SEARCH

SEARCH BY CITATION

Keywords:

  • Positron emission tomography;
  • Periventricular nodular heterotopia;
  • Epilepsy;
  • Serotonin

Summary

  1. Top of page
  2. Methods
  3. Results
  4. Discussion
  5. Acknowledgments
  6. References

Background:α-[11C]methyl-L-tryptophan (α-MTrp) positron emission tomography (PET) is a promising tool in the localization of the epileptogenic area in selected group of focal epilepsy patients. Electrophysiological evidence suggests the involvement of the neocortex in periventricular nodular heterotopia (PVNH).

Purpose: To determine whether α-MTrp PET can detect neocortical changes in patients with PVNH.

Methods: Four patients (2 male, mean age 28, range 23–35 years) with PVNH and intractable seizures were studied. The functional image in each patient was compared with those from 21 healthy controls (mean age 34.6 ± 14.2 years) by using statistical parametric mapping (SPM). The location of increased α-MTrp uptake was compared with the location of the EEG focus. A significant cluster was defined as a cluster with a height p = 0.005 and an extent threshold 100.

Results:α-MTrp PET revealed increased cortical uptake in two of four patients. The area of increased α-MTrp uptake in one patient was widespread. In the other patient, the area of increased uptake did not include the region where most seizures were generated on EEG. α-MTrp PET did not show increased uptake in the heterotopic nodules in any of the patients.

Conclusions:α-MTrp PET suggests abnormal metabolism of tryptophan in the neocortex. The increased uptake may be diffuse and may not co-localize with the EEG focus. This preliminary study suggests that α-MTrp PET may be useful, in conjunction with other evaluations, in localizing epileptic focus in patients with PVNH and refractory seizures.

Periventricular nodular heterotopia (PVNH) is a commonly identified malformation of cortical development (Barkovich & Kuziecky, 2000). Although the association between PVNH and epilepsy is well known (Dubeau et al., 1995; Battaglia et al., 1997), there is still controversy concerning the localization and the extend of the epileptogenic area (Dubeau et al., 1995; Kothare et al., 1998; Aghakhani et al., 2005; Battaglia et al., 2005; Tassi et al., 2005). Some evidence suggests involvement of the neocortex (Dubeau et al., 1995; Aghakhani et al., 2005; Battaglia et al., 2005; Tassi et al., 2005).

α-[11C]methyl-L-tryptophan (α-MTrp) has been developed as a tracer for PET measurement of serotonin synthesis in vivo (Diksic et al., 1990). Chugani et al. and we recently demonstrated that α-MTrp with positron emission tomography (PET) is a useful tool in the localization of the epileptogenic area in partial epilepsy, especially in patients with tuberous sclerosis or cortical dysplasia (Chugani et al., 1998; Fedi et al., 2001; Juhasz et al., 2003). However, in pathological conditions this tracer probably accumulates locally via the kynurenine pathway (Saito et al., 1993; Chugani & Muzik, 2000), in which case its uptake and trapping would not be related to the serotonin synthesis.

To determine whether metabolism in the brain serotonergic system, including the kynurenine pathway, is involved in the epileptic process associated with PVNH, we performed α-MTrp PET studies in patients with PVNH.

Methods

  1. Top of page
  2. Methods
  3. Results
  4. Discussion
  5. Acknowledgments
  6. References

Study population

Four patients (2 male, mean age 28 ± 5 years, range 23–35 years) with PVNH and intractable partial seizures (mean age of onset 16 ± 5 years) were studied. All received antiepileptic medications at the time of the study. None had a history of intellectual disability. They underwent a comprehensive presurgical workup including prolonged video-EEG monitoring. Three of them underwent intracranial EEG recordings 4–7 months after the PET studies. These three patients had had intractable seizures for more than 10 years. Demographic, clinical, and EEG information of all four patients are summarized in Table 1.

Table 1.  Patient demography and clinical data
Patient/Sex/AgeDuration of epilepsy (y)MRIInterictal EEG focusIctal onsetFDG PETα-MTrp PET increased uptake
  1. Bil, bilateral; Post, posterior; PVNH, periventricular nodular heterotopia; R, right; L, left; T, temporal; ND, not done; F, frontal.

1/F/3519Bil Post PVNHR or L TR post T, R PVNH, L post TNDR T-F, L F
2/M/2713Bil diffuse PVNH, L F encephalomalaciaL or R TL FNDR T, L O
3/F/2311Bil F PVNH, L F subcortical heterotopiaL or R TBil T R T, R Post TNormalNone
4/M/264L body PVNHNormalNot recordedNormalBil, nonlocalized

The control group used for statistical parametric mapping (SPM) analysis consisted of 21 healthy volunteers (10 men, mean age 34.6 ± 14.2 years). Control subjects were not taking any medication and had no history of neurological or psychiatric disorder. All had normal MRIscans.

