Abnormal sulcation as an early sign for migration disorders

The cerebral cortex develops in three overlapping stages: proliferation and differentiation of neuronal precursors, migration of postmitotic immature neurons, and formation of a mature cortex by lamination, synaptogenesis and apoptosis. Neuronal migration is a sequential process that starts around the seventh week of gestation and ends around the twentieth week. During this time, neurons migrate from the germinal matrix in the periventricular zone, on radial glial guides, towards the pial surface1. The migration waves form an ‘inside-out’, six-layered cortex in which the deeper cortical layers consist of the first migrating neurons. Gyration/sulcation is also an orderly process that is not temporally related to the migration (it mainly occurs after the end of migration) and is due to a rapid increase of the outer cortical surface without a similar increase of the inner one2. Disruption of these stages of normal development can result in a wide range of congenital and acquired malformations of cortical development (MCD)3. Children with MCD usually present with epilepsy, developmental delay and/or neurological deficits and are referred for magnetic resonance imaging (MRI). This procedure has enabled a rapid increase in our knowledge of different MCD. Current classifications are based on clinical3, 4, imaging3, 4 and genetic5 criteria. Despite the advances in postnatal diagnosis of MCD, many fetuses escape prenatal diagnosis for the following reasons: the relatively late development of the brain during pregnancy when ultrasound is not routinely performed; technical limitations precluding a detailed study of the fetal brain; the limited use of fetal MRI, which is the modality of choice for detection of MCD; and incomplete knowledge of the prenatal development of the cortex. Another limitation in the expansion of knowledge in prenatal detection of MCD is infrequent feedback on pathological findings of the brain after termination of pregnancy due to maceration. Currently the prenatal diagnosis of MCD is usually limited to patients at high risk for such malformations (history of a previous child with MCD) or to patients who undergo a detailed neurosonographic or MRI examination because of other more common brain anomalies (ventriculomegaly, agenesis of the corpus callosum or vermian hypoplasia). Some cases are detected either by an astute or lucky ultrasonographer. In this issue of the Journal, Toi et al.6, using the transabdominal approach, describe the normal appearance and the development pattern of cerebral sulcation (parieto-occipital fissure, calcarine, cingulate and convexity sulci and the Sylvian fissure) between 15.6 and 29.6 weeks of gestation. The great importance of this study lies in the fact that the authors prove the feasibility of diagnosing some migration disorders using standard techniques of ultrasonographic screening, without the need of extensive training. In another paper in this issue the same group from Canada, Fong et al.7, report the ultrasonographic findings in a group of seven patients with postnatal cytogenetic diagnosis of lissencephaly associated with Miller–Dieker syndrome (MDS). In this retrospective study the authors reviewed the prenatal ultrasonographic findings and found that in six fetuses mild ventriculomegaly was present at the time of the first ultrasound examination performed between 20 and 33 weeks of gestation. A prenatal diagnosis of MCD was obtained in three fetuses with the additional use of MRI. The retrospective analysis of the same fissures and sulci as proposed by Toi et al.6 demonstrated that they were absent or abnormal in all the seven fetuses by the time of the first ultrasound examination. Fong et al.7 describe the in-utero identification of lissencephaly due to MDS, however there are other genetic causes for the agyria–pachygyria spectrum. Since the discovery of deletions of chromosome 17p13.3 in MDS, five other genes that cause or contribute to human lissencephaly have been discovered: LIS1, 14-3-3 epsilon, DCX, RELN and ARX8. Therefore, when lissencephaly is identified by ultrasound it is important to include these genetic etiologies in the differential diagnosis. Currently, anomalies due to abnormal migration are classified into three principal subgroups: lissencephaly/subcortical band heterotopia spectrum, cobblestone complex, and heterotopia. Abnormal migration may also be found in patients with microcephaly, macrocephaly, hemimegalencephaly and anomalies due to abnormal cortical organization such as polymicrogyria and schizencephaly3. The prenatal diagnosis of some of these conditions is possible by ultrasound, MRI and/or cytogenetic studies. The studies of Toi et al. and Fong et al. demonstrate that lissencephaly can be diagnosed in utero by ultrasonography based on the knowledge of the prenatal development of normal sulcation. This method has not been applied to the diagnosis of other MCD. It is plausible that disorders involving generalized agyria/pachygyria will also be accurately detected when only delayed development of gyration is evaluated; however, when the malformation is localized (i.e. focal pachygyria, heterotopias, polymicrogyria and cortical dysplasias) it may be missed by this method. In our experience, the first clue to the presence of more subtle forms of MCD in the fetus may be the visualization not of absent or delayed sulci but rather the presence of abnormally developed sulci and gyri appearing at an earlier than expected stage of cortical development9. It is conceivable that future studies will identify other ultrasonographic features that will enable the in-utero diagnosis of many MCD. The implementation of new methods and techniques into established screening procedures, enabling widespread availability of prenatal diagnosis of MCD by ultrasonography, will raise important issues regarding medicolegal aspects of failure to visualize these malformations. It will require the use of additional imaging or laboratory modalities to confirm or refine the suspicion of MCD, and consultation at a prenatal clinic staffed by a multidisciplinary team that includes a geneticist and pediatric neurologist. The patients will still need to be informed of the limitations of ultrasound diagnosis with particular reference to limitations in central nervous system diagnosis9. We believe that any new screening procedure needs to be proven to be useful by means of methodologically sound prospective studies. Until that time, the diagnosis of seemingly abnormal or absent sulcation needs to be undertaken with extreme caution and referral to centers with experience in neurosonographic and MRI diagnosis must be considered.

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