What is the likely diagnosis for a patient with numbness on the medial aspect of the hand and forearm, normal motor study results, and reduced amplitude of the ulnar sensory response on nerve conduction study?

Ulnar Neuropathy at the Elbow (Sensory Predominant)

The most likely diagnosis is ulnar neuropathy at the elbow with predominantly sensory involvement, given the isolated reduction in ulnar sensory nerve action potential amplitude, normal motor studies, and the sparing of the medial antebrachial cutaneous nerve.

Key Diagnostic Features Supporting This Diagnosis

The clinical and electrodiagnostic pattern strongly localizes the lesion to the ulnar nerve at the elbow level:

• Reduced ulnar sensory response amplitude with normal motor conduction indicates a sensory-predominant or pure sensory ulnar neuropathy 1, 2
• Normal medial antebrachial cutaneous nerve bilaterally excludes lower brachial plexus or C8 root pathology, as this nerve branches from the medial cord proximal to the ulnar nerve formation 1
• Normal ulnar motor study suggests either early/mild disease or selective sensory fascicle involvement at the elbow 1, 2

Anatomical Localization Logic

The electrodiagnostic findings create a specific anatomical signature:

• The ulnar nerve at the elbow is the second most common site of compressive neuropathy 3
• Sensory fibers are often affected earlier or more severely than motor fibers in ulnar neuropathy at the elbow 1, 2
• The medial antebrachial cutaneous nerve arises from the medial cord (C8-T1) and travels independently of the ulnar nerve below the axilla, making its preservation critical for excluding more proximal lesions 1
• Pure sensory ulnar neuropathy represents approximately 6% of ulnar nerve lesions at the wrist but can also occur at the elbow with selective fascicular involvement 4, 2

Electrodiagnostic Pattern Analysis

The nerve conduction findings fit a sensory axonal pattern:

• Reduced amplitude of sensory nerve action potentials indicates axonal loss rather than demyelination 5, 1
• Normal motor studies one week after symptom onset suggest either: (1) sensory fibers are selectively affected, or (2) motor involvement is too mild to detect at this early stage 1, 2
• The amplitude ratio of ulnar sensory nerve action potentials across the elbow segment would be the most sensitive test to confirm the lesion site, with a cutoff value of 0.65 for above-elbow to below-elbow stimulation 1

Clinical Correlation

The distribution of numbness provides additional localization:

• Medial hand and forearm involvement in the ulnar nerve distribution (fifth digit and medial half of fourth digit on palmar surface, plus medial hand dorsum) 3, 6
• Absence of motor weakness at one week post-onset is consistent with either pure sensory involvement or early-stage disease where motor axons remain functional 1, 2
• Many cases of mild-to-moderate ulnar neuropathy at the elbow appear to remit spontaneously, which is relevant for prognosis 3

Important Diagnostic Caveats

Several pitfalls must be avoided in this clinical scenario:

• Do not diagnose C8 radiculopathy: The preserved medial antebrachial cutaneous nerve excludes this, as C8 root lesions would affect both nerves 1
• Consider anatomical variants: Approximately 6-9% of individuals have variant innervation of the medial hand dorsum from the superficial radial nerve rather than the dorsal ulnar cutaneous nerve 6
• Early timing matters: At one week post-onset, Wallerian degeneration may not be complete, potentially underestimating the severity of motor involvement 3
• Ultrasound imaging can provide superior localization compared to electrodiagnostics alone, distinguishing cubital tunnel entrapment from retro-epicondylar groove injury 3

Recommended Next Steps

To confirm diagnosis and guide management:

• Perform segmental sensory nerve conduction studies across the elbow with amplitude ratio calculations (normal cutoff for above-elbow/below-elbow is 0.65) 1
• Consider nerve ultrasound to identify specific anatomical pathology and distinguish cubital tunnel syndrome from other elbow-level compression sites 3
• Reassess motor function at 3-4 weeks if symptoms persist, as motor changes may become more apparent with complete Wallerian degeneration 3, 2
• Document dorsal ulnar cutaneous nerve function to assess for anatomical variants that might confound clinical examination 6