Neuromuscular disorders: peripheral neuropathy, myopathy, neuromuscular junction, motor neuron disease
This domain tests anatomical pattern recognition within the peripheral nervous system: which structure is affected, whether the process is axonal or demyelinating, and whether it is acquired, inherited, or immune-mediated. Local epidemiology matters - leprosy and diabetic neuropathy are common in Egypt.
Peripheral neuropathy
Length-dependent symmetrical sensory loss beginning in the feet suggests diabetes, alcohol, B12 deficiency, uraemia, or a toxic cause. Guillain-Barré syndrome (AIDP) follows infection by 1-3 weeks, produces ascending flaccid weakness with areflexia, albuminocytological dissociation in CSF, and a nadir within four weeks; treat with IVIG 0.4 g/kg/day for five days or plasma exchange - steroids are ineffective. Monitor vital capacity (intubate around 20 mL/kg or falling rapidly), swallowing and autonomic instability. CIDP is the chronic counterpart, relapsing beyond eight weeks, and responds to steroids, IVIG or plasma exchange. Vasculitic neuropathy is painful, asymmetric and multiplex. Hereditary motor and sensory neuropathy type 1A (PMP22 duplication) gives pes cavus, areflexia and a family history.
Myopathy
Proximal weakness with normal sensation and reflexes preserved until late. Inflammatory myopathies: dermatomyositis (heliotrope rash, Gottron papules, anti-Mi-2, malignancy screen), polymyositis, immune-mediated necrotising myopathy (anti-SRP, anti-HMGCR). Statins, hypothyroidism, alcohol and corticosteroids cause toxic myopathy. Duchenne and Becker dystrophies reflect dystrophin defects with calf pseudohypertrophy and markedly raised creatine kinase.
Neuromuscular junction
Myasthenia gravis: fluctuating ptosis and diplopia, bulbar and proximal weakness worsening with activity; anti-acetylcholine receptor antibodies in about 85% of generalised cases, anti-MuSK in a minority; decremental response on repetitive stimulation; single-fibre jitter is most sensitive; screen for thymoma with chest imaging. Treat with pyridostigmine, corticosteroids, azathioprine or mycophenolate, and thymectomy in selected cases. Lambert-Eaton syndrome is presynaptic with anti-voltage-gated calcium channel antibodies, proximal leg weakness, hyporeflexia, autonomic symptoms, an incremental response and small-cell lung cancer. Botulism causes descending weakness with pupillary involvement.
Motor neuron disease
ALS combines upper and lower motor neuron signs with no sensory loss and no sphincter involvement; fasciculations, wasting, brisk reflexes and extensor plantars may coexist. Diagnosis is clinical supported by EMG. Riluzole 50 mg twice daily modestly prolongs survival; edaravone and tofersen (SOD1) have niche roles. Manage spasticity, secretions, nutrition (gastrostomy), and non-invasive ventilation.
Worked example
A 45-year-old has two weeks of ascending leg weakness, areflexia, and mild bulbar symptoms after diarrhoea. CSF shows protein 0.9 g/L with 2 cells. Give IVIG, admit for monitoring of vital capacity and swallow, and counsel that nadir occurs within four weeks and recovery is usually gradual. Steroids should not be given.
Traps
Do not give steroids in Guillain-Barré. Do not miss myasthenic crisis (respiratory failure from weakness) or confuse it with cholinergic crisis (salivation, bradycardia, diarrhoea). Sensory sparing is the key discriminator in ALS. An incremental response means Lambert-Eaton, not myasthenia.