Shorter time between seizure onset and radiotracer injection improves ictal SPECT localization in pediatric patients with pharmacologically intractable epilepsy

Objectives Determination of a seizure focus in pharmacologically intractable epilepsy can guide surgical intervention. The ability of ictal SPECT to identify epileptogenic cortex may be related to the timing of radiotracer injection relative to seizure onset. This study investigated the relationship between this time interval and SPECT localization.

Methods EEG and ictal SPECT studies for 95 pediatric patients with pharmacologically intractable focal epilepsy, age 1 - 20 years (mean = 10.2, SD = 5.4 years), who underwent ictal brain SPECT were retrospectively reviewed. EEG findings were used to identify seizure onset time and to determine the time interval until radiotracer injection. Brain SPECT was performed with ^99mTc-ECD or ^99mTc-HMPAO. The time between electrographic seizure onset and radiotracer injection was compared to localization by SPECT.

Results Of the 95 patients, 60 had SPECT localization to a specific lobe or smaller region while 35 did not localize. Based on their respective distributions of injection times, 70% of positive localizations were associated with injection times <= 25 seconds (median = 22.5 seconds), whereas 66% of non-localizing SPECT studies were associated with injection times >= 30 seconds (median = 35 seconds). Overall, the injections times in positive and negative SPECT localizations were statistically distinct (p<0.001).

Conclusions Early radiotracer injection results in a higher rate of seizure focus localization by ictal SPECT. There may be an injection timing window of about 20 seconds from electrographic seizure onset, in which positive localization is probable. Injection delay beyond 30 seconds significantly decreases the likelihood of SPECT localization.