Incomplete Hippocampal Inversion and Its Relationship to Hippocampal Subfield Volumes and Aging

Background and Purpose: Incomplete hippocampal inversion (IHI) is an atypical anatomical pattern presented by the hippocampus. It is associated with several neuropathological conditions and is thought to be a factor of susceptibility to hippocampal sclerosis and loss of volume. The volume loss of hippocampus is an inevitable consequence of aging, and when accelerated it is commonly considered an imaging biomarker of Alzheimer’s disease dementia.

Methods: We have studied the relationship between IHI and hippocampal subfield volumes in a cohort of 60 healthy participants of 49 to 87 years of age. The presence and severity of IHI and hippocampal subfield volumes were quantified from T2 MR images acquired at 3T.

Results: It was found that IHI presented in 23.3% of participants. Right unilateral IHI was rare (2 cases, 3.3%) in comparison to left unilateral IHI (9 cases, 15%), with 3 (5%) of participants showing bilateral IHI. No significant relationships between the whole hippocampal volumes and IHI was observed. Instead, significant relationships between the volumes of the left and right Cornu Ammonis subfield-1 (CA1) and IHI scores were evident.

Conclusions: The rates of IHI prevalence in the current cohort are similar to those previously reported in healthy cohorts. The IHI severity is related to hippocampal subfield volumes, most notably the CA1, which is a novel finding with potential implications in research on aging and dementia.