The Nerve Palsy Localizer: Median, Radial, Ulnar, Peroneal, and the Brachial Plexus

The Localizer Method: Movement, Skin, Reflex, Mechanism

Four steps, in order, on every palsy. The boards grade the order: name the movement, map the skin, check the reflex, then ask how it happened.

Step one is the lost movement, and one movement has one owner. Wrist drop is the radial nerve, the broken OK sign is the anterior interosseous nerve, the claw is the ulnar nerve, foot drop is the common peroneal nerve, and the winged scapula is the long thoracic nerve. Two nerves can share a gross function, like wrist flexion, but each specific motion answers to one nerve, and boards test the specific motion.

Step two is the skin patch, the fingerprint the movement cannot give you. The median owns the palmar thumb, index, and middle fingers; the radial owns the dorsoradial hand and forearm; the ulnar owns the ring and little fingers on both sides; the common peroneal owns the dorsum of the foot; the femoral reaches the medial leg through the saphenous nerve. The discriminator that recurs: a root lesion follows a dermatome that crosses nerve borders, while a nerve lesion stays inside one patch.

Tap each site on the arm and read the territory it owns.

Step three is the reflex, the segmental anchor. The biceps and brachioradialis arcs live at C5-C6, the triceps at C7, the patellar at L4, and the Achilles at S1. A peripheral nerve lesion knocks out the one arc that rides that nerve; a root lesion can fade every arc at its segment. That is why quadriceps weakness with a dead patellar reflex points above the muscle, and why a pure radial palsy at the spiral groove spares the triceps reflex while a C7 root does not. Step four is the mechanism, which sets the management: compression gets time and a splint, entrapment gets splinting and electrodiagnostics, traction gets reduction and protection, and laceration gets repair.

The Median Nerve: Carpal Tunnel, the Broken OK Sign, and the Spared Palm

The median is tested as two stories: the sensory tunnel at the wrist and the pure motor branch below the elbow. The palmar cutaneous branch is the level detector for both.

Carpal tunnel syndrome is compression of the median nerve under the transverse carpal ligament. The classic history is nocturnal burning and tingling in the thumb, index, and middle fingers that wakes the patient, relieved by shaking the hand, with dropping objects as the grip fades. Tinel sign on percussion and Phalen sign on sustained wrist flexion reproduce it. Late disease shows wasting of the thenar eminence as the recurrent thenar branch to the abductor pollicis brevis fails. The discriminator boards love: sensation over the palmar base of the thumb stays intact, because the palmar cutaneous branch leaves the median above the flexor retinaculum. Sensory loss over the thenar eminence itself means the lesion is above the wrist, not in the tunnel.

The anterior interosseous nerve is the pure motor median branch below the elbow. It feeds the flexor pollicis longus and the index slip of the flexor digitorum profundus, so its failure breaks the OK sign: the circle becomes a pad-to-pad pinch, and there is no sensory loss anywhere. It appears after forearm fractures, heavy use, or a backpack strap, and most cases recover with activity change and time. Pronator syndrome is the elbow-level entrapment between the heads of pronator teres: median symptoms that worsen with resisted pronation rather than at night, often with palmar sensation involved and thenar bulk preserved.

Read the vignette and commit before the reveal.

The Radial Nerve: Wrist Drop and the Spiral Groove

The great extensor nerve fails as a dropped wrist, and the site of the drop is decided by the triceps and the sensory patch. PIN is the motor-only masquerader.

Wrist drop is the radial signature: wrist and finger extension fail and the wrist hangs in flexion when the arm is held out. Grip weakens because the wrist cannot lock in extension, and the brachioradialis reflex fades. Saturday night palsy is compression in the spiral groove, classically the arm draped over a chair back during deep sleep or intoxication: wrist drop with sensory loss over the dorsoradial hand and forearm and, critically, triceps spared, because the branches to the triceps leave the nerve above the groove. A midshaft humerus fracture injures the same site: the humerus nerve ladder runs axillary at the surgical neck, radial at the midshaft, and median at the distal shaft.

