Focal Neuropathies, Radiculopathy & Plexopathy
Pinpoint the nerve, the root or the plexus from a handful of exam findings
Upper-Limb Compression Neuropathies
Ulnar neuropathy at the elbow. The second most common entrapment after carpal tunnel syndrome, caused by leaning on the elbows, prolonged elbow flexion or old fractures compressing the nerve in the cubital tunnel. Sensation is lost over the little finger, the ulnar half of the ring finger and the dorsal ulnar hand. Weak interossei impair finger spreading, weak adductor pollicis produces the Froment sign (the thumb IP joint flexes during a paper pinch as flexor pollicis longus substitutes), and an abducted little finger (Wartenberg sign) and clawing of fingers 4–5 follow. Elbow padding and night extension splints come first; decompression or transposition is for progressive weakness.
Ulnar lesions at the wrist (Guyon canal — cyclists, hook-of-hamate fracture) spare dorsal hand sensation and the ulnar flexor digitorum profundus, so clawing is paradoxically worse than with an elbow lesion.
Radial neuropathy at the spiral groove. Classic after sleeping with the arm draped over a chair while intoxicated ("Saturday night palsy") or after a midshaft humerus fracture. Wrist and finger drop, weak brachioradialis and numbness of the dorsal first web space follow, but the triceps and its reflex are spared because their branches leave above the groove. Most lesions are neurapraxic and recover over weeks to months with a cock-up wrist splint. Axillary compression (crutches) also weakens the triceps, and a posterior interosseous nerve lesion causes finger drop without sensory loss, with radially deviated wrist extension because extensor carpi radialis longus is spared.
| Nerve and site | Typical cause | Weakness | Sensory loss |
|---|---|---|---|
| Median at wrist (carpal tunnel) | Repetitive wrist use, pregnancy, hypothyroidism | Thumb abduction (thenar) | Palmar thumb to radial ring finger |
| Ulnar at elbow | Leaning, elbow flexion, old fracture | Interossei, hypothenar, adductor pollicis | Little finger, ulnar ring finger, dorsal ulnar hand |
| Radial at spiral groove | Saturday night palsy, humerus fracture | Wrist and finger extensors, brachioradialis; triceps spared | Dorsal first web space |
Lower-Limb Compression Neuropathies
Common fibular (peroneal) neuropathy. The nerve winds superficially around the fibular neck, where habitual leg crossing, prolonged squatting, casts, braces, bed rest and rapid weight loss (bariatric surgery, anorexia) compress it. The deep branch lesion causes foot drop and a steppage gait, the superficial branch lesion weak eversion, with numbness of the lateral leg and dorsum of the foot and often a Tinel sign at the fibular neck. Inversion (tibialis posterior, supplied by the tibial nerve), hip abduction and the ankle jerk are preserved, and there is no back pain. Management is removing pressure, padding and an ankle-foot orthosis; most recover over months.
| Finding | Common fibular nerve | L5 root | Sciatic nerve |
|---|---|---|---|
| Dorsiflexion, eversion | Weak | Weak | Weak |
| Inversion | Normal | Weak | Often weak |
| Hip abduction | Normal | Weak | Normal |
| Knee flexion, plantarflexion | Normal | Normal or mildly weak | Often weak |
| Setting | Painless; leg crossing, weight loss | Back pain radiating to the great toe | Hip surgery, gluteal injection |
Meralgia paresthetica. The purely sensory lateral femoral cutaneous nerve (L2–L3) is compressed under the inguinal ligament near the anterior superior iliac spine by obesity, pregnancy, tight belts, waistbands or tool belts. Burning or numbness of the anterolateral thigh occurs with normal strength and knee reflex. Diagnosis is clinical; looser clothing, weight loss and NSAIDs usually suffice, with a local nerve block for persistent cases. Femoral neuropathy (retroperitoneal hematoma on anticoagulation, prolonged lithotomy positioning) differs by weakening the quadriceps and abolishing the knee jerk.
Tarsal tunnel syndrome. Compression of the tibial nerve under the flexor retinaculum behind the medial malleolus (ankle trauma, flat feet, ganglia, varicosities, tenosynovitis) causes burning pain and paresthesia of the sole, worse with standing and walking, and a Tinel sign behind the malleolus. Orthotics, NSAIDs or injection come first; surgical release is for refractory cases.
