Ventilatory and cardiovascular responses to hypercapnia and hypoxia in multiple-system atrophy

Cerebrovascular Reactivity Following Spinal Cord Injury

Background

Spinal cord injuries (SCI) often result in cardiovascular issues, increasing the risk of stroke and cognitive deficits.

Objectives

This study assessed cerebrovascular reactivity (CVR) using functional magnetic resonance imaging (fMRI) during a hypercapnic challenge in SCI participants compared to noninjured controls.

Methods

Fourteen participants were analyzed (n = 8 with SCI [unless otherwise noted], median age = 44 years; n = 6 controls, median age = 33 years). CVR was calculated through fMRI signal changes.

Results

The results showed a longer CVR component (tau) in the grey matter of SCI participants (n = 7) compared to controls (median difference = 3.0 s; p < .05). Time since injury (TSI) correlated negatively with steady-state CVR in the grey matter and brainstem of SCI participants (RS = -0.81, p = .014; RS = -0.84, p = .009, respectively). Lower steady-state CVR in the brainstem of the SCI group (n = 7) correlated with lower diastolic blood pressure (RS = 0.76, p = .046). Higher frequency of hypotensive episodes (n = 7) was linked to lower CVR outcomes in the grey matter (RS = -0.86, p = .014) and brainstem (RS = -0.89, p = .007).

Conclusion

Preliminary findings suggest a difference in the dynamic CVR component, tau, between the SCI and noninjured control groups, potentially explaining the higher cerebrovascular health burden in SCI individuals. Exploratory associations indicate that longer TSI, lower diastolic blood pressure, and more hypotensive episodes may lead to poorer CVR outcomes. However, further research is necessary to establish causality and support these observations.

Cerebrovascular reactivity measurements using 3T BOLD MRI and a fixed inhaled CO2 gas challenge: Repeatability and impact of processing strategy

Introduction: Cerebrovascular reactivity (CVR) measurements using blood oxygen level dependent (BOLD) magnetic resonance imaging (MRI) are commonly used to assess the health of cerebral blood vessels, including in patients with cerebrovascular diseases; however, evidence and consensus regarding reliability and optimal processing are lacking. We aimed to assess the repeatability, accuracy and precision of voxel- and region-based CVR measurements at 3 T using a fixed inhaled (FI) CO₂ stimulus in a healthy cohort. Methods: We simulated the effect of noise, delay constraints and voxel- versus region-based analysis on CVR parameters. Results were verified in 15 healthy volunteers (28.1±5.5 years, female: 53%) with a test-retest MRI experiment consisting of two CVR scans. CVR magnitude and delay in grey matter (GM) and white matter were computed for both analyses assuming a linear relationship between the BOLD signal and time-shifted end-tidal CO₂ (EtCO₂) profile. Results: Test-retest repeatability was high [mean (95% CI) inter-scan difference: -0.01 (-0.03, -0.00) %/mmHg for GM CVR magnitude; -0.3 (-1.2,0.6) s for GM CVR delay], but we detected a small systematic reduction in CVR magnitude at scan 2 versus scan 1, accompanied by a greater EtCO2 change [±1.0 (0.4,1.5) mmHg] and lower heart rate [-5.5 (-8.6,-2.4] bpm]. CVR magnitude estimates were higher for voxel- versus region-based analysis [difference in GM: ±0.02 (0.01,0.03) %/mmHg]. Findings were supported by simulation results, predicting a positive bias for voxel-based CVR estimates dependent on temporal contrast-to-noise ratio and delay fitting constraints and an underestimation for region-based CVR estimates. Discussion: BOLD CVR measurements using FI stimulus have good within-day repeatability in healthy volunteers. However, measurements may be influenced by physiological effects and the analysis protocol. Voxel-based analyses should be undertaken with care due to potential for systematic bias; region-based analyses are more reliable in such cases.

Nocturnal Pulse Event Frequency Is Reduced in Multiple System Atrophy

Risk of sudden death in multiple system atrophy (MSA) is greatest during sleep with unknown mechanisms. We compared nocturnal pulse event frequency in 46 MSA patients and age-/sex-matched controls undergoing overnight pulse oximetry. Nocturnal oxyhemoglobin desaturation indices and pulse event indices (PEIs) were recorded, and relationships between pulse oximetry variables and survival were analyzed. MSA patients had lower PEI (3.1 ± 5.3 vs. 12.8 ± 10.8, p < 0.001) despite greater hypoxic burden and similar frequency of respiratory events. Nocturnal pulse events were not associated with severity of daytime autonomic failure. Two MSA patients had suspected sudden death, both with severely reduced PEI. MSA patients have fewer nocturnal pulse events compared with controls, despite similar respiratory event frequency, suggesting abnormal cardiac responses to sleep-disordered breathing. Whether this contributes to sudden death in MSA requires further study. ANN NEUROL 2023;93:205-212.

