ICTAL AND INTERICTAL SPECT IN PATIENTS WITH FRONTAL LOBE EPILEPSY

Article Nº AJ03-3

 

SUMMARY

The frontal lobe seizures present variable clinical and electroencephalographic features that play some difficulties in the presurgical work-up for epilepsy. The aim of the present study is to determine the role of ictal and interictal SPECT in the localization of the epileptogenic zone in frontal lobe epilepsies. We evaluated 13 patients, 6 men and 7 women, with mean age of 16yr, and with medically intractable frontal lobe seizures. All patients were submitted to ictal SPECT and 11 also to interictal scans. Ictal SPECT studies were contributive in 54% of patients, and nonlateralized and nonlocalized, or contralateralized in the remaining patients. Interictal SPECT showed normal perfusion in 45,5% of patients, hypoperfusion localized in the suspected epileptogenic zone in 45,5%, and diffuse findings in the other 9%. We conclude that the sensitivity of ictal and interictal SPECT studies in frontal lobe epilepsies is lower than that for temporal lobe seizures, thus confirming previous data.

Key words: Frontal lobe epilepsy, frontal lobe seizures, ictal SPECT, interictal SPECT, rCBF, extratemporal lobe epilepsy.

INTRODUCTION

Frontal lobe epilepsy (FLE) is an heterogeneous group of disorders which are characterized by a variety of epileptic seizures, electroencephalographic (EEG) patterns and etiologies (Harvey et al ¹). The epileptic zone localization based on scalp EEG is often negative, and invasive evaluation may be necessary. An additional neuroimaging method such as the ictal and interictal single-photon emission computerized tomography (SPECT) can sometimes be helpful.

Contrarily to the contribution of ictal SPECT in temporal lobe seizures, the positive rate of ictal SPECT in FLE is much lower due to several reasons. The relative inaccessibility of frontal cortex to electrodes, propagation of spike and wave discharges throughout both hemispheres and hypercinetic nature of ictal behavior obscuring EEG records. Recordings with intracranial electrodes may be misleading because frontal lobe has a large area and volume. Among neuroimaging methods, Magnetic Resonance Imaging (MRI) often does not show structural abnormality and computed tomography has less spatial resolution. Ictal Single Photon Emission Computed Tomography (SPECT) may present regional enhancement of cerebral blood flow (rCBF) in the epileptogenic zone, and because its non-invasive nature, may avoid invasive EEG. The aim of this study is to determine the ictal and interictal SPECT contribution to presurgical work-up of medically intractable FLE.

PATIENTS AND METHODS

Patients with medically intractable FLE were included after informed consent was signed. Video-EEG was continuously performed with 10-20 system plus intermediate electrodes. Seizures were analyzed by long-term Video-EEG monitoring system VANGARDâ . MRI was performed in a 1,5T Siemens device, and processed with T1 and T2 weighted, MPR and FLAIR sequences. Syndromic and topographic diagnosis were made based on the summation of all these informations.

SPECT scans were performed after ictal and interictal intravenous injections of 1110 MBq of 99mTc-ethyl cysteinate dimer (99mTc-ECD). Ictally, the injection was performed as soon as technologists and nurses perceived the first clinical or electroencephalographic signs of seizures. It was considered ictal when the injection was made until the most evident clinical or EEG findings end. Interictal SPECT was carried out in a supine position with eyes open, ears uncovered in a quiet and darkness room, with previous venous puncture. SPECT data were acquired in 64 projections with 360 degree rotation, with single head camera (Orbiter-Siemens) equipped with parallel-hole, high resolution and low energy collimator. Reconstructed slices were displayed on 64x64 matrix. Attenuation correction was performed using Chang 1^st order method. Images were blindly and independently analyzed by two observers (MK, LWA).

RESULTS

Thirteen patients were submitted to 16 ictal SPECT scans, 6 men and 7 women, mean age 16yrs (2 to 35yrs). All had Video-EEG recorded at CIREP between October 1996 and May 1998. All patients performed ictal SPECT and 11 had interictal SPECT as well. Seven patients had symptomatic FLE, 5 patients cryptogenic FLE and 1 patient with a suspected familiar FLE. From the symptomatic group MRI was able to detect Tuberous Sclerosis (3/7), tumor (1/7), gliosis (2/7) and cortical dysplasia (1/7).

Ictal injections were performed with a mean latency of 37sec (10sec – 1min 7sec) from clinical onset, and 38sec (5sec - 1min 18sec) from electroencephalographic seizure onset. The mean duration of clinical and electroencephalographic seizures was 56sec (6sec - 2min 1sec) and 1min 4sec (4sec – 2min 12sec), respectively. From tracer injection, the time lag was 22sec (00-54sec) for clinical, and 31sec (00sec – 1min 6sec) for electroencephalographic seizure duration. The mean time lag for image acquisition was 1h 43min (49min – 3h 27min).

