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Acquired neuromyotonia (Isaacs' syndrome) is a rare disorder characterized by hyperexcitability of peripheral motor nerves. The cardinal features consist of myokymia, pseudomyotonia and contracture of hands and feet. The diagnosis of Isaacs' syndrome is based on the clinical features and classic electromyographic findings. Serum antibodies against Voltage-Gated Potassium Channels (VGKCs) are detected in some cases. The authors report a 17 year-old man presented with difficulty in walking, writing and respiratory discomfort for 7 months. His body weight had decreased from 120 to 70 kilograms during that period. Physical examination was remarkable for profound sweating. Muscles were in a state of contraction, action myotonia without percussion myotonia, myokymia and carpopedal spasm. Electromyography showed classical neuromyotonic and myokymic discharges. The investigations for conditions associated with Isaacs' syndrome were unrevealing. VGKCs antibody were not performed. Treatment with carbamazepine resulted in substantial improvement of the symptoms within 7 days.

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Whereas paralysis is the hallmark for paralytic rabies, the precise pathological basis of paralysis is not known. It is unclear whether weakness results from involvement of anterior horn cells or of motor nerve fibers. There is also no conclusive data on the cause of the neuropathic pain which occurs at the bitten region, although it has been presumed to be related to sensory ganglionopathy. In this study, six laboratory-proven rabies patients (three paralytic and three furious) were assessed clinically and electrophysiologically. Our data suggests that peripheral nerve dysfunction, most likely demyelination, contributes to the weakness in paralytic rabies. In furious rabies, progressive focal denervation, starting at the bitten segment, was evident even in the absence of demonstrable weakness and the electrophysiologic study suggested anterior horn cell dysfunction. In two paralytic and one furious rabies patients who had severe paresthesias as a prodrome, electrophysiologic studies suggested dorsal root ganglionopathy. Postmortem studies in two paralytic and one furious rabies patients, who had local neuropathic pain, showed severe dorsal root ganglionitis. Intense inflammation of the spinal nerve roots was observed more in paralytic rabies patients. Inflammation was mainly noted in the spinal cord segment corresponding to the bite in all cases; however, central chromatolysis of the anterior horn cells could be demonstrated only in furious rabies patient. We conclude that differential sites of neural involvement and possibly different neuropathogenetic mechanisms may explain the clinical diversity in human rabies.

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Furious rabies is a well-recognized clinical disorder in humans but the paralytic form is not as easily identified. The mechanisms responsible for the weakness and longer survival periods are not clear. Several hypotheses have been proposed, including rabies virus variants associated with a particular vector, location of wounds, incubation period, influence of prior rabies vaccination, and virus localization in the central nervous system (CNS). However, none of these have been substantiated. Regarding molecular analyses of rabies viruses isolated from both furious and paralytic rabies patients, only minor genetic variations with no specific patterns in glyco- (G), phospho- (P), and nucleoprotein (N) sequences have been identified and arginine 333 in G protein was present in all samples. Regional distribution of rabies virus antigenin rabies patients whose survival periods were 7 days or less and magnetic resonance imaging (MRI) of the CNS indicated brainstem and spinal cord as predilection sites regardless of clinical presentations. There are clinical, electrophysiological, and pathological indications that peripheral nerve dysfunction is responsible for weakness in paralytic rabies whereas in furious rabies, even in the absence of clinical weakness, abundant denervation potentials with normal sensory nerve conduction studies and proximal motor latencies suggest anterior horn cell dysfunction. The lack of cellular immunity to rabies virus antigen accompanied by an absence of cerebrospinal fluid (CSF) rabies neutralizing antibody in most paralytic rabies patients may argue against role of an immune response against rabies virus-positive axons. Aberrant immune responses to peripheral nerve antigen, in particular those mediated by one or more cellular-dependent mechanisms, may be involved as is supported by the absence of putative anti-ganglioside antibodies commonly found in immune-mediated peripheral nerve diseases. Longer survival period in paralytic rabies may possibly be related to currently unidentified mechanism(s) on neuronal gene expression, required for virus transcription/replication and for maintaining neuronal survival.

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BACKGROUND AND PURPOSE: Whether human rabies of different forms, encephalitic (furious) and paralytic (dumb), share similar MR imaging patterns is unknown. We assessed the diagnostic value of MR imaging in both forms of the disease and compared the clinical and neuroimaging findings. METHODS: Three patients with paralytic and two with encephalitic rabies were examined during preserved or deteriorated levels of consciousness. Six MR examinations of the brain, three of the spinal cord, and one of the brachial plexus were performed with a 1.5-T superconducting magnet. RESULTS: No difference was noted between the MR findings in both clinical forms of human rabies. Nonenhancing, ill-defined, mild hyperintensity changes in the brain stem, hippocampi, hypothalami, deep and subcortical white matter, and deep and cortical gray matter were demonstrated on T2-weighted images in the noncomatose patients with rabies. Enhancement along the brachial plexus of the bitten arm was noted in one patient with encephalitic rabies who at that time had only local neuropathic pain symptoms. Enhancement with gadolinium-based contrast material was seen at the hypothalami, brain stem nuclei, spinal cord gray matter, and intradural cervical nerve roots only when the patients became comatose. CONCLUSION: Both forms of human rabies share a similar MR imaging pattern. Such pattern and the lack of enhancement in a noncomatose patient with suspected encephalitis may differentiate rabies from other viral encephalitides.

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