Neurobiological evidence for epilepsy-induced interictal disturbances

It is not in the best interest of persons with epilepsy to deny the possibility that seizures could cause enduring behavioral disturbances. Rather, it is essential to pursue clinical and animal investigations in order to identify any such changes that might occur and to elucidate their mechanisms. Many testable hypotheses can be developed from existing evidence. Antiepileptic medication may produce interictal behavioral disturbances in patients with epilepsy by indirect mechanisms. Some aberrant behaviors could be due to medication-induced systemic disorders, neuroendocrine dysfunction, or REM deficit, whereas depression following successful treatment with drugs, as well as with surgery, may be related more specifically to cessation of seizures. The underlying neuropathological process also induces neurological and mental deficits, but it is not always possible to differentiate those behavioral disturbances due to destructive effects of the lesion from those due to recurrent epileptic seizures. Behavioral disturbances are associated more frequently with epileptogenic lesions in limbic structures than with those elsewhere in the brain, but a relationship between hemispheric lateralization of the epileptogenic lesion and specific interictal behavioral symptoms remains controversial. When considering the effects of seizures per se on interictal behavior, it is important to realize that some "interictal" behavioral disturbances may actually be ictal events. Prolonged affective, autonomic, and psychic disturbances can occur in clear consciousness with unilateral limbic seizures that are not associated with scalp EEG changes. When epilepsy is acquired as a result of cerebral damage, the epileptogenic process takes time to develop before spontaneous seizures appear. It is more reasonable to assume that this progressive process continues than to postulate that it stops completely at the time the first seizure occurs. Epilepsy-induced protective homeostatic mechanisms that act to terminate ictal events, prevent ictal spread, and maintain the interictal state may also disrupt interictal function. Furthermore, seizures could indirectly influence interictal behavior as a result of their effects on neuroendocrine function and sleep. Because of confounding biological factors, it is difficult to document the association of any epilepsy disorder, by itself, with progressive behavioral disturbances in humans. Secondary epileptogenesis, protective homeostatic mechanisms, and epilepsy-induced disturbances in development can be readily demonstrated, however, in experimental animal models. In experimental animals, endogenous opoids are released during seizures and mediate some postictal behaviors. A physiological dependency on high levels of endogenous opioids released during seizures could produce depression as a withdrawal symptom interictally or when seizures no longer occur as a result of successful therapy. Experimental animal models of depression exist to test hypotheses concerning pro- and antidepressant effects of epileptogenesis.(ABSTRACT TRUNCATED AT 400 WORDS)