Atonic phenomena in focal seizures: Nomenclature, clinical findings and pathophysiological concepts

Atonic seizures have traditionally been described in patients with generalized epilepsies; however, ictal atonia is increasingly recognized as a phenomenon of focal seizures. Recognition of atonia as a manifestation of focal seizures is crucial in order to not mislabel these events as non-epileptic. In addition, a large number of patients, who undergo presurgical investigations due to focal pharmacoresistant epilepsy, may present with atonia. Therefore, knowledge about the clinical manifestation of atonic seizures and pathological and anatomical insight about the generators of atonia are pivotal to form a hypothesis about the seizure onset zone in epilepsies presenting with such seizures. First, we aim to outline the nomenclature and the definitions in use to describe this enigmatic phenomenon. Second, this paper provides insight into the clinical manifestation and summarizes the cases reported in the literature that meet the criteria of focal atonic seizures. Third, we provide an overview of the current pathophysiological concepts surrounding focal atonia based on the clinical manifestation. This comprehensive review will provide in depth understanding of focal atonic seizures, which will aid in general neurological practice and in the presurgical work up of patients with pharmacoresistant epilepsy manifesting as atonic seizures.