The two sites boards make you separate: triceps full means the lesion is below the axilla, and triceps weak means the lesion is at the axilla, the crutch palsy. The posterior interosseous nerve adds the third rung: compression of the deep motor branch at the supinator spares wrist extension, because extensor carpi radialis longus is innervated above it, and carries no sensory loss, because the superficial radial branch separated above. Finger drop with a live wrist and a silent hand is PIN every time.

Flip between the two radial lesions and hold the split.

Management follows the mechanism. Compression gets time: remove the pressure, splint the wrist in extension to keep the fingers moving, and recheck over 6 to 12 weeks; most compressive radial palsies recover. A fracture gets fracture care plus nerve observation, with electrodiagnostics at 3 to 4 weeks if recovery is not underway. A laceration gets repair. The wrist splint is the one move that belongs to every radial palsy, because it prevents contracture while the nerve heals.

The Ulnar Nerve: The Claw, Froment, and the Dorsal Hand Split

The ulnar is the nerve of the intrinsics, and it is tested at two sites with one sensory question: is the dorsal hand numb or spared?

Loss of the intrinsics makes the claw hand: the ring and little fingers hyperextend at the metacarpophalangeal joints and flex at the interphalangeal joints. The Froment sign completes the story: with adductor pollicis weak, the patient pinches paper with a flexed thumb IP joint, because the median-innervated flexor pollicis longus takes over. The Wartenberg sign is the little finger sitting abducted, pulled away by the extensor digiti minimi once the adducting palmar interosseous dies. All three signs, one nerve.

Site one is the cubital tunnel at the elbow behind the medial epicondyle: tingling in the ring and little fingers with numbness over the dorsal AND palmar medial hand, from leaning on the elbow or chronic flexion. Site two is the Guyon canal at the wrist between the pisiform and the hook of hamate: a hook of hamate fracture or handlebar pressure crushes the nerve there, and the dorsal hand stays spared because the dorsal ulnar cutaneous branch left the nerve above the wrist. The paradox: a wrist-level claw is worse than an elbow-level claw, because at the elbow the ulnar half of the flexor digitorum profundus is paralyzed too, so the fingertips do not flex as hard. Dorsal numbness picks the elbow; dorsal sparing picks the wrist; the severe claw agrees with the wrist.

Open each ulnar mechanism and hold the level test.

The Plexus Ladder: Roots to Branches, Erb to Klumpke

The brachial plexus is a five-rung ladder, and the boards test it as two named falls: the waiter tip of C5-C6 and the claw with Horner of C8-T1.

The ladder runs roots to trunks to divisions to cords to terminal branches: the C5 through T1 roots leave the cord, form three trunks behind the scalenes, split into anterior and posterior divisions, reorganize into the lateral, medial, and posterior cords around the axillary artery, and end in the named nerves. The axillary and radial come off the posterior cord, the ulnar off the medial cord, and the median forms from the lateral and medial cords together. The level of the injury is the level of the ladder, and the pattern of lost branches reads it.

Erb-Duchenne palsy is the upper injury, C5-C6, from shoulder dystocia at delivery, a fall on the shoulder, or lateral traction on the head. The arm hangs adducted and internally rotated with the elbow extended and the wrist flexed, the waiter tip posture, because the deltoid, external rotators, and biceps fail together. Most infants recover with gentle range of motion; failure of the biceps to return by 3 to 6 months earns a plexus surgical evaluation. Klumpke palsy is the lower injury, C8-T1, from upward traction on the arm or an apical lung tumor: an ulnar claw hand with medial hand and forearm numbness, and if T1 is involved, ipsilateral Horner syndrome with ptosis, miosis, and anhidrosis. Horner with a hand palsy is T1, and an apical lung mass is the adult cause to name.

Build the ladder in order.

Tap each letter of the ladder mnemonic.

The Foot Drop Bench: Peroneal, Sciatic, L5, or Tibial

The same dropped foot localizes to four levels, and one question settles most of them: is inversion strong or weak?

Common peroneal nerve injury is the default foot drop. The nerve wraps the fibular head, so fibular head fracture, leg crossing, tight casts, prolonged squatting, and rapid weight loss all hit it. Ankle dorsiflexion and toe extension fail, the foot slaps with each step, and the patient lifts the knee high to clear the toes: the steppage gait. The two facts that seal it: inversion stays strong, because tibialis posterior is tibial nerve, and the ankle reflex stays intact. Sensation fades over the dorsum of the foot and the lateral leg. An ankle-foot orthosis clears the toes while the nerve recovers.

Sciatic nerve injury is the combined story: hamstring weakness plus everything below the knee, so foot drop AND weak plantar flexion AND a dead ankle reflex together, from a gluteal injection or a posterior hip dislocation. L5 radiculopathy weakens dorsiflexion AND inversion, because both tibialis anterior and tibialis posterior share L5, spares plantar flexion, and carries radicular pain with paraspinal denervation on EMG. The tibial nerve is the masquerader in reverse: it takes plantar flexion, toe walking, and the ankle reflex, never dorsiflexion, and its entrapment is tarsal tunnel syndrome behind the medial malleolus. Above the knee, the femoral nerve fails as a buckling knee with an absent patellar reflex, classically from a psoas hematoma in the anticoagulated patient with the hip held flexed.

Read each foot-drop finding and commit to the level.

Meralgia and Tarsal Tunnel: The Numb Extremes

Burning that hugs the outer thigh or the medial ankle is often a tunnel, not a spine. The reflex and the weakness pattern draw the line.

Meralgia paresthetica is entrapment of the lateral femoral cutaneous nerve, a pure sensory nerve. Obesity, pregnancy, tight belts, and prolonged standing compress it, producing burning or numbness over the lateral thigh that worsens with standing and vanishes lying down. Because the nerve carries no motor fibers, strength and the patellar reflex stay normal; weakness or a depressed reflex redirects the diagnosis to an L2-L3 radiculopathy or a femoral neuropathy.

Tarsal tunnel syndrome is entrapment of the tibial nerve behind the medial malleolus: burning along the medial ankle and sole, worse at night, with a positive Tinel sign at the tunnel. The Achilles reflex stays intact, which separates it from S1 radiculopathy. Treatment is unloading the tunnel; plantar fasciitis hurts under the heel with weight bearing and has no numbness, which keeps the two apart.

Localize the burning: lateral thigh, medial ankle, or spine.

The EMG/NCS Readout: Axonal Loss, Demyelination, and the Paraspinal Stamp

The last cut between nerve and root is electrodiagnostic, and it reads in two numbers: amplitude and velocity, then the needle.

Nerve conduction studies split the lesion by what is lost. Low compound muscle action potential amplitude with a normal velocity is axonal loss: too few fibers remain to fire, and recovery is slow because axons must regrow. Slow conduction velocity with a preserved amplitude is demyelination: the myelin is gone but the axons survive, conduction crawls across the segment, and remyelination brings recovery. The classic cubital tunnel study shows exactly that: slow conduction across the elbow with a live response.

Needle EMG adds the location and the date. Fibrillations and positive sharp waves are the electrical signature of muscle that has lost its axon, and they need about 2 to 3 weeks to appear, so a study run too early looks falsely normal. The single most important split: denervation in the paraspinal muscles means the root, because the dorsal rami leave the root right at the spine, and a nerve lesion never touches them. Foot drop with denervation in the tibialis anterior but silent paraspinals is a peroneal nerve; the same fibrillations reaching the paraspinals move the lesion to L5.

Tick each finding and read the classification it earns.

Walkthrough: localize the palsy

Original practice scenarios, one at a time. Choose an answer, then open any option to work its reasoning.