Ulnar Nerve Distribution (C8-T1 Dermatomes)
A left-sided Pancoast tumor causes reduced sensation along the medial (ulnar) aspect of the forearm and hand, affecting the fourth and fifth fingers (ring and pinky fingers), corresponding to C8-T1 dermatomes. 1
Anatomical Basis
Pancoast tumors (superior sulcus tumors) characteristically invade the lower trunk of the brachial plexus, which is formed by the C8 and T1 nerve roots 1. This anatomical relationship explains the specific sensory distribution affected:
- Medial forearm: Along the ulnar border
- Medial hand: Ulnar side of the palm
- Fourth and fifth digits: Ring finger and pinky finger
- Medial aspect of the hand and forearm: Classic ulnar nerve distribution 2, 3, 4
Associated Clinical Features
Beyond sensory changes, expect these accompanying findings in left-sided Pancoast syndrome:
- Motor deficits: Weakness and atrophy of intrinsic hand muscles, particularly the hypothenar eminence (small muscles of the hand) 2, 5
- Horner syndrome: Ptosis, miosis, and anhidrosis from sympathetic trunk involvement 6, 5
- Severe shoulder pain: Radiating toward the axilla, scapula, and along the ulnar distribution of the upper arm 5, 4
- C8-T1 myotome weakness: Distal hand weakness affecting grip strength 6
Diagnostic Confirmation
Electrodiagnostic studies will demonstrate:
- Reduced or absent sensory nerve action potentials in C8-T1 territories 6, 3
- Reduced distal compound muscle action potentials from affected nerves 6
- Severe C8 radiculopathy pattern (though isolated radiculopathy presentation is less common) 3
Critical Pitfall
Do not mistake this for isolated cervical radiculopathy or carpal tunnel syndrome. The combination of ulnar-distribution sensory loss, intrinsic hand muscle weakness, shoulder pain, and Horner syndrome should immediately raise suspicion for apical lung pathology 3, 4, 7. Always include the lung apices on any shoulder or cervical spine radiographs when evaluating unilateral arm pain with ulnar nerve distribution symptoms 4, 7.