ulnar nerve palsy

**Anatomy (C8–T1, medial cord → ulnar nerve):** - Runs posterior to the medial epicondyle (cubital tunnel) → between the two heads of flexor carpi ulnaris → along the forearm with the ulnar artery → wrist via Guyon canal (between pisiform and hook of hamate). - Motor: flexor carpi ulnaris, medial half of flexor digitorum profundus (digits 4–5), hypothenar muscles, adductor pollicis, deep head of flexor pollicis brevis, all dorsal and palmar interossei, and the medial 2 lumbricals (digits 4–5). - Sensory: palmar and dorsal medial 1.5 digits (5th and medial half of 4th) plus the hypothenar eminence. The palmar cutaneous branch (palm) and dorsal cutaneous branch (dorsum of hand) arise proximal to Guyon canal.

**Common sites/causes:** 1. **Elbow (most common)**: cubital tunnel syndrome, from prolonged elbow flexion, leaning on the elbow, or repetitive trauma. Fracture of the medial epicondyle (typically a supracondylar fracture in children) and "funny bone" trauma are also causes. Also seen with arthritis and cubital valgus. 2. **Wrist (Guyon canal)**: hook of hamate fracture, cyclists ("handlebar palsy"), ganglion cyst, repetitive pressure. 3. Less common: diabetic neuropathy, leprosy, thoracic outlet syndrome (lower trunk/C8–T1; Klumpke palsy), and medial cord injury.

**Clinical findings:** - **Sensory loss**: medial 1.5 digits (palmar and dorsal). Wrist (Guyon) lesions spare the dorsal hand sensation because the dorsal cutaneous branch comes off proximally. Elbow lesions involve the dorsal ulnar hand. - **Motor**: weak finger abduction/adduction (interossei), weak grip, atrophy of the hypothenar and interosseous muscles (guttering between metacarpals), and wasting of the first dorsal interosseous. - **Claw hand (ulnar claw)**: hyperextension at the MCP joints and flexion at the PIP/DIP joints of digits 4–5, from loss of the medial lumbricals (which normally flex MCP and extend IP joints). - **"Ulnar paradox"**: the claw is *less* pronounced with proximal (elbow) lesions because the medial FDP is also paralyzed, so the DIP flexion is lost. Distal (wrist) lesions spare the FDP, so the claw is more obvious. - **Froment sign**: when holding paper between thumb and index finger, the patient flexes the thumb IP joint (using the median-innervated FPL) to compensate for weak adductor pollicis. - **Wartenberg sign**: persistent abduction of the 5th digit due to weak palmar interosseous (unopposed extensor digiti minimi). - **Card test**: inability to hold a card between extended fingers (weak palmar interossei adduction). - **Elbow lesions** additionally cause: radial deviation of the wrist on flexion (weak FCR... i.e., loss of FCU) and weak DIP flexion of digits 4–5. - **Tinel sign** at the cubital tunnel, positive elbow flexion test.

**Differentiation (high-yield):** - Ulnar claw (distal lesion; "can't extend fingers at the IP joints") vs. **"Hand of benediction"** (median nerve injury at the proximal forearm/elbow; digits 2–3 can't flex when asked to make a fist) vs. **Ape hand** (median, thenar atrophy). - C8–T1 radiculopathy / Klumpke palsy can mimic this but also involves median-innervated thenar muscles and may have Horner syndrome (T1 sympathetic).

**Diagnosis:** Clinical exam, confirmed with EMG/nerve conduction studies (slowed conduction across the elbow). Imaging (ultrasound/MRI/X-ray, CT for hook of hamate) as indicated.

**Management:** Conservative therapy first (avoid elbow flexion/pressure, night splinting in extension, padding, NSAIDs, activity modification). Surgical decompression ± anterior transposition of the nerve for refractory or severe cases (motor weakness, atrophy, or denervation on EMG). Wrist lesions: remove the compressive cause (e.g., excise a ganglion, hamate fracture treatment).

**Board pearls:** - Ulnar nerve = most commonly compressed at the elbow; the second most common compression neuropathy after carpal tunnel (median). - Medial epicondyle fracture → ulnar nerve injury. - Hook of hamate fracture → ulnar nerve (deep branch) injury. - Radial nerve: wrist drop; median nerve: ape hand/hand of benediction; ulnar nerve: claw hand.