α-MTrp PET procedure

Patients fasted for 8 h prior to injection of the tracer. They underwent PET scanning in the interictal state while continuing to receive their habitual antiepileptic drugs. Scans were performed in the awake resting state. During the scans, EEG was recorded in order to identify any epileptiform discharges and electrographic seizures. Two venous lines were placed, one for tracer injection, the other for collection of blood samples. After a 12-min transmission scan, an average of 445 MBq of α-[11C]MTrp was injected intravenously over 2 min. Following this, data were acquired for 60 min with an ECAT HR+ scanner (Siemens, Canada). Sixty-three slices with an intrinsic resolution of 5.0 × 5.0 × 5.0 mm full width at half maximum were obtained for each frame. During image acquisition, thirteen 1.5 ml venous blood samples were drawn, contralateral to the side of tracer injection, at progressively longer intervals (every 15–30 s for the first 2 min, every minute for the next 3 min, every 3–5 min up to 20 min, and every 10 min for the rest of the scanning period).

This study was performed as a part of PET studies of patients with intractable partial epilepsy approved by the Research Ethics Board of the Montreal Neurological Institute and Hospital of McGill University. The research protocol was explained, and informed consent obtained from all patients.

MRI procedure

High-resolution MRI scans were acquired on a 1.5 T Gyroscan (Philips Medical Systems, Eindhoven, The Netherlands) or a 1.5 T Siemens Vision Magnetom (Erlangen, Germany), using a T1-weighted gradient echo sequence (repetition time, 18 ms; echo time, 10 ms; 1 signal average; flip angle 30°; matrix 256 × 256; field of view 250 mm; slice thickness, 1 mm). Approximately 170 slices with an isotropic voxel size of 1 mm3 were acquired. Patients were asked to lie quietly in a supine position during the examination, with their head held in place by a padded strap.

Data analysis

PET and MRI data analysis was performed on a Silicon Graphics workstation (Mountain View, CA, U.S.A.).

PET-MRI coregistration

Coregistration of individual PET and MRI images was performed using an automatic procedure with averaged tissue activity images obtained between 20 and 60 min. The MRI images from each subject were automatically transferred into Talairach space. Using parameters obtained from the automatic coregistration and transformation, PET images were resampled linearly into the stereotaxic coordinates space of Talairach and used for comparison in the SPM program (13) (http://www.fil.ion.ucl.ac.uk/spm/software/spm99/).

Calculation of functional images of α-MTrp PET

For each pixel, the net α-MTrp uptake constant K*[μL/g/min] was calculated from the time activity curve and the input function estimated from the venous sinus radioactivity and blood samples, using our previously described constrained linear regression method (Kumakura et al., 2001).

Statistical analysis

The PET data were analyzed with SPM 99, developed and distributed by the Hammersmith Hospital (London, England) (Friston et al., 1995). K* images after standardization into Talairach space were converted to a file format that can be applied to the SPM program. Before SPM comparison, all images were smoothed using a Gaussian filter (14 mm FWHM final isotropic resolution). The proportional scaling was performed to remove the effect of global differences in regional values among subjects (Okazawa et al., 2000). The t-test was applied pixel by pixel to compare the regional differences in K* images between each patient and 21 normal control subjects. To identify the regions considered to show a significant difference, two criteria were used: (1) Cluster has p = 0.005, two-tailed. (2) Extent threshold set at 100 voxels.

Comparison of PET findings with structural and EEG abnormalities

After identifying the regions showing significant difference on α-MTrp PET, we compared the location of these regions with the MRI and EEG abnormalities in each patient by evaluating if the PVNH, the overlying cortex or areas of seizure focus on EEG showed increased K*.

Results

  1. Top of page
  2. Methods
  3. Results
  4. Discussion
  5. Acknowledgments
  6. References

The results in all patients are summarized in Table 1. MRI showed PVNH in all patients, but no obvious cortical abnormalities except for left frontal encephalomalacia in patient 2 (Fig. 1B).

image

Figure 1. MRI findings of all patients. (A) Patient 1 had bilateral posterior PVNH (arrows). (B) Patient 2 had bilateral diffuse PVNH (arrows) and additional left frontal encephalomalacia (white arrow). (C) Patient 3 had bilateral frontal PVNH (arrows). This patient also had an additional left frontal focal subcortical heterotopia. (D) In Patient 4, PVNH was seen only in the body of the left lateral ventricle (arrow).

