Progressive muscular atrophy (PMA), also known as Duchenne-Aran muscular atrophy and Duchenne-Aran disease — is a rare subtype of motor neuron disease (MND) that affects only the lower motor neurones. PMA is thought to account for around 4% of all MND cases. This is in contrast to amyotrophic lateral sclerosis (ALS), the most common form of MND, which affects both the upper and lower motor neurones, or primary lateral sclerosis, another rare MND variant, which affects only the upper motor neurons. The distinction is important because PMA is associated with a better prognosis than classic ALS.