Ulnar Neuropathy at the Elbow: Localizing the Lesion When Segments Disagree
A 47-year-old machinist reports 5 months of right hand weakness and intermittent numbness of the little finger. Examination shows mild wasting of the first dorsal interosseous, Medical Research Council grade 4/5 abduction of the little finger, and preserved flexor carpi ulnaris bulk. You perform ulnar motor studies recording from the abductor digiti minimi with stimulation at the wrist, below the elbow (3 cm distal to the medial epicondyle), above the elbow (8 cm proximal to the below-elbow site), and at the axilla. Results: wrist latency 3.1 ms, amplitude 8.0 mV; below-elbow latency 7.0 ms, amplitude 7.8 mV; above-elbow latency 9.4 ms, amplitude 5.1 mV; axilla latency 11.6 ms, amplitude 4.9 mV. Forearm conduction velocity is 59 m/s. Limb temperature is 31.5 degrees C at the forearm.
1. Calculate the across-elbow conduction velocity and the conduction block percentage, and state whether these findings localize the lesion to the elbow segment.
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Across-elbow velocity: distance 8 cm divided by the latency difference (9.4 - 7.0 = 2.4 ms) = 80 mm / 2.4 ms = 33 m/s, well below the 50 m/s lower limit for the ulnar elbow segment.
Conduction block: amplitude falls from 7.8 mV to 5.1 mV, a drop of 2.7 mV = 35%, which exceeds the commonly used 20% amplitude-drop criterion for definite partial motor conduction block (with a normal distal amplitude, so this is not simply a low-amplitude baseline).
Interpretation: both focal slowing and a significant amplitude drop occur only across the elbow; the forearm segment is normal at 59 m/s and the axilla-to-above-elbow segment is normal (11.6 - 9.4 = 2.2 ms over the measured axillary distance). The lesion is therefore localized to the elbow segment. The preserved flexor carpi ulnaris with denervation of the first dorsal interosseous fits a lesion at or just distal to the cubital tunnel rather than a high humeral-groove lesion.
2. The referring physician asks whether the amplitude drop could be explained by something other than conduction block. What technical and physiologic confounders must you exclude, and what additional study would you add?
Read the worked answer
Before accepting conduction block, exclude:
- Anomalous innervation - a Martin-Gruber anastomosis in the forearm can send fibers to the thenar muscles that are stimulated above the elbow but not at the wrist, producing a spurious amplitude drop. Check the median nerve with ulnar stimulation and look for an intermediate thenar compound muscle action potential.
- Co-stimulation or submaximal stimulation - supramaximal stimulation must be confirmed at each site; a poor above-elbow response from a thick arm or poor electrode contact mimics block.
- Volume conduction / phase cancellation - measure amplitude from baseline to negative peak and consider that temporal dispersion broadens and lowers the waveform; a drop with marked dispersion is less specific for true block.
- Temperature - the forearm at 31.5 degrees C slows conduction and can exaggerate latency differences; warm the limb and repeat.
- Distance measurement error - remeasure the across-elbow distance along the nerve, not the skin surface, since an overestimated distance falsely slows velocity.
Add: a short-segment (2 cm incremental) study across the elbow to confirm a focal velocity drop, plus needle EMG of flexor carpi ulnaris and first dorsal interosseous to confirm the anatomic level and look for active denervation.