Does peripheral nerve degeneration affect circulatory responses to head-up tilt in spinal cord-injured individuals?

Responses to Valsalva's maneuver in spinal cord injury do not broadly relate to vasoconstrictor capacity

PURPOSE

A blood pressure stabilization during late phase II of Valsalva's maneuver may be utilized to confirm sympathetic vasoconstrictor control after a spinal cord injury. This study investigated whether Valsalva response was predictive of hemodynamics during tilt or isometric handgrip.

METHODS

Presence/absence of Valsalva response was compared to heart rate, mean arterial pressure, leg blood flow, and vascular resistance during head-up tilt and isometric handgrip to fatigue in 14 adults with spinal cord injury from C7 to T12 and 14 controls. Statistics were performed with two-way repeated measure analysis of variance (ANOVA), post hoc t-tests for between-group comparisons, and Mann-Whitney U tests for within-group.

RESULTS

In total, six participants with spinal cord injury lacked a blood pressure stabilization for Valsalva's maneuver. However, this was not related to vasoconstrictor responses during the other tests. The groups had similar heart rate and blood pressure changes during tilt, though leg blood flow decreases and vascular resistance increases tended to be smaller at 20° tilt in those with spinal cord injury (p = 0.07 and p = 0.11, respectively). Participants with spinal cord injury had lower heart rates and markedly smaller blood pressure increases during handgrip (both p < 0.05). There were no group differences in leg blood flow, but those with spinal cord injury demonstrated a blunted vascular resistance increase by the final 10% of the handgrip (p < 0.01).

CONCLUSIONS

Valsalva response was not consistent with hemodynamics during other stimuli, but some individuals evidence increases in sub-lesional vascular resistance to isometric handgrip comparable to controls, suggesting a sympathoexcitatory stimulus may be critical to provoke hemodynamic responses after spinal cord injury.

Position- and posture-dependent vascular imaging-a scoping review

OBJECTIVES

Position- and posture-dependent deformation of the vascular system is a relatively unexplored field. The goal of this scoping review was to create an overview of existing vascular imaging modalities in different body positions and postures and address the subsequent changes in vascular anatomy.

METHODS

Scopus, Medline, and Cochrane were searched for literature published between January 1, 2000, and June 30, 2022, incorporating the following categories: image modality, anatomy, orientation, and outcomes.

RESULTS

Out of 2446 screened articles, we included 108. The majority of papers used ultrasound (US, n = 74) in different body positions and postures with diameter and cross-sectional area (CSA) as outcome measures. Magnetic resonance imaging (n = 22) and computed tomography (n = 8) were less frequently used but allowed for investigation of other geometrical measures such as vessel curvature and length. The venous system proved more sensitive to postural changes than the arterial system, which was seen as increasing diameters of veins below the level of the heart when going from supine to prone to standing positions, and vice versa.

CONCLUSIONS

The influence of body positions and postures on vasculature was predominantly explored with US for vessel diameter and CSA. Posture-induced deformation and additional geometrical features that may be of interest for the (endovascular) treatment of vascular pathologies have been limitedly reported, such as length and curvature of an atherosclerotic popliteal artery during bending of the knee after stent placement. The most important clinical implications of positional changes are found in diagnosis, surgical planning, and follow-up after stent placement.

CLINICAL RELEVANCE STATEMENT

This scoping review presents the current state and opportunities of position- and posture-dependent imaging of vascular structures using various imaging modalities that are relevant in the fields of clinical diagnosis, surgical planning, and follow-up after stent placement.

KEY POINTS

• The influence of body positions and postures on the vasculature was predominantly investigated with US for vessel diameter and cross-sectional area. • Research into geometrical deformation, such as vessel length and curvature adaptation, that may be of interest for the (endovascular) treatment of vascular pathologies is limited in different positions and postures. • The most important clinical implications of postural changes are found in diagnosis, surgical planning, and follow-up after stent placement.

