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SPECT perfusion changes during complex partial seizures in patients with hippocampal sclerosis

W. Van Paesschen , P. Dupont , G. Van Driel , H. Van Billoen , A. Maes
DOI: http://dx.doi.org/10.1093/brain/awg108 1103-1111 First published online: 1 May 2003

Summary

Cerebral perfusion changes reliably reflect changes in neuronal activity. Our aim was to obtain new insights into the pathophysiology of complex partial seizures (CPS) in patients with hippocampal sclerosis (HS) using interictal and ictal single photon emission computed tomography (SPECT). We studied 24 patients with refractory temporal lobe epilepsy (TLE) associated with HS. All had an interictal and ictal SPECT with early injection during a CPS. Images were normalized and co‐registered. Using statistical parametric mapping (SPM99), brain regions with significant ictal perfusion changes were determined. To assess possible interrelationships between these regions, Pearson correlation coefficients were calculated. The temporal lobe ipsilateral to the seizure focus, the border of the ipsilateral middle frontal and precentral gyrus, both occipital lobes and two small regions in the contralateral postcentral gyrus showed ictal hyperperfusion. The frontal lobes, contralateral posterior cerebellum and ipsilateral precuneus showed hypoperfusion. Further exploratory analysis suggested an association between ipsilateral temporal lobe hyperperfusion and ipsilateral frontal lobe hypoperfusion, and an inverse association between seizure duration and hyperperfusion in the ipsilateral anterior cerebellum and contralateral postcentral gyrus. We conclude that there is a network of perfusion changes during CPS in patients with HS. Studying a particular seizure type in patients with HS with peri‐ictal SPECT performed during a defined time window will allow further analysis of the cerebral network activities, and excitatory, inhibitory and gating mechanisms during seizures associated with HS.

  • Keywords: complex partial seizure; gating mechanism; hippocampal sclerosis; network; SPECT
  • Abbreviations: CPS = complex partial seizure; HS = hippocampal sclerosis; Pcor = significance threshold corrected for multiple comparisons; SISCOM = subtraction ictal SPECT co‐registered to MRI; SPECT = single photon emission computed tomography; SPM = statistical parametric mapping; TLE = temporal lobe epilepsy

Introduction

Hippocampal sclerosis (HS) is the most frequent pathological substrate of refractory mesial temporal lobe epilepsy (TLE) (Williamson et al., 1993), and up to 70% of patients who undergo epilepsy surgery have HS (Wiebe et al., 2001). Complex partial seizures (CPS) are the predominant seizure type associated with HS (French et al., 1993).

Ictal single photon emission computed tomography (SPECT) is an established technique in the presurgical evaluation of patients with intractable partial epilepsy. The localizing value of ictal SPECT performed with cerebral perfusion imaging agents in patients with partial epilepsy is based on a focal increase in cerebral blood flow associated with an ictal increase in neuronal metabolic activity (Schwartz and Bonhoeffer, 2001). The sequence of perfusion changes in temporal lobe CPS has been well documented. During the ictus, there is hyperperfusion of the whole temporal lobe. Up to 2 min postictally, there is hyperperfusion of the mesial temporal structures with hypoperfusion of lateral temporal structures, and from 2–15 min postictally there is hypoperfusion of the whole temporal lobe. A return to normal is seen in 10 to 30 min (Duncan et al., 1993; Newton et al., 1995). Depending on the aetiology, differential patterns of ictal hyperperfusion in TLE have been described (Ho et al., 1996).

