What is the Relationship Between Arachnoid Cysts and Seizure Foci?

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1 Epilepsin, 38( 10): , 1997 Lippincott-Raven Publishers, Philadelphia 0 International League Against Epilepsy What is the Relationship Between Arachnoid Cysts and Seizure Foci? Santiago Arroyo and Joan Santamaria Servicio de Neurolog fa, Hospital Clinic i Provincial de Barcelona, Barcelona, Spain Summary: Purpose: Arachnoid cysts are sometimes encountered in MRIs performed for a variety of reasons. In patients with epilepsy, particularly those with refractory epilepsy, arachnoid cysts are often assumed to be related to their seizure focus. We conducted a study to investigate this putative relationship. Methods: A retrospective study on the incidence of arachnoid cysts was performed in patients seen in our Epilepsy Clinic who had CT or MRI scans, interictal EEGs or ictal EEGS. Locations of seizure foci in these patients were defined from clinical and electrophysiologic data. Results: Seventeen of 867 patients had arachnoid cysts. Twelve patients had temporal lobe cysts and only 3 of them had temporal lobe seizures. Four patients had frontal lobe cysts and only 1 had frontal lobe seizures ipsilateral to the cyst. One patient had a cerebello-pontine angle cyst and frontal lobe seizures. Thus, clinical manifestations of seizures and EEG findings (interictal and/or ictal) indicated that the seizure focus was adjacent to the cysts in only 4 patients (23.5%). Conclusions: Our findings suggest that arachnoid cysts are often an incidental finding in patients with epilepsy and do not necessarily reflect the location of the seizure focus. Key Words: Arachnoid cysts-frontal lobe seizures-temporal lobe seizures-complex partial seizures-epilepsy surgery. Arachnoid cysts are frequently encountered in association with various neurological disorders, including epilepsy (1-3). In a recent study of 27 patients with arachnoid cysts in the middle cranial, 10 had neurologic symptoms not referable to the cyst, 17 had neurologic symptoms which were possibly related to the cyst, and 7 of these 17 patients had epilepsy (3). Arachnoid cysts are congenital lesions of unknown causes and consist of a reduplication of the arachnoid membrane within which fluid collects as a cyst (4). The etiology of the cysts is controversial, but it has been ascribed to congenital malformation, infection, trauma, or increased intracranial pressure. Their most common locations in the sylvian or choroidal (3). In most cases, cysts are not associated with progressive symptoms related to raised intracranial pressure, and frequently are detected incidentally, or following hemorrhagic complications due to minor trauma. A number of anecdotal reports have yielded inconsistent conclusions about the relationship of arachnoid cysts and epilepsy. A patient with Landau-Kleffner syndrome was reported to have an area of hypometabolism around an arachnoid cyst. Shunting of the cyst led to seizure Accepted May 7, Address correspondence and reprint requests to Dr. S. Arroyo at Servicio de Neurologia, Hospital Clinic i Provincial de Barcelona, cnillarroel 170, Barcelona 08036, Spain. improvement and reduction of the hypometabolism (5). However, bilateral temporal lobe cysts were described in a patient with multifocal epilepsy and Lennox-Gastaut syndrome, suggesting that arachnoid cysts in themselves are not necessarily related to a specific epileptogenic region (2). Moreover, a recent study (l), has shown that epilepsy surgery in selected patients with temporal lobe cysts does not significantly reduce seizure frequency. In view of this controversy, we conducted a retrospective study of those patients in our clinic with both epilepsy and arachnoid cysts, in order to investigate the neuroanatomic, and possibly neurophysiologic, relationship between these neurological entities. An understanding of this relationship will help in determining whether the presence of an arachnoid cyst provides consistently reliable information regarding the location of the seizure focus in candidates for resective surgery. It might also shed light on the relationship between structural lesions of this type and epileptogenesis. METHODS Patients were selected from the Epilepsy Database (which includes all patients who are seen in the Epilepsy Unit) at the Hospital Clinic of Barcelona (University Hospital, referral center). In this database, there is a specific table for MRI results, from which MRI data can be 1098

