Interictal dysphoric disorder – the bridge between epilepsy and bipolar disorder

Introduction Psychiatric disorders are frequent among patients with epilepsy. The association between epilepsy and mood disorders is recognized since the classical antiquity. Recent studies demonstrated that the prevalence of bipolar symptoms in epilepsy patients is more significant than previously expected. In the first half of the twentieth century, Kraeplin and Bleuler were the first to describe a pleomorphic pattern of symptoms claimed to be typical of patients with epilepsy and recently Blumer coined the term interictal dysphoric disorder to identify this condition. Although for some authors, the existence of this condition as a diagnostic entity is still doubtful, for others, it represents a phenotypic copy of bipolar disorder. Objectives In this work, we start from the phenomenological similarities between the interictal dysphoric disorder and the bipolar disorder, to explore the neurobiological underpinnings that support a possible link between epilepsy and bipolar disorder. Methods Research of articles published in PubMed and other databases. Results Interictal dysphoric patients have features that resemble the more unstable forms of bipolar II disorder and benefit from the same therapy used in bipolar depression. Epilepsy and bipolar disorder share features like episodic course, the kindling phenomenon as possible pathogenic mechanisms and the response to antiepileptic drugs. The study of possible common biological processes like neurogenesis/neuroplasticity, inflammation, brain-derived-neurotrophic-factor, hypothalamus pituitary adrenal axis, provided promising but not consensual results. Conclusions Further efforts to understand the link between epilepsy and bipolar disorder could provide the insight needed to find common therapeutic targets and improve the treatment of both illnesses.