Cerebral perfusion changes in partial seizures other than the ictal hyperperfusion and postictal hypoperfusion of the epileptic focus have received little attention. Ictal frontal lobe hypoperfusion during temporal lobe CPS has been described (Rabinowicz et al., 1997; Menzel et al., 1998). Ictal ipsilateral or contralateral cerebellar hyperperfusion (Bohnen et al., 1998; Shin et al., 2001), ipsilateral thalamic (Spencer, 2002) and ipsilateral or contralateral basal ganglia hyperperfusion has been reported (Shin et al., 2001; Spencer, 2002) in TLE. In these studies, inclusion of patients with different aetiologies and epilepsy syndromes, ictal SPECT studies of different seizure types, inclusion of late ictal and postictal injections, and visual or region of interest analysis of the SPECT images made detection of systematic perfusion changes and a comparison of studies difficult.

In the present study we selected patients with refractory TLE associated with HS who had an ictal SPECT with early injection during a typical mesial temporal lobe CPS (Commission on Classification and Terminology of the International League Against Epilepsy, 1989; Williamson et al., 1998). Brain regions with significant perfusion changes, either hyper‐ or hypoperfusion, were determined using a voxel‐based statistical analysis. We also explored interrelationships between these regions and possible neural network activities during CPS (Spencer, 2002). The aim of the present work was to obtain new insights into the pathophysiology of CPS associated with HS.

Methods

Inclusion criteria were: (i) adults who underwent a presurgical evaluation for intractable mesial TLE; (ii) unilateral HS on high resolution MRI of the brain; (iii) ictal SPECT injection during a CPS with electroclinical features characteristic of mesial TLE (Risinger et al., 1989; French et al., 1993; Williamson et al., 1993, 1998); (iv) initiation of ictal SPECT injection within 30 s of seizure onset; (v) ongoing seizure activity after initiation of ictal SPECT injection for at least 30 s in order to obtain true ictal SPECT studies (Berkovic, 2000); (vi) ictal hyperperfusion in the temporal lobe ipsilateral to the HS (Ho et al., 1995; Newton et al., 1995; Lee et al., 2000) (in this manuscript, this temporal lobe will be referred to as the ipsilateral side, the other side as the contralateral one); (vii) patients considered good surgical candidates after full presurgical evaluation.

The following data were obtained for all patients: clinical history, neurological and physical examination, interictal and ictal EEG recordings, video analysis of seizures, neuropsychological assessment, interictal and ictal SPECT and subtraction ictal SPECT co‐registered to MRI (SISCOM), and standard MRIs of the brain, including T1‐ and T2‐weighted sequences, fluid‐attenuated inversion recovery (FLAIR) images and a magnetization prepared rapid gradient echo (MPRAGE) sequence: 9.7/4/300/1 [repetition time (TR)/echo time (TE)/inversion time (TI)/number of excitations (NEX), flip angle 12°, matrix size 256 × 256, field of view (FOV) 256] on a 1.5‐T Siemens Vision scanner.

A diagnosis of HS was made on visual inspection of the MRIs, and was based on a combination of unilateral hippocampal atrophy and an increased T2 signal in the atrophic hippocampus (Jackson et al., 1993).

All patients had an interictal and ictal SPECT study. Technetium‐99m‐ethyl cysteinate dimer (99mTc‐ECD) (Leveille et al., 1992) (600–1000 MBq) was used as tracer. The set‐up for ictal SPECT injection has previously been described in detail (Vanbilloen et al., 1999; Van Paesschen et al., 2000). A fresh dose of the tracer was delivered to the video‐EEG monitoring suite twice a day (at 09:00 and 13:00). Ictal SPECT injection was performed during video‐EEG monitoring. The duration of the seizure and the time of initiation of ictal SPECT injection were determined during analysis of the video‐EEG recordings of the CPS. Seizure onset was defined on clinical or EEG grounds, whichever was first. Injection of the tracer for the interictal SPECT was performed with surface EEG monitoring. Patients were injected in the EEG suite, with eyes open in a dimly lit and quiet environment. Image acquisition and processing were performed in a similar way for both the interictal and ictal SPECT studies. SPECT imaging commenced within 1 h of tracer injection. Images were acquired using a triple head gamma camera (Trionix Triad) equipped with low energy, ultra high‐resolution collimators and a rotation of 360° per head. Attenuation correction was performed according to the Chang method, using an attenuation coefficient of 0.12/cm. Images were reconstructed using a filtered back projection algorithm with a Butterworth filter (0.6 cycles/cm, power 10).