Radiculopathy
Localization. A compressed root causes dermatomal pain and numbness, myotomal weakness and reflex loss. The Spurling maneuver (neck extension and rotation toward the painful side with downward pressure) reproduces arm pain in cervical radiculopathy, and a straight-leg raise reproducing radicular pain at about 30–70° supports L5 or S1 involvement; the crossed straight-leg raise is more specific. A posterolateral L4–L5 disc (or foraminal stenosis) compresses the L5 root, and an L5–S1 disc compresses S1. C7 is the most commonly affected cervical root.
| Root | Sensory loss | Weakness | Reflex |
|---|---|---|---|
| C6 | Thumb, index finger | Elbow flexion, wrist extension | Biceps, brachioradialis |
| C7 | Middle finger | Triceps, wrist flexion | Triceps |
| C8 | Little finger, medial forearm | Grip, hand intrinsics | None reliable |
| L4 | Medial leg | Knee extension | Knee jerk |
| L5 | Dorsum of foot, great toe | Great-toe extension, dorsiflexion, inversion, hip abduction | None reliable |
| S1 | Lateral foot, sole | Plantarflexion (toe-walking) | Ankle jerk |
Management. Without red flags, disc-related radiculopathy usually improves within 6–12 weeks with NSAIDs, staying active and physical therapy. MRI is reserved for severe or progressive motor deficit, suspected cancer or infection, or persistent pain when surgery or an injection is being considered.
Saddle anesthesia, urinary retention or bilateral leg deficits signal cauda equina syndrome — obtain emergency MRI and surgical decompression.
Plexopathy and Mononeuritis Multiplex
Neuralgic amyotrophy (Parsonage-Turner syndrome). An inflammatory brachial plexus neuritis that often follows infection, vaccination, surgery or childbirth. Days to weeks of severe shoulder and arm pain give way, as the pain eases, to patchy weakness and wasting — winged scapula (long thoracic nerve), weak supraspinatus and infraspinatus (suprascapular nerve), or a lost "OK" sign (anterior interosseous nerve) — with little sensory loss. EMG shows patchy denervation. Treatment is analgesia and physical therapy; recovery takes months to years.
Pancoast tumor and radiation. An apical lung tumor invading the lower trunk (C8–T1) and sympathetic chain causes shoulder pain radiating down the medial arm, wasted hand intrinsics, ulnar-side numbness and ipsilateral Horner syndrome. After breast or lung irradiation, painful lower-trunk plexopathy with Horner favors tumor, whereas painless upper-trunk weakness with myokymic discharges on EMG favors radiation injury.
Diabetic lumbosacral radiculoplexus neuropathy. Also called diabetic amyotrophy, it affects older adults with type 2 diabetes that is often well controlled. Severe unilateral hip or thigh pain and marked weight loss precede proximal weakness, quadriceps wasting and a lost knee jerk, often spreading to the other leg. Nerve biopsy shows ischemic microvasculitis. The course is monophasic with slow, incomplete recovery; treatment is pain and glucose control plus rehabilitation.
Mononeuritis multiplex. Painful, stepwise deficits in two or more separate named nerves reflect nerve infarction. Causes include vasculitis (polyarteritis nodosa with hepatitis B, EGPA, microscopic polyangiitis, rheumatoid vasculitis, cryoglobulinemia with hepatitis C), diabetes and leprosy, the leading cause worldwide (thickened nerves, anesthetic hypopigmented patches). Workup includes ESR/CRP, ANCA, hepatitis serologies, cryoglobulins, glucose, EMG/NCS and nerve biopsy; vasculitic neuropathy is treated with glucocorticoids plus cyclophosphamide or rituximab. Painless recurrent palsies at pressure sites instead suggest hereditary neuropathy with liability to pressure palsies (PMP22 deletion).
Electrodiagnostic Testing
Timing. Right after an injury, conduction distal to the lesion is still normal. Wallerian degeneration lowers distal motor and sensory amplitudes over about 1–2 weeks, and fibrillations and positive sharp waves appear on EMG by about 3 weeks, so studies are most informative at least 3 weeks after injury; reinnervation later produces large, polyphasic motor units.
Interpretation. Nerve conduction studies separate axonal loss (low amplitudes) from demyelination (slowed velocity, conduction block), and needle EMG maps denervated muscles to a root, plexus or nerve; paraspinal denervation places the lesion at the root.
Radiculopathy numbs the skin yet leaves sensory nerve action potentials normal because the lesion lies proximal to the dorsal root ganglion; plexopathies and peripheral nerve lesions reduce them.