Late infantile epileptic encephalopathy: A distinct developmental and epileptic encephalopathy syndrome

Objective Within the spectrum of developmental and epileptic encephalopathy (DEE), there are a group of infants with features that are distinct from the well‐recognized syndromes of early infantile developmental and epileptic encephalopathy (EIDEE), infantile epileptic spasm syndrome (IESS), and Lennox–Gastaut syndrome (LGS). We refer to this condition as late infantile epileptic encephalopathy (LIEE). Our objective was to highlight the characteristics of this group by analyzing patients who exhibit prototypical features. Methods From July 2022 to May 2023, we searched for LIEE features in pediatric patients who underwent epilepsy follow‐up at the University of Chicago Comer Children's Hospital. Results Out of 850 patients evaluated, thirty patients (3.5%) were identified with LIEE based on electroclinical characteristics. These patients had an average onset of epilepsy at 6.8 months and an average onset of LIEE features at 18.1 months. The epilepsy etiology was most commonly genetic and metabolic (50%), followed by congenital cortical malformations (23%), acquired structural abnormalities (20%), and unknown (7%). The predominant seizure types were myoclonic–tonic (70%), spasm–tonic (50%), epileptic spasms (47%), tonic (43%), and myoclonic (43%) seizures. All patients reported a history of either spasm–tonic or myoclonic–tonic seizures in addition to other types. All patients had EEGs showing discontinuity, electrodecrements, or both along with diffuse slowing, background voltages between 100 and 300 μV, and superimposed multifocal, diffuse epileptiform discharges. Every patient, except one, fulfilled the definition of drug‐resistant epilepsy, and all reported either moderate‐to‐severe or severe developmental delay. Significance Late infantile epileptic encephalopathy (LIEE) is characterized by several unique clinical and electrographic features. Typically, LIEE manifests in patients during the second year of life and occurs before two years of age, hence late infantile onset. The condition is commonly observed in infants with symptomatic epilepsy. Myoclonic–tonic and spasm–tonic seizures are the quintessential seizure types. The inter‐ictal EEG exhibits more organization and lower voltages than seen with hypsarrhythmia and lacks the defining EEG characteristics of EIDEE, IESS, or LGS. We propose that LIEE is a distinct electroclinical syndrome within the spectrum of developmental and epileptic encephalopathies.