Incomplete Hippocampal Inversion in Normal Individuals- A Retrospective Observational Study using Magnetic Resonance Imaging

Abstract

Introduction: Hippocampal inversion is a normal process that happens between 11 and 21 weeks of gestation. In some individuals the process is incomplete and is termed as Incomplete Hippocampal Inversion (IHI). There are differing observations recorded, of its incidence in normal individuals- from as high as 17% to nil making it difficult to come to a conclusion as to whether IHI is a normal anatomical variant or a marker of a wide spread disorder in brain. Aim: To determine the incidence of IHI in the hippocampus of individuals who don’t suffer from any form of epilepsy and for whom Magnetic Resonance Imaging (MRI) brain has been performed and seen to be normal. Materials and Methods: A retrospective observational study was conducted in a tertiary centre using T1fl 3D, check globally isovoxel 0.86 mm images of the brain and incidence of IHI in normal individuals without seizures for whom MRI brain was normal otherwise was calculated. The MRI images of a total of 197 individuals qualified for the study. Analysis was done by four Radiologists using subjective visual criteria. Results: It was seen that 1.015-1.52% was the incidence of IHI in otherwise normal MRI brain in present study. A 1.52% is the incidence placing a study classified as partial IHI under IHI and 1.015% not placing it under IHI. All were unilateral, on left side. Conclusion: The results of present study point that there was very less incidence of IHI in normal individuals. Such a less incidence will make it difficult to suggest that IHI is a normal anatomical variant. Present study also encountered potential pitfalls with the use of T1 weighted 3 Dimensional (3D) images to determine of presence or absence of IHI.