Download figure to PowerPoint

FDG-PET was performed in two patients, and did not reveal cortical abnormalities. FDG uptake in PVNH was lower than that of cortex.

α-MTrp PET revealed focal increased uptake in the cortex of two patients as detailed below. No patient showed increased uptake within the PVNH.

Patient 1 had bilateral posterior PVNH (Fig. 1A). α-MTrp PET showed increased uptake in the right temporal and frontal neocortex that included the seizure onset region (Fig. 2). SPM analysis revealed increased uptake in the left fronto-polar areas and also the right fronto-temporal cortex (Fig. 3). This patient had electrodes implanted in both temporal lobes including posterior temporal structures and periventricular nodules in the temporal and occipital horns. The majority (9 of 10) of clinical and EEG seizures originated in the right posterior temporal neocortex with or without simultaneous involvement of the underlying gray matter heterotopia. One seizure originated in the contralateral homologous posterior temporal neocortical structures. The EEG findings showed a prominent zone of epileptogenicity in the right temporal region maximum in the posterior temporal neocortex, which was included in the areas of increased α-MTrp uptake. However, the areas of increased α-MTrp uptake were more widespread and covered the contralateral frontopolar area as well.

image

Figure 2. MRI and K*[μL/g/min] images of α-MTrp PET in patient 1. α-MTrp PET shows increased uptake in the right frontotemporal cortex (white arrowheads). There was no increased uptake of α-MTrp within the PVNH (black arrows).

Download figure to PowerPoint

image

Figure 3. K*[μL/g/min] image of patient 1 compared with normal controls by SPM, using the threshold p = 0.005 and extend threshold set at 100 voxels. Significantly increased uptake of α-MTrp is observed in the right frontotemporal cortex, which includes the seizure onset region and also in the left frontopolar area.

Download figure to PowerPoint

Patient 2 had bilateral diffuse PVNH and additional left frontal encephalomalacia (Fig. 1B). α-MTrp PET showed increased uptake in the right temporal and left occipital cortex by SPM analysis. The patient had electrodes implanted in both occipital, temporal and frontal lobe structures. The main seizure onset zone was found in the left frontal lobe (5 seizures recorded). Although seizures with left occipital onset were also recorded, the left occipital EEG findings were likely to be due to an acute lesion related to the implanted electrode. Increased α-MTrp uptake was not seen in the area corresponding to the left frontal focus.

Patient 3 had bilateral frontal PVNH and an additional left frontal focal subcortical heterotopia (Fig. 1C). On α-MTrp PET, we could not find increased uptake by SPM. He had electrodes implanted only in temporal lobe structures bilaterally. Of 34 seizures recorded, 25 had onset in both perihippocampal areas, 5 had onset in the right perihippocampal region, and 4 had onset in the right posterior temporal neocortex. No EEG–PET correlation could be made.

In Patient 4, three PVNH were seen only in the body of the left lateral ventricle (Fig. 1D) α-MTrp PET showed nonlocalized multiple areas of increased uptake in both hemispheres. This patient was not investigated with depth electrodes. No interictal or ictal abnormalities could be recorded during prolonged video-EEG monitoring.

Areas of decreased α-MTrp uptake in each patient were also determined by SPM analysis with the same criteria. In patient 1 and 3, areas of decreased α-MTrp were not detected. In patient 2, SPM showed decreased uptake in the left frontal lobes, but it corresponded to the defect in gray and white matter due to the encephalomalacia (Fig. 1B). In Patient 4, bilateral multiple small clusters were seen, but no specific areas of increased uptake were detected.

Discussion

  1. Top of page
  2. Methods
  3. Results
  4. Discussion
  5. Acknowledgments
  6. References

This is the first study to use α-MTrp PET to evaluate possible abnormalities in tryptophan metabolism in the neocortex of patients with PVNH. We and others previously reported that α-MTrp PET is useful in detecting the epileptic focus in patients with partial epilepsy, in particular those related to tuberous sclerosis or focal cortical dysplasia (Chugani et al., 1998; Fedi et al., 2001; Juhasz et al., 2003). This is the first report focusing on α-MTrp PET findings in patients with PVNH.

α-MTrp PET revealed focal increased uptake in two out of four patients with PVNH. In Patient 1, the seizure onset was located in the neocortex overlying the PVNH. However, the area of abnormal α-MTrp PET was more widespread than that of the seizure onset. Interictal and ictal EEGs showed bilateral epileptic foci, although the majority of seizures originated on the right side. The widespread increase in α-MTrp uptake possibly indicated a more widespread epileptogenic zone with altered metabolism of tryptophan.

