Hexosaminidase A deficiency manifesting as spinal muscular atrophy of late onset.

Proximal lower limb weakness and fasciculations were the only clinical manifestations of hexosaminidase A (Hex A) deficiency in a 39-year-old woman. Hex A activity in serum and leukocytes was 0 to 4% by standard heat inactivation fluorogenic substrate methods, and 5% when Hex A isoenzymes were fractionated by diethylaminoethanol cellulose chromatography. Computed tomography of the brain showed cerebellar atrophy. We suggest that Hex A activity studies be done in patients with typical as well as atypical spinal muscular atrophy.