Progressive muscular atrophy

Progressive muscular atrophy (PMA) is a rare subtype of motor neuron disease (MND) that affects only the lower motor neurons. PMA is thought to be 4% of all motor neurone disease cases. 
This is in different 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.