Analysis was carried out on SUN SPARC computers (SUN microsystems, Mountain View, CA, USA) using statistical parametric mapping (SPM) software (Wellcome Department of Cognitive Neurology, London, UK), version SPM99 (http://www.fil.ion.ucl.ac.uk/spm), implemented in MATLAB (Mathworks Inc., Sherborn, MA, USA).

The SPECT images of patients with right HS were flipped in order to align the epileptogenic zone on the same side, i.e. the ipsilateral side, for all images (no differences in perfusion patterns were present between patients with right and left HS). The two SPECT scans from each subject were co‐registered using mutual information (Maes et al., 1997) and the mean image was calculated. This latter image was used to determine the spatial transformation (Friston et al., 1995a) to the Montreal Neurological Institute (MNI) template space. This transformation was then applied to the two single SPECT images. Finally, images were smoothed with a three‐dimensional isotropic Gaussian kernel of 16 mm full‐width half maximum (FWHM). Images were normalized by global scaling (which does not impose any a priori constraint on specific regions as opposed to normalizing with respect to cerebellar counts, as is also often used in nuclear medicine) to correct for differences in administered tracer dose. The condition and subject effects were estimated according to the general linear model at each voxel (Worsley et al., 1992; Friston et al., 1995b). Two contrasts (ictal versus interictal SPECT and the reverse) were tested (fixed effect model) and for each contrast the resulting set of voxel values constituted a statistical parametric map of the t‐statistic SPM{t}. The significance threshold was set at Pcor < 0.2 (corrected for multiple comparisons) for peak height for screening purposes, and at Pcor < 0.05 (corrected for multiple comparisons) to identify statistically significant perfusion changes (Worsley et al., 1996). The differences between ictal and interictal SPECT scan for the identified local maxima within the SPM{t} were calculated for all 24 patients, and used to assess possible interrelationships between these points. SPSS release 10.0.5 was used (www.SPSS.com) to calculate Pearson correlation and partial correlation coefficients. This part of the study was exploratory, and in this context, the probabilities for the correlation coefficients were used as an indicator of a possible association (as opposed to statistics obtained from a small number of a priori hypotheses). Accordingly, no adjustment has been made for multiple comparisons. A P value of <0.05 was used as an indicator of an association, whereas a value between 0.05 and 0.10 was taken as a weak indicator of an association.

Results

Forty‐eight consecutive patients with unilateral HS, who underwent a presurgical evaluation and had an ictal SPECT, were considered for this study. Twenty‐three patients were excluded for the following reasons. First, ictal SPECT injection during: (i) a partial seizure with extratemporal lobe features (n = 6); (ii) a secondarily generalized seizure (n = 3); (iii) a simple partial seizure (n = 2); (iv) a neocortical temporal lobe seizure (n = 1); (v) a non‐epileptic seizure (n = 1); and (vi) a non‐convulsive status epilepticus (n = 1). Secondly, nine patients with initiation of ictal SPECT injection >30 s after seizure onset, or seizure activity that lasted <30 s after termination of ictal SPECT injection. One patient was excluded because the SPECT was technically of poor quality.

