PLS belongs to a group of disorders known as motor neuron diseases / is a form of motor neurone disease (see IATE:2250577 ).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 (ALS - see IATE:1685830 ), which it resembles. Diagnosis of PLS is often delayed because it is mistaken for ALS.