In Patient 2, α-MTrp PET showed increased uptake outside the main focus, but not in the left frontal region. It is possible that decreased α-MTrp uptake caused by encephalomalacia at the focus affected the result because of neuronal loss and partial volume effect.

In Patient 3, we could not find areas of increased α-MTrp uptake.

In Patient 4 with unilateral PVNH, areas of increased α-MTrp uptake were found bilaterally in several cortical regions. This patient had no seizures and no epileptiform discharges during the prolonged EEG monitoring. Therefore, it is possible that the lack of an active focus may have correlated with the nonlocalized PET finding. The significance of the multiple small clusters of increased or decreased α-MTrp uptake in this patient is not clear.

The association between PVNH and seizures is well known (Raymond et al., 1994; Dubeau et al., 1995). However, whether seizures originate from heterotopia itself or from other areas is still a matter of discussion (Dubeau et al., 1995; Kothare et al., 1998; Aghakhani et al., 2005; Battaglia et al., 2005; Tassi et al., 2005). Aghakhani et al. (2005) performed intracranial EEG recordings in eight patients with PVNH. Four of these had seizure onsets with synchronous involvement of mesial temporal lobe structures, heterotopia and portions of temporal and extratemporal neocortex. The neocortical extension of the epileptic activity correlated and paralleled the distribution of the heterotopia. Our finding of increased uptake of α-MTrp in the cortex suggests epileptogenicity of the neocortex overlying the PVNH.

Seizure outcome after surgery has been reported to be unsatisfactory in patients with PVNH (Dubeau et al., 1995; Li et al., 1997), particularly in those with bilateral PVNH (Tassi et al., 2005). It is tempting to speculate that a widespread increase of α-MTrp uptake extending beyond the MRI-visible lesion and involving several neocortical areas may be an indicator for poor outcome after surgery. However, this hypothesis should be tested prospectively in future studies involving patients with PVNH undergoing surgical treatment for medically intractable seizures. At the present time, α-MTrp PET should be used only as one of several different modalities for localizing focus in patients with PVNH and refractory seizures. The main reason for this is the fact that the tracer uptake region did not always correlate with the EEG foci of the seizure onset.

The pathological abnormality underlying the widespread alteration of cortical tryptophan metabolism extending beyond the MRI-visible lesion remains unclear. Previous imaging studies indicate possible aberrant connectivity of the heterotopic nodules with distant cortical structures (Aghakhani et al., 2005). These abnormal networks may cause excitatory interactions with the overlying neocortex. A defect in cortical migration may also result in abnormal cortical organization of the overlying neocortex that causes abnormal cortical excitability and possibly increased serotoninergic innervation (Trottier et al., 1996). Therefore, the increased α-MTrp uptake observed in our patients may represent a serotonin-mediated compensatory mechanism with the purpose of decreased cortical excitability, and/or increased activity of the kynurenine pathway with formation of pro-convulsant products (Chugani & Muzik, 2000; Saito et al., 1993).

We did not find increased α-MTrp uptake within the heterotopiae themselves in the present group of patients. On the other hand, previous isolated reports demonstrated that at least in some patients, the main seizure generator is located within the heterotopic malformation (Dubeau et al., 1995; Kothare et al., 1998). Future studies combining α-MTrp PET with other imaging modalities and electrophysiological data are needed to fully explore the pattern of epileptogenic activity in PVNH.

Acknowledgments

  1. Top of page
  2. Methods
  3. Results
  4. Discussion
  5. Acknowledgments
  6. References

This research was supported in part by the Canadian Institute for Health Research (MOP-42438) and the National Institutes of Health, USA (R01-NS29629) grants. The authors thank the staff of the Positron Emission Tomography and Cyclotron-Radiochemistry Units for their dedication in performing this study.

Conflict of interest: We confirm that we have read the Journal's position on issues involved in ethical publication and affirm that this report is consistent with those guidelines. The authors report no conflict of interest.

