Peripheral Nerve and Muscle Pathology
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Direct answer
Peripheral nerve and muscle biopsy serve the small set of neuromuscular diseases that cannot be settled by clinical and electrophysiological means — vasculitic neuropathy, leprous neuritis, amyloid neuropathy and the inflammatory myopathies. Nerve pathology divides into axonal degeneration (Wallerian change, regeneration clusters, asymmetric loss in vasculitis) and demyelination (segmental demyelination, onion-bulb formations in chronic inflammatory demyelinating polyneuropathy), while muscle pathology divides into denervation change, inflammatory infiltrates and structural dystrophies. Because these are pattern diagnoses, the biopsy earns its place only when the pattern changes management — the question FMGE asks by presenting a patient with a specific stainable finding.
What you must remember
- Nerve biopsy site: the sural nerve, a pure sensory nerve sacrificed at the ankle; muscle (usually quadriceps or biceps) is sampled on the weak side.
- Vasculitic neuropathy: necrotising vessel-wall inflammation with fibrinoid necrosis, asymmetric axonal loss and regeneration clusters — often the presenting feature of polyarteritis or ANCA disease.
- Demyelinating pattern: segmental demyelination with remyelination; onion bulbs (concentric Schwann cell processes) indicate repeated demyelination-remyelination, as in CIDP.
- Leprous neuritis: Mycobacterium leprae invades Schwann cells; lepromatous disease shows foamy macrophages teeming with bacilli, tuberculoid disease shows epithelioid granulomas destroying the nerve — the thickened, tender ulnar or great auricular nerve of Indian clinics.
- Denervation sequence: angular atrophic fibres first, then group atrophy, and fibre-type grouping once reinnervation redistributes fibres — the last proves chronic neurogenic change.
- Inflammatory myopathies: dermatomyositis shows perifascicular atrophy with perivascular inflammation; polymyositis shows endomysial CD8 cells invading non-necrotic fibres; inclusion body myositis adds rimmed vacuoles and targets finger flexors in the elderly.
- Structural markers: dystrophin absent in Duchenne, mosaic in Becker and carrier muscle; ragged red fibres on modified Gomori trichrome mark mitochondrial disease; nemaline rods appear in congenital myopathies.
Where students slip
The three inflammatory myopathies are memorised as words without location: perifascicular atrophy belongs to dermatomyositis (its pathology is vascular), endomysial T-cell invasion to polymyositis, and rimmed vacuoles with finger-flexor weakness in an older patient to inclusion body myositis, which responds poorly to steroids. Onion bulbs get labelled as tumours; they are hyperplastic Schwann cell whorls from repetitive demyelination. And in Indian exams, never leave out leprosy when a question hands you a thickened nerve — it is the world's commonest treatable neuropathy and appears in every FMGE cycle.
Frequently asked questions
Which nerve is biopsied and why?
The sural nerve, being purely sensory, so its removal leaves only numbness while revealing vasculitis, amyloid, leprosy or demyelinating patterns.
What are onion-bulb formations?
Concentric whorls of Schwann cell processes around axons from repeated episodes of demyelination and remyelination, hallmarking chronic inflammatory demyelinating polyneuropathy.
What does perifascicular atrophy indicate?
Dermatomyositis, reflecting complement-mediated microangiopathy that injures the watershed periphery of fascicles, often with perivascular inflammation.
How does dystrophin staining separate Duchenne and Becker?
Duchenne muscle shows complete absence of sarcolemmal dystrophin, while Becker shows patchy, reduced-size staining — and carriers a mosaic pattern.
Which myopathy produces rimmed vacuoles?
Inclusion body myositis, in elderly patients with asymmetric finger-flexor and quadriceps weakness, poorly responsive to immunosuppression.