Neuromuscular Disorders and Peripheral Neuropathy
This domain tests pattern recognition across nerve, neuromuscular junction and muscle, plus the emergencies that kill patients.
Peripheral neuropathy patterns
- Length-dependent axonal: diabetes, vitamin B12 deficiency, alcohol, uraemia. Symmetrical distal sensory loss first, then weakness.
- Demyelinating: Guillain-Barre syndrome, chronic inflammatory demyelinating polyneuropathy, anti-MAG neuropathy, hereditary CMT1.
- Mononeuritis multiplex: vasculitis, diabetes, leprosy.
Guillain-Barre and CIDP
Progression over four weeks or less with areflexia and albuminocytological dissociation after the first week. Treat with intravenous immunoglobulin or plasma exchange; steroids are ineffective. Admit if vital capacity falls below about 20 mL/kg, if bulbar weakness appears, or if progression is rapid.
Myasthenia gravis
Fatigable ptosis and diplopia with normal pupils, bulbar symptoms and fluctuating weakness. Confirm with anti-acetylcholine receptor antibodies, repetitive nerve stimulation showing decrement, or an ice-pack test. Screen for thymoma with chest imaging. Lambert-Eaton syndrome instead shows areflexia, autonomic symptoms, an increment on high-frequency stimulation and an association with small cell lung cancer.
Motor neuron disease
Amyotrophic lateral sclerosis combines upper and lower motor neuron signs with no sensory loss, often with split hand wasting. Riluzole and edaravone modestly slow progression; non-invasive ventilation and gastrostomy improve survival and quality of life.
Worked example
A 60-year-old has three months of drooping eyelids worse in the evening, diplopia, nasal speech and normal pupils, with normal reflexes. Repetitive stimulation shows a 15 percent decrement. This is generalised myasthenia gravis; arrange chest CT for thymoma, start pyridostigmine and corticosteroids, and monitor for crisis.
Traps
Missing respiratory failure in Guillain-Barre syndrome; diagnosing myasthenia in a patient with absent reflexes and dry mouth, which suggests Lambert-Eaton syndrome.