Characteristics of cardiovascular responses to an orthostatic challenge in trained spinal cord-injured individuals

Background

We investigated cardiovascular responses to an orthostatic challenge in trained spinal cord-injured (SCI) individuals compared to able-bodied (AB) individuals.

Methods

A total of 23 subjects participated, divided into three groups: seven were trained as spinal cord-injured (Tr-SCI) individuals, seven were able-bodied individuals trained as runners (Tr-AB), and nine were untrained able-bodied individuals (UnTr-AB). We measured the cardiovascular autonomic responses in all three groups during each 5-min head-up tilt (HUT) of 0°, 40°, and 80°. Stroke volume (SV), heart rate (HR), and cardiac output (Qc) as cardiovascular responses were measured by impedance cardiography. Changes in deoxyhemoglobin (∆[HHb]) and total hemoglobin (∆[Hb_tot]) concentrations of the right medial gastrocnemius muscle were measured using near-infrared spectroscopy (NIRS).

Results

As the HUT increased from 0° to 80°, Tr-SCI group showed less change in SV at all HUT levels even if HR increased significantly. Mean arterial pressure (MAP) also did not significantly increase as tilting increased from 0° to 80°. Regarding peripheral vascular responses, the alterations of ∆[Hb_tot] from 0° to 80° were less in Tr-SCI group compared to AB individuals.

Conclusion

There is a specific mechanism whereby blood pressure is maintained during a HUT in Tr-SCI group with the elicitation of peripheral vasoconstriction and the atrophy of the vascular vessels in paraplegic lower limbs, which would be associated with less change in SV in response to an orthostatic challenge.

Changes in sympathetic neurovascular function following spinal cord injury

The effects of spinal cord injury (SCI) on sympathetic neurovascular transmission have generally been ignored. This review describes changes in sympathetic nerve-mediated activation of arterial vessels to which ongoing sympathetic activity has been reduced or silenced following spinal cord transection in rats. In all vessels studied in rats, SCI markedly enhanced their contractile responses to nerve activity. However, the mechanisms that augment neurovascular transmission differ between the rat tail artery and mesenteric artery. In tail artery, the enhancement of neurovascular transmission cannot be attributed to changes in sensitivity of the vascular muscle to α₁- or α₂-adrenoceptor agonists. Instead the contribution of L-type Ca²⁺ channels to activation of the smooth muscle by nerve-released noradrenaline is greatly increased following SCI. By contrast, mesenteric arteries from SCI rats had increased sensitivity to phenylephrine but not to methoxamine. While both phenylephrine and methoxamine are α₁-adrenoceptor agonists, only phenylephrine is a substrate for the neuronal noradrenaline transporter. Therefore the selective increase in sensitivity to phenylephrine suggests that the activity of the neuronal noradrenaline transporter is reduced. While present evidence suggests that sympathetic vasoconstrictor neurons do not contribute to the normal regulation of peripheral resistance below a complete SCI in humans, the available evidence does indicate that these experimental findings in animals are likely to apply after SCI in humans and contribute to autonomic dysreflexia.

Asymptomatic lower extremity acrocyanosis: report of two cases and review of the literature

Vasospastic disorders affecting the lower extremities are unusual entities. Two cases of asymptomatic foot discoloration with lower extremity dependency are presented. In the first case, an elderly gentleman was admitted to the hospital with a contralateral foot infection. His physical exam revealed pulse examination within normal limits and his blood work failed to identify any known causes for vasospasm. His foot infection was successfully treated with sensitivity-directed antibiotics. The second case is that of a young man with a history of quadriplegia secondary to a remote diving accident, who presented with a three-month history of foot discoloration while sitting up in his wheelchair, which resolved with recumbent leg positioning. A review of the possible diagnoses for these patients' presentation is presented herein.

