Disorders of neuronal migration: Lissencephaly, Cobblestone Malformations and Heterotopia

The original classification scheme for MCD by Barkovich in 1996 was based upon the earliest developmental step at which the process was disturbed. Since then, the classification scheme has been revised multiple times due to the identification of many new genes and syndromes resulting in MCD ^{1, 2}. Although overlap exists between the three groups, cortical abnormalities in Group 1 are malformations secondary to proliferative disorders, abnormal neuronal and glial proliferation or apoptosis.  Included in Barkovich Group 1 are microlissencephaly (decreased proliferation), megalencephaly and hemimegalencephaly (increased proliferation) and focal cortical dysplasia (abnormal proliferation). Group II malformations are due to abnormal neuronal migration and is further divided into four sub-categories. The first of these is related to abnormalities of the neuroependyma (ventricular epithelium), which result in the formation of periventricular nodular heterotopias. The second describes generalized abnormalities of trans-mantle migration, mainly lissencephalies. The third describes localized abnormalities of trans-mantle migration, mainly subcortical heterotopias; and the fourth describes abnormalities due to abnormal terminal migration/defects in pial limiting membrane, mainly cobblestone malformation³. Group III malformations are secondary to abnormal post-migrational development, such as polymicrogyria and schizencephaly.

Lissencephaly

The term “lissencephaly” is derived from the Greek words lissos, meaning “smooth”

and enkephalos, meaning “brain”. Lissencephaly is caused by neuronal under-migration.

Lissencephaly spectrum includes agyria (complete lack of gyri), pachygyria (few broad gyri and shallow sulci) and subcortical band heterotopia (SBH, see below)⁴. The most common, known as classical lissencephaly or Type 1 lissencephaly, features a very thick and smooth cortex (10–20 mm) and no other major brain malformations⁵. It can be isolated or can accompany multiple congenital anomaly syndromes, including Miller-Dieker, Baraitser-Winter cerebrofrontofacial syndrome and X-linked lissencephaly with abnormal genitalia or XLAG^6-8. Miller-Dieker (17p13.3 mutation) is the most severe of these, with typical facies and a lack of sulcation.  The other known genes responsible for lissencephaly Type 1 include LIS1, ARX, RELN, VLDLR, ACTB, ACTG1, DCX, DYNC1H1, KIF2A, TUBA1A, TUBB2B, and TUBG1⁹. 

Since the Barkovich classification was last revised, several additional types of lissencephalies have been recognized. Not only the pattern and gradient of gyral anomaly, but cortical thickness, callosal anomalies and posterior fossa involvement can be utilized in phenotype-genotype classification. This expanded classification attempts to offer linkage of various lissencephaly phenotypes to certain genotypes ^{10, 11}.

Cortical thickness is an important characteristic of lissencephaly. While the normal cortex is comprised of six layers with an overall thickness of 4 mm, in lissencephaly there are two to four layers⁹, resulting in either a thick (10-20 mm), thin (5-10mm) or variable cortex, with the former being most common.

Gradient is another important feature of lissencephaly, with posterior predominant lissencephaly more common than anterior predominant or diffuse lissencephaly⁶.

The gradient of involvement in the gyral pattern can help differentiate the other LIS disorders. Involvement may be partial or demonstrate pachy/agyria with frontal or posterior severity reflecting the mutation. Frontal pachygyria and posterior agyria is the most common pattern and suggests mutations of the LIS1 gene. For example, the two most common causal genes, DCX (Doublecortin) and LIS1 can be differentiated on imaging as the former results in amore severe anterior  than posterior gradient of gyral abnormality, while the latter should be suspected with a  more severe posterior  than anterior gradient. The less involved lobes may have a more normal 6-layer cortex, with a transitional zone between that and the more involved,  disorganized lobes, which exhibit 4-layers.

Gender related differences also occur. DCX mutations lead to classic lissencephaly in males, but may exhibit subcortical band heterotopia in females^{6, 10, 12}. The band or double cortex, as can be seen in DCX mutations, would be difficult to confirm on prenatal imaging, although the dysgyria and vermian hypoplasia are helpful prenatal imaging features⁵.

The cerebellum may also be involved in lissencephaly¹³.  For example, a RELN gene mutation should be suspected when lissencephaly is associated with cerebellar hypoplasia^{10, 14}, while an ARX gene mutation should be suspected in chromosomal males with X-linked lissencephaly and ambiguous genitalia (XLAG).Vermian hypoplasia may be seen in DCX mutations, while thin, asymmetric or bent brainstem in association with callosal deficiency, abnormal basal ganglia and hypoplastic cerebellum should suggest a tubulinopathy.

