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Genetic disorder resulting in familial branchial myoclonus, spastic paraparesis, and cerebellar ataxia.

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Approximately 13 cases have been reported. The original description in 1988 reported a family with six affected individuals in two generations. No other definite case reports have been published.

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Autosomal dominant.

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The precise mechanism leading to this progressive degenerative condition remains poorly understood. The levels of the serotonin metabolite 5-hydroxyindoleacetic acid in the cerebrospinal fluid (CSF) are low, and an abnormality in serotonin-mediated neurotransmission has been proposed as a possible cause.

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Made by clinical features consistent with the syndrome. CT and MRI scans demonstrate severe atrophy of medulla, brainstem, and cervical spinal cord, and mild atrophy of the cerebral and cerebellar cortex, with normal pons and olives. CSF studies reveal a markedly reduced concentration in the serotonin metabolite 5-hydroxyindoleacetic acid. Visual auditory and somatosensory evoked responses, electroencephaloand electromyogram are normal.

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Characterized by a rhythmic branchial myoclonus (affecting palate, pharynx, larynx, lower face, and neck) resulting in dysphagia, dysphonia, choking spells, spastic paraparesis (preserved, muscle strength brisk reflexes, normal sensation), cerebellar ataxia (truncal), nystagmus, and normal mental status. The age at onset usually is between 30 and 50 years. The condition is progressive, leading to death or severe disability within 5 to 10 years of onset.

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Detailed history and examination are necessary to determine the progression of neurologic impairment (bulbar palsy, degree of spastic paraparesis and cerebellar ataxia). Assess speech, swallowing, and glottic competency (gastrostomy feeding, tracheostomy). Pulmonary assessment (especially with a history of frequent aspiration episodes) by chest radiography, spirometry, and arterial blood gas analysis. Evaluate motor impairment (wheelchair bound).

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There is an increased risk of aspiration as a result of laryngeal incompetence (consider awake fiberoptic or a rapid sequence induction for tracheal intubation). Spastic paraparesis may lead to fixed limb deformities with difficulty in positioning and venous access. These patients have normal sensation in affected limbs (and hence normal analgesic requirements). The potential risk for postoperative ventilation, tracheostomy, and feeding gastrostomy should be considered.

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Suxamethonium should be avoided (hyperkalemia, rhabdomyolysis, arrhythmia, myoglobinuria, cardiac arrest). Acid aspiration prophylaxis is recommended (H2 blockers, antacids). Treatment with 5-hydroxytryptophan and carbidopa at highest tolerated doses may improve ataxia, but not myoclonus.

De Yebenes JG, Vazquez A, Rabano J, et al: Hereditary branchial myoclonus with spastic paraparesis and cerebellar ataxia: A new autosomal dominant disorder. Neurology 38:569, 1988.
Howard RS, Greenwood R, Gawler J, et al: A familial disorder associated with palatal myoclonus, other brainstem signs, tetraparesis, ataxia and Rosenthal fiber formation. J Neurol Neurosurg Psychiatry 56:977, 1993.  [PubMed: 8410038]

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