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Difficulty: MediumPhysical Examination Findings

A 45-year-old man is evaluated in the outpatient clinic for right hand clumsiness and numbness that has progressively worsened over the past four weeks. He recently began training for a long-distance cycling event and spends up to 15 hours a week riding his bicycle. He has no other medical conditions and takes no medications. Vital signs are within normal limits. On physical examination, there is decreased pinprick and light touch sensation over the palmar aspect of the right fifth digit and the medial half of the fourth digit. Sensation is completely intact over the dorsal aspect of the medial hand and the medial aspect of the forearm. Motor examination reveals weakness with adduction and abduction of the digits. Finger flexion strength is symmetric and normal bilaterally. Based on these physical examination findings, which of the following is the most likely anatomic site of this patient's nerve compression?

  1. A
    Groove between the medial epicondyle and the olecranon
  2. B
    Space deep to the transverse carpal ligament
  3. Canal between the pisiform and the hook of the hamateAnswer
  4. D
    Spiral groove of the mid-shaft of the humerus
  5. E
    Triangle formed by the anterior and middle scalene muscles

Answer

Canal between the pisiform and the hook of the hamate (Guyon canal)
The correct answer is the canal between the pisiform and the hook of the hamate, clinically known as Guyon canal. This patient has physical examination findings consistent with an ulnar neuropathy (cyclist's palsy) localized precisely to the wrist. The ulnar nerve provides sensation to the palmar aspect of the fifth digit and medial half of the fourth digit, and motor innervation to the intrinsic hand muscles (interossei and lumbricals). Crucially, the physical examination reveals completely intact sensation over the dorsal medial hand and preserved finger flexion. The dorsal cutaneous branch of the ulnar nerve branches off from the main nerve in the distal forearm, approximately 5 cm proximal to the wrist, to supply the dorsal medial hand. Therefore, compression at the wrist (Guyon canal) will spare dorsal sensation, whereas compression at the elbow (cubital tunnel) will impair both palmar and dorsal sensation. Furthermore, motor branches to the flexor carpi ulnaris and the medial half of the flexor digitorum profundus originate in the proximal forearm; their functional sparing on physical exam further confirms a distal (wrist) rather than proximal (elbow) compression site.

Step-by-Step Solution

1
Identify the affected peripheral nerve based on the general sensory and motor deficits.
The sensory deficit on the palmar fifth and medial fourth digits, combined with weakness of finger adductors and abductors (interossei muscles), confirms an ulnar nerve injury.
The ulnar nerve provides the specific sensory distribution to the medial one and a half digits and motor innervation to the intrinsic hand muscles.
2
Localize the exact level of the nerve lesion using specific spared physical examination findings.
Sensation on the dorsal medial hand is intact, and finger flexion is preserved. This localizes the compression to the wrist rather than the elbow.
The dorsal cutaneous branch of the ulnar nerve arises proximal to the wrist. Sparing of this branch indicates the lesion must be distal to its takeoff.

Key Concept

Localization of ulnar nerve lesions using physical examination findings (differentiating Guyon canal from cubital tunnel compression).
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Topics

Physical Examination Findings

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