Progressive muscular atrophy

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Progressive muscular atrophy (PMA) is a rare subtype of Motor neurone disease (MND) which affects only the lower motor neurones. This is contrast to the most common form of MND, amyotrophic lateral sclerosis, which affects both the upper and lower motor neurones.

As a result of lower motor neurone degeneration, the symptoms of PMA include:

However, in contrast to amyotrophic lateral sclerosis or primary lateral sclerosis, PMA is distinguished by the absence of:

brisk reflexes
spasticity
Babinski's sign
Emotional lability

To be diagnosed with PMA one must have the LMN symptoms described above in the absence of UMN symptoms for a given period of time. Some authors recommend 3 years, others 5 years or even up to 7 years. However it is worth bearing in mind that PMA can progress into typical ALS at any time, even as much as 26 years after the initial diagnosis.

The significance of diagnosing PMA as opposed to ALS is twofold. Crucially, the prognosis is better. The typical prognosis for ALS is approximately 2-5 years after initial diagnosis. In PMA survival is in the order of 5-10 years. More recently evidence has shown that patients with PMA do not suffer from the cognitive change identified in at least a subgroup of patients with ALS.

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