A 28-year-old female graphic designer from Bangalore presents with a 2-week history of progressive weakness and wasting of the muscles of her right hand, particularly affecting the thenar and hypothenar eminences. She also reports clumsiness with fine motor tasks and loss of sensation over the medial one-and-a-half fingers. On examination, there is marked atrophy of the first dorsal interosseous muscle and the adductor pollicis. Intrinsic hand muscles are weak, but extrinsic hand muscles (wrist and finger extensors) are normal. Which nerve is most likely compressed?

Explanation

## Clinical Syndrome Analysis **Key Point:** The combination of atrophy of both thenar AND hypothenar muscles with intrinsic hand weakness and sensory loss over the medial one-and-a-half fingers indicates a **ulnar nerve lesion at the elbow**, which affects all ulnar-innervated intrinsic hand muscles. ## Anatomical Innervation of Hand Muscles | Muscle Group | Innervation | Nerve Compression Site | |--------------|-------------|------------------------| | **Thenar muscles (APB, FPB, OPP)** | Median nerve (recurrent branch) | Wrist (carpal tunnel) | | **Adductor pollicis** | Ulnar nerve (deep branch) | Elbow or wrist | | **Hypothenar muscles** | Ulnar nerve (superficial + deep) | Elbow or wrist | | **Dorsal interossei** | Ulnar nerve (deep branch) | Elbow or wrist | | **Lumbricals (medial 2)** | Ulnar nerve (deep branch) | Elbow or wrist | | **Lumbricals (lateral 2)** | Median nerve | Wrist (carpal tunnel) | ## Differential Diagnosis: Ulnar Nerve Compression Sites ```mermaid flowchart TD A["Ulnar Nerve Compression"]:::outcome --> B{"Clinical findings?"}:::decision B -->|"Atrophy: APB, FPB, OPP + Adductor pollicis + hypothenar + interossei"| C["Ulnar at elbow"]:::action B -->|"Atrophy: Adductor pollicis + hypothenar + interossei only<br/>APB/FPB/OPP spared"| D["Ulnar at wrist<br/>Guyon's canal"]:::action B -->|"Atrophy: APB, FPB, OPP only<br/>Adductor pollicis spared<br/>Hypothenar spared"| E["Median at wrist<br/>Carpal tunnel"]:::action C --> F["Sensory loss: medial 1.5 fingers<br/>+ dorsal hand":::outcome] D --> G["Sensory loss: medial 1.5 fingers<br/>Dorsal hand spared":::outcome] E --> H["Sensory loss: lateral 3.5 fingers<br/>Dorsal thumb/index spared":::outcome] ``` ## Why Ulnar at Elbow? 1. **Bilateral thenar AND hypothenar atrophy:** Only ulnar nerve compression can cause hypothenar atrophy. Thenar atrophy here is from loss of adductor pollicis (ulnar-innervated), not from median nerve compression. 2. **Adductor pollicis weakness:** This muscle is innervated by the deep branch of the ulnar nerve. It is NOT affected by median nerve compression at the wrist. 3. **First dorsal interosseous atrophy:** This is a classic sign of ulnar nerve compression; it is one of the earliest muscles to atrophy. 4. **Sensory loss in medial 1.5 fingers:** Consistent with ulnar nerve distribution. 5. **Extrinsic muscles spared:** The wrist and finger extensors (radial nerve territory) are intact, ruling out radial nerve compression. **Clinical Pearl:** The **adductor pollicis** is the key discriminator. It is ulnar-innervated (deep branch) and is spared in carpal tunnel syndrome (median compression). Presence of adductor pollicis atrophy = ulnar nerve problem. **High-Yield:** **Froment's sign** (flexion of thumb IP joint when pinching paper between thumb and index finger due to adductor pollicis paralysis) is a classic bedside test for ulnar nerve compression affecting the adductor pollicis. **Mnemonic:** **"LOAF"** = Lateral 2 lumbricals, Opponens pollicis, Abductor pollicis brevis, Flexor pollicis brevis → all **median nerve**. Everything else intrinsic = **ulnar nerve**.