Hippocampal Malrotation

Hippocampal malrotation, perhaps more properly referred to as incomplete hippocampal inversion, is the constellation of:

• Incomplete inversion of the hippocampus with with an abnormally round shape
• Unilateral involvement of the whole hippocampus: Most often left-sided
• Normal hippocampal signal intensity and size
• Blurred hippocampal internal structure
• Abnormal angle of the collateral sulcus: The normal collateral sulcus angle is flat at the level of the body and tail of the hippocampus, and more vertical in cases of hippocampal malrotation
• Abnormal position and size of the fornix: Normally symmetrical, the fornix may be inferiorly displaced in patients with hippocampal malrotation. However, a low fornix can also be seen in subjects without seizures and without hippocampal malrotation, suggesting that it may be an incidental finding.
• Normal size of the temporal lobe.
• Enlargement and particular configuration of the temporal horn seen with agenesis of the corpus callosum but with a normal corpus callosum.

The full constellation is reportedly not present in patients without seizures, while others have found no causality between temporal lobe epilepsy and hippocampal malrotation. Raininko and Bajic (2010) suggest that hippocampal malrotation is a common variant that can be a sign of disturbed cerebral development, which in turn may affect other parts of the brain, leading to epilepsy. Gamss et al (2009) suggest that like mesial temproal sclerosis, hippocampal malrotation is uncommon in people without seizures and should elevate suspicion for an underlying epileptogenic disorder.

References

• Barsi P, Kenéz J, Solymosi D, Kulin A, Halász P, Rásonyi G, Janszky J, Kalóczkai A, Barcs G, Neuwirth M, Paraicz E, Siegler Z, Morvai M, Jerney J, Kassay M, Altmann A. Hippocampal malrotation with normal corpus callosum: a new entity? Neuroradiology. 2000 May;42(5):339-45.
• Gamss RP, Slasky SE, Bello JA, Miller TS, Shinnar S. Prevalence of hippocampal malrotation in a population without seizures. AJNR Am J Neuroradiol. 2009 Sep;30(8):1571-3.
• Raininko R, Bajic D. "Hippocampal Malrotation": No Real Malrotation and Not Rare. AJNR Am J Neuroradiol. 2010 Jan 14.