Primary lateral sclerosis (PLS) causes weakness in your voluntary muscles, such as those you use to control your legs, arms and tongue. Primary lateral sclerosis is a type of motor neuron disease that causes muscle nerve cells to slowly break down, causing weakness.
Primary lateral sclerosis can happen at any age, but it usually occurs between ages 40 and 60. A subtype of primary lateral sclerosis, known as juvenile primary lateral sclerosis, begins in early childhood and is caused by an abnormal gene passed from parents to children.
Primary lateral sclerosis is often mistaken for another, more common motor neuron disease called amyotrophic lateral sclerosis (ALS). However, primary lateral sclerosis progresses more slowly than ALS, and in most cases isn't fatal.