Ictal Hypoxemia in Localization-Related Epilepsy: Analysis of Incidence, Severity and Risk Factors

Lisa M. Bateman; Chin-Shang Li; Masud Seyal


Brain. 2008;131(12):3239-3245. 

In This Article


We studied 57 consecutive patients with intractable localization-related epilepsy admitted for in-patient video-EEG telemetry to assess candidacy for epilepsy surgery. Anti-epileptic medications were tapered and/or discontinued during the monitoring period to provoke seizures in the majority of patients (n = 52). In four patients, anti-epileptic therapy was not altered due to the occurrence of frequent daily seizures. One patient was not on anti-epileptic therapy at the time of admission. EEG was recorded from scalp locations corresponding to the standard International 10-20 system, including T1 and T2 locations and bilateral mandibular notch surface electrodes, except in six patients who had intracranial electrodes. Synchronized video and single-channel electrocardiogram (using right and left lateral infraclavicular electrodes) were recorded. Oxygen saturation values were recorded using digital pulse oximetry (Nellcor N-395 oximeter) and displayed every second on a dedicated channel of a BMSI 6000 video-EEG recording unit. The accuracy of the pulse oximetry data at very low saturation levels is unspecified by the manufacturer. Therefore, all nominal saturation values of <50% were truncated at 50% for data analysis. Body position and state of the patient at seizure onset (awake or asleep), seizure localization at onset and time to contralateral spread when present, duration of seizure, duration of desaturation (defined as the interval from saturation dropping below 90% to subsequent return above 90%), time to onset of desaturation from seizure onset, oxygen desaturation nadir and ictal heart rate change were recorded. The heart rate was measured in 10-s epochs relative to seizure onset. We monitored nasal airflow and recorded abdominal excursions. The BMI for each patient was calculated.

One of the 57 patients had only simple partial seizures without EEG change and was excluded from further analysis. In one other patient with frontal lobe seizures, 98 brief, stereotyped seizures were recorded without oxygen desaturation. In this patient, only the first 10 consecutive seizures were analyzed. A total of 381 partial onset seizures were captured and analysed in the group of 56 patients.

Summary statistics were reported as mean ± SD (median; range). The generalized estimating equation (GEE) approach (Liang and Zeger, 1986) was used for repeated measurements analysis of binary outcomes to investigate the association between a risk factor (explanatory variable) of interest and the binary response variable. Logistic regression was used to study the association between a risk factor and the binary response variable. A linear mixed-effects model was used for repeated measurement analysis of continuous outcomes to investigate the association between a risk factor and the continuous response variable. All analyses were performed with SAS Version 9.1.

The majority of patients with localization-related epilepsy admitted to the epilepsy monitoring unit have temporal lobe epilepsy. To determine whether seizure localization was associated with desaturation below 90% we obtained an estimate of the sample size that would allow a reasonably robust detection of a difference between temporal and extratemporal seizures. The Fisher's exact test was used to test the null hypothesis that the probability (p1) of a desaturation below 90% in the temporal seizure group equals the probability (p2) of desaturation below 90% in the extratemporal seizure group versus the alternative hypothesis that p1 does not equal p2. We determined that a sample size of 85 seizures in each group (temporal onset seizures and extratemporal onset seizures) would test the null hypothesis p1 = p2 = 0.12 versus the alternative hypothesis p1 ≠ p2 = 0.43 providing a power of 0.995 at a significance level of α = 0.05. Data collection was therefore stopped when 85 extratemporal onset seizures with oxygen saturation data were acquired. At that point, we had recorded a total of 381 partial onset seizures.

In seven patients (19 seizures), in addition to the parameters noted above, end-tidal CO2 (ETCO2) was recorded simultaneously with nasal cannulae using a capnograph (BCI, Inc, Waukesha, WI, USA). The time on the capnograph was synchronized with that on the BMSI 6000. Digitized ETCO2 values, averaged over four breaths, were displayed every 4 s and recorded. ETCO2 was recorded in this small subset of patients primarily to demonstrate the validity of pulse oximetry during seizures. We anticipated that seizure-related hypoventilation resulting in oxygen desaturation would be accompanied by a concomitant rise in ETCO2. Conversely, artefactual seizure-related drops in pulse oximetry values would not be accompanied by increases in ETCO2.


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