Ulnar Neuropathy at the Elbow: True Conduction Block or a Technical Artifact?
38-year-old man has six weeks of numbness in the right small and ring fingers, worse with elbow flexion. Examination shows mild intrinsic hand wasting and a positive Froment sign; sensation is reduced in the ulnar digit distribution, with normal dorsal hand sensation. Ulnar motor study recording abductor digiti minimi: wrist 9.0 mV, below-elbow 8.8 mV, above-elbow 4.0 mV, axilla 3.8 mV; distal latency 3.2 ms; forearm conduction velocity 55 m/s and across-elbow velocity 32 m/s. Ulnar sensory response to digit V is 4 microvolts; dorsal ulnar cutaneous response is 15 microvolts. Median motor and sensory studies are normal. The technologist says the study proves ulnar neuropathy at the elbow with conduction block and asks whether to stop.
1. Is this enough to diagnose ulnar neuropathy at the elbow with conduction block? What technical and anatomic checks must precede that conclusion?
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Not yet. The amplitude drop from 8.8 to 4.0 mV is about 55%, which meets a common amplitude criterion for partial conduction block, but only if stimulation was supramaximal and the CMAP morphology is stable. I would confirm supramaximal stimulation at each site, inspect for co-stimulation of the median nerve or flexor carpi ulnaris, measure the across-elbow segment with the elbow flexed about 70-90 degrees, and ensure the below-elbow site is truly below the elbow. I would also check CMAP area and duration, not amplitude alone, and look for a Martin-Gruber anastomosis by stimulating the median nerve while recording the abductor digiti minimi. The normal dorsal ulnar cutaneous SNAP is discordant with a typical elbow lesion and raises the possibility of a distal lesion, sensory sparing, or an anatomic anomaly. Inching and ultrasound can resolve the level.
2. How do the normal dorsal ulnar cutaneous SNAP and a possible Martin-Gruber anastomosis change localization and planning?
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The dorsal ulnar cutaneous nerve branches from the ulnar nerve proximal to the wrist, so a normal response argues against a lesion in the ulnar tunnel but does not exclude ulnar neuropathy at the elbow; sensory fibers can be spared or the lesion can be predominantly motor. If a Martin-Gruber anastomosis is present, above-elbow ulnar stimulation will not activate median-origin fibers that join the ulnar nerve in the forearm, producing a false amplitude drop. Median nerve stimulation with recording at the abductor digiti minimi should be performed. If a median-to-ulnar CMAP is present, I would interpret the across-elbow drop cautiously and use inching, needle EMG, and ultrasound. The study should not be stopped; it should be modified.
3. What would you do if above-elbow stimulation produced a forearm twitch and changed the CMAP waveform?
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That suggests co-stimulation or movement artifact rather than true conduction block. I would reposition the stimulator, reduce interelectrode distance, ensure the active cathode is over the ulnar nerve, and increase stimulus intensity only to supramaximal. I would check that the elbow is flexed and the arm relaxed. If the waveform changes reproducibly, I would compare area and duration, repeat below-elbow and above-elbow trials, and use inching. Needle EMG of flexor carpi ulnaris and first dorsal interosseous can help confirm an elbow lesion. If the artifact cannot be eliminated, I would report the limitation and not diagnose conduction block.