Twenty‐four patients were included (13 females, 11 males). The median age was 37 years (range 30–53 years). All had refractory TLE. Sixteen patients (67%) had a history of febrile seizures, two (8%) of meningo‐encephalitis, one (4%) of cerebral trauma and five (21%) had no clear aetiology. High‐resolution MRI scan of the brain showed unilateral HS, which was left‐sided in 14 patients (58%) and right‐sided in 10 (42%). One of these patients had dual pathology, i.e. HS and a post‐traumatic scar. HS was considered the epileptogenic zone in all patients. Visual inspection of the ictal SPECTs and SISCOMs showed the most significant hyperperfusion in the ipsilateral temporal lobe in all. Interictal EEGs showed ipsilateral temporal lobe spikes, sharp waves or both in 22 patients (92%), contralateral temporal lobe spikes in one (4%) and bitemporal temporal lobe spikes in one (4%). Ictal EEGs showed ipsilateral temporal lobe epileptic activity in 20 (82%), bitemporal in two (8%) and contralateral in two (8%). All patients had been operated on. Pathological examination of the resected specimen confirmed HS in all. Median follow‐up after surgery was 29 months (range 1–41 months). Twenty patients had a follow‐up of at least 1 year. The Engel’s seizure outcome classification at their last visit was I for 12 of these 20 patients (60%), II for seven patients (35 %) and IV for one patient (5%). (I = seizure free, auras only or convulsions only on drug withdrawal; II = rare disabling seizures or nocturnal seizures only; III = worthwhile improvement; IV = no improvement.) This last patient had been seizure‐free for 1.5 years after surgery, when she developed extratemporal lobe seizures that had not been present before. The patient with the dual pathology has remained seizure‐free for >2 years after surgery.

The ictal SPECT study was during a CPS in all patients. The median duration of the CPS was 74 s (range 49–221 s). The median time of initiation of ictal SPECT injection was 16 s (range 0–29 s) after seizure onset. Medical personnel performed the injections in 21 patients, and three patients performed a self‐injection ictal SPECT (Van Paesschen et al., 2000). The median duration of ictal activity after the initiation of the injection was 56 s (range 33–203 s).

Ictal cerebral blood flow changed significantly compared with the interictal blood flow in several brain regions (Fig. 1 and Table 1). Ipsilateral temporal lobe hyperperfusion was present in all 24 patients. Three clusters of hyperperfusion were observed in the ipsilateral temporal lobe (superior temporal gyrus/uncus, medial temporal gyrus and fusiform/inferior temporal gyrus). One cluster of hyperperfusion in the ipsilateral frontal lobe (border of ipsilateral middle frontal and precentral gyrus) was present. Also, both visual cortices and two small regions in the contralateral postcentral gyrus showed hyperperfusion. Hypoperfusion was present in the frontal lobes, the contralateral cerebellum and ipsilateral precuneus. Eight clusters of frontal lobe hypoperfusion were observed (bilateral inferior, middle and superior frontal gyrus).

Fig. 1 Brain regions with significant ictal cerebral perfusion changes during CPS in 24 patients with HS are shown on a surface rendering of a brain MRI scan. (A) Anterior view; (B) posterior view; (C) contralateral view; (D) ipsilateral view; (E) inferior view; (F) superior view. Ictal hyperperfusion is in red and ictal hypoperfusion is in blue. There was ictal hyperperfusion in the ipsilateral temporal lobe, the border of the ipsilateral middle frontal gyrus and precentral gyrus, the occipital lobes and two small regions in the contralateral postcentral gyrus. Ictal hypoperfusion was present in the frontal lobes and the contralateral cerebellum. The hypoperfused ipsilateral precuneus is not visible on this image.