Ictal SPECT scans showed: (1) Lateralized and localized epileptogenic zone in 5 patients (39%); (2) lateralized but nonlocalized in 2 patients (15%); (3) neither localized nor lateralized in 4 patients (31%) and (4) contralateralized in other 2 patients (15%). Summing up all series with positive lateralization, SPECT was helpful in 54% of the cases. SPECT demonstrated spatial resolution good enough to find the epileptogenic foci and be comparable to MRI findings.

Interictal scans were performed in 11/13 patients and showed: Normal perfusion in 5/11 patients (45,5%), ipsilateral frontal lobe hypoperfusion in 5/11 patients (45,5%), and a diffuse hypoperfusion in the last one patient (9%). From the 5 patients with hypoperfusion in the suspected frontal lobe, 3 were of the symptomatic group.

Patient 1

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A 20-yr-old man with seizures since 4mo of age. Video-EEG showed daily simple partial seizures and occasionally secondary generalization starting with clonus in the mouth at a right. Scalp EEG was inconclusive and MRI showed less evident atrophy. Invasive EEG showed seizure onset in left fronto-temporal regions. The epileptogenic foci was then determined as being frontal after multiple ictal SPECT`s. Ictal scans showed left focal frontal hyperperfusion, while interictal SPECT was normal. Patient underwent left frontal lobectomy and is seizure free.

Patient 2

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A 28-year-old female patient with seizures since she was 5yo. Tonic seizures with abduction, raising of upper limbs, sometimes preceded by partial complex seizures. Video-EEG showed diffuse cortical rhythm with peak amplitude in left frontal lobe. Seizure frequency was 4-5 times a day, predominantly at night. MRI disclosure dysplasia in the middle and inferior left frontal lobe. Neuropsychologic examination showed nonlateralized frontal lobe dysfunction. Ictal SPECT showed accentuated hyperperfusion in dorso-lateral and anterior left frontal region.

Patient 3

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A 28-year-old male patient with seizure onset at 7mo of age. He had generalized tonic-clonic seizure while sleeping. Mean seizure frequency of 2-3 times by night. He had good neuromotor development and nowadays shows sligth bradipsyquism. Video-EEG showed seizures with onset in right frontal lobe. Ictal SPECT showed hyperperfusion foci in right orbito-frontal region next to basal ganglia.

DISCUSSION

The most reliable application of SPECT in epilepsy is that for temporal lobe seizures. In this setting the result of ictal / postictal SPECT studies show increased rCBF in the ipsilateral temporal lobe in ~95% of patients (Berkovic et al. ², Newton et al. ^{3, 4}). Otherwise, this so high result has not been reproduced for frontal lobe epilepsies. For this reason we aimed to determine how much the ictal / interictal SPECT`s were collaborative in suggest the localization of the epileptogenic zone in FLE.

We found that the ictal scans allowed the localization of foci in 39%, and the lateralization of rCBF changes in other 15%. Summing up these two rates, 54% of the patients were benefited from those data. In the majority of series, in literature, extratemporal epilepsy has had different degrees of ictal SPECT detection. Stefan et al. ⁵ reported 3 patients with frontal lobe epilepsy where ictal SPECT was localized in 2 patients (66%). Marks et al. ⁶, also reported 3 patients with frontal lobe seizures and obtained similar results, a positive rate in 2/3 patients (66%). In these two studies, however the sample was little. Harvey et al. ¹, however, correlated ictal SPECT scans with electroclinical data, and showed that the first was informative in 20/22 (91%) children. In spite of being high rates, these would be expected due the homogeneous sample of patients, children, and almost all had partial or behavioral simple partial seizures.

Interictal SPECT showed ipsilateral hypoperfusion in 45,5% of our patients. Stefan et al ⁵, report 3 patients with frontal lobe seizures and interictal SPECT, and none of them showed localized frontal hypoperfusion. Marks et al ⁶ reported localized frontal hypoperfusion in only one of their five patients with FLE. Harvey et al. ¹ demonstrated localized, unilateral frontal hypoperfusion concordant with lateralization from clinical, EEG, MRI, and pathologic data in only two of 22 children (9%).

Our results and those previously reported by other authors demonstrated that ictal and interictal SPECT have incongruent sensibilities. This may possibly be due different amount of patients in samples, distinct tracer used (HMPAO vs ECD) which can have different latencies in quickly availability besides the inpatient, and the complexity of neurophysiology and anatomy of the frontal lobes. Devous et al ⁷ concluded in their meta-analysis about SPECT brain imaging in epilepsy that there is great variability in the methods and standards of reporting data from SPECT studies in the literature. He suggest some useful recommendations for future studies approaching the application of this neuroimaging technique in epilepsy from which are of prominence: to describe results of EEG, SPECT and CT/MRI, and the correlation of these resources, describe criteria used to define ictal, postictal or interical states, and describe EEG morphology and semiology (behavior) at the time of the injections. These are welcomed issues to be stated in future studies.

 

REFERENCES

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