Abstract
Hippocampal malrotation (HIMAL) is an increasingly recognized neuroimaging feature but the clinical correlation and significance in epilepsies remain under debate. It is characterized by rounded hippocampal shape, deep collateral, or occipitotemporal sulcus, and medial localization of the hippocampus. In this review, we describe the embryonic development of the hippocampus and HIMAL, the qualitative and quantitative diagnosis issues, and the pathological findings of HIMAL. HIMAL can be bilateral or unilateral and more on the left side. Furthermore, the relevance of HIMAL diagnosis in clinical practice, including its role in epileptogenesis and the impact on the pre-surgical decision are reviewed. Finally, the relationship between HIMAL and hippocampal sclerosis (HS) and the possible role of genetics in the etiology of HIMAL are discussed. The evidence so far suggested that HIMAL does not have a significant role in epileptogenesis or surgical decision. HIMAL could be a genetic developmental imaging feature that represents a more diffuse but subtle structural error during brain development. Many questions remain to be explored, such as possible cognitive alteration associated with HIMAL and whether HIMAL predisposes to the development of HS. Further studies using high-quality MRI, unified consensus qualitative and quantitative diagnostic criteria, and comprehensive cognitive assessment are recommended.
Highlights
Hippocampal malrotation (HIMAL), termed incomplete inversion of the hippocampus or hippocampal “malformation,” is an increasingly recognized neuroimaging finding of undetermined clinical significance
11/19 patient with left HIMAL has left-sided EEG focus 3/4 patients with right HIMAL, EEG focus were on the left side 3/6 bilateral HIMAL, EEG focus on the left side
HIMAL was not related to the side of the EEG focus
Summary
Hippocampal malrotation (HIMAL), termed incomplete inversion of the hippocampus or hippocampal “malformation,” is an increasingly recognized neuroimaging finding of undetermined clinical significance. Since the hippocampus is one of the most important brain structures associated with epilepsy, HIMAL has been implicated in the development of epilepsy; the relationship is not clear cut. 2. Terminology The term “hippocampal malformation” is somewhat confusing in the pathological and imaging literature because it has been used in different ways, including a broader concept of any abnormality during hippocampal development, which included HIMAL and hippocampal hypoplasia. In HIMAL, the hippocampus usually has a normal T2 signal but appears to be rounded or pyramidal in shape. It is unclear whether these different findings are just different variants of the same abnormal development of the hippocampus or represent completely different etiologic entities. The term HIMAL refers to those cases fulfilling specific MRI features instead of the ambiguous term “hippocampal malformation.”
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