[A patient with GM2 gangliosidosis presenting with motor neuron disease symptom in his forties].

Here, we report a Japanese man with adult Sandhoff disease who presented with a motor neuron disease phenotype with slow progression. At the age of 42, he noticed weakness in his legs. At the age of 46, he was admitted to our hospital. Neurological examination revealed muscle weakness and atrophy of the upper and lower extremities, and hyperreflexia of the upper extremities. Magnetic resonance imaging showed very mild cerebellar atrophy. We diagnosed him as having atypical amyotrophic lateral sclerosis. Because of the atypical course of motor neuron disease, hexosaminidase activity in peripheral leukocytes was indicated. Asseys of hexosaminidase A and hexosaminidase B showed low activities, and we found a membranous cytoplasmic body in the submucosal nerve, leading to the diagnosis of Sandhoff disease. This is the second case of a Japanese adult with Sandhoff disease presenting with a motor neuron disease phenotype, and to our knowledge, this is the latest age of onset in Japan.