A sister and brother, now aged 7 and 9 years, presented with developmental 
arrest, gait disturbance, dementia, and a progressive myoclonic epilepsy 
syndrome with hyperacusis in the second year of life. Then, spastic 
quadriparesis led to a decerebrate state. In the absence of macular or retinal 
degeneration, organomegaly, and somatic-facial features suggesting 
mucopolysaccharidosis, the presence of hyperacusis together with sea-blue 
histiocytes in bone marrow biopsies and deficient beta-galactosidase activity 
but normal glucosidase, hexosaminidase, and neuraminidase activity on lysosomal 
enzyme assays constitutes the clinical-pathologic-biochemical profile of GM1 
gangliosidosis type 2. This is a rare, late infantile onset, progressive 
gray-matter disease in which beta-galactosidase deficiency is largely localized 
to the brain, though it can be demonstrated in leukocytes and cultured skin 
fibroblasts. It must be distinguished from the Jansky-Bielschowsky presentation 
of neuronal ceroid lipofuscinosis, mitochondrial encephalopathy, lactic 
acidosis, strokelike episodes (MELAS) and myoclonic epilepsy with ragged-red 
fibers (MERRF) syndromes, atypical presentations of GM2 gangliosidoses 
(Tay-Sachs and Sandhoff's diseases), primary sialidosis (neuraminidase 
deficiency), galactosialidosis, and Alpers' disease.