View this table:
Table 1

Brain regions with ictal perfusion changes during CPS in patients with HS

Anatomic region (Brodmann area)SidePerfusion patternCoordinates (mm)TCorrected P‐valuen (%)
x y z
Temporal lobe
Superior temporal gyrus, uncus     (38, 28, 36) i + –30 4 –26 13.83 0.000 24 (100%)
Medial/superior temporal gyrus     (21, 20, 38) i + –44 4 –34 11.99 0.000 24 (100%)
Fusiform/inferior temporal gyrus     (20) i + –46 –30 –6 9.05 0.000 23 (96%)
 Superior temporal gyrus (38)c+5414–345.460.06620 (83%)
Occipitoparietal lobe
Primary visual cortex (17) c + 20 –76 8 8.05 0.001 23 (96%)
Cuneus (18) i + –4 –88 12 8.03 0.001 22 (92%)
Postcentral gyrus (3,1,2) c + 28 –32 80 6.00 0.024 20 (83%)
Postcentral gyrus (2) c + 52 –32 60 5.89 0.029 22 (92%)
 Postcentral gyrus (4 or 2)c+60–14465.350.08120 (83%)
Frontal lobe
Border of middle frontal and     precentral gyrus (6) i + –40 –14 28 5.82 0.034 21 (87.5%)
Cerebellum
 Anterior cerebellumi+–18–46–225.030.14421 (87.5%)
Frontal lobe
Middle frontal gyrus (10) i –26 52 20 12.68 0.000 24 (100%)
Inferior/middle frontal gyrus     (46) i –34 42 18 11.81 0.000 24 (100%)
Superior frontal gyrus, medial     part (6) 0 16 50 10.30 0.000 23 (96%)
Superior frontal gyrus,     medial part (8) 0 32 36 9.04 0.000 23 (96%)
Middle frontal gyrus (10) c 34 58 10 8.43 0.000 23 (96%)
Superior frontal gyrus, medial     part (10) c 10 58 12 7.73 0.001 24 (100%)
Medial frontal gyrus (9) 2 44 18 7.56 0.001 23 (96%)
Inferior frontal gyrus (45) c 52 24 6 6.59 0.008 23 (96%)
 Middle frontal gyrus (8)c3426525.510.06020 (83%)
 Cingulate gyrus (31)i–434404.930.17223 (96%)
 Middle frontal gyrus (6)c2616604.880.18921 (87.5%)
Cerebellum
Posterior cerebellum, crus II of     the lateral cerebellar hemisphere c 42 –64 –44 9.15 0.000 24 (100%)
 Posterior cerebellumi–36–74–365.080.13121 (87.5%)
Occipital lobe
Precuneus (7) i –6 –46 30 5.75 0.038 22 (92%)

Pcorrected <0.2 was used for screening purposes and Pcorrected <0.05 (bold) was used to identify brain regions with statistically significant perfusion changes. Coordinates x, y and z are according to the AC–PC (anterior commissure–posterior commissure) system [x = 0 at the midline (+/– = contralateral/ipsilateral to seizure focus); y = 0 at the anterior commissure (+/– = anterior/posterior); z = 0 at the AC–PC level (+/– = superior/inferior)]. i/c = ipsilateral/contralateral to seizure focus; +/– = ictal hyperperfusion/hypoperfusion; T = statistical t‐value; n = number of patients.

We explored interrelationships between perfusion changes in brain regions of Table 1, time of injection of the ictal SPECT and the logarithm of the seizure duration (Table 2). An inverse association was found between ipsilateral ictal frontal lobe (–26, 52, 20) hypoperfusion and ipsilateral ictal temporal lobe (–30, 4, –26) hyperperfusion (r = –0.47; P = 0.02) (Fig. 2), and between ipsilateral ictal anterior cerebellar (–18, –46, 22) hyperperfusion and contralateral ictal postcentral gyrus (28, –32, 80) hyperperfusion (r = 0.41; P = 0.05).

Fig. 2 Inverse association (r = –0.47; P = 0.02) between ipsilateral ictal frontal lobe (–26, 52, 20) hypoperfusion and ipsilateral ictal temporal lobe (–30, 4, –26) hyperperfusion. Notice that all 24 patients showed ipsilateral temporal lobe hyperperfusion and ipsilateral frontal lobe hypoperfusion. The difference between the ictal and interictal regional brain perfusion is given in arbitrary units (ictal and interictal brain perfusion values were normalized for total counts to correct for differences in injected activity and scaled in such a way that the average perfusion in the brain equals 50).

