The occurrence of various types and MRI appearance of hippocampal abnormalities in 220 patients with malformations of cortical development (MCD) were studied in various centers and large public hospital outpatient clinics in Germany and Austria. Patients' ages ranged from 2 years to 76 years, mean age 31. The data were obtained by visual analysis of MRI findings recorded by 3 independent raters. Types of hippocampal abnormalities were partially infolded/hypoplastic, sclerosis, malrotated, and enlarged. Malformations of cortical development were focal cortical dysplasia (27%), polymicrogyria (21%), developmental tumors (15%), and periventricular nodular heterotopia (14%). Hippocampal abnormalities were found in 69/220 (31%) patients. Hypoplastic hippocampus was the most common hippocampal abnormality, occurring in 34 (49%) of 69 patients. Hippocampal sclerosis, the second most frequent type, occurred in 18/69 (26%). MCD patients with hippocampal abnormalities (41/69 [52%]) had a higher rate of learning disabilities and delayed developmental milestones than those without (56/151 [37%]). Hypoplastic hippocampus was associated with symptomatic generalized epilepsies (West and Lennox-Gastaut syndromes) (11/34 [32%]) and a high rate of learning disability (27/34 [79%]), neurologic deficits (25/34 [73%]), and delayed developmental milestones (23/34 [68%]). Family history of epilepsy, febrile seizures, and response to antiepileptic drugs did not differ with type of hippocampal abnormality. A history of febrile seizures was present in 5/69 (7%) patients. 
COMMENT. One third of patients with malformations of cortical development have hippocampal abnormalities, and approximately one half have temporal lobe epilepsy. Hypoplasia, the most common hippocampal abnormality, is associated with the most severe neurological, developmental, and cognitive abnormalities. In preterm infants, small hippocampal volume is correlated with white matter injury, exposure to postnatal steroids, treatment with indomethacin, and impairment of cognitive and psychomotor development (Ped Neur Briefs 2008;22:42-43). 
Hippocampal sclerosis is often regarded as an acquired abnormality, but its precise etiology as a sequela of a prolonged febrile seizure or febrile status epilepticus is debated [3, 4]. One theory favors hippocampal sclerosis as a developmental abnormality that facilitates febrile seizures in predisposed families and leads to subsequent typical mesial temporal sclerosis. A long-term study of 200 patients with prolonged febrile seizures should clarify the relation of the febrile seizure to mesial temporal sclerosis and temporal lobe epilepsy.