Ulnar nerve pressure: Influence of arm position and relationship to somatosensory evoked potentials

Background: Although the ulnar nerve is the most frequent site of perioperative neuropathy, the mechanism remains undefined. The ulnar nerve appears particularly susceptible to external pressure as it courses through the superficial condylar groove at the elbow, rendering it vulnerable to direct compression and ischemia. However, there is disagreement among major anesthesia textbooks regarding optimal positioning of the arm during anesthesia. Methods: To determine which arm position (supination, neutral orientation, or pronation) minimizes external pressure applied to the ulnar nerve, we studied 50 awake, normal volunteers using a computerized pressure sensing mat. An additional group of 15 subjects was tested on an operating table with their arm in 30°, 60°, and 90°of abduction, as well as in supination, neutral orientation, and pronation. To determine the onset of clinical paresthesia compared to the onset and severity of somatosensory evoked potential (SSEP) electrophysiologic changes, we studied a separate group of 16 male volunteers while applying intentional pressure directly to the ulnar nerve. Data are presented as mean (median; range). Results: Supination minimizes direct pressure over the ulnar nerve at the elbow (2 mmHg [0; 0-23]; n = 50), compared with both neutral forearm orientation (69 mmHg [22; 0-220]; P < 0.0001), as well as pronation (95 mmHg [61; 0-220]; P < 0.0001). Neutral forearm orientation also results in significantly less pressure over the ulnar nerve compared to pronation (P 0.04). The estimated contact area of the ulnar nerve with the weight-bearing surface was significantly (P < 0.0001) smaller in the supine position (2.2 cm² [0.5; 0- 9]; n = 50) compared with both neutral orientation (5.5 cm² [5.0; 0-13]) and pronation (5.8 cm² [6; 0-12]). With the forearm in neutral orientation, ulnar nerve pressure decreased significantly (P ≤ 0.01; n = 15) as the arm was abducted at the shoulder from 0°to 90°. In the 16 male subjects tested, notable alterations in ulnar nerve SSEP signals (decrease ≥ 20% in N9-N9' amplitude) were detected in 15 of 16 awake males during application of intentional pressure to the ulnar nerve. However, eight of these subjects did not perceive a paresthesia, even as SSEP waveform amplitudes were decreasing 23-72%. Two of these eight subjects manifested severe decreases in SSEP amplitude (≥ 60%). Conclusions: Extrapolating these results to the clinical setting, the supinated arm position is likely to minimize pressure over the ulnar nerve. With the forearm in neutral orientation, pressure over the ulnar nerve decreases as the arm is abducted between 30°and 90°. In addition, up to one half of male patients may fail to perceive or experience clinical symptoms of ulnar nerve compression sufficient to elicit SSEP changes.