Neuroimaging methods to evaluate the etiology and consequences of epilepsy

Magnetic resonance imaging (MRI) is widely available and is generally the imaging method of first choice for identifying the structural basis of seizure disorders, having both sensitivity and specificity. Positron emission tomography (PET) and single photon emission computed tomography (SPECT) scans may be more sensitive in some patients when MRI is unremarkable, but do not confer specificity of etiological diagnosis. Methods to assess the consequences of epilepsy on the brain need to be quantitative, reliable, reproducible and safe. They must be acceptable to patients and to a healthy control group for repeated use, and the acquisition and analytical methods must be stable over years. Longitudinal studies are necessary to determine whether secondary cerebral damage occurs as a consequence to the epilepsies. Patient groups will be heterogeneous in this regard and analysis will need to be not only of changes in group means, but also of the number of patients who show significant changes in imaging parameters, that exceed the limits of test-retest reliability and of changes in age-matched controls. MRI is an attractive tool to evaluate the presence and development of cerebral damage in patients with epilepsy as it is readily available, non-invasive, and acceptable to patients and controls. MRI volumetry is reliable and reproducible, but the sensitivity of the method to detect subtle abnormalities has not yet been established. Preliminary analysis of longitudinal studies of patients with newly diagnosed and chronic active epilepsy suggests that 10% of newly diagnosed patients and 25% of those with chronic active epilepsy develop significant cerebral, hippocampal or cerebellar atrophy over 3.5 years. MR spectroscopy may be more sensitive for detecting abnormalities, but the test-retest reliability is less good. Other MRI tools such as diffusion tensor imaging (DTI) may be useful methods for evaluating secondary cerebral damage acutely and chronically.