Neuromuscular disorders, peripheral neuropathy and myology
This domain tests lesion localisation along the neuraxis (anterior horn, root, plexus, nerve, neuromuscular junction, muscle) and the treatment of the treatable disorders. Electrophysiology questions are common.
Localisation
LMN: weakness, atrophy, fasciculations, hyporeflexia. UMN: spasticity, hyperreflexia, Babinski. Neuromuscular junction: fatigable weakness with normal sensation and reflexes. Myopathy: proximal weakness, myalgia, normal reflexes, elevated CK.
Myasthenia gravis
Antibodies to acetylcholine receptor in about 85% of generalised cases, MuSK in a minority (bulbar and neck weakness, poor response to pyridostigmine). Repetitive nerve stimulation at 3 Hz shows a decrement >10%; single-fibre EMG shows increased jitter. Chest CT is mandatory to exclude thymoma (10–15%). Treatment: pyridostigmine, corticosteroids, steroid-sparing agents, rituximab in MuSK disease, thymectomy in AChR-positive non-thymomatous patients aged 18–65. Crisis is treated with IVIG or plasma exchange. Avoid aminoglycosides, quinolones and beta-blockers.
Lambert–Eaton myasthenic syndrome
Presynaptic, voltage-gated calcium channel antibodies, proximal leg weakness, hyporeflexia, autonomic features; incremental response on high-frequency stimulation; screen for small-cell lung cancer.
Guillain–Barré syndrome
Progressive weakness peaking within 4 weeks, areflexia, albuminocytological dissociation. AIDP is demyelinating; Miller Fisher syndrome is ophthalmoplegia, ataxia, areflexia with anti-GQ1b. Treat with IVIG 0.4 g/kg for 5 days or plasma exchange; steroids are not indicated. Monitor vital capacity (intubate around <20 mL/kg or <1 L).
Myopathies
Duchenne: X-linked dystrophin defect, Gowers sign, calf pseudohypertrophy, CK 10–100× normal, steroids and exon-skipping. Myotonic dystrophy type 1: CTG repeat in DMPK, distal weakness, myotonia, cataracts, cardiac conduction defects, anticipation. Inclusion body myositis: quadriceps and finger flexor weakness, poor response to immunosuppression.
EMG patterns
Fibrillations and positive sharp waves indicate active denervation; large polyphasic motor units with reduced recruitment indicate chronic reinnervation; short, small polyphasic units indicate myopathy; myotonic discharges sound like a dive bomber.
Worked example
A 55-year-old has fluctuating ptosis and diplopia worse in the evening, normal pupils and normal sensation. Fatigable ocular weakness with normal findings elsewhere suggests myasthenia gravis; confirm with AChR antibodies and 3 Hz repetitive stimulation, then image the thymus.
Traps
Giving steroids for Guillain–Barré syndrome; forgetting respiratory monitoring; missing a thymoma; labelling a myopathy as neuropathy without EMG; ignoring cardiac involvement in myotonic dystrophy.