Excerpted from the NIH/NINDS website
Primary lateral sclerosis (PLS) is a rare neuromuscular disease characterized by progressive muscle weakness in the voluntary muscles. PLS belongs to a group of disorders known as motor neuron diseases. Motor neuron diseases develop when the nerve cells that control voluntary muscle movement degenerate and die, causing weakness in the muscles they control. Onset of PLS usually occurs after age 50. Symptoms may include difficulty with balance, weakness and stiffness in the legs, and clumsiness. Other symptoms may include spasticity (sudden, involuntary muscle spasms) in the hands, feet, or legs; foot dragging, and speech problems due to involvement of the facial muscles. The disorder usually begins in the legs, but it may also start in the tongue or the hands. The disease-which scientists believe is not hereditary-progresses gradually over a number of years, or even decades. In PLS, there is no evidence of the degeneration of spinal motor neurons or muscle wasting (amyotrophy) that occurs in amyotrophic lateral sclerosis or ALS (Lou Gehrig's disease), which it resembles. Diagnosis of PLS is often delayed because it is mistaken for ALS.