Leg vasoconstriction during head-up tilt in patients with autonomic failure is not abolished

Maintaining blood pressure during orthostatic challenges is primarily achieved by baroreceptor-mediated activation of the sympathetic nervous system, which can be divided into preganglionic and postganglionic parts. Despite their preganglionic autonomic failure, spinal cord-injured individuals demonstrate a preserved peripheral vasoconstriction during orthostatic challenges. Whether this also applies to patients with postganglionic autonomic failure is unknown. Therefore, we assessed leg vasoconstriction during 60° head-up tilt in five patients with pure autonomic failure (PAF) and two patients with autonomic failure due to dopamine-β-hydroxylase (DBH) deficiency. Ten healthy subjects served as controls. Leg blood flow was measured using duplex ultrasound in the right superficial femoral artery. Leg vascular resistance was calculated as the arterial-venous pressure gradient divided by blood flow. DBH-deficient patients were tested off and on the norepinephrine pro-drug l-threo-dihydroxyphenylserine (l-DOPS). During 60° head-up tilt, leg vascular resistance increased significantly in PAF patients [0.40 ± 0.38 (+30%) mmHg·ml−1·min−1]. The increase in leg vascular resistance was not significantly different from controls [0.88 ± 1.04 (+72%) mmHg·ml−1·min−1]. In DBH-deficient patients, leg vascular resistance increased by 0.49 ± 0.01 (+153%) and 1.52 ± 1.47 (+234%) mmHg·ml−1·min−1 off and on l-DOPS, respectively. Despite the increase in leg vascular resistance, orthostatic hypotension was present in PAF and DBH-deficient patients. Our results demonstrate that leg vasoconstriction during orthostatic challenges in patients with PAF or DBH deficiency is not abolished. This indicates that the sympathetic nervous system is not the sole or pivotal mechanism inducing leg vasoconstriction during orthostatic challenges. Additional vasoconstrictor mechanisms may compensate for the loss in sympathetic nervous system control.

The isolated sympathetic spinal cord: Cardiovascular and sudomotor assessment in spinal cord injury patients: A literature survey

OBJECTIVES

To present a comprehensive approach to the assessment of the severity of the autonomic lesion in spinal cord injury (SCI) patients, with regard to the level of lesion. To discuss how to assess an isolated sympathetic spinal cord that has lost supraspinal control (sympathetically complete lesion).

METHOD

PubMed was searched for articles related to cardiovascular (mainly cold pressor test, respiratory and postural challenges) and sudomotor (sympathetic skin responses) tests that have been used. The results of these evaluations are analysed with regard to the site of stimulation (above or below the lesion) according to three types of SCI that offer typical autonomic reactions (tetraplegics, paraplegics at T6 and at T10).

RESULTS

Non-invasive cardiovascular and sudomotor testing allows the assessment of the isolated sympathetic spinal cord in SCI patients. Typical responses are found in relation with the level of the sympathetic lesion. Its definition would allow comparison with the somatic motor and sensory level of lesion of SCI patients and provide additional aid to the classification of those patients.

CONCLUSION

For research purposes on the integrity of the spinal sympathetic pathways, a battery of test approach is probably needed, using a combination of stimuli above and below the lesion, evaluating both cardiovascular and sudomotor pathways.

The role of the alpha-adrenergic receptor in the leg vasoconstrictor response to orthostatic stress

AIM

The prompt increase in peripheral vascular resistance, mediated by sympathetic alpha-adrenergic stimulation, is believed to be the key event in blood pressure control during postural stress. However, despite the absence of central sympathetic control of the leg vasculature, postural leg vasoconstriction is preserved in spinal cord-injured individuals (SCI). This study aimed at assessing the contribution of both central and local sympathetically induced alpha-adrenergic leg vasoconstriction to head-up tilt (HUT) by including healthy individuals and SCI, who lack central sympathetic baroreflex control over the leg vascular bed.