View this table:
Table 2

Associations between regional cerebral ictal perfusion changes, logarithm of seizure duration and time of injection of ictal SPECT

i Superior temporal gyrus, uncus (–30, 4, –26)i Middle frontal gyrus (–26, 52, 20)c Middle frontal gyrus (34, 58, 10)c Posterior cerebellum (42, –64, –44)i Anterior cerebellum (–18, –46, –22)i Posterior cerebellum (–36, –74, –36)c Postcentral gyrus (28, –32, 80)i Precentral gyrus (–40, –14, 28) i Precuneus (–6, –46, 30)Time of ictal SPECT injection(s)
i Middle frontal gyrus (–26, 52, 20)r= –0.47; P= 0.02
c Middle frontal gyrus (34, 58, 10)n.s.n.s.
c Posterior cerebellum (42, –64, –44)n.s.n.s.n.s.
i Anterior cerebellum (–18, –46, –22)n.s.n.s.n.s.r = 0.35; P = 0.09
i Posterior cerebellum (–36, –74, –36)n.s.n.s.n.s.n.s.n.s.
c Postcentral gyrus (28, –32, 80)n.s.n.s.n.s.n.s.r= 0.41; P= 0.05n.s.
i Precentral gyrus (–40, –14, 28) n.s.n.s.n.s.n.s.n.s.n.s.n.s.
i Precuneus (–6, –46, 30)n.s.n.s.n.s.n.s.n.s.n.s.n.s.n.s.
Time of ictal SPECT injection (s)r = 0.37; P = 0.07n.s.n.s.r= –0.44; P= 0.03n.s.n.s.n.s.n.s.n.s.
Logarithm of seizure durationn.s.n.s.n.s.n.s.r = –0.37; P = 0.07n.s.r= –0.41; P= 0.05n.s.n.s.n.s.

Pearson correlation coefficient (r) and P value (two‐tailed), not corrected for multiple comparisons. For the frontal and temporal lobes, only regions with the most significant perfusion changes are shown. i = ipsilateral; c = contralateral; n.s. = non‐significant association. Associations (P < 0.05) are printed in bold, with weak associations (P < 0.1) in normal font.

Time of ictal SPECT injection was inversely associated with contralateral ictal posterior cerebellar (42, –64, –44) hypoperfusion (r = –0.44; P = 0.03) and weakly associated with ipsilateral ictal temporal lobe hyperperfusion (–30, 4, –26) (r = 0.37; P = 0.07). A weak association was found between ipsilateral ictal anterior cerebellar (–18, –46, –22) hyperperfusion and contralateral ictal posterior cerebellar (42, –64, –44) hypoperfusion (r = 0.35; P = 0.09). The logarithm of the duration of the seizure showed an inverse association with contralateral ictal postcentral gyrus (28, –32, 80) hyperperfusion (r = –0.41; P = 0.05) and a weak inverse association with ipsilateral ictal anterior cerebellar (–18, –46, –22) hyperperfusion (r = –0.37; P = 0.07). This inverse association between seizure duration and ipsilateral ictal anterior cerebellar (–18, –46, –22) hyperperfusion was stronger using a partial correlation coefficient, controlling for time of injection and contralateral ictal posterior cerebellar hypoperfusion (rpartial = –0.49; P = 0.01) (Fig. 3).

Fig. 3 Partial correlation coefficient (rpartial = –0.49; P = 0.01) between the logarithm of the duration of the seizure and the ictal perfusion changes in the ipsilateral anterior cerebellum (–18, –46, –22), controlled for time of injection of the ictal SPECT and contralateral cerebellar perfusion changes (42, –64, –44). The graph was obtained by plotting the residuals of two regression equations, the first predicting the logarithm of the seizure duration from time of injection of the ictal SPECT and contralateral ictal cerebellar perfusion changes, the second predicting ipsilateral ictal anterior cerebellar perfusion changes from time of injection and contralateral ictal posterior cerebellar perfusion changes.

