Beyond dichotomy: patterns and amplitudes of SSEPs and neurological outcomes after cardiac arrest

Abstract

Background: We hypothesized that the absence of P25 and the N20-P25 amplitude in somatosensory evoked potentials (SSEPs) have higher sensitivity than the absence of N20 for poor neurological outcomes, and we evaluated the ability of SSEPs to predict long-term outcomes using pattern and amplitude analyses.

Methods: Using prospectively collected therapeutic hypothermia registry data, we evaluated whether cortical SSEPs contained a negative or positive short-latency wave (N20 or P25). The N20-P25 amplitude was defined as the largest difference in amplitude between the N20 and P25 peaks. A good or poor outcome was defined as a Glasgow-Pittsburgh Cerebral Performance Category (CPC) score of 1-2 or 3-5, respectively, 6 months after cardiac arrest.

Results: A total of 192 SSEP recordings were included. In all patients with a good outcome (n = 51), both N20 and P25 were present. Compared to the absence of N20, the absence of N20-P25 component improved the sensitivity for predicting a poor outcome from 30.5% (95% confidence interval [CI], 23.0-38.8%) to 71.6% (95% CI, 63.4-78.9%), while maintaining a specificity of 100% (93.0-100.0%). Using an amplitude < 0.64 μV, i.e., the lowest N20-P25 amplitude in the good outcome group, as the threshold, the sensitivity for predicting a poor neurological outcome was 74.5% (95% CI, 66.5-81.4%). Using the highest N20-P25 amplitude in the CPC 4 group (2.31 μV) as the threshold for predicting a good outcome, the sensitivity and specificity were 52.9% (95% CI, 38.5-67.1%) and 96.5% (95% CI, 91.9-98.8%), respectively. The predictive performance of the N20-P25 amplitude was good, with an area under the receiver operating characteristic curve (AUC) of 0.94 (95% CI, 0.90-0.97). The absence of N20 was statistically inferior regarding outcome prediction (p < 0.05), and amplitude analysis yielded significantly higher AUC values than did the pattern analysis (p < 0.05).

Conclusions: The simple pattern analysis of whether the N20-P25 component was present had a sensitivity comparable to that of the N20-P25 amplitude for predicting a poor outcome. Amplitude analysis was also capable of predicting a good outcome.

Keywords: Evoked potentials; Heart arrest; Induced hypothermia; Prognosis.

Fig. 1

Pattern categories according to the presence of N20 or P25 on cortical somatosensory evoked potential recordings. Type I (n = 91): 51 cases of CPC 1–2 and 40 cases of CPC 3–5; type II (n = 58): all CPC 3–5; type III (n = 2): all CPC 4–5; and type IV (n = 41): all CPC 4–5. Erb, Erb’s point; FZ, frontal pole electrode; CII, C2 spinous process; C3’ and C4’, contralateral somatosensory cortexes; CPC, Cerebral Performance Category

Fig. 2

Flow chart for inclusion of the study patients. TH, therapeutic hypothermia; SSEP, somatosensory evoked potential

Fig. 3

a Amplitudes of the N20–P25 component according to Cerebral Performance Category (CPC). Two patients in the CPC 3 group and 3 patients in the CPC 5 group had an amplitude above the upper limit for CPC 4 (> 2.31 μV). b Y-axis restricted to low amplitudes. The lower limit for CPC 1 and 2 was N20–P25 amplitudes > 0.64 μV

Fig. 4

Scatter plots illustrating the associations between the peak levels of neuron-specific enolase between 48 and 72 h after the return of spontaneous circulation and the cortical amplitudes (n = 160)

Fig. 5

The receiver operating characteristic curves for Cerebral Performance Category scale scores 3–5 at 6 months showing the predictive powers of various SSEP analyses, the pattern of DWI, and the highest serum level of NSE. SSEP, somatosensory evoked potential; DWI, diffusion-weighted imaging; NSE, neuron-specific enolase