Common Mononeuropathies: A High-Yield USMLE Review
A mononeuropathy produces weakness and sensory loss in the distribution of one specific peripheral nerve—not a dermatomal or stocking-glove pattern. Localizing the lesion means matching the compression site to the exact motor and sensory deficits and the classic history. This review covers the four highest-yield compression neuropathies: median at the carpal tunnel, ulnar at the cubital tunnel, radial at the spiral groove, and common peroneal at the fibular head.
Median Nerve — Carpal Tunnel Syndrome
The median nerve is compressed at the wrist within the carpal tunnel. Motor involvement produces thenar weakness with impaired thumb abduction and opposition. Sensory loss covers the palmar surface of the thumb, index, middle, and the lateral half of the ring finger. The classic history is nocturnal paresthesias that prompt the patient to shake the hand for relief.
Ulnar Nerve — Cubital Tunnel Syndrome
The ulnar nerve is entrapped at the elbow within the cubital tunnel. Motor deficits involve the interossei, impairing finger abduction and adduction. Sensory loss affects the medial hand, the pinky, and the medial half of the ring finger. The classic history involves leaning on the elbow or repetitive elbow flexion.
Radial Nerve — Saturday Night Palsy
The radial nerve is compressed at the spiral groove of the humerus. This produces wrist drop from loss of wrist and finger extension. Sensory loss localizes to the dorsal web space between the thumb and index finger. The classic history is an arm draped over a chair while intoxicated, producing prolonged compression.
Common Peroneal (Fibular) Nerve — Foot Drop
The common peroneal (fibular) nerve is compressed at the fibular head at the knee. Motor loss impairs dorsiflexion and eversion, causing foot drop. Sensory loss covers the dorsal foot and lateral leg. The classic history includes habitual leg crossing, prolonged squatting, or recent weight loss.
High-yield
- Localize before you diagnose: mononeuropathy follows a single nerve's distribution, not a dermatome or a symmetric stocking-glove pattern.
- Median = carpal tunnel at the wrist → thenar weakness + palmar thumb/index/middle/lateral ring sensory loss + nocturnal shaking for relief.
- Ulnar = cubital tunnel at the elbow → interossei weakness + medial hand/pinky/medial ring sensory loss.
- Radial = spiral groove of humerus → wrist drop + dorsal thumb-index web space sensory loss + 'Saturday night palsy.'
- Common peroneal = fibular head at the knee → foot drop (dorsiflexion + eversion loss) + dorsal foot/lateral leg sensory loss.
- A stocking-glove, symmetric, sensory-predominant pattern signals polyneuropathy (diabetes, alcohol, B12 deficiency, GBS)—not a single mononeuropathy.
Pitfalls
- Confusing the sensory territories of the median (thumb, index, middle, lateral ring) and ulnar (pinky, medial ring, medial hand) nerves.
- Mislocalizing wrist drop—it reflects radial nerve compression at the spiral groove, not a wrist or forearm problem.
- Attributing foot drop reflexively to a spinal root when compression at the fibular head is the classic peripheral cause.
- Overlooking recent weight loss as a risk factor for common peroneal palsy at the fibular head.
- Interpreting a symmetric ascending, areflexic, stocking-glove pattern as mononeuropathy when it points to a polyneuropathy such as Guillain-Barré.
Clinical pearls
- Shaking the hand at night to relieve tingling is the classic clue for median nerve carpal tunnel syndrome.
- An arm draped over a chair while intoxicated points straight to radial palsy at the spiral groove.
- Habitual leg crossing plus foot drop localizes to the common peroneal nerve at the fibular head.
- Interossei weakness with medial-hand numbness after elbow leaning is ulnar cubital tunnel syndrome.
Frequently asked
How do I distinguish median from ulnar neuropathy on an exam question?
Match the sensory territory: median covers the palmar thumb, index, middle, and lateral ring finger with thenar (thumb abduction/opposition) weakness, while ulnar covers the medial hand, pinky, and medial ring finger with interossei (finger abduction/adduction) weakness. Site also differs—median at the wrist (carpal tunnel), ulnar at the elbow (cubital tunnel).
Why does radial nerve compression cause wrist drop?
The radial nerve supplies wrist and finger extensors. Compression at the spiral groove of the humerus knocks out extension, so the wrist and fingers droop—the classic wrist drop of Saturday night palsy.
What is the classic history for radial 'Saturday night palsy'?
An intoxicated person draping the arm over the back of a chair, compressing the radial nerve at the spiral groove for a prolonged period, then awakening with wrist drop and dorsal web-space numbness.
Where is the common peroneal nerve compressed, and what deficit results?
At the fibular head at the knee. This causes foot drop from loss of dorsiflexion and eversion, plus sensory loss over the dorsal foot and lateral leg.
What are the risk factors for common peroneal neuropathy?
Habitual leg crossing, prolonged squatting, and recent weight loss—each increases external compression of the superficial nerve at the fibular head.
How do I know a presentation is a mononeuropathy and not a polyneuropathy?
A mononeuropathy follows the distribution of one specific nerve. A polyneuropathy is symmetric, affects the longest nerves first (feet then hands, 'stocking-glove'), and is usually sensory-predominant—think diabetes, alcohol, B12 deficiency, or Guillain-Barré.
Which mononeuropathy presents with nocturnal symptoms relieved by shaking the hand?
Median nerve carpal tunnel syndrome—patients wake with paresthesias and shake the hand ('flick sign') for relief, with numbness over the palmar thumb, index, middle, and lateral ring finger.
Turn this into reasoning you can use on exam day — practice Common Mononeuropathies on branching cases where your decisions shape the patient.