Information provided by the National Institutes of Health (NIH)

What is Primary Lateral Sclerosis?
Primary lateral sclerosis (PLS) is a rare neuromuscular disease characterized by progressive muscle weakness in the voluntary muscles. Onset of the disorder generally 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 which produces slowness and stiffness of movement, dragging of the feet (leading to inability to walk), and facial involvement resulting in dysarthria (poorly articulated speech). The disorder usually begins in the legs, however, it may also begin in the tongue or the hands. PLS belongs to a group of disorders known as motor neuron diseases. It occurs when specific nerve cells in the cerebral cortex that control voluntary movement gradually degenerate, causing the muscles under their control to weaken. The disease—which scientists believe is not hereditary—progresses gradually over a number of years, or even decades, leading to stiffness and weakening of the affected muscles. In PLS, there is no evidence of the degeneration of spinal motor neurons or muscle wasting (amyotrophy) that occurs in a similar disorder called amyotrophic lateral sclerosis or ALS (Lou Gehrig's disease). For the first few years (3-5) after symptoms begin there is a possibility that the true diagnosis is ALS, which can have some of the same symptoms as PLS. For this reason, most neurologists follow the affected individual's clinical course for a few years before making a diagnosis of PLS.