Neuroimaging in the Epilepsies:
Modern neuroimaging technology now allows noninvasive diagnosis of a wide spectrum of brain lesions which may cause epilepsy. In the past, imaging of the brain relied on indirect visualization of brain lesions, using skull roentgen-ograms, cerebral angiography and pneumoencephalography. This article reviews the contribution of magnetic resonance imaging techniques to our understanding of the epilepsies. A subsequent article will discuss the use of functional neuroimaging with PET and SPECT.
Since its introduction a decade ago, magnetic resonance imaging (MRI) has provided increasingly superb definition of the internal structure of the brain. Many lesions responsible for partial epilepsy can now be easily and non-invasively diagnosed. MRI is superior to computerized x-ray tomography (CT) because it has better spatial resolution and better ability to detect abnormal tissue characteristics. In addition, CT cannot visualize the temporal lobes as well as MRI, because of bony artifact from the middle cranial fossa.
Mesial temporal sclerosis is the most common pathology in temporal lobe epilepsy. Hippocampal volume loss can now be detected both visually and quantitatively on MRI using newer techniques such as turboFLASH which rapidly acquire 2 mm thick coronal slices of the entire brain. A number of studies have shown that hippocampal atrophy on MRI correlates well with EEG lateralization (sensitivity 76-100 percent, specificity 86-100 percent), histological mesial temporal sclerosis, and is a good predictor of seizure- freedom after temporal lobectomy (97 percent satisfactory outcome).
Disorders of cortical development can now be diagnosed by MRI. High-resolution studies may reveal small areas of neuronal heterotopia, abnormal gyral patterns, or hamartomas. Detection of cortical dysplasia may allow a medically refractory patient to be considered for epilepsy surgery.
New MRI techniques have been developed to take advantage of the increased T2 signal often seen in pathological tissue. Fast fluid-attenuated inversion-recovery (FLAIR) is a sequence which acquires a highly T2-weighted image of the brain parenchyma, while reducing the bright signal of fluid-containing spaces (e.g. the cerebral ventricles) which often obscure parenchymal abnormalities in standard T2-weighted images. Recent studies have shown that FLAIR in-creases the conspicuity of abnormalities in mesial temporal sclerosis as well as dysplasia, glioma, and enceph-alomalacia, increasing the yield of MRI-detected abnormalities by 30 percent or more. Our experience with FLAIR has been quite promising and we are using it routinely for all epilepsy MRI studies.
Magnetic resonance spectroscopy (MRS) is a technique which studies the magnetic resonances of atoms other than those of protons in water (which make up the standard clinical MRI image). Proton MRS can measure small amino acids, creatine, choline, and lactate. This technique shows promise in the detection of epileptogenic areas of the brain. Decrease in the signal of N-acetyl aspartate, an amino acid found in neurons, is highly correlated with epileptiform EEG activity in the same temporal lobe, as well as with decreased hippocampal volume and good outcome after temporal lobectomy. Focal elevation of brain lactate has been demonstrated by proton spectroscopy after temporal lobe seizures in humans, and may prove to be a metabolic marker for the epileptogenic region. Proton MRS can also detect signals from small amino acids such as glutamate and GABA, and may eventually help us to understand the function of these neurotransmitters in normal brains and in epilepsy.
Use of Neuroimaging in Clinical Practice
All patients with partial-onset seizures should have a high quality imaging study to rule out a lesion such as tumor or AVM which may require surgery. Patients with generalized tonic-clonic seizures but focal abnormalities on neurological examination or EEG definitely require imaging. Patients who have difficult to control seizures of any type may benefit from referral to a comprehensive epilepsy center, where a high-quality imaging study may reveal abnormalities not seen in "routine" scans. Detection of hippocampal atrophy, cortical dysplasia or other non-tumorous lesions may allow medically refractory patients to be considered for epilepsy surgery. Patients whose clinical history and EEG pattern is diagnostic of a primary (idiopathic) generalized epilepsy, such as juvenile myoclonic epilepsy or absence epilepsy, generally do not require imaging.
MRI is the study of choice. The only role of CT is to rule out intracranial hemorrhage or mass effect in an emergency setting. The routine use of gadolinium contrast has not been shown to increase diagnostic yield on initial studies and is probably not necessary if high-quality noncontrast images are available. However, contrast is indicated if the clinical history is suggestive of tumor. Hippocampal abnormalities, the most common finding in patients with partial epilepsy, are best seen in thin-section images made in a coronal plane perpendicular to the axis of the temporal lobe. There is a wide variety of MRI pulse sequences available, and advances in MRI technology continue. Therefore, patients with a non-diagnostic MRI study in the past may benefit from repeat imaging with the highest resolution techniques if they have medically refractory seizures or have a change in their seizure type or frequency.
New Study Seeks Participants
The Cleveland Clinic Epilepsy Center is currently participating in two studies of the investigational AED Tiagabine in pediatric patients and adults with partial seizures. For more information, contact Adina Chirla, Research nurse clinician at 216/444-5559.
Managing Pediatric Epilepsy with the Ketogenic Diet
The ketogenic diet has proven very successful in allowing some children with severe epilepsy to have better seizure control. Consisting of foods that are high-fat, low-carbohydrate and low-protein, the ketogenic diet is generally reserved for children whose seizures have not stopped with medication. For more information, please call the ketogenic diet coordinator at 216/445-3447.
Further Reading
Jack CR Jr. MRI-based hippocampal volume measurements in epilepsy. Epilepsia 1994;35(Suppl 6):S21-S29.
Kuzniecky R. Magnetic resonance imaging in developmental disorders of the cerebral cortex. Epilepsia 1994;35(Suppl 6):S44-S56.
Kuzniecky RI, Jackson GD. Magnetic Resonance in Epilepsy. New York: Raven Press, 1995.
Ng TC, Comair YG, Xue M, et al. Temporal lobe epilepsy: Presurgical localization with proton chemical shift imaging. Radiology 1994;193:465-472.
Ruggieri P et al. The utility of fast FLAIR imaging in epilepsy. Epilepsia 1995;36(Suppl 4):25.
To Find Out More...
If you would like to discuss a case with an epilepsy specialist or want to schedule an appointment please call,
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800/223-2273, ext. 45559
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Reprinted with permission by: Med Help International