“Hippocampal Malrotation”: No Real Malrotation and Not Rare

Published ahead of print on January 14, 2010
doi: 10.3174/ajnr.A2013

American Journal of Neuroradiology 31:E39, April 2010
© 2010 American Society of Neuroradiology

R. Raininkoa and D. Bajica
aDepartment of Radiology Uppsala University Uppsala, Sweden

We read with great interest the article relating to hippocampal malrotation (HIMAL) by Gamss et al.1 We did, however, find some of the terminology questionable. In particular, we thought that “incomplete hippocampal inversion” (IHI) would be a better descriptive term because the hippocampus is not “malrotated,” but rather probably the inversion was never completed. The authors themselves acknowledge this possibility when they write that “in cases of hippocampal malrotation, hippocampal inversion fails to occur.” Why then, we wonder, should this condition be called “malrotation”?

Various terms have been used for IHI, and the authors may not have checked for terms other than HIMAL in their literature search. There are earlier articles relating to hippocampal form in seizure-free populations, and this form variant is relatively common in these articles. Bronen and Cheung2 described configurations other than oval in 12/58 hippocampi of healthy volunteers. Wefound IHI (a round or pyramidal hippocampus and a vertical collateral sulcus in each case) in 18%–19% of the subjects in populations of healthy volunteers and patients without epilepsy or obvious developmental brain anomalies.3,4 The differences in prevalence may depend on the criteria used in interpretation (ie, the inversion can be incomplete in a part of the hippocampus but completein the rest). As was the case in the material of Gamss et al,1 IHI was most often unilateral and left-sided. In our work, we have not found an association between age and the rounded appearance of the hippocampus, but the IHI prevalence seems to be the same as in adulthood from gestational week 25 onwards (D. Bajic, unpublished data, September 2009).

Comparison of the prevalence in 2 studies by different researchers is not an optimal method to investigate the relationships between IHI and seizures. In a recent blind study, we also found a statistically significant difference between the populations without and with seizures (18% versus 30%, P < .05),4 but this difference was not as great as that presented by Games et al.1 In our study, the IHI frequency was very high in some epileptic syndromes (ie, in cryptogenic generalized epilepsy), but there was no statistically significant difference between the patients having temporal lobe epilepsy and the control group when IHI was the only deviating finding in the temporal lobe. There was no correlationbetween electroencephalography and IHI laterality. Our conclusion was that there was no causality between temporal lobe epilepsy and IHI.

IHI, a common morphologic variant of the hippocampus, is not an etiologic factor in epilepsy but can be a sign of disturbedcerebral development that may affect other parts of the brain, leading to epilepsy.

References

  1. Gamss PR, Slasky SE, Bello JA, et al. Prevalence of hippocampal malrotation in a population without seizures.AJNR Am J Neuroradiol 2009;30: 1571–73[Abstract/Free Full Text]
  2. Bronen RA, Cheung G. MRI of the normal hippocampusMagn Reson Imaging 1991;9: 497–500[CrossRef][Medline]
  3. Bajic D, Wang C, Kumlien E, et al. Incomplete hippocampal inversion-a common developmental anomalyEur Radiol 2008;18: 138–42[CrossRef][Medline]
  4. Bajic D, Kumlien E, Mattsson P, et al. Incomplete hippocampal inversion-is there a relationship to epilepsy? Eur Radiol 2009;19: 2544–50[CrossRef][Medline]

Reply

Published ahead of print on January 14, 2010
doi: 10.3174/ajnr.A2031

American Journal of Neuroradiology 31:E40, April 2010
© 2010 American Society of Neuroradiology

J.A. Belloa and T.S. Millera
aDepartment of Radiology

S. Shinnarb
bDepartments of Neurology, Pediatrics, and Epidemiology and Population Health Albert Einstein College of Medicine Montefiore Medical Center Bronx, New York

We thank Drs Raininko and Bajic for their interest in our recent article and welcome the opportunity to clarify certain issues.1 We would like to emphasize the distinction between the collection of 10 findings termed “hippocampal malrotation” (HIMAL) by Barsi et al2 and the isolated finding of an under-rotated hippocampus or “incomplete hippocampal inversion.” We do not dispute theidea that abnormal hippocampal rotation is a failure of the complete rotation that is an accepted part of normal brain development.3,4 Variation in other authors’ terminology for this isolated finding is acknowledged in the article by Bajic et al.5

The differences between our lower reported rate in individuals without epilepsy and those of Bajic et al5 may be due to the different criteria used. We used the stricter criteria requiring all the elements to be present, whereas they accepted partial forms, which will clearly result in higher rates and may also account for the frequency of bilateral findings, which are rare when stricter criteria are used.1

Given that there is still a debate about whether HIMAL represents a finding of pathologic significance, we believe that a strict definition is more appropriate because if HIMAL is of pathologic significance, the stricter criteria are more appropriate for identifying associated pathology. If HIMAL becomes established as a pathologic finding, then expanding the spectrum to partialforms and clarifying whether they are also associated with pathology will be the next step. However, if one expands the criteria to partial forms, one may well lose the association between HIMAL and epilepsy. In any case, that would have to be identified in a case-control study by using the same criteria.

References

  1. Games PR, Slasky SE, Bello JA, et al. Prevalence of hippocampal malrotation in a population without seizuresAJNR Am J Neuroradiol 2009;30: 1571–73[Abstract/Free Full Text]
  2. Barsi P, Kenez J, Solymosi D, et al. Hippocampal malrotation with normal corpus callosum: a new entity?Neuroradiology 2000;42: 339–45[CrossRef][Medline]
  3. Kier EL, Kim JH, Fulbright RK, et al. Embryology of the human fetal hippocampus: MR imaging, anatomy, and histologyAJNR Am J Neuroradiol 1997;18: 525–32[Abstract]
  4. Baker LL, Barkovich AJ. The large temporal horn: MR analysis in developmental brain anomalies versus hydrocephalusAJNR Am J Neuroradiol 1992;13: 115–22[Abstract]
  5. Bajic D, Kumlien E, Mattsson P, et al. Incomplete hippocampal inversion: is there a relationship to epilepsy? Eur Radiol 2009;19: 2544–50[CrossRef][Medline]
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