Progressive muscular atrophy (PMA) is a rare subtype of amyotrophic lateral sclerosis (ALS) or motor neurone disease (MND) which affects only the lower motor neurones. PMA is thought to account for around 4% of all ALS/MND cases. This is in contrast to the most common form of ALS/MND, amyotrophic lateral sclerosis, which affects both the upper and lower motor neurones, or another rare form of ALS/MND, primary lateral sclerosis, which affects only the upper motor neurones. The distinction is important because PMA is associated with a better prognosis than classical ALS/MND.