a rare disease that causes progressive weakness in voluntary muscles such as in the legs, hands, and tongue. PLS is one of the diseases, along with amyotrophic lateral sclerosis, that are grouped together as motor neuron diseases.
Diagnosis is based on the observance of the muscle weakness and the progressive worsening of the weakness. The diagnosis can be delayed because the disease is mistaken for ALS.
Motor neuron diseases like primary lateral sclerosis develop because the nerve cells that normally control the movement of voluntary muscles degenerate and die.
The symptoms of the disorder become progressively worse, with muscles typically affected in the following order: legs and feet, main part of the body (the trunk), arms and hands, and face.