2 SEIZURES AND ARACHNOID CYSTS I099 extracted. From the Epilepsy Database, which included 867 patients ( ), we extracted the names of those patients with arachnoid cysts (17 patients). Patients with MRI showing arachnoid cysts who were not referred to the Epilepsy Unit are not included in the database. All had 2 or more interictal electroencephalograms (EEGs). Electrodes were placed, using the Standard 10/20 System. All patients also had brain MRIs (General Electric 1.5 T). Eight of them had more than one MRI. In most cases, clinical characteristics of seizures were taken from the patient histories except for 6 patients for whom video-eeg recording was obtained. EEG localization was performed with knowledge of the location of the arachnoid cyst. Seizures were classified as frontal, temporal, or parietal, based upon clinical phenomenology and EEG recordings (interictal and/or ictal), according to the standard International League Against Epilepsy (ILAE) classification of the epilepsies (6). RESULTS There were 6 men and 11 women. Mean age was 29.9 f 11.3; mean age of seizure was 16.1 * Possible etiologic or predisposing factors for epilepsy were identified in 6 patients. These factors included mild mental retardation (4 patients); pyridoxine-dependent epilepsy, significant perinatal asphyxia, developmental delay and mental retardation (1 patient); and kernicterus, purulent meningitis, developmental delay and mental retardation (1 patient). had a history of severe head trauma. All patients except 2 had intractable epilepsy. One patient had benign rolandic epilepsy and another had had only 1 seizure. The location and size of the cysts and the type of seizures are displayed in Table 1. Only 1 patient had more than 1 cyst (bilateral sylvian cysts). In all patients, cysts were non-enhancing and did not communicate with the ventricles. of them had progressive symptoms, suggesting an increase in intracranial pressure. There were 9 choroidal cysts (Fig. l), 3 cysts in the temporal (Fig. 2), 2 sylvian cysts (1 with bilateral cysts), 2 frontal lobe cysts, and 1 cyst in the pontocerebellar angle. Three patients also had lesions suggestive of heterotopias, and 1 had a dysembryoplastic neuroectodermal tumor (DNT). The presumed location of the seizure focus was not associated with the arachnoid cyst location in most patients. Of the 12 patients with choroidal or large temporal lobe cysts, only 3 (nos. 3, 7, and 10) had probable temporal lobe seizures, with interictal EEG in 2 of them showing frontotemporal spikes or temporal lobe. Localization of seizures in the 9 remaining patients was performed with video-eeg recording in 4 (nos. 1, 4, 6, ll), and none of them had temporal lobe foci. In the other 5 patients, localization was performed using clinical characteristics and interictal EEG features: One (no. 5) had benign rolandic epilepsy with typical clinical and EEG characteristics; 1 (no. 2) had complex partial seizures with frontal spikes and a bilateral frontal and parietal band heterotopia suggesting an extratemporal lobe focus; 1 (no. 8) had secondarily generalized seizures preceded by left sided ascending paresthesias, which suggested a parietal lobe focus; and 2 (nos. 9 and 12) had generalized seizures without history of complex partial seizures, 1 of them with interictal EEG showing bursts of right frontal spike-wave discharges. Thus, in our patients, epileptogenic cortex usually was not adjacent to the patient s arachnoid cyst. Patients with cysts in the sylvian or frontal lobe (4) had: 2 frontal