Primary Motor Neuron Disease

The signs and symptoms characteristic of many of these diseases (e.g., PMA, PLS) eventually evolve in most patients to demonstrate mixed upper and lower motor neuron involvement and thus a diagnosis of ALS. Notable exceptions include HSP, SMA, and spinal bulbar muscular atrophy.

Amyotrophic lateral sclerosis (known as “Lou Gehrig disease” in the nonmedical literature in the United States) characteristically involves a mixture of lower motor neuron findings (weakness, atrophy, fasciculations) and upper motor neuron features (spasticity, brisk reflexes, upgoing toes). Estimates suggest that this disease affects 3 to 6 of every 100,000 persons, with a male to female ratio of about 1.6 : 1. Onset is insidious and usually in middle to late life, although symptoms rarely begin in the second or third decade. About 10% of cases in adults are hereditary, primarily of dominant inheritance.

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