Discussion

The aim of the present study was to determine in an objective manner perfusion changes during CPS associated with HS, and to explore interrelationships between regions of hyper‐ and hypoperfusion. Our finding of ipsilateral frontal lobe hypoperfusion corroborates the observations of Rabinowicz and colleagues (Rabinowicz et al., 1997) and Menzel and colleagues (Menzel et al., 1998), who reported ipsilateral ictal frontal lobe hypoperfusion in three out of four (75%) and 12 out of 17 patients (71%) with TLE, respectively, using a semiquantitative region‐of‐interest analysis technique. In our study, using a more objective and sensitive analysis technique, we found ipsilateral, and to a lesser extent contralateral, frontal lobe hypoperfusion in all 24 patients (100%).

We suggest three hypotheses to explain the ictal frontal lobe hypoperfusion, which are not mutually exclusive. The frontal lobe hypoperfusion could represent a steal phenomenon, i.e. a shunt of blood towards the temporal lobe at the expense of frontal lobe perfusion (Rabinowicz et al., 1997). Secondly, ictal frontal lobe hypoperfusion could also reflect the absence of active frontal lobe cognitive processes, which are present during the interictal state (Fuster, 2001). Thirdly, frontal lobe hypoperfusion could represent an ictal surround inhibition (Prince and Wilder, 1967). Ictal surround inhibition has been shown to be present in the cortex surrounding an epileptic focus using optical imaging, which is a functional imaging modality based on the same principles as SPECT, i.e. coupling of focal alterations in metabolism and blood flow (Schwartz and Bonhoeffer, 2001). In this hypothesis, some interneurons in the hyperperfused temporal lobe undergo active synaptic inhibition with downstream decreased synaptic activity in the ipsilateral frontal lobe, which is the most common route of spread of mesial temporal lobe seizures (Lieb et al., 1991). Frontal lobe ictal surround inhibition may prevent seizure propagation (Schwartz and Bonhoeffer, 2001) and represent a defensive mechanism against secondary generalization. The observation of frontal lobe hyperperfusion, i.e. failed inhibition, during secondarily generalized temporal lobe seizures (Shin et al., 2002) corroborates this hypothesis.

The only other region of cerebral hypoperfusion was in the ipsilateral precuneus. As for the frontal lobes, this could be due to a steal phenomenon, decreased activity, surround inhibition or a combination of these three factors.

We found bi‐occipital hyperperfusion during CPS associated with HS. We believe that this occipital hyperperfusion is physiological, and could be explained by a relative occipital hypoactivation during interictal injection with eyes open in a dimly lit environment as opposed to ictal injection during bright daylight. On visual inspection of the ictal SPECTs and SISCOMs, the occipital hyperperfusion was always less than the ipsilateral temporal lobe hyperperfusion. This is important, because ictal occipital hyperperfusion that is more intense than temporal lobe hyperperfusion or is the only site of hyperperfusion in a patient with HS may indicate the presence of an occipitotemporal epilepsy (Palmini et al., 1993; Aykut‐Bingol and Spencer, 1999). In our initial study population of 48 patients with HS who had an ictal SPECT, we excluded three patients who had occipitotemporal epilepsy. Ictal SPECT in these patients with HS showed the most intense hyperperfusion in the occipital region in two patients.

We consider the ipsilateral ictal hyperperfusion at the border of the middle frontal and precentral gyrus as seizure propagation from the ipsilateral temporal lobe.

Ictal hyperperfusion in basal ganglia and thalamus has been reported previously (Shin et al., 2001, 2002; Spencer, 2002), which we did not find. Blumenfeld and colleagues studied ictal perfusion changes during CPS in patients with HS with ictal SPECT injection times ranging from 60–90 s, instead of the 0–30 s range used in our study (Blumenfeld et al., 2002). They found hyperperfusion in the temporal lobe, ventral basal ganglia, medial thalamus and brainstem, and hypoperfusion in the frontal and parietal association cortices. These findings suggest that basal ganglia, thalamus, brainstem and parietal association cortices are involved during later stages of CPS, and may reflect dynamic changes during the course of CPS. These results stress the importance of using selection criteria with respect to ictal SPECT injection times, seizure types, epilepsy syndromes and the aetiology of epilepsy in order to understand the neurobiology and pathophysiology of partial seizures better.

