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
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.