METHODS

In 10 controls and nine SCI the femoral artery was cannulated for drug infusion. Upper leg blood flow (LBF) was measured bilaterally using venous occlusion strain gauge plethysmography before and during 30 degrees HUT throughout intra-arterial infusion of saline or the non-selective alpha-adrenergic receptor antagonist phentolamine respectively. Additionally, in six controls the leg vascular response to the cold pressor test was assessed during continued infusion of phentolamine, in order to confirm complete alpha-adrenergic blockade by phentolamine.

RESULTS

During infusion of phentolamine HUT still caused vasoconstriction in both groups: leg vascular resistance (mean arterial pressure/LBF) increased by 10 +/- 2 AU (compared with 12 +/- 2 AU during saline infusion), and 13 +/- 3 AU (compared with 7 +/- 3 AU during saline infusion) in controls and SCI respectively.

CONCLUSION

Effective alpha-adrenergic blockade did not reduce HUT-induced vasoconstriction, regardless of intact baroreflex control of the leg vasculature. Apparently, redundant mechanisms compensate for the absence of sympathetic alpha-adrenoceptor leg vasoconstriction in response to postural stress.

Sympathetic vasomotor control does not explain the change in femoral artery shear rate pattern during arm-crank exercise

During lower limb exercise, blood flow through the resting upper limbs exhibits a change characterized by increased anterograde flow during systole, but also large increases in retrograde diastolic flow. One explanation for the retrograde flow is that increased sympathetic nervous system (SNS) tone and concomitant increased peripheral resistance generate a rebound during diastole. To examine whether the SNS contributes to retrograde flow patterns, we measured femoral artery blood flow during arm-crank exercise in 10 healthy men (31 ± 4 yr) and 10 spinal cord-injured (SCI) subjects who lack sympathetic innervation in the legs (33 ± 5 yr). Before, and every 5 min during 25-min arm-crank exercise at 50% maximal capacity, femoral artery blood flow and peak anterograde and retrograde shear rate were assessed using echo Doppler sonography. Femoral artery baseline blood flow was significantly lower in SCI compared with controls. Exercise increased femoral artery blood flow in both groups (ANOVA, P < 0.05), whereas leg vascular conductance did not change during exercise in either group. Mean shear rate was lower in SCI than in controls ( P < 0.05). Peak anterograde shear rate was higher in SCI than in controls ( P < 0.05), whereas peak retrograde shear rate did not differ between groups. Arm-crank exercise induced an increase in peak anterograde and retrograde shear rate in the femoral artery in controls and SCI subjects ( P < 0.05). This suggests that the SNS is not obligatory to change the flow pattern in inactive regions during exercise. Local mechanisms may play a role in the arm-crank exercise-induced changes in flow pattern in the femoral artery.

Venous response to orthostatic stress

Head-up tilt (HUT) induces a reduction in preload, which is thought to be restored through sympathetic venoconstriction, reducing unstressed volume (V(u)) and venous compliance (VeC). In this study, we assessed venous inflow and outflow responses and their reproducibility and determined the relation with autonomic function during HUT. Eight healthy non-pregnant women were subjected to 20 degrees head-down tilt to 60 degrees HUT at 20 degrees intervals. At each rotational step, we randomly assessed forearm pressure-volume (P-V) curves (venous occlusion plethysmography) during inflow (VeC(IN)) and outflow [venous emptying rate (VER(OUT))]. VeC(IN) was defined as the ratio of the slope of the volume-time curve and pressure-time curve, with direct intravenous pressure measurement. VER(OUT) was determined using the derivate of a quadratic regression model using cuff pressure. We defined V(u) as the y-intercept of the P-V curve. We calculated, for both methods, the coefficients of reproducibility (CR) and variation (CV). Vascular sympathetic activity was determined by spectral analysis. VeC(IN) decreased at each rotational step compared with the supine position (P<0.05), whereas VER(OUT) increased. CR of VeC(IN) was higher in the supine position than VER(OUT) but lower during HUT. CV varied between 19% and 25% (VeC(IN)) and between 12% and 21% (VER(OUT)). HUT decreased V(u). The change in VeC(IN) and VER(OUT) correlated with the change in vascular sympathetic activity (r= -0.36, P<0.01, and r=0.48, P<0.01). This is the first study in which a reproducible reduction in VeC(IN) and V(u) and a rise in VER(OUT) during HUT are documented. The alterations in venous characteristics relate to changes in vascular sympathetic activity.