Medullary and Hypothalamic Functional Magnetic Imaging During Acute Hypoxia in Tracing Human Peripheral Chemoreflex Responses

[Figure: see text].

Stunning of pigs with different gas mixtures: Behavioural and physiological reactions

The present study used thirty-one pigs to investigate induction of unconsciousness and behavioural reactions in different gas mixtures: 80% CO₂/air, 90 s; 40% CO₂/30% O₂/air, 180 s; 70% N₂O/30% CO₂, 90 s. All pigs lost consciousness. All presented respiratory difficulties and most pigs involuntary muscle contractions, often before loss of standing posture. Between mixtures, average latencies of certain behaviours and delays between behaviours differed. Following immersion, blood pH was lower than normal. The low pH induced by the CO₂/O₂/air mixture was physiologically associated with hyperoxemia. Relationships between blood gases, different behavioural and heart rate responses are discussed. In conclusion, all mixtures caused discomfort due to respiratory difficulties and the addition of O₂ or N₂O to the CO₂ mixture did not present an advantage.

Quantitative assessment of near-infrared spectroscopy time course under hypercapnia using an a priori model-based fitting

BACKGROUND

Partial arterial pressure of carbon dioxide (CO₂) modulates cerebral blood flow through a vasoreactivity mechanism. Near infrared spectroscopy (NIRS) can be used to record these changes in cerebral hemodynamics. However, no laterality comparison of the NIRS signal has been performed despite being a prerequisite for the use of such a method in a vasoreactivity monitoring context. We propose to investigate the NIRS signal laterality in response to a CO₂-inhalation-based hypercapnia paradigm in healthy volunteers.

METHODS

Eleven healthy volunteers (6 women, 5 men, mean age: 31 ± 11) underwent a 3-block-design inhalation paradigm: normoxia (5min, "baseline") - hypercapnia (2min, "stimulation") - normoxia (5min, "post-stimulation"). NIRS signal was measured using a two-channel oximeter (INVOS 5100C, Medtronic, USA) with sensors placed symmetrically on both left and right sides on each subject's forehead. Additional heart rate (HR) monitoring was performed simultaneously. Based on the NIRS mean signal pattern, an a priori model of parametric identification was applied for each channel to quantify parameters of interest (amplitude, time delay, excitation and post-stimulation time) for each inhalation block.

RESULTS

HR increased significantly during the stimulation block. The quality of the model was satisfactory: mean absolute errors between modeled and experimental signals were lower than the resolution of the device. No significant lateralization was found between left and right values of most of the parameters.

CONCLUSION

Due to the lack of lateralization, this parametric identification of NIRS responses to hypercapnia could bring light to a potential asymmetry and be used as a biomarker in patients with cerebrovascular diseases.

The Logic of Carotid Body Connectivity to the Brain

The carotid body has emerged as a therapeutic target for cardio-respiratory-metabolic diseases. With the expansive functions of the chemoreflex, we sought mechanisms to explain differential control of individual responses. We purport a remarkable correlation between phenotype of a chemosensory unit (glomus cell-sensory afferent) with a distinct component of the reflex response. This logic could permit differential modulation of distinct chemoreflex responses, a strategy ideal for therapeutic exploitation.

Sudden unexpected nocturnal death in Chiari type 1 malformation and potential role of opioid analgesics

Background:

Chiari malformation type 1 (CM1) is a common congenital anomaly of the craniocervical junction. CM1 is reported to run a usually benign course and patients typically experience no symptoms or chronic, slowly progressive symptoms. However, recent reports indicate that a subset of patients with CM1 may present with acute deterioration and sudden unexpected death (SUD). We report a case of SUD during sleep in a young man with CM1, which we believe was related to the administration of common and therapeutic doses of narcotic analgesics for the management of pain. We will clarify the pathophysiology of acute deterioration and SUD in CM1 and the possibility that the adverse effects of opiate analgesics likely were the leading cause of death in our patient.

Case Description:

In this review, we present a 29-year-old male with worsening headache secondary to previously diagnosed CM1. The patient died suddenly and unexpectedly after administration of common and therapeutic doses of narcotic analgesics for the management of pain.

Conclusion:

The mechanism(s) of acute neurological deterioration and sudden death in patients with CM1 remains poorly understood. We believe the rapid fatal deterioration in our patient following administration of opioids suggests that this category of medication may cause sudden unexpected “neurogenic” cardiac death in CM1 patients by inducing sleep-related breathing difficulties and associated hypercapnia. Hypercapnia by further increasing intracranial pressure can result in a sudden pressure-induced decompensation of the cardiopulmonary control centers in the brain stem and cause instantaneous cardiorespiratory arrest.

Degeneration of brainstem respiratory neurons in dementia with Lewy bodies

BACKGROUND

Respiratory dysfunction, including sleep disordered breathing, is characteristic of multiple system atrophy (MSA) and may reflect degeneration of brainstem respiratory nuclei involved in respiratory rhythmogenesis and chemosensitivity, including the pre-Bötzinger complex (preBötC), nucleus raphe pallidus (RPa), and nucleus raphe obscurus (ROb). However, impaired ventilatory responses to hypercapnia have also been reported in dementia with Lewy bodies (DLB), suggesting that these nuclei may also be affected in DLB.

OBJECTIVES

To determine whether there is involvement of the preBötC, RPa, and ROb in DLB.

DESIGN

We applied stereological methods to analyze sections immunostained for neurokinin-1 receptor and tryptophan hydroxylase in neuropathologically confirmed cases of DLB, MSA, and controls.

RESULTS

Reduction of neuronal density occurred in all three nuclei in DLB, as well as in MSA. The magnitude of neuronal depletion in ROb was similar in DLB and MSA (49% versus 56% respectively, compared to controls, P < 0.05), but neuronal loss in the preBötC and RPa was less severe in DLB than in MSA (40% loss in preBötC of DLB, P < 0.05 and 68% loss in MSA, P < 0.0001, compared to controls; 46% loss in RPa of DLB, P < 0.05 and 73% loss in MSA P < 0.0001, compared to controls).

CONCLUSIONS

Medullary respiratory nuclei are affected in dementia with Lewy bodies but less severely than in multiple system atrophy. This may help explain differences in the frequency of sleep disordered breathing in these two disorders.

Breathing variability and brainstem serotonergic loss in a genetic model of multiple system atrophy

Breathing disorders like sleep apnea, stridor, and dysrythmic breathing are frequent in patients with multiple system atrophy (MSA). These observations have been related to neurodegeneration in several pontomedullary respiratory nuclei and may explain the occurrence of sudden death. In this study, we sought to determine whether these functional and neuropathological characteristics could be replicated in a transgenic model of MSA. Mice expressing human wild-type α-synuclein under the control of the proteolipid promoter (PLP-αSYN) were compared with age-matched controls. Using whole-body, unrestrained plethysmography, the following breathing parameters were measured: inspiratory and expiratory times, tidal volume, expiratory volume, peak inspiratory and expiratory flows, and respiratory frequency. For each category, the mean, coefficient of variation, and irregularity score were analyzed. Brains were then processed for stereological cell counts of pontomedullary respiratory nuclei. A significant increase in the coefficient of variation and irregularity score was observed for inspiratory time, tidal volume, and expiratory volume in PLP-αSYN mice (P < 0.05). Glial cytoplasmic inclusions were found in the medullary raphe of PLP-αSYN mice, together with a loss of serotonergic immunoreactivity in the raphe obscurus (P < 0.001) and pallidus (P < 0.01). There was a negative correlation between α-synuclein burden and raphe pallidus cell counts (P < 0.05). There was no significant neuronal loss in the pre-Botzinger complex. The PLP-αSYN mouse model replicates the breathing variability and part of the neuronal depletion in pontomedullary respiratory nuclei observed in patients with MSA. Our findings support the use of this model for future candidate drugs in the breathing disorders observed in MSA.

From REM sleep behaviour disorder to status dissociatus: insights into the maze of states of being

Sleep is a coordinated process involving more or less simultaneous changes in sensory, motor, autonomic, hormonal, and cerebral processes. On the other hand, none of the changes occurring with sleep are invariably coupled to sleep. EEG synchrony, heat loss, sleep-related hormone secretion, and even REM-related motoneuron paralysis may occur independent of the parent state. In REM sleep behaviour disorder (RBD) the muscle tone of wakefulness intrudes into REM sleep, allowing the release of dream-enacting behaviours. Status dissociatus (SD) is a condition in which brain and mind are in disarray along the boundaries of sleep and wakefulness. The existence of such dissociated behaviours shows that they have separate neuronal control systems and indicates that the whole organization of sleep is an emergent property of the collective neuronal systems to synchronize. Insults to the brain can drastically alter the circuitries responsible for maintaining the integrity of wakefulness, NREM sleep, and REM sleep. As a consequence, the basic states of existence can become admixed and interchanged with striking disturbances of consciousness, brain electrophysiology, and the behavioural and polygraphic expression of sleep and wakefulness. The evolution of RBD into SD may result from a disarray of (brainstem) structures that orchestrate the whole brain wake-sleep conditions, but with preserved discrete systems and dissociable strategies to still place navigation in wake and sleep. Advances in the fields of genetics, neuroimaging, and behavioural neurology will expand the understanding of the mechanisms underlying the organization of the states of being along with their somatic/behavioural manifestations.