Late-onset lower motor neuronopathy: A new autosomal dominant disorder

Characterization of a new type of late-onset autosomal dominant lower motor neuron disease. Patients from 2 families underwent detailed neurologic, electrophysiologic, muscle biopsy, and laboratory investigations. MRI of lower limbs was performed in selected patients. DNA samples from leukocytes were used for molecular genetic linkage studies. First symptoms were muscle cramps and fasciculations after age 25-30, followed by a slowly progressive proximal and distal weakness without overt atrophy during the first decades of symptoms. Nerve conduction velocities were within normal range and EMG showed widespread neurogenic alterations. Muscle biopsy revealed characteristic neurogenic findings: fiber type grouping and group atrophy. MRI showed diffuse fatty-degenerative changes, marked in medial gastrocnemius. Exactly the same clinical phenotype has not previously been described, and linkage studies showed exclusion of known chromosomal loci for hereditary motor neuropathies, suggesting the disease we report may represent a new disorder.