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Entrapment neuropathies
An entrapment neuropathy damages one nerve at a known narrow site, so it is diagnosed from that nerve's sensory and muscle map: median at the wrist, ulnar at the elbow, radial in the arm, peroneal at the fibular head.
Why it matters in the exam
Questions present numbness in certain fingers, weakness of a defined muscle, or foot drop, and ask where the lesion is and what separates it from a root lesion. Classic examples: carpal tunnel syndrome in pregnancy, hypothyroidism or amyloidosis; "Saturday night palsy" of the radial nerve after sleeping with the arm over a chair back; and peroneal palsy after sitting cross-legged or rapid weight loss. A key point is telling foot drop from the peroneal nerve (inversion preserved) from an L5 root lesion (weak inversion). In ulnar neuropathy at the elbow, sensation on the back of the hand is affected, unlike a lesion in Guyon's canal at the wrist.
Key points
- An entrapment neuropathy is diagnosed from the sensory and muscle map of one nerve.
- Carpal tunnel: thumb to half of the ring finger, at night; linked to pregnancy, thyroid, diabetes and amyloidosis.
- Ulnar neuropathy at the elbow also affects sensation on the back of the hand, unlike Guyon's canal.
- Peroneal foot drop preserves inversion; in an L5 lesion it is weak.
- Conservative care first; wasting or failure — surgery.
Summary table
| Nerve and site | Sensation | Weakness and signs | Treatment |
|---|---|---|---|
| Median at the wrist (carpal tunnel) | Thumb to half of the ring finger on the palm side, worse at night | Weakness and wasting of the thenar muscles; Tinel and Phalen signs | Night splint, steroid injection; surgery for wasting or failed treatment |
| Ulnar at the elbow | Little finger and half of the ring finger, including the back of the hand | Weak intrinsic hand muscles, Froment sign, "claw" hand | Avoid pressure and prolonged flexion; surgery for advanced weakness |
| Radial in the arm (spiral groove) | Back of the hand between thumb and index | Wrist drop; triceps spared | Splint; usually recovers within weeks |
| Peroneal at the fibular head | Dorsum of the foot and outer leg | Foot drop, weak dorsiflexion and eversion; inversion preserved | Avoid crossing legs, ankle splint |
| Lateral femoral cutaneous nerve (meralgia paraesthetica) | Burning and numbness of the outer thigh | No weakness | Weight loss, avoid tight belts |
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Sources
- AAOS Clinical Practice Guideline: Management of Carpal Tunnel Syndrome, 2024 — Diagnosis and treatment
- AANEM Practice Parameter: Electrodiagnostic Studies in Ulnar Neuropathy at the Elbow, 1999 — Localisation criteria
Sources are given for further reading; the text is written afresh and not copied from them.
Reviewed by the Examqo AI Professor · 2026-11-06. The text may contain errors; when in doubt, check the source.