatility of blood flow. Active experimental modification of ICP pulsations would allow investigation of potential active effects on blood and CSF flow and potentially create a new platform for the treatment of acute and chronic low blood flow states as well as a method of CSF substance clearance and delivery. This study presents a novel method and device for altering the ICP waveform via cardiac-gated volume changes. METHODS The novel device used in this experiment (named Cadence) consists of a small air-filled inelastic balloon (approximately 1.0 ml) implanted into the intracranial space and connected to an external programmable pump, triggered by an R-wave detector. Balloons were implanted into the epidural space above 1 of the hemispheres of 19 canines for up to 10 hours. When activated, the balloons were programed to cyclically inflate with the cardiac cycle with variable delay, phase, and volume. The ICP response was measured in both hemispheres. Additionally, cerebral blood flow (heat diffusion and laser Doppler) was studied in 16 canines. RESULTS This system, depending on the inflation pattern of the balloon, allowed a flattening of the ICP waveform, increase in the ICP waveform amplitude, or phase shift of the wave. This occurred with small mean ICP changes, typically around ± 2 mm Hg (15%). Bilateral ICP effects were observed with activation of the device: balloon inflation at each systole increased the systolic ICP pulse (up to 16 mm Hg, 1200%) and deflation at systole decreased or even inverted the systolic ICP pulse (-0.5 to -19 mm Hg, -5% to -1600%) in a dose-(balloon volume) dependent fashion. No aphysiological or deleterious effects on systemic pressure (≤ ±10 mm Hg; 13% change in mean pressure) or cardiac rate (≤ ± 17 beats per minute; 16% change) were observed during up to 4 hours of balloon activity. CONCLUSIONS The results of these initial studies using an intracranially implanted, cardiac-gated, volume-oscillating balloon suggest the Cadence device can be used to modify ICP pulsations, without physiologically deleterious effects on mean ICP, systemic vascular effects, or brain injury. This device and technique may be used to study the role of ICP pulsatility in intracranial hemo- and hydrodynamic processes and introduces the creation of a potential platform of a cardiac-gated system for treatment of acute and chronic low blood flow states, and diseases requiring augmentation of CSF substance clearance or delivery.

Lymphatic Clearance of the Brain: Perivascular, Paravascular and Significance for Neurodegenerative Diseases

The lymphatic clearance pathways of the brain are different compared to the other organs of the body and have been the subject of heated debates. Drainage of brain extracellular fluids, particularly interstitial fluid (ISF) and cerebrospinal fluid (CSF), is not only important for volume regulation, but also for removal of waste products such as amyloid beta (Aβ). CSF plays a special role in clinical medicine, as it is available for analysis of biomarkers for Alzheimer's disease. Despite the lack of a complete anatomical and physiological picture of the communications between the subarachnoid space (SAS) and the brain parenchyma, it is often assumed that Aβ is cleared from the cerebral ISF into the CSF. Recent work suggests that clearance of the brain mainly occurs during sleep, with a specific role for peri- and para-vascular spaces as drainage pathways from the brain parenchyma. However, the direction of flow, the anatomical structures involved and the driving forces remain elusive, with partially conflicting data in literature. The presence of Aβ in the glia limitans in Alzheimer's disease suggests a direct communication of ISF with CSF. Nonetheless, there is also the well-described pathology of cerebral amyloid angiopathy associated with the failure of perivascular drainage of Aβ. Herein, we review the role of the vasculature and the impact of vascular pathology on the peri- and para-vascular clearance pathways of the brain. The different views on the possible routes for ISF drainage of the brain are discussed in the context of pathological significance.

Direct-trauma model of posttraumatic syringomyelia with a computer-controlled motorized spinal cord impactor

OBJECTIVE The pathogenesis of posttraumatic syringomyelia remains enigmatic and is not adequately explained by current theories. Experimental investigations require a reproducible animal model that replicates the human condition. Current animal models are imperfect because of their low reliability, severe neurological deficits, or dissimilar mechanism of injury. The objective of this study was to develop a reproducible rodent model of posttraumatic syringomyelia using a spinal cord impactor that produces an injury that more closely mimics the human condition and does not produce severe neurological deficits. METHODS The study consisted of 2 parts. Seventy animals were studied overall: 20 in Experiment 1 and 48 in Experiment 2 after two rats with severe deficits were killed early. Experiment 1 aimed to determine the optimal force setting for inducing a cystic cavity without neurological deficits using a computer-controlled motorized spinal cord impactor. Twenty animals received an impact that ranged from 50 to 150 kDyn. Using the optimal force for producing an initial cyst determined from Experiment 1, Experiment 2 aimed to compare the progression of cavities in animals with and those without arachnoiditis induced by kaolin. Forty-eight animals were killed at 1, 3, 6, or 12 weeks after syrinx induction. Measurements of cavity size and maximum anteroposterior and lateral diameters were evaluated using light microscopy. RESULTS In Experiment 1, cavities were present in 95% of the animals. The duration of limb weakness and spinal cord cavity size correlated with the delivered force. The optimal force chosen for Experiment 2 was 75 kDyn. In Experiment 2, cavities occurred in 92% of the animals. Animals in the kaolin groups developed larger cavities and more vacuolations and enlarged perivascular spaces than those in the nonkaolin groups. CONCLUSIONS This impact model reliably produces cavities that resemble human posttraumatic syringomyelia and is suitable for further study of posttraumatic syringomyelia pathophysiology.

Inwardly rectifying potassium channel 4.1 expression in post-traumatic syringomyelia

Post-traumatic syringomyelia (PTS) is a serious neurological disorder characterized by fluid filled cavities that develop in the spinal cord. PTS is thought to be caused by an imbalance between fluid inflow and outflow in the spinal cord, but the underlying mechanisms are unknown. The ion channel Kir4.1 plays an important role in the uptake of K(+) ions from the extracellular space and release of K(+) ions into the microvasculature, generating an osmotic gradient that drives water movement. Changes in Kir4.1 expression may contribute to disturbances in K(+) homeostasis and subsequently fluid imbalance. Here we investigated whether changes in Kir4.1 protein expression occur in PTS. Western blotting and immunohistochemistry were used to evaluate Kir4.1 and glial fibrillary acidic protein (GFAP) expression in a rodent model of PTS at 3 days, 1, 6 or 12 weeks post-surgery. In Western blotting experiments, Kir4.1 expression increased 1 week post-surgery at the level of the cavity. Immunohistochemical analysis examined changes in the spinal parenchyma directly in contact with the syrinx cavity. In these experiments, there was a significant decrease in Kir4.1 expression in PTS animals compared to controls at 3 days and 6 weeks post-surgery, while an up-regulation of GFAP in PTS animals was observed at 1 and 12 weeks. This suggests that while overall Kir4.1 expression is unchanged at these time-points, there are many astrocytes surrounding the syrinx cavity that are not expressing Kir4.1. The results demonstrate a disturbance in the removal of K(+) ions in tissue surrounding a post-traumatic syrinx cavity. It is possible this contributes to water accumulation in the injured spinal cord leading to syrinx formation or exacerbation of the underlying pathology.