Nerve Injury – Clinical Scenarios
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Direct answer
Nerve injuries are graded by what has been disrupted. Neurapraxia (Sunderland first degree) is conduction block without structural break — the compressed "Saturday night" radial palsy that recovers in days to weeks. Axonotmesis (Sunderland second to fourth degree) breaks the axon, triggering Wallerian degeneration distally while the connective tissue scaffold survives variably: second degree spares endoneurium, third disrupts it, fourth leaves only epineurium with internal scarring (neuroma-in-continuity), and regeneration, when it occurs, proceeds at roughly 1-3 millimetres per day. Neurotmesis (fifth degree) severs the whole nerve; without surgical repair, no useful recovery. Diagnosis is clinical patterning plus timing, confirmed by serial examination and nerve conduction studies: a Tinel sign marching distally marks regenerating axons; a Tinel fixed at the injury site marks a neuroma blocking them.
What you must remember
- Seddon-Sunderland ladder: neurapraxia (block, recovers fully); axonotmesis grades 2-4 (axon broken, scaffold variably intact); neurotmesis grade 5 (complete transection) — say the grade, then the prognosis.
- Regeneration arithmetic: axons regrow about 1-3 mm per day (commonly rounded to a millimetre a day); children and proximal injuries regenerate better, and muscles denervated beyond about 12-18 months fibrose irreversibly — the reason for early referral.
- Wallerian degeneration: distal axon and myelin fragment over the first days; the distal stump conducts for a few days before failing — so a normal conduction study immediately after injury proves nothing.
- Ulnar paradox: a proximal ulnar lesion (at the elbow) produces less clawing than a distal one (at the wrist), because the flexor digitorum profundus to the medial two fingers — an ulnar-innervated claw-setter — is still working in the distal lesion; examiners adore asking why the "worse" injury looks milder.
- Classic pattern set: radial nerve at the spiral groove — wrist drop with preserved elbow extension (triceps branch given off higher); common peroneal at the fibular neck — foot drop with intact ankle inversion (tibialis posterior); median at the carpal tunnel — night paraesthesia, thenar wasting, thenar sparing of the superficial head? Note the lumbricals and thenar weakness pattern; axillary — deltoid paralysis after shoulder dislocation; long thoracic — winged scapula; Erb (C5-C6) — waiter's tip; Klumpke (C8-T1) — claw hand with Horner syndrome.
- Froment's sign: thumb interphalangeal flexion on pinching paper — adductor pollicis (ulnar) replaced by flexor pollicis longus (median).
- Management mapping: neurapraxia — observe; clean lacerations — primary epineurial repair within days; closed injuries with no recovery by 3-6 weeks electrophysiology or no clinical return by 3-6 months — explore; grade 4 lesions — resect the neuroma and graft.
Working through a night on call
A young man arrives after a motorcycle crash: closed humeral shaft fracture, wrist drop, loss of thumb and finger extension. Two diagnostic questions settle the frame. Is it neurapraxia or axonotmesis? Early conduction studies cannot tell (the distal stump still conducts for days); so you wait — radial neurapraxia from the spiral groove usually begins recovering within six to eight weeks, and if it does not, exploration with grafting follows. Second, is the injury open or closed? A glass laceration across the wrist with median nerve division is repaired primarily within 72 hours; a traction injury from a humeral fracture is watched. Now apply the arithmetic: a brachial plexus traction injury at C5-6 that regenerates must cover, say, 40 centimetres to the shoulder muscles — nearly six months at best; the same distance from wrist to fingertips is 3-4 weeks, which is why low median and ulnar repairs regain sensation quickest. Along the way, map recovery with a Tinel sign: a point of tingling advancing down the limb month by month is axons under the skin; a tingle that never moves is scar.
How the exam frames it
Indian practicals stage this as a short case: a patient with foot drop, and the examiner asks "which nerve, where, and what is spared?" — the expected answer names the common peroneal at the fibular neck, inversion of the ankle preserved by tibialis posterior (tibial nerve), and the mechanism as squatting, crossed legs or cast pressure. The ulnar paradox is the single favourite trap: be ready to explain that wrist lesions claw more than elbow lesions because only in the former does the uninjured flexor digitorum profundus medial half flex the ulnar two fingers against a paralysed interossei-lumbrical balance. Theory papers add the timeline question — axons 1-3 mm/day, motor end plates degenerating beyond recovery at about a year — and the Seddon-Sunderland correspondence, expecting "fourth degree" to be volunteered with the phrase neuroma-in-continuity. One viva line worth rehearsing: after repair, the sensory return follows the same rule but never fully, because axons mismatch their original receptors.
Frequently asked questions
What distinguishes neurapraxia from axonotmesis?
Neurapraxia is conduction block without axonal break and recovers in days to weeks; axonotmesis interrupts the axon with Wallerian degeneration and recovers only by slow regeneration.
At what rate do severed axons regenerate?
Roughly 1-3 millimetres per day, so clinical recovery time can be estimated from the distance between injury and target muscle.
Why is clawing worse in distal than proximal ulnar nerve injury?
In the ulnar paradox, a proximal lesion also paralyses the ulnar half of flexor digitorum profundus, relaxing the fingers and reducing clawing that a wrist lesion preserves.
What does a migrating Tinel sign indicate?
Regenerating axons advancing along the nerve — a Tinel fixed at the injury site suggests a neuroma or block needing surgical exploration.
Which nerve injury causes foot drop with preserved ankle inversion?
Common peroneal nerve compression at the neck of the fibula; tibialis posterior (tibial nerve) still inverts the foot, excluding an L5 radiculopathy from a pure peroneal palsy.