Ulnar nerve pressure - Influence of arm position nd relationship to somatosensory evoked potentials

Citation
Rc. Prielipp et al., Ulnar nerve pressure - Influence of arm position nd relationship to somatosensory evoked potentials, ANESTHESIOL, 91(2), 1999, pp. 345-354
Citations number
34
Categorie Soggetti
Aneshtesia & Intensive Care","Medical Research Diagnosis & Treatment
Journal title
ANESTHESIOLOGY
ISSN journal
00033022 → ACNP
Volume
91
Issue
2
Year of publication
1999
Pages
345 - 354
Database
ISI
SICI code
0003-3022(199908)91:2<345:UNP-IO>2.0.ZU;2-N
Abstract
Background: Although the ulnar nerve is the most frequent site of periopera tive neuropathy, the mechanism remains undefined. The ulnar nerve appears p articularly susceptible to external pressure as it courses through the supe rficial condylar groove at the elbow, rendering it vulnerable to direct com pression and ischemia. However, there is disagreement among major anesthesi a 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 st udied 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 degrees, 60 degrees, and 90 degrees of abduction, as well a s in supination, neutral orientation, and pronation. To determine the onset of clinical paresthesia compared to the onset and severity of somatosensor y evoked potential (SSEP) electrophysiologic changes, we studied a separate group of 16 male volunteers while applying intentional pressure directly t o the ulnar nerve. Data are presented as mean (median; range). Results: Supination minimizes direct pressure over the ulnar nerve at the e lbow (2 mmHg [0; 0-23]; n = 50), compared with both neutral forearm orienta tion (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 significan tly less pressure over the ulnar nerve compared to pronation (P less than o r equal to 0.04). The estimated contact area of the ulnar nerve with the we ight-bearing surface was significantly(P < 0.0001) smaller in the supine po sition (2.2 cm(2) [0.5; 0-9]; n = 50) compared with both neutral orientatio n (5.5 cm(2) [5.0; 0-13]) and pronation (5.8 cm(2) [6; 0-12]). With the for earm in neutral orientation, ulnar nerve pressure decreased significantly ( P less than or equal to 0.01; n = 15) as the arm was abducted at the should er from 0 degrees to 90 degrees. In the 16 male subjects tested, notable al terations in ulnar nerve SSEP signals (decrease greater than or equal to 20 % in N9-N9' amplitude) were detected in 15 of 16 awake males during applica tion of intentional pressure to the ulnar nerve. However, eight of these su bjects did not perceive a paresthesia, even as SSEP waveform amplitudes wer e decreasing 23-72%. Two of these eight subjects manifested severe decrease s in SSEP amplitude (greater than or equal to 60%). Conclusions Extrapolating these results to the clinical setting, the supina ted 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 degrees and 90 degrees. In addition, up to one half of male patients may fail to perceive or experience clinical sy mptoms of ulnar nerve compression sufficient to elicit SSEP changes.