Neuromuscular and Peripheral Nerve Disorders
What to know
Localize weakness to anterior horn cell, nerve, neuromuscular junction, or muscle. Myopathic weakness is proximal with Gowers sign and elevated CK; neuropathic weakness is distal with sensory loss, areflexia, and fasciculations.
Duchenne muscular dystrophy (X-linked, dystrophin) presents at ages 2–5 with proximal weakness, calf pseudohypertrophy, Gowers sign, and CK elevated 10–100 fold; cognitive impairment and dilated cardiomyopathy occur, with respiratory failure later. Corticosteroids prolong ambulation; exon-skipping and stop-codon therapies apply to selected mutations. Becker is milder, with ambulation beyond 16 years. Spinal muscular atrophy (SMN1 homozygous deletion) produces hypotonia, tongue fasciculations, bell-shaped chest, and preserved facial expression and cognition; types 1–4 vary by onset. Treatment includes nusinersen, risdiplam, and gene therapy.
Myasthenia gravis causes fatigable ptosis and diplopia; congenital myasthenic syndromes present earlier. Guillain-Barré syndrome is ascending areflexic weakness with albuminocytologic dissociation - which may be absent in the first week, so normal CSF does not exclude it; treat with IVIG or plasma exchange and monitor vital capacity. Miller Fisher variant is ophthalmoplegia, ataxia, and areflexia (anti-GQ1b). CIDP evolves over more than 8 weeks. Charcot-Marie-Tooth disease causes pes cavus, hammer toes, and areflexia; CMT1A is a PMP22 duplication.
Traps
Normal CK does not exclude myopathy. Mildly elevated CK occurs in SMA, so do not use it to rule out anterior horn cell disease. Succinylcholine can cause hyperkalemic arrest in dystrophinopathy.
Rule of thumb
Weakness plus calf hypertrophy plus high CK equals dystrophinopathy; do genetic testing first, not biopsy.