Progressive Muscular Atrophy

Progressive muscular atrophy (PMA) is an adult onset motor neurone disease with isolated lower motor neurone degeneration.

• Accounts for 5 to 10% of motor neurone disease; male predominance, onset in the 50s.

• Many later develop upper motor neurone signs (about 30% convert to ALS); survival is longer than ALS (median 5 years or more).

• Distinct from spinal muscular atrophy (genetic), Kennedy disease, multifocal motor neuropathy.

• Asymmetric distal limb weakness and wasting, fasciculations, cramps, hyporeflexia.

• Bulbar involvement later. No sensory loss.

• Respiratory muscle weakness in late disease.

• EMG: widespread denervation and reinnervation without upper motor neurone signs on examination.

• Exclude treatable mimics: multifocal motor neuropathy (nerve conduction block, anti-GM1), Kennedy disease (CAG repeats in AR), spinal muscular atrophy (SMN1), post polio syndrome, inclusion body myositis, paraneoplastic and lymphoma related disease, hexosaminidase A deficiency, lead toxicity.

• Blood: CK, immunofixation, thyroid, B12, HIV. MRI spine.

• Neurofilament and repeat examination for upper motor neurone signs.

• Multidisciplinary supportive care as for ALS: riluzole 50 mg twice daily may be tried.

• Multifocal motor neuropathy must be excluded and treated with IVIG 2 g/kg over 2 to 5 days monthly.

• Cramps: baclofen, mexiletine 150 mg twice daily.

• Respiratory monitoring (FVC, sniff pressure) and non invasive ventilation; nutritional support.

• Physiotherapy, occupational therapy, mobility devices.

• Genetic counselling if familial. Review every 3 to 6 months for emergence of upper motor neurone signs.