Introduction
Neuromuscular diseases affect the lower motor unit: the anterior horn cell, the peripheral nerve, the neuromuscular junction or the muscle. They present as weakness, low tone and absent or reduced reflexes. Cerebral palsy, by contrast, comes from the brain (upper motor neurone) and gives stiffness and brisk reflexes.
Spinal muscular atrophy (SMA)
Autosomal recessive loss of the SMN1 gene (chromosome 5q); anterior horn cells die. The number of SMN2 copies sets severity.
- Type 1 (onset under 6 months): never sits; severe floppiness, tongue fasciculations, absent reflexes, bell-shaped chest with belly breathing; untreated death before 2 years.
- Type 2: sits, never walks. Type 3: walks, later weakness. Intellect normal.
- Treatment: nusinersen (intrathecal), risdiplam (oral), onasemnogene abeparvovec (one-off gene therapy); best given before symptoms, hence newborn screening.
Duchenne muscular dystrophy (DMD)
X-linked recessive, affects boys (about 1 in 3500); no dystrophin, so muscle fibres break down.
- Late walking, clumsy, falls, waddling gait, toe-walking; starts at 2–5 years.
- Gowers sign: climbs up his legs with his hands to stand; calf pseudohypertrophy.
- Creatine kinase very high (10–100 times normal); genetic test confirms.
- Loses walking around 10–12 years, then scoliosis, cardiomyopathy, respiratory failure; survival now into the 30s with care.
- Treatment: corticosteroids (prednisolone, deflazacort, vamorolone) slow the decline; physiotherapy, night splints, cardiac drugs, night ventilation, spinal surgery; exon-skipping drugs for certain mutations.
Becker dystrophy is the milder form (some dystrophin): later onset, walking into adulthood.