Leg intravenous pressure during head-up tilt

Leg vascular resistance is calculated as the arterial-venous pressure gradient divided by blood flow. During orthostatic challenges it is assumed that the hydrostatic pressure contributes equally to leg arterial, as well as to leg venous pressure. Because of venous valves, one may question whether, during orthostatic challenges, a continuous hydrostatic column is formed and if leg venous pressure is equal to the hydrostatic pressure. The purpose of this study was, therefore, to measure intravenous pressure in the great saphenous vein of 12 healthy individuals during 30 degrees and 70 degrees head-up tilt and compare this with the calculated hydrostatic pressure. The height difference between the heart and the right medial malleolus level represented the hydrostatic column. The results demonstrate that there were no differences between the measured intravenous pressure and the calculated hydrostatic pressure during 30 degrees (47.2 +/- 1.0 and 46.9 +/- 1.5 mmHg, respectively) and 70 degrees head-up tilt (83.9 +/- 0.9 and 85.1 +/- 1.2 mmHg, respectively). Steady-state levels of intravenous pressure were reached after 95 +/- 12 s during 30 degrees and 161 +/- 15 s during 70 degrees head-up tilt. In conclusion, the measured leg venous pressure is similar to the calculated hydrostatic pressure during orthostatic challenges. Therefore, the assumption that hydrostatic pressure contributes equally to leg arterial as well as to leg venous pressure during orthostatic challenges can be made.

Vascular mechanism of axonal degeneration in peripheral nerves in hemiplegic sides after cerebral hemorrhage: An experimental study

Background

Though retrograde neuronal death and vascular insufficiency have been well established in plegics following intracerebral hemorrhage, the effects of plegia on arterial nervorums of peripheral nerves have not been reported. In this study, the histopathological effects of the intracerebral hemorrhage on the dorsal root ganglions and sciatic nerves via affecting the arterial nervorums were investigated.

Methods

This study was conducted on 13 male hybrid rabbits. Three animals were taken as control group and did not undergo surgery. The remaining 10 subjects were anesthetized and were injected with 0.50 ml of autologous blood into their right sensory-motor region. All rabbits were followed-up for two months and then sacrificed. Endothelial cell numbers and volume values were estimated a three dimensionally created standardized arterial nervorums model of lumbar 3. Neuron numbers of dorsal root ganglions, and axon numbers in the lumbar 3 nerve root and volume values of arterial nervorums were examined histopathologically. The results were analyzed by using a Mann-Whitney-U test.

Results

Left hemiplegia developed in 8 animals. On the hemiplegic side, degenerative vascular changes and volume reduction in the arterial nervorums of the sciatic nerves, neuronal injury in the dorsal root ganglions, and axonal injury in the lumbar 3 were detected. Statistical analyses showed a significant correlation between the normal or nonplegic sides and plegic sides in terms of the neurodegeneration in the dorsal root ganglions (p < 0.005), axonal degeneration in the lumbar 3 nerve roots (p < 0.005), endothelial cell degeneration in the arterial nervorums (p < 0.001), and volume reduction in the arterial nervorums (p < 0.001).

Conclusion

Intracerebral hemorrhage resulted in neurodegeneration in the dorsal root ganglion and axonolysis in the sciatic nerves, endothelial injury, and volume reduction of the arterial nervorums in the sciatic nerves. The interruption of the neural network connection in the walls of the arterial nervorums in the sciatic nerves may be responsible for circulation disorders of the arterial nervorums, and arterial nervorums degeneration could result in sciatic nerves injury.

