The neuropsychiatric disorders of focal epilepsy

Abstract

Neuropsychiatric disorders commonly co-exist with focal epilepsy. Despite intense investigation there remains significant limitations to our current understanding of the aetiological relationship between these conditions, as well as the clinical and radiological factors that are associated with the development of mental illness in epileptic patients.

To address these questions I assembled a large cohort of consecutive focal epilepsy patients reviewed at a large tertiary referral centre over an 11-year period, and analysed relevant clinical, radiological and demographic findings using both cross-sectional (baseline evaluation) and longitudinal (serial assessments) study designs. For cross-sectional analyses, psychiatric and epilepsy comorbidity were comprehensively identified and compared to baseline MRI-determined Deep Brain Structure (DSB) volumes. In the prospective study, standardised psychiatric and quality of life assessments were obtained in a subset of patients and the development of mental illness compared to interval changes in DSB volumes. I focus on depression and psychosis, as they represent the two most prevalent psychiatric disorders identified, and classify focal epilepsy patients according to the site of seizure origin and presence of a lesion.

Contrary to the common medical belief, I found that the rates of neuropsychiatric disorders in temporal lobe epilepsy were equivalent to those in extratemporal lobe epilepsy. There were no differences between those with and without psychiatric disorder by age, gender, and laterality of seizure, epilepsy severity or duration. Patients with non-lesional epilepsy, both temporal and extratemporal, have double the rate of depression compared to those with lesional focal epilepsy. There were high rates of quality of life difficulties in people with epilepsy and comorbid psychiatric disorders but the pattern of subjective concerns does not match objective clinician ratings.

The hippocampal and amygdalae volumes of epilepsy patients were reduced compared to normal controls. People with co-existing epilepsy and depression had a trend towards smaller reductions in temporal lobe structures, which may represent differential expressions of progression through inflammation, trauma or emotional processing needs with either relative sparing or volumetric increases. People with psychosis and epilepsy have bilaterally reduced hippocampi compared to those with epilepsy, where reductions are predominantly ipsilateral to seizure focus. There is less evidence for amygdala change in psychosis, but a small relative increased volume was observed compared to those with epilepsy alone. There was no evidence of progression over time at a population level over 3.9 years, although there was greater variability in size in all epilepsy subjects compared to normal controls.

These results convincingly argue against assumptions about the primary role of temporal lobe foci in the pathogenesis of psychiatric comorbidities in patients with epilepsy, whilst allowing for the possibility that disruption to frontotemporal networks may be a component in the development of psychiatric disorders. Progression is not an inevitable part of the natural history, at least over a 4 year period but it is possible that individuals may exhibit marked changes. They highlight the as yet unexplored possibility that the absence of a lesion as an epileptic site may be associated with greater risks of neuropsychiatric illness and allow for speculation that this may be related to inhibitory surround impacts, more extensive underlying diffuse abnormalities and disruption to frontotemporal connectivity.