Primary lateral sclerosis (PLS) is a rare, neuromuscular disorder that affects the central motor neurons and is characterized by painless but progressive weakness and stiffness of the muscles of the legs. Such weakness may progress to affect the arms and the muscles at the base of the brain (bulbar muscles). Less frequently, the muscles of the face are affected.
Further assuming an average disease duration of 20 years (close to the reported median of 19 y), this prevalence would translate into an annual PLS incidence rate of 1 per 10 million (0.01 case per 100,000 population per year), which is approximately 0.5% of that for ALS. The tentative nature of these estimates should be emphasized. They are consistent with a conservative estimate that not more than 500 people with PLS.
There is no treatment that will reverse or slow the progress of PLS. Symptoms may be treated with drugs to reduce muscle spasticity or cramps, physical therapy to reduce joint immobility, and speech therapy to help those whose facial muscles are affected. Baclofen and tizanidine may be prescribed for spasticity, quinine for cramps, and diazepam, a drug that relaxes muscles, for muscular contractions.