Electrodiagnostic Error with Ulnar Nerve Conduction Studies at Full Elbow Extension
Performing nerve conduction studies of the ulnar nerve at the elbow in full extension will underestimate the cross-sectional area of the nerve and may fail to detect focal slowing or conduction abnormalities that become apparent only with elbow flexion, leading to false-negative results.
The Core Problem: Nerve Geometry Changes with Elbow Position
The ulnar nerve's cross-sectional area (CSA) is significantly larger at the medial epicondyle when the elbow is extended compared to when it is flexed 1. Specifically:
- At elbow extension: Average CSA at the medial epicondyle is 5.95 ± 0.74 mm², with measurements 2 cm distal being 6.27 ± 0.92 mm² and 2 cm proximal being 5.92 ± 0.73 mm² 1
- At elbow flexion: Average CSA at the medial epicondyle drops to 5.70 ± 0.83 mm², with 2 cm distal being only 5.23 ± 0.87 mm² and 2 cm proximal being 5.73 ± 0.71 mm² 1
- The difference is statistically significant (p < 0.001) at all three measurement points 1
Why This Matters for Nerve Conduction Studies
Nerve Stretching and Conduction Velocity
When the elbow is flexed, the ulnar nerve becomes stretched and elongated, which can unmask focal conduction abnormalities that are not apparent in extension 2. The stretched position increases mechanical stress on already compromised nerve segments, making pathology more detectable 2.
Normal Values Vary by Position
Normal reference values for motor conduction velocity differ between elbow positions 3. Studies examining 380 upper extremities in 227 healthy volunteers with the elbow flexed at 90° found motor nerve conduction velocity (MNCV) across the elbow to be 57.1 ± 5.9 m/s 4. Using extension-based measurements against flexion-based normative data (or vice versa) creates diagnostic errors 3.
Detection of Conduction Block
The sensitivity for detecting different types of ulnar neuropathy varies, with conduction block having lower detection rates (55% sensitivity) compared to axonal changes (93% sensitivity) 5. Testing in full extension may particularly miss conduction blocks that become apparent only when the nerve is under tension during flexion 2, 5.
Recommended Testing Position
The elbow should be flexed at 90° from horizontal for ulnar nerve conduction studies 4. This position:
- Provides standardized, reproducible measurements 4
- Maximizes detection of focal abnormalities by placing the nerve under physiologic tension 2
- Aligns with methodology recommended by the American Association of Neuromuscular & Electrodiagnostic Medicine (AANEM) 4
Clinical Pitfall to Avoid
Do not assume that normal findings at elbow extension rule out ulnar neuropathy at the elbow. If clinical suspicion remains high despite normal studies in extension, repeat the examination with the elbow flexed at 90° 3, 4. The yield of abnormal findings is greatest when measuring absolute motor conduction velocity across the elbow rather than comparing velocity differences between adjacent segments 3.
Perioperative Context
The American Society of Anesthesiologists notes that extension of the elbow in an anesthetized patient beyond the normal comfortable range may increase the risk of median neuropathy 6. While the literature is insufficient to definitively evaluate the impact of elbow flexion versus extension on ulnar neuropathy risk perioperatively 6, specific padding at the elbow may decrease ulnar neuropathy risk 6.