Interobserver variation in the interpretation of SSEPs in anoxic–ischaemic coma
Introduction
Bilateral absence of the N20 cortical potential of median nerve somatosensory evoked potentials (SSEP) has been identified as the most valid predictor of poor outcome in patients who are unconscious after circulatory arrest (Robinson et al., 2003, Zandbergen et al., 1998). Its prognostic value has been studied more extensively than that of any other factor, and it appears less susceptible to sedatives and metabolic changes than the clinical neurological examination and the EEG (Coulthard and Rood, 1993, Koht et al., 1988, Sloan et al., 1990). Since absence of the N20 is often used as a basis for non-treatment decisions in these patients, it is important to know whether these SSEPs can be interpreted reliably.
In earlier studies of the prognostic value of SSEPs in anoxic–ischaemic and nontraumatic coma Madl et al. mentioned good reproducibility of the SSEPs, tested by repeated measurements (Madl et al., 1993, Madl et al., 1996). In another study two physicians reviewed recordings independently (Madl et al., 2000), but interobserver variation was not mentioned. We therefore, studied the interobserver variation in the interpretation of SSEP-recordings in patients with anoxic–ischaemic coma.
Section snippets
Study design
The study consisted of two phases: a pilot phase and the actual interobserver study. The pilot phase aimed to define guidelines for the interpretation of SSEPs in patients with anoxic–ischaemic coma. In this phase, a convenient sample of SSEPs from 20 patients with anoxic–ischaemic coma of 3 days' duration was collected. Inclusion criteria were: registration after median nerve stimulation on both sides; registration over Erb's point, the cervical spine and the cerebral cortex, and absence of
Pilot phase
Absolute agreement among all 5 observers for the primary question (bilateral absence of N20, or not) was found for 17 of the 20 SSEPs. A separate consideration of left- and right-sided tests yielded 40 recordings (20 from each side). Agreement among all 5 observers on the question ‘N20 absent or not?’ was reached in 32. The most important sources of disagreement among observers in the remaining 8 recordings were related to confusion concerning variant stimulation sites (in a few cases with
Discussion
The average interobserver agreement of the interpretation of SSEP-recordings was only moderate (K=0.52).
The main source of disagreement was related to the noise level. In spite of the guidelines agreed upon, some observers felt they could reliably conclude the N20 to be absent in some recordings with a considerable noise level, consequently increasing the interobserver variation for such recordings. In cases with low noise levels (noise <0.25 μV), the observer agreement was substantial (K=0.74).
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