A 52-year-old male carpenter from Delhi presents with a 3-month history of progressive weakness and wasting of the intrinsic muscles of his right hand. He reports pain and paresthesias along the medial border of his hand and the medial one-and-a-half fingers. On examination, he has a claw hand deformity affecting the 4th and 5th digits, with weakness of finger abduction and adduction. Sensation is diminished over the hypothenar eminence. There is no sensory loss over the dorsal aspect of the hand. What is the most likely site of ulnar nerve compression?

Explanation

## Clinical Presentation Analysis **Key Point:** The combination of intrinsic hand muscle weakness WITH preserved dorsal hand sensation localizes the ulnar nerve lesion to the wrist — specifically at Guyon's canal — NOT at the elbow. ### Motor Findings - Claw hand deformity of digits 4 and 5 (hyperextension at MCP joints, flexion at IP joints) - Weakness of intrinsic hand muscles (interossei, medial lumbricals) - Weakness of finger abduction and adduction (interossei function) ### Sensory Findings - Paresthesias along medial border of hand and medial one-and-a-half fingers (volar surface) - Diminished sensation over hypothenar eminence - **Preserved dorsal sensation** — this is the critical localizing finding ### Anatomical Correlation The **dorsal cutaneous branch of the ulnar nerve** arises approximately 5–8 cm proximal to the wrist crease, on the posteromedial aspect of the forearm. It supplies sensation to the dorsal ulnar aspect of the hand and the dorsal surfaces of the little finger and medial half of the ring finger. | Site of Lesion | Motor Loss | Volar Sensory Loss | Dorsal Sensory Loss | |---|---|---|---| | Cubital tunnel (elbow) | Yes | Yes | **Yes** | | Arcade of Struthers | Yes | Yes | **Yes** | | Wrist proximal to pisiform (before Guyon's) | Yes | Yes | **Yes** | | **Guyon's canal** | **Yes** | **Yes** | **No (preserved)** | **High-Yield:** Because the dorsal cutaneous branch leaves the ulnar nerve 5–8 cm above the wrist, any lesion at or proximal to this point (cubital tunnel, Arcade of Struthers, or wrist proximal to the pisiform) will cause BOTH volar AND dorsal sensory loss. A lesion within Guyon's canal occurs DISTAL to the origin of the dorsal cutaneous branch, so dorsal hand sensation is **preserved** — exactly as described in this vignette. **Clinical Pearl:** The **absence of dorsal hand sensory loss** is the single most important localizing sign that distinguishes Guyon's canal compression from more proximal ulnar nerve lesions. This patient's preserved dorsal sensation definitively rules out the cubital tunnel, Arcade of Struthers, and the wrist level proximal to the pisiform as the site of compression. ### Why Guyon's Canal? Guyon's canal is a fibro-osseous tunnel at the wrist bounded by the pisiform (medially), the hook of hamate (laterally), the flexor retinaculum (floor), and the palmar carpal ligament (roof). Compression here is seen in: - Cyclists (handlebar palsy) - Carpenters and manual laborers using vibrating tools - Ganglion cysts or hook of hamate fractures At this level, the dorsal cutaneous branch has already departed, so dorsal hand sensation is **intact**, while both motor and volar sensory fibers are affected — perfectly matching this clinical picture. *(Reference: Gray's Anatomy; Snell's Clinical Neuroanatomy; Aids to the Examination of the Peripheral Nervous System, 5th ed.)*