|Year : 2019 | Volume
| Issue : 2 | Page : 448--449
Malformations of cortical development and incomplete hippocampal inversion
Bhaskara Rao Malla
Department of Neurosurgery, National Institute of Mental Health and Neurosciences, Bengaluru, Karnataka, India
Dr. Bhaskara Rao Malla
Department of Neurosurgery, National Institute of Mental Health and Neurosciences, Hosur Road, Bengaluru, Karnataka
|How to cite this article:|
Malla BR. Malformations of cortical development and incomplete hippocampal inversion.Neurol India 2019;67:448-449
|How to cite this URL:|
Malla BR. Malformations of cortical development and incomplete hippocampal inversion. Neurol India [serial online] 2019 [cited 2020 Jul 4 ];67:448-449
Available from: http://www.neurologyindia.com/text.asp?2019/67/2/448/258014
Malformations of Cortical Development
Malformations of cortical development (MCDs) are the most common and important causes of childhood drug-resistant epilepsy. The clinical features of MCD are often heterogeneous. In addition to epilepsy, they are frequently associated with cognitive deficits and behavioral alterations. MCDs result from alterations in the progenitor proliferation, neuronal migration, and differentiation.
Incomplete Hippocampal Inversion
Incomplete hippocampal inversion (IHI) is also known as hippocampal malrotation. It is an atypical anatomical pattern of the hippocampus. Incomplete inversions were clearly more frequent in the left than in the right hemisphere. It has been reported in healthy subjects as well as in people with epilepsy. Extensive characterization of IHI in a large sample has not yet been performed. It is not clear whether IHIs are restricted to the medial temporal lobe or are associated with more extensive anatomical changes.
Incidence and Prevalence of Mcd and Ihi
Although the exact incidence of MCDs is not known, they account for up to approximately 40% of cases of drug-resistant childhood epilepsies, and at least 75% of the patients with MCDs will have epilepsy. Similarly, asymmetric development of the hippocampus is known and is commonly encountered. The right hippocampus develops at a faster pace in a vast majority of cases. The hippocampal inversion, as well as closing of the hippocampal sulcus, may occur earlier in the right hemisphere. If the hippocampal inversion process is stopped at a specific time, it may be incomplete only in the left hemisphere.
In a person presenting with epilepsy and MCD or IHI, the causal relationship between the clinical presentation and the neuroimaging findings has to be established through the presurgical evaluation. In normal adults, studies have shown an asymmetry in the hippocampal volumes, with the right hippocampus being larger than the left one. There are also functional differences between the two hippocampi: the right is predominantly involved in memory modulation for locations within an environment, whereas the left hippocampus plays a central role in context-dependent episodic memory or in autobiographical memory perception. Unlike foreign tissue lesions, MCD and IHI may be present in the functional and eloquent tissues. Hence, in the people harboring these malformations, a detailed evaluation is required not only to identify the epileptogenicity but also the functional impairment.
Literature Review from India
There are very few studies in the literature, especially from India, regarding the prevalence of MCD and IHI in drug-resistant epilepsy and surgical specimens. Manjari et al., reviewed 57 cases of focal cortical dysplasia (FCD) evaluated and operated at the All India Institute of Medical Sciences, New Delhi, between 1995 and 2006, and concluded that FCDs had a good outcome if evaluated and managed with concordant electrical and imaging modalities. Among the 57 cases, there was only 1 case of FCD associated with mesial temporal sclerosis who had Engel grade IV surgical outcome.
Thomas et al., reported 34 cases of MCDs studied at the National Institute of Mental Health and Neurosciences (NIMHANS), Bangalore, over a period of 5 years from 1998 to 2003. The clinical features, seizure semiology, as well as electroencephalogram and neuroimaging findings were discussed in this cohort. The authors concluded that epilepsy caused by MCDs usually responds poorly to medical management and that surgery may be a better option for the carefully selected patients, but the latter modality of treatment requires an extensive presurgical evaluation and good surgical skills.
Subsequently, Chaturvedi et al., analyzed the presurgical evaluation, surgical strategy, and outcome of 52 people who underwent resective surgery for drug-resistant epilepsy with FCD at NIMHANS, Bangalore, between January 2008 and December 2016. After a median follow-up of 3.7 years following surgery, 84% of patients had an Engel's Ia outcome. The Quality of Life in Epilepsy-89 inventory scores improved from 38.33 ± 4.7 (31.14–49.03) before surgery to 75.21 ± 8.44 (56.49–90.49) after surgery (P < 0.001).
In this context, the publication by Bhoopathy et al., is a welcome addition. The authors have studied the prevalence and clinical characteristics of MCD and IHI in adults with intractable seizures at a tertiary referral center. In this prospective study, amongst a cohort of 3220 people with epilepsy and 416 people with drug-resistant epilepsy, MCDs were observed in 46 (11.05%) patients and IHIs were observed in 39 (9.37%) patients. The authors proposed that among people with drug resistant epilepsy (DRE) and normal intelligence, volumetric studies have to be carried out to identify IHI. The authors have also proposed that as hippocampal malrotation is a gyrational developmental defect, it has to be included as one of the varieties of MCD and further histopathological studies may be performed in these patients to confirm IHI.
These conclusions are debatable. The findings reported by Tsai and colleagues support the concept that hippocampal malrotation in temporal lobe epilepsy is a normal variant, unrelated to the epilepsy that the patient might have been having and, therefore, is not an indicator of focal pathology. These authors make a compelling argument that this neuroanatomic variant must not influence surgical decision-making in adults with intractable focal-onset epilepsy, particularly in those with magnetic resonance imaging (MRI)-negative temporal lobe epileptogenic networks.
People with epilepsy who fulfill the criteria of drug resistance and have the potential to be cured or be significantly relieved of their seizures by surgery, corresponding to the cohort studied by Bhoopathy et al., should undergo a detailed presurgical evaluation. The risks and benefits accrued in the context of surgical intervention provided to the selected group of patients in this cohort will be based upon their multimodality evaluation.
IHIs are frequently found in normal subjects, predominantly in the left hemisphere. IHI is associated with extrahippocampal morphological changes, in particular, in the sulci of the limbic lobe. IHI is an anatomic variant commonly found in controls. IHI is also an incidental nonpathologic finding in adult MRI-negative temporal lobe epilepsy and should not influence surgical decision-making. To substantiate the conclusions made by Bhoopathy et al., a sizable number of people with drug-resistant epilepsy with MCD and IHI need to undergo surgery, and the harvested tissues subjected to detailed histopathological studies. One option to get comprehensive data on this subject is to pool databases across epilepsy surgery centers in various parts of India.
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