A 35-year-old man presents with weakness of finger abduction and adduction, along with sensory loss over the medial 1.5 fingers. Grip strength is preserved. Which feature best distinguishes a lesion at the ulnar tunnel (Guyon's canal) from a lesion at the medial epicondyle?

## Discriminating Feature: Flexor Carpi Ulnaris (FCU) Involvement ### Anatomical Basis The ulnar nerve has a long course from the medial epicondyle to the hand. The key to distinguishing proximal (medial epicondyle) from distal (Guyon's canal) lesions is the relationship to FCU. **Key Point:** Flexor carpi ulnaris is innervated by the ulnar nerve proximal to the medial epicondyle. A lesion at or above the medial epicondyle will damage FCU, but a distal lesion in Guyon's canal will spare it. ### Comparison Table | Feature | Medial Epicondyle Lesion | Guyon's Canal Lesion | | --- | --- | --- | | **FCU weakness** | Present | Absent | | **Intrinsic hand muscles** | Affected | Affected | | **Dorsal cutaneous branch** | Affected (sensory loss dorsal hand) | Spared (purely motor distal) | | **Hypothenar atrophy** | Yes | Yes | | **Interossei atrophy** | Yes | Yes | | **Grip strength** | Reduced | Preserved | ### Clinical Pearl **High-Yield:** The **dorsal cutaneous branch** of the ulnar nerve arises proximal to Guyon's canal and supplies sensation to the dorsal medial hand. This branch is spared in pure Guyon's canal lesions, making dorsal sensory loss a marker of more proximal injury. ### Why This Matters - **Medial epicondyle lesion** → FCU weak + intrinsics weak + dorsal sensory loss - **Guyon's canal lesion** → FCU normal + intrinsics weak + dorsal sensation intact FCU weakness is the single best discriminator because it is present only in proximal lesions and absent in distal (Guyon's canal) lesions. [cite:Clinically Oriented Anatomy 8e Ch 6]