References

  1. Top of page
  2. Methods
  3. Results
  4. Discussion
  5. Acknowledgments
  6. References
  • Aghakhani Y, Kinay D, Gotman J, Soualmi L, Andermann F, Olivier A, Dubeau F. (2005) The role of periventricular nodular heterotopia in epileptogenesis. Brain 128:641651.
  • Barkovich AJ, Kuziecky RI. (2000) Gray matter heterotopia. Neurology 55:16031608.
  • Battaglia G, Franceschetti S, Chiapparini L, Freri E, Bassanini S, Giavazzi A, Finardi A, Taroni F, Granata T. (2005) Electroencephalographic recordings of focal seizures in patients affected by periventricular nodular heterotopia: role of the heterotopic nodules in the genesis of epileptic discharges. J Child Neurol 20:369377.
  • Battaglia G, Granata T, Farina L, D'Incerti L, Franceschetti S, Avanzini G. (1997) Periventricular nodular heterotopia: epileptogenic findings. Epilepsia 38:11731182.
  • Chugani DC, Chugani HT, Muzik O, Shah JR, Shah AK, Canady A, Mangner TJ, Chakraborty PK. (1998) Imaging epileptogenic tubers in children with tuberous sclerosis complex using alpha-[11C]methyl-L-tryptophan positron emission tomography. Ann Neurol 44:858866.
  • Chugani DC, Muzik O. (2000) Alpha[C-11]methyl-L-tryptophan PET maps brain serotonin synthesis and kynurenine pathway metabolism. J Cereb Blood Flow Metab 20:29.
  • Diksic M, Nagahiro S, Sourkes TL, Yamamoto YL. (1990) A new method to measure brain serotonin synthesis in vivo. I. Theory and basic data for a biological model. J Cereb Blood Flow Metab 10:112.
  • Dubeau F, Tampieri D, Lee N, Andermann E, Carpenter S, Leblanc R, Olivier A, Radtke R, Villemure JG, Andermann F. (1995) Periventricular and subcortical nodular heterotopia. A study of 33 patients. Brain 118(Pt 5):12731287.
  • Fedi M, Reutens D, Okazawa H, Andermann F, Boling W, Dubeau F, White C, Nakai A, Gross DW, Andermann E, Diksic M. (2001) Localizing value of alpha-methyl-L-tryptophan PET in intractable epilepsy of neocortical origin. Neurology 57:16291636.
  • Friston K, Holmes AP, Worsley K, Poline JP, Frith CD, Frackowiak R. (1995) Statistical parametric maps in functional imaging: a general linear approach. Hum Brain Mapping 2:189210.
  • Juhasz C, Chugani DC, Muzik O, Shah A, Asano E, Mangner TJ, Chakraborty PK, Sood S, Chugani HT. (2003) Alpha-methyl-L-tryptophan PET detects epileptogenic cortex in children with intractable epilepsy. Neurology 60:960968.
  • Kothare SV, VanLandingham K, Armon C, Luther JS, Friedman A, Radtke RA. (1998) Seizure onset from periventricular nodular heterotopias: depth-electrode study. Neurology 51:17231727.
  • Kumakura Y, Natsume J, Toussaint P, Rosa P, Nakai A, Mzengeza S, Meyer E, Diksic M. (2001) Generation of functional images of the tissue trapping constant for α-[11C]methyl-L-tryptophan using constrained linear regression and venous sinus normalized imput function. European Association of Nuclear Medicine Annual Congress, Napoli, Italy, August 25-29, 2001. Eur J Nucl Med Mol Imaging 28:1107.
  • Li LM, Dubeau F, Andermann F, Fish DR, Watson C, Cascino GD, Berkovic SF, Moran N, Duncan JS, Olivier A, Leblanc R, Harkness W. (1997) Periventricular nodular heterotopia and intractable temporal lobe epilepsy: poor outcome after temporal lobe resection. Ann Neurol 41:662668.
  • Okazawa H, Leyton M, Benkelfat C, Mzengeza S, Diksic M. (2000) Statistical mapping analysis of serotonin synthesis images generated in healthy volunteers using positron-emission tomography and alpha-[11C]methyl-L-tryptophan. J Psychiatry Neurosci 25:359370.
  • Raymond AA, Fish DR, Stevens JM, Sisodiya SM, Alsanjari N, Shorvon SD. (1994) Subependymal heterotopia: a distinct neuronal migration disorder associated with epilepsy. J Neurol Neurosurg Psychiatry 57:11951202.
  • Saito K, Nowak TS Jr, Suyama K, Quearry BJ, Saito M, Crowley JS, Markey SP, Heyes MP. (1993) Kynurenine pathway enzymes in brain: responses to ischemic brain injury versus systemic immune activation. J Neurochem 61:20612070.
  • Tassi L, Colombo N, Cossu M, Mai R, Francione S, Lo Russo G, Galli C, Bramerio M, Battaglia G, Garbelli R, Meroni A, Spreafico R. (2005) Electroclinical, MRI and neuropathological study of 10 patients with nodular heterotopia, with surgical outcomes. Brain 128:321337.
  • Trottier S, Evrard B, Vignal JP, Scarabin JM, Chauvel P. (1996) The serotonergic innervation of the cerebral cortex in man and its changes in focal cortical dysplasia. Epilepsy Res 25:79106.