lobe complex partial seizures recorded with video-eeg (nos. 15 and 16), one of them with seizure contralateral to the cyst. One patient (no. ) had generalized seizures without clinical absences or complex partial seizures, with interictal bursts of generalized polyspikes and spike-and-wave, suggesting a generalized epilepsy. The other (no. 14) had probable frontal lobe seizures with interictal bursts of generalized spikes contralateral to the cyst, although we cannot exclude false localization, because the patient had hemiatrophy. Thus, only in one of them (no. 16) was the presumed seizure focus possibly associated with the cyst. Despite the probable origin of the video-eeg-recorded seizures in the frontal lobe in this patient, we decided to resect the tumor to analyze its pathology. Pathology showed a DNT. The patient has continued to have seizures, suggesting that the origin of the seizures was the frontal lobe. Finally, 1 patient (no. 17) had a cerebello-pontine angle cyst and simple partial seizures with motor manifestations, most likely arising from the frontal lobe. DISCUSSION The findings of this study suggest that the location of an arachnoid cyst in a patient with epilepsy rarely coincides with the location of the epileptogenic area. Specifically, temporal lobe arachnoid cysts usually are not associated with temporal lobe epilepsy. Most of the patients in our series had frontal or parietal lobe seizures despite having temporal lobe cysts, suggesting that in most cases the cysts are an incidental finding. Moreover, the population included individuals with both partial and generalized epilepsies. In fact, 2 patients (nos. 5 and 6) had epilepsies not related to structural abnormalities: one had benign rolandic epilepsy and the other had pyridoxine-dependent epilepsy. These data support our conclusion that arachnoid cysts may not be related to a specific seizure focus in a significant fraction of cases. In 4 patients, there was indeed an association between the sei-

3 1100 S. ARROYO AND J. SANTAMARIA TABLE 1. Characteristics of the patients and arachnoid cysts Location Size of Patient of the the cyst Other MRI Type of no. Age Onset cyst (cm) abnormalities seizures Interictal EEG Ictal EEG I neonate months 12 and left sylvian Left sylvian orbito-frontal mesial frontal pontocerebellar angle 1 x 0.6 x 0.5 Parietal CP Bursts of slow spike-wave, maximum right frontal-parietal- 0.5 x 0.5 x 0.5 Bilateral band heterotopia (frontal and parietal) temporal frontal spikes 0.3 x 0.3 x 0.3 posterior periventricular nodular heterotopia (1.5 cm) Temporal CP 0.5 x 0.5 x 0.5 Frontal CP and sec. Left frontal spikes, bursts of generalized spikes, diffuse central and temporal spikes and delta 5 seizures: right parietal lobe 4 seizures: frontal lobe (not definite side of preference) 1.5 x 1.5 x.i5 Frontal (motor) SP (benign rolandic epilepsy) 0.5 x 0.5 x 0.5 Frontal CP and Left fronto-temporal spikes 2 seizures: right 1.5 x 1 x 1 pyridoxine dependent epilepsy Temporal CP 1 x 0.5 x x 0.5 x x 3.5 x x 3 x 2.5 Parietal CP and sec. (seizures preceded by left sided sensory symptoms) CP and sec. GTC (undetermined localization) Frontal CP and sec. fronto-temporal spikes 1 x 3 x 2.5 frontal R: 2 x 3 x 2.5; L: 0.5 x 0.5 x 0.5 2X2X2 0.5 x 0.5 x 0.5 Left hemiatrophy 0.5 x 0.5 x 0.5 mesial temporal dysembrioplastic neuroepithelial tumor l x l x l frontal (subcortical) heterotopia and right temporal parietal cortical dysplasia with paquigira spike-wave Bursts of generalized spikes and polyspikes and 3.5 Hz spike-wave spikes, maximum right Frontal CP and sec. Hz spike-wave and polyspike and wave Frontal (sensitive) SP and sec. Slow spike-wave, right frontal maximum fronto-central 2 seizures: right frontal lobe 2 seizures: left frontal lobe 2 seizures: diffuse (bilateral fronto-temporal) MRI, Magnetic Resonance Imaging; EEG, Electroencephalogram; SP, Simple partial seizures; CP, complex partial seizures; CP and sec., complex partial and secondarily generalized seizures; GTC, generalized seizures.