Contralateral ictal posterior cerebellar hypoperfusion was present in all 24 patients (100%), which has not been reported before. Three SPECT studies reporting ictal cerebellar perfusion changes only addressed cerebellar hyperperfusion and not hypoperfusion (Won et al., 1996; Bohnen et al., 1998; Shin et al., 2001). Interictal contralateral cerebellar hypometabolism associated with ipsilateral frontal lobe hypometabolism on [18F]fluorodeoxyglucose‐PET has been reported in patients with mesial TLE (Savic et al., 1996), which is similar to the ictal pattern of our study. We postulated that the contralateral cerebellar hypoperfusion could be explained by crossed cerebellar diaschisis (Kurthen et al., 1990; Won et al., 1996; Sagiuchi et al., 2001) related to the ipsilateral frontal lobe hypoperfusion. However, we did not find an association between the contralateral cerebellar hypoperfusion and the ipsilateral frontal lobe hypoperfusion.

We found ipsilateral anterior cerebellar hyperperfusion in 21 of 24 patients (87.5%). Ictal cerebellar hyperperfusion has been reported in patients with refractory partial epilepsy, mainly when unilateral clonic motor activity (Bohnen et al., 1998) or frontal hyperperfusion (Shin et al., 2001) was present, and in frontal lobe seizures (Seto et al., 1997), but also in temporal lobe seizures without frontal lobe hyperperfusion (Shin et al., 2001). Our study is the first to suggest a weak association between ictal contralateral posterior cerebellar hypoperfusion and the degree of perfusion changes (mainly hyperperfusion) in the ipsilateral anterior cerebellum.

The mechanisms underlying seizure duration and termination are little understood. Our observational study offered a unique possibility to determine associations between ictal activity in particular brain areas and seizure duration, i.e. to assess gating functions during seizures. We found an inverse association between ipsilateral ictal anterior cerebellar hyperperfusion and seizure duration, i.e. the more hyperperfusion in the ipsilateral anterior cerebellum, the shorter the seizure. Both animal studies (Mutani et al., 1969; Maiti and Snider, 1975; Snider and Maiti, 1975; Paz et al., 1985) and anterior cerebellar stimulation in the treatment of epilepsy in humans (Cooper, 1973; Cooper et al., 1973; Levy and Auchterlonie, 1979; Wright et al., 1984; Krauss and Fisher, 1993) have implicated the anterior cerebellum or paleocerebellum as a seizure inhibiting structure. Our findings further suggest that contralateral ictal postcentral gyrus hyperperfusion is also inversely associated with seizure duration, and may be part of a seizure gating network, together with the ipsilateral anterior cerebellum. Identification of brain structures that can inhibit seizure activity in specific epilepsy syndromes, such as mesial TLE associated with HS, could lead to a more scientific foundation of electrical stimulation as a treatment for refractory epilepsy.

Our study showed that ictal SPECT studies may reveal network activity during seizures, and that selection of aetiology, seizure type and timing of ictal injection is important. Our study population was rather small. Larger studies will be required to confirm our exploratory findings, and could reveal more detail of the network activity during different partial seizures types associated with HS and improve our understanding of the pathophysiology of seizures associated with HS.

Acknowledgements

This work was funded by the Research Fund Katholieke Universiteit Leuven Interdisciplinair Onderzoeksprogramma (IDO; grant /99/5). P.D. is a postdoctoral researcher of the Fonds voor Wetenschappelijk Onderzoek‐Vlaanderen (FWO).

References

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