Local vasoconstriction in spinal cord-injured and able-bodied individuals

Local vasoconstriction plays an important role in maintaining blood pressure in spinal cord-injured individuals (SCI). We aimed to unravel the mechanisms of local vasoconstriction [venoarteriolar reflex (VAR) and myogenic response] using both limb dependency and cuff inflation in SCI and compare these with control subjects. Limb blood flow was measured in 11 male SCI (age: 24–55 yr old) and 9 male controls (age: 23–56 yr old) using venous occlusion plethysmography in forearm and calf during three levels of 1) limb dependency, and 2) cuff inflation. During limb dependency, vasoconstriction relies on both the VAR and the myogenic response. During cuff inflation, the decrease in blood flow is caused by the VAR and by a decrease in arteriovenous pressure difference, whereas the myogenic response does not play a role. At the highest level of leg dependency, the percent increase in calf vascular resistance (mean arterial pressure/calf blood flow) was more pronounced in SCI than in controls (SCI 186 ± 53%; controls 51 ± 17%; P = 0.032). In contrast, during cuff inflation, no differences were found between SCI and controls (SCI 17 ± 17%; controls 14 ± 10%). Percent changes in forearm vascular resistance in response to either forearm dependency or forearm cuff inflation were equal in both groups. Thus local vasoconstriction during dependency of the paralyzed leg in SCI is enhanced. The contribution of the VAR to local vasoconstriction does not differ between the groups, since no differences between groups existed for cuff inflation. Therefore, the augmented local vasoconstriction in SCI during leg dependency relies, most likely, on the myogenic response.

Orthostatic hypotension and autonomic pathways after spinal cord injury

Individuals with spinal cord injury (SCI) are prone to orthostatic hypotension (OH). We aimed to develop a simple bedside test to evaluate autonomic control following chronic SCI, and to identify those most at risk of OH and cardiovascular dysfunction. We studied 14 subjects with cervical SCI, 11 with thoracic SCI, and 17 able-bodied controls. We continuously recorded heart rate (HR; ECG) and beat-to-beat systolic (SAP), diastolic (DAP) and mean (MAP) arterial pressures (Finometer) while supine, and following the passive assumption of an upright seated position. Stroke volume (SV), cardiac output (CO), and total peripheral resistance (TPR) were calculated. Plasma catecholamines were determined. Motor and sensory loss was assessed using the American Spinal Injury Association (ASIA) impairment scale. Autonomic pathways were assessed from sympathetic skin responses (SSR). Cervical SCI subjects had lower supine HR, SAP, and noradrenaline levels than thoracic SCI and controls (p < 0.05), and lower DAP and MAP than controls (p < 0.05). When upright, HR increased in all groups (p < 0.05); SAP, DAP, and MAP increased (p < 0.01) in thoracic SCI and controls, but not in cervical SCI. Cervical SCI had larger postural falls in SV (p < 0.05) and CO, with smaller increases in TPR than the other two groups. Upright catecholamine levels were lower in cervical SCI (p < 0.05) than thoracic SCI and controls. Completeness of SCI assessed by ASIA scale did not necessarily correlate with autonomic completeness assessed by SSR. Cardiovascular control during orthostasis was impaired and OH was common in cervical SCI, but not thoracic SCI. SSR may identify those at greatest risk of orthostatic hypotension and impaired cardiovascular control. We advocate that assessments of autonomic function be included in the neurological evaluation of SCI, in addition to the ASIA assessment.

Leg blood flow measurements using venous occlusion plethysmography during head-up tilt

We tested whether venous occlusion plethysmography (VOP) is an appropriate method to measure calf blood flow (CBF) during head-up tilt (HUT). CBF measured with VOP was compared with superficial femoral artery blood flow as measured by Doppler ultrasound during incremental tilt angles. Measurements of both methods correlated well (r = 0.86). Reproducibility of VOP was fair in supine position and 30° HUT (CV: 11%–15%). This indicates that VOP is an applicable tool to measure leg blood flow during HUT, especially up to 30° HUT.