Radiological Studies on Hippocampal Development : Morphological Variants and their Relationship to Epilepsy

Abstract: During fetal development, the hippocampal structures are folded forming the hippocampal sulcus which penetrates into the temporal lobe and then the entity rotates.  During this process, the hippocampal sulcus will be closed and the inverted hippocampus takes a rounded form. After complete inversion, the hippocampus has an oval form in a plane perpendicular to its long axis. If this process has not been completed the hippocampus remains the rounded form. That condition is called incomplete hippocampal inversion (IHI). The aims of this study was to evaluate the frequency of IHI in non-epileptic and epileptic children and adults and to explore the development of the hippocampal region by studying premature neonates and fetuses. Magnetic resonance (MR) images of 201 epilepsy patients and 150 non-epileptic subjects were evaluated without knowing clinical data. IHI was found in 19 % in seizure free controls (20 left-sided and 8 bilateral). 30% of the 201 epilepsy patients had IHI (40 left-sided, 4 right-sided, 16 bilateral). The difference was statistically significant (p<0.02). 25% of the temporal lobe epilepsy patients had IHI. The frequency was not significantly higher than in controls. There is no causality between temporal lobe epilepsy and IHI. 44% of the Rolandic epilepsy patients and 57% of the cryptogenic generalized epilepsy patients had IHI. IHI can be a sign of possible disturbed cerebral development in other parts of the brain. Cranial ultrasound examinations of 160 premature children were analyzed. The age at examination was 23-24 GW in 24 children, 25-28 GW in 72 children, and 29-36 GW in 64 children. IHI was found in 50%, 25% and 14%, respectively. The frequency difference between the children < 25 GW and > 25 GW was statistically significant (p< 0.001). From 25 GW onwards, the frequency and laterality of IHI is similar to that in the adult population. MRIs of 63 fetuses without intracranial pathology were reviewed independently by two radiologists. Three MRIs were performed post mortem at gestation week (GW) 17-18 and 60 in utero at GW 19-35. The hippocampal sulcus was open, bi- or unilaterally, in 35 fetuses at GW 17-32. The oldest of them was at GW 32.  The sulcus was closed at GW 21 at the earliest, unilaterally, and always from GW 33 onwards bilaterally. In 26/63 fetuses (41%), the hippocampal development was asymmetric and in 23 fetuses, the right side had developed faster.