February 2000

Ulnar nerve pressure: influence of arm position and relationship to somatosensory evoked potentials.
Prielipp RC, Morell RC, Walker FO, Santos CC, Bennett J, Butterworth J.
Anesthesiology. 1999; 91:345-54.

Will we ever understand perioperative neuropathy? A fresh approach offers hope and insight (editorial).
Caplan RA.
Anesthesiology. 1999; 91:335-6

Commentary by David Lubarsky, MD

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[ see abstracts below ]

When you read one of our commentaries it should leave you thinking or doing something different in your practice. This great study gives the best evidence yet of how to prevent ulnar nerve injury. The ultimate definitive study would put into practice these suggestions and prove a decreased incidence. Assuming nerve injury is due to pressure (that's the flaw, but incredibly likely), then systematically studying how to minimize ulnar nerve pressure is the logical approach. Why did it take to the end of the millennium to do this. To Dr. Prielipp et al. we say: "better late than never." (Actually I wished I had thought of this).

Basically, using a pressure-sensing mat, the pressure over the ulnar groove (i.e. funny bone) was measured in 3 positions at varying degrees of abduction. Supination was best (palm upward), neutral second, and pronation (palm down) worst. Interestingly, as the arm was moved from the side (30 degrees) out to 90 degrees, the pressure decreased with the arm in a neutral position, but did not affect a supinated arm. The effect on the brachial plexus with this maneuver was not noted.

Missing (and hopefully forthcoming) is which type of cradle is best, how do cradles (gel vs. foam) affect the nerve, and will systematic use of this information actually decrease reported injury claims.

Also unclear from this study are the best options when positioning lateral/prone. An academic's work is never done to the complete satisfaction of other academics. BUT I, for one, am happy to have this initial piece of information. More than 90% of my patients are supine, and they will ALL go to sleep with the arm in the supine position if I can manage it.

As noted in the accompanying editorial, nerve injury (since we've pretty much solved so many of the bigger problems like esophageal intubation) is big, accounting for 28% of all claims in the closed claims database, with many patients with ulnar neuropathy having symptoms for >1 year. Prospective analysis notes a 1:200 incidence. For most of us doing about 1000 cases per year, that's several per year per anesthesiologist or anesthetist. Prielipp et al give us some information that should allow us to do better-- We should act on it!

ABSTRACT 1. Ulnar nerve pressure: influence of arm position and relationship to somatosensory evoked potentials. AUTHORS: Prielipp RC; Morell RC; Walker FO; Santos CC; Bennett J; Butterworth J. SOURCE: Anesthesiology. 1999 Aug;91(2):345-54 ABSTRACT: 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 degrees, 60 degrees, and 90 degrees 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 < or = 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 < or = 0.01; n = 15) as the arm was abducted at the shoulder from 0 degrees to 90 degrees. In the 16 male subjects tested, notable alterations in ulnar nerve SSEP signals (decrease > or = 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 (> or = 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 degrees and 90 degrees. 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. 2. Will we ever understand perioperative neuropathy? A fresh approach offers hope and insight (editorial). AUTHORS: Caplan RA. SOURCE: Anesthesiology. 1999; 91:335-6 ABSTRACT: None available.