4 SEIZURES AND ARACHNOID CYSTS 1101 Another limitation of our study is that localization was not performed in all cases, using video-eeg. However, clinical characteristics and interictal EEG features suggested the location of their seizures with a good degree of certainty, or at least excluded their spatial association to the cyst. In most cases, cysts were small and none of them showed signs of being under pressure. We do not believe that deep dipoles projecting to distant areas can account for the apparent localization of the seizure focus. It is interesting that 3 of our patients also had heterotopias, located far from the arachnoid cysts, which could also be responsible for seizures. One of our patients also had a DNT. To our knowledge, the relationship between arachnoid cysts and gray-matter heterotopias has not been described previously. The occurrence of developmental pathology in some of these patients could be coincidental. In our database, we have 36 patients with developmental pathology (dysplasia, cortical heterotopia or schizencephaly) and only 3 of them have arachnoid cysts. In any case, the presence of a cyst should prompt physicians to search for heterotopias elsewhere, and to consider whether these heterotopias might be better markers for the epileptogenic region (7). Recently, there has been discussion about the relative value of anatomical studies (MRI) versus functional tests, such as EEG, Positron Emission Tomography, (PET) Single Photon Emission Tomography (SPET) etc. for localization of the seizure focus (8). To some practitioners, epilepsy is reliably related to structural lesions, FIG. 1. TI-weighted coronal MRI images from patient no. 2, showing a right choroidal arachnoid cyst (narrow arrow) and bilateral band heterotopias affecting frontal parietal regions (large arrows). zure focus and arachnoid cysts, but our data led us to believe that there is no causal relationship in this association. Conclusions from this study are limited by the small number of patients and the selection bias of our clinic population-i.e., most of our patients had intractable epilepsy and were referred to an Epilepsy Center. However, it is these patients in whom surgical management often is contemplated, particularly given the presence of a lesion, and in whom a better understanding of pathophysiologic between arachnoid cysts and is most relevant. FIG. 2. TI-weighted axial MRI image from patient no. 11, showing a large arachnoid cyst in the right temporal. This patient had right frontal lobe seizures on video EEG recording.

5 1102 S. ARROYO AND J. SANTAMARIA and when such lesions are present, functional studies are not necessary. However, there is widely accepted agreement that structural imaging elicits only part of the information necessary to reach firm localization of the epileptogenic region. It is not rare, in fact, for the lesion to be distant from the seizure focus. There also may be more than 1 seizure focus (9-1 1). In our study, we found that the dissociation between structural and functional pathology is particularly likely in patients with arachnoid cysts, in whom there is usually no relationship to be found between the seizure focus and the cyst. Thus we emphasize that a comprehensive evaluation is of utmost importance to the firm localization of seizures in these patients, notwithstanding their obvious MRI/CT scan lesions. Patients with other lesions or with cysts under pressure may or may not need a comprehensive evaluation before surgery. Acknowledgment: We are grateful to Dr. Nathan Crone and Dr. Ronald P. Lesser for their helpful comments. REFERENCES 1. Koch CA, Voth D, Kraemer G, Schwarz M. Arachnoid cysts: Does surgery improve epileptic seizures and headaches? Neurosurg Rev 1995;18: Pascual-Castroviejo I, Pascual-Pascual SI. Bilateral arachnoid cysts, seizures and severe encephalopathy: Case report. Neuropediatrics 1994;25 : Passero S, Filosomi G, Cioni R, Venturi C, Volpini B. Arachnoid cysts of the middle cranial : A clinical, radiological and follow-up study. Acta Neurol Scand 1990;82: Krawchenko J, Collins GH. Pathology of an arachnoid cyst: Case report. J Neurosurg 1979;50: De Volder AG, Michel C, Thauvoy C, Willems G, Ferriere G. Brain glucose utilisation in acquired childhood aphasia associated with a sylvian arachnoid cyst: Recovery after shunting as demonstrated by PET. J Neurol Neurosurg Psychiatry 1994;57: Commission on Classification and Terminology of the International League Against Epilepsy: Proposal for revised classification of epilepsies and epileptic syndromes. Epilepsia 1989;30: Raymond AA, Fish DR, Sisodiya SM, Alsanjari N, Stevens JM, Shorvon SD. Abnormalities of gyration, heterotopias, tuberous sclerosis, focal cortical dysplasia, mycrodysgenesis, dysembrioplastic neuroepithelial tumour and dysgenesis of the archicortex in epilepsy: Clinical, EEG and neuroimaging features in 100 adult patients. Brain 1995;118: Stevens JM. Epilepsy: Structural or functional? AJNR 1996;17: Berger MS, Kincaid J, Ojemann GA, Lettich E. Brain mapping techniques to maximize resection, safety, and seizure control in children with brain tumors. Neurosurgery 1989;25: Yeh HS, Privitera MD: Secondary epileptogenesis in cerebral artenovenous malformations. Arch Neurol 1991;48: Awad IA, Rosenfeld J, Ah1 J, Hahn JF, Luders HO. Intractable epilepsy and structural lesions of the brain: Mapping, resection strategies, and seizure outcome. Epilepsia 1991;32: