Peripheral Neuropathies & Acute Nerve Entrapments
Comprehensive emergency evaluation and protocolized management of acute peripheral nerve entrapments, compressive neuropraxias, and mononeuritis multiplex: Radial nerve compression in the spiral groove ('Saturday night palsy'/'honeymoon palsy', wrist drop, sensory loss over first dorsal webspace), Common Peroneal (fibular) nerve compression at the fibular neck (acute foot drop, loss of dorsiflexion and eversion, steppage gait), Median nerve (carpal tunnel syndrome, Phalen/Tinel signs, thenar atrophy), Ulnar nerve at the cubital tunnel ('claw hand'), and Diabetic CN III pupil-sparing diabetic mononeuropathy vs. compressive posterior communicating artery aneurysm.
Resuscitation Quick Actions • First 2 Minutes
Radial 'Saturday Night Palsy'
Wrist drop + finger extensor weakness + 1st dorsal web sensory loss; TRICEPS EXTENSION IS SPARED; cock-up wrist splint
Peroneal Foot Drop Rule
Weak dorsiflexion/eversion; distinguish from L5 radiculopathy by checking FOOT INVERSION (intact in peroneal palsy, weak in L5)
Pupil-Sparing CN III Rule
Diabetic CN III = pupil spared (normal reactive pupil); DILATED/BLOWN PUPIL = PCOM ANEURYSM (STAT CTA HEAD)
Carpal Tunnel Acute Phalen
Median nerve: numbness of thumb, index, middle, and radial half of ring finger + thenar weakness; volar wrist splint in neutral position
Cubital Tunnel Ulnar Palsy
Medial elbow compression: numbness of 5th digit + ulnar half of 4th digit + weak intrinsic hand muscles (Froment's sign)
Mononeuritis Multiplex
Asymmetric, painful multifocal peripheral nerve infarctions; suspect systemic vasculitis (Polyarteritis Nodosa, Churg-Strauss, Cryoglobulinemia)
Bottom-Line Clinical Pearl
Acute peripheral nerve palsies are categorized by anatomical vulnerability to mechanical compression (neuropraxia). (1) Radial Nerve ('Saturday Night Palsy'): compression against the spiral groove of the humerus produces acute WRIST DROP, finger extensor paralysis, and numbness over the first dorsal web space, but TRICEPS FUNCTION IS PRESERVED (the triceps motor branch arises proximal to the spiral groove). Splint the wrist in 20° extension; neuropraxia resolves over 6–12 weeks. (2) Common Peroneal Nerve: compression against the superficial fibular neck (tight casts, crossing legs, prolonged squatting) produces acute FOOT DROP (inability to dorsiflex or evert foot; sensory loss over lateral lower leg and dorsal foot). Distinguish from L5 radiculopathy: foot inversion (tibialis posterior) is INTACT in peroneal palsy (innervated by the tibial nerve), but IMPAIRED in L5 radiculopathy! (3) Diabetic CN III Palsy: microvascular ischemia spares the superficial parasympathetic pupillary fibers (PUPIL-SPARING); any CN III palsy with a DILATED, NON-REACTIVE PUPIL is a compressive surgical aneurysm of the posterior communicating artery until proven otherwise.
| Nerve & Compression Site | Mechanism of Injury/Eponym | Motor Deficits | Sensory Deficits & Splinting |
|---|---|---|---|
| Radial Nerve (Spiral Groove) | Prolonged direct pressure over the middle third of the humerus (intoxicated sleep with arm draped over a chair: 'Saturday Night Palsy'; sleeping on partner's arm: 'Honeymoon Palsy') | Wrist Drop; paralysis of wrist extensors, finger extensors at MCP joints, and thumb abductors. TRICEPS FUNCTION IS PRESERVED (branch arises proximal to groove in axilla). | Sensory loss confined to the dorsal first web space (between thumb and index finger). Management: Cock-up wrist splint in 20° extension; physical therapy; full recovery expected within 6–12 weeks. |
| Median Nerve (Carpal Tunnel) | Compression under the transverse carpal flexor retinaculum; repetitive motion, pregnancy, hypothyroidism, rheumatoid arthritis | Weakness of thumb abduction (abductor pollicis brevis) and opposition; severe chronic cases show thenar muscle atrophy. | Paresthesias and burning pain in palmar aspects of thumb, index, middle, and radial half of ring finger. Positive Phalen's and Tinel's tests. Management: Volar wrist splint in neutral position. |
| Ulnar Nerve (Cubital Tunnel) | Compression behind the medial epicondyle at the elbow (leaning on elbows, prolonged flexion) | Weakness of interossei and hypothenar muscles; positive Froment's sign (compensatory thumb IP flexion by flexor pollicis longus when pinching paper); severe chronic palsy yields 'Ulnar Claw' hand. | Sensory loss confined to the entire 5th digit (pinky) and medial half of the 4th digit (ring finger). Management: Elbow pad, avoidance of flexion, nocturnal extension splint. |
The common peroneal (fibular) nerve curves superficial to the bone directly around the fibular neck, making it exceptionally vulnerable to external compression (tight plaster casts, knee braces, crossing legs, prolonged bed rest, or fibular head fractures).
| Clinical Parameter | Common Peroneal (Fibular) Nerve Palsy | L5 Lumbar Radiculopathy (Herniated Disc) |
|---|---|---|
| Ankle Dorsiflexion (Tibialis Anterior) | WEAK/PARALYZED (producing foot drop and steppage gait) | WEAK/PARALYZED (foot drop) |
| Foot Eversion (Peroneus Longus/Brevis) | WEAK/PARALYZED | WEAK |
| Foot Inversion (Tibialis Posterior) | COMPLETELY INTACT (NORMAL)<br>(Tibialis posterior is innervated by the Tibial Nerve/L4-L5) | WEAK/IMPAIRED<br>(L5 nerve root supplies both peroneal and tibial branches) |
| Sensory Loss Distribution | Confined to the lateral lower leg and the dorsum of the foot; plantar aspect of foot is completely spared | Radiates down posterolateral thigh, calf, and dorsum of foot to great toe; associated with low back pain and positive Straight Leg Raise. |
Critical Pitfall / Contraindication
DIABETIC PUPIL-SPARING CN III PALSY VS. COMPRESSIVE ANEURYSM: The pupillomotor parasympathetic fibers of Cranial Nerve III (oculomotor) run along the extreme OUTSIDE/SUPERFICIAL surface of the nerve, where they are supplied by pial blood vessels. Microvascular ischemia (diabetes mellitus, hypertension) infarcts the deep central core of the nerve, producing severe ptosis and a 'down-and-out' eye with a COMPLETELY NORMAL, REACTIVE PUPIL (pupil-sparing). In contrast, extrinsic mechanical compression (e.g., expanding Posterior Communicating Artery [PCOM] aneurysm or uncal herniation) crushes the superficial parasympathetic fibers first, producing an acutely DILATED, BLOWN, NON-REACTIVE PUPIL. Any third nerve palsy with pupillary dilation is an emergent PCOM aneurysm requiring immediate CTA Head and neurosurgical intervention.
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