Time jitter of somatosensory evoked potentials in recovery from hypoxic-ischemic brain injury

Abstract

Impaired neural conductivity shown by delayed latency and reduced amplitude of characteristic peaks in somatosensory evoked potentials (SSEPs), has been used to monitor hypoxic-ischemic brain injury after cardiac arrest (CA). However, rather than characteristic peak deferral and suppression, the time jitter of the peak in SSEP related with time-variant neurological abnormalities is diminished by the commonly used ensemble average method. This paper utilizes the second order blind identification (SOBI) technique to extract characteristic peak information from one trial of SSEPs. Sixteen male Wistar rats were subjected to 7 or 9 min of asphyxial CA (n=8 per group). The SSEPs from median nerve stimulation were recorded for 4h after CA and then for 15 min periods at 24, 48 and 72 h. Neurological outcomes were evaluated by neurologic deficit score (NDS) at 72 h post-CA. The SSEP signal was analyzed offline with SOBI processing in Matlab. The N10 feature of SSEP was compared between good (NDS≥50) and bad (NDS<50) outcomes. After processed by SOBI, the N10 detection rate was significantly increased (p<0.001) from 90 min post-CA. Statistical difference of the latency variance of the N10 between good and bad outcome groups existed at 24, 48 and 72 h post-CA (p≤0.001). Our study is the first application using SOBI detecting variance in neural signals like SSEP. N10 latency variance, related with neurophysiological dysfunction, increased after hypoxic-ischemic injury. The SOBI technique is an efficient method in the identification of peak detection and offers a favorable alternative to reveal the neural transmission variation.

Fig. 1

(A) Source components decomposed by SOBI from multi-channel SSEP. Channels 1 and 3 contain peak information around 10 msec which are recognized as N10, while channels 2 and 4 do not have short-latency components. (B) Original SSEP signal resulting from stimulation of contralateral side forelimb channel. Notice only N7 is visible. (C) SOBI reconstructed signal from stimulation of contralateral side forelimb channel. We see that now both the N7 and N10 peaks are visible after SOBI processing.

Fig. 2

Ratio of detectable N10 peaks from raw data (dark grey bar) and data processed by SOBI (light grey bar) in baseline, at 90min, 120min, 150min, 180min, 210min, 24h, 48h and 72h post-CA (Mean ± SD). Statistically significant differences denoted by asterisks (**p<0.01, ***p<0.001).

Fig. 3

Normalized N10 peak latency (3A) versus normalized N10 peak variance (3B) between good and bad outcome groups at 90 min, 120 min, 150 min, 180 min, 210 min, 24h, 48h and 72h post-CA (Mean ± SD). The dark grey bar represents the group with good outcome while the light grey bar represents bad outcome. Statistically significant differences denoted by asterisks (**p<0.01, ***p<0.001).