Spinal Muscular Atrophy (SMA) is an heterogeneous group of disorders characterized by anterior horn cell degeneration leading to progressive weakness and muscle atrophy. Phenotypical variability depends mainly on age of onset and clinical progression; moreover, some patients with atypical clinical features are described within the so called "SMA plus". We describe a 13 years old female in which clinical, neurophysiological and neuropathological data are consistent with chronic lower motor neuron disorder associated with progressive myoclonus epilepsy (PME). Genetic investigations failed to demonstrate deletions or point mutations in SMN gene, while EPM1 analysis for mutations of the cystatin B gene showed the dodecamer repeat expansion in one allele. To date only 10 patients presenting the association of a juvenile SMA with a clinical picture of PME are described; PME usually begin after the onset of weakness, and can be accompanied by mental decline and other neurological symptoms as nerve deafness. We report the first evidence of an heterozygote expansion in the gene for Cistatin B, in a patient with a "non5q-SMA" or "SMA plus" phenotype. Association of SMA type III and PME is a rare condition, but similarities to the literature cases suggest that it might represent a distinct nosological entity.
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