Neuronuclear Assessment of Patients With Epilepsy

Epilepsy is a common chronic neurological disorder that is controlled with medication in approximately 70% of cases. When partial seizures are recurrent despite the use of antiepileptic drugs, resection of the epileptogenic cortex may be considered. Nuclear medicine plays an important role in the presurgical assessment of patients with refractory epilepsy. Single-photon emission computed tomography (SPECT) and positron emission tomography (PET) techniques are used to determine the seizure onset zone, which needs to be resected to render a patient seizure free. Correct localization of the ictal onset zone with the use of SPECT or PET is associated with a better surgical outcome. Ictal perfusion SPECT imaging with ^99mTc-ethyl cysteinate dimer (ECD) or ^99mTc-hexamethylpropyleneamine oxime (HMPAO) enables one to detect the seizure onset zone in a majority of cases, especially in patients with temporal lobe epilepsy. Interictal SPECT imaging, which is more widely available, is unreliable to determine the ictal onset zone and is usually only used for comparison with ictal SPECT images. Assessment of the ictal onset zone using subtracted ictal and interictal studies, overlayed on structural imaging has proven to be more sensitive and more specific compared with visual assessment. Video-electroencephalography monitoring in combination with ictal SPECT imaging, however, is only available in specialized centers. It is important to inject the perfusion tracer as early as possible after the beginning of a seizure and to be aware of patterns of seizure propagation. Interictal ¹⁸F-fluorodeoxyglucose (FDG)-PET is routinely used to detect brain areas of hypometabolism, which usually encompass, but tend to be larger than, the seizure onset zone. Also, for assessment of FDG-PET, it is advisable to use an automated technique comparing the patient's images to a normal database in addition to visual interpretation of the images, since automated techniques have proven to be more accurate. In view of the thickness of the cortical ribbon, which may be below the resolution of the PET camera, posthoc partial volume correction or PET reconstruction incorporating the anatomical information of magnetic resonance imaging (MRI), may be useful for optimal assessment of glucose metabolism. Perfusion SPECT and interictal FDG-PET are able to demonstrate areas of abnormal perfusion and metabolism at a distance from the ictal onset zone, which may be associated with cognitive and psychiatric comorbidities, and may represent the functional deficit zone in epilepsy. Part of the functional deficit zone is a dynamic seizure-related process, which may resolve with cessation of seizures. In recent years, novel PET tracers have been developed to visualize not only glucose metabolism but also a wide variety of specific receptor systems. In patients with epilepsy, changes in the γ-amino-butyric acid_A receptor, opioid receptor, 5-HT_1A serotonin receptor, nicotinic acetylcholine receptor systems, and others have been described. Because these tracers are not widely available and the superiority of studying these receptor systems over glucose metabolism in the presurgical evaluation of patients with refractory epilepsy remains to be proven, their use in clinical practice is limited at the moment. Finally, advances in small animal PET scanning allow the in vivo study of the process of epileptogenesis, starting from an initial brain insult to the development of seizures, in animal models of epilepsy. Potential new therapeutic targets may be discovered using this translational approach.