Mechanisms of "Escape Phenomenon" of spinal cord and brainstem in human rabies and study of neuronal cytoskeletion and myelin integrity

Abstract

Two classical forms of human rabies, furious or encephalitic and dump or paralytic rabies, share a similar pattern of regional virus antigen distribution in the central nervous system (CNS) with a predilection of brainstem and spinal cord during the early clinical phase. Magnetic resonance imaging (MRI) of the brain also appeared similar in both forms. Besides abnormalities in brainstem, spinal cord, thalamus, basal ganglia and hippocampus, subcortical and deep white matter were another areas where hyperintensity T2 signals can be demonstrated. Nevertheless, clinical manifestations are totally different. Paralytic rabies patient exhibited lower motor neuron type of weakness of all limbs. Despite the abundant presence of rabies virus antigen in the spinal cord motoneuron or anterior horn cell, clinical weakness, on the other hand, appears to be the result of peripheral nerve dysfunction. Weakness of the extremities does not appear in furious rabies patient until coma develops. Another intriguing feature in human rabies is the preservation of consciousness until the pre-terminal phase. This is clearly different to other forms of viral encephalitides in which degradation of consciousness is a hall mark of the disease. Since pathology of the CNS in human rabies is unremarkable with scant inflammation and necrosis, this project aimed to determine whether spinal cord and brainstem (major controller of consciousness level) of both forms of human rabies are more resistant to the process of apoptosis extrinsic and intrinsic pathways. Further, integrity of white matter structures were also sought out to determine whether there were any abnormalities in axon or myelin to explain those abnormal signals in white matter areas. In order to fulfill objectives, immunohistochemical (IHC) and terminal deoxynucleotidyl transferase-mediated dUTP-digoxigenin nick end labeling (TUNEL) techniques were used to demonstrate and quantify rabies virus antigen and evidence of cytochrome C leakage [representing mitochondrial outer membrane permeabilization (MOMP)] and apoptosis respectively. Analysis of white matter components, tubulin and myelin basic protein (MBP), was done by IHC technique. Of 15 CNS regions from 7 rabies patients (4 furious and 3 paralytic), there was a relative absence of cytochrome C leakage in spinal cord and brainstem despite a massive degree of apoptosis (TUNEL). Owing to the fact that cytochrome C is a major inducer of apoptosis and caspase cascade, absence of MOMP may result to delay in neuronal death is such regions. Apoptosis at later stage may be unavoidable as a consequence of other apoptotic mechanisms, hypoxic insult and postmortem changes. There was no correlation between the degree of apoptosis and MOMP and survival periods and clinical forms of the disease. Tubulin and myelin basic protein (MBP) appeared intact suggesting that such abnormal MRI signals might be explained by other underlying mechanisms.