From Primary Lateral Sclerosis (PLS)

Primary lateral sclerosis (PLS) affects the upper motor neurons of the arms, legs, and face. It occurs when specific nerve cells in the motor regions of the cerebral cortex (the thin layer of cells covering the brain which is responsible for most high-level brain functions) gradually degenerate, causing the movements to be slow and effortful. The disorder often affects the legs first, followed by the body trunk, arms and hands, and, finally, the bulbar muscles. Speech may become slowed and slurred. When affected, the legs and arms become stiff, clumsy, slow and weak, leading to an inability to walk or carry out tasks requiring fine hand coordination. Difficulty with balance may lead to falls. Speech may become slow and slurred. Affected individuals commonly experience pseudobulbar affect and an overactive startle response. PLS is more common in men than in women, with a very gradual onset that generally occurs between ages 40 and 60. The cause is unknown. The symptoms progress gradually over years, leading to progressive stiffness and clumsiness of the affected muscles. PLS is sometimes considered a variant of ALS, but the major difference is the sparing of lower motor neurons, the slow rate of disease progression, and normal lifespan. PLS may be mistaken for spastic paraplegia, a hereditary disorder of the upper motor neurons that causes spasticity in the legs and usually starts in adolescence. Most neurologists follow the affected individual’s clinical course for at least 3 to 4 years before making a diagnosis of PLS. The disorder is not fatal but may affect quality of life.

From NIH Motor Neuron Diseases Fact Sheet