Brainstem involvement can also be seen in the dystroglycanopathies and features may overlap with the tubulinopathies (Di Donato 2017). Cobblestone lissencephaly (formerly called Type 2 lissencephaly) is genetically, embryologically, and pathologically distinct from type 1 lissencephaly. It results from defects in dystroglycan glycosylation, which affects linkage of radial glial cells with the pial limiting membrane, leading to neuronal over-migration through pial gaps³. The dystroglycanopathies with brain and eye anomalies, include Walker–Warburg syndrome (WWS), muscle–eye–brain disease (MEB), and Fukuyama muscular dystrophy. In these conditions the brain surface lacks normal sulcation and has an uneven appearance, thus the name cobblestone cortex. Many patients have cerebellar and ocular abnormalities and congenital muscular dystrophy. WWS is considered the most severe of the disorders, but there is considerable overlap. The currently known genes responsible for Cobblestone malformation include FKTN, POMT1, POMT2, POMGnT1, FKRP, CRPPA, TMEM5, ISPD and LARGE ^{3, 15}.

Heterotopia

Heterotopia occurs when neurons originating in the periventricular (subependymal) region fail to migrate, leaving tracks or nodules of normal neurons in abnormal locations adjacent to the ependymal lining (periventricular nodular heterotopia) or in subcortical topography (SBH)¹⁶. In subcortical band heterotopia (SBH), bilateral bands of grey matter are found interposed in the white matter between the cortex and the lateral ventricles¹⁷. This may appear as a solid band of heterotopic tissue or as numerous islands of radially oriented grey matter separated by white matter¹⁸. SBH always have a genetic origin, and abnormalities in the DCX and LIS1 genes account for the majority of cases ¹⁹. Again, the predominant location (anterior or posterior) is helpful in suggesting the mutation involved.

Periventricular nodular heterotopia (PNH), consists of nodules of grey matter located along the walls of the lateral ventricles. PNH can be highly variable in extent, ranging from isolated single nodules to confluent bilateral lesions. These neurons failed to migrate into the cortex² usually due to a clastic event and not secondary to a motility defect³. Consequently, the majority are not linked to a gene mutation. Two genes have been identified – x linked PNH due to Filamin A (FLNA) gene mutation and the rarer autosomal recessive ARFGEF2 gene mutation²⁰. An additional form, not yet related to known mutations, consists of peri-atrial PHN in association with cerebellar dysplasia²¹.

All types of lissencephaly and heterotopias are rare. The fetal prevalence is unknown. Classical lissencephaly has a reported prevalence of about 12 per million births²².

The prevalence of WWS is unknown but it is estimated to affect 1 in 60,500 newborns. It is equally distributed between males and females. Fukuyama congenital muscular dystrophy is the second most common form of muscular dystrophy in the Japanese population and is caused by mutations in the fukutin (FKTN) gene²³.

Malformations of neuronal proliferation and migration occur between gestational weeks 8 and 25²⁴.

Cortical maturation and gyral formation follow an age specific temporo-spatial schedule. Familiarity with this schedule allows for the diagnosis of abnormal maturation. Both dedicated neurosonography and MRI can depict fetal cortical malformations.  Neurosonography is the most important imaging tool for prenatal malformation screening as it is widely available and safe for both mother and fetus. It can detect lissencephalies and PNH, but MRI is a superior imaging modality for this purpose thanks to its optimal delineation of grey and white matter²⁵ and topographical characterization of development of cortical gyri and sulci. Most cortical abnormalities will be detected after 24 weeks, but some, particularly the severe lissencephaly seen in Miller-Dieker patients, can be diagnosed prior to 24 weeks^{26, 27}. With high resolution ultrasound transducers focused on the cortical rim, the more severe malformations can be detected even prior to 20 weeks.^{28, 29}

Lissencephaly type 1

The diagnosis of lissencephaly type 1 can be suspected already at 23 weeks with appearance of mild ventriculomegaly and delayed operculization ^{30, 31}. This diagnosis however is usually made after 27 weeks, due to absence of the Rolandic fissure which should be visualized from 26 weeks ³². Other fetal neuroimaging signs that can be seen include a smooth and thick cortex and abnormal lamination³³, ventriculomegaly, a sulcation gradient, and associated cerebellar and/or callosal anomalies. The cortical findings are usually symmetric³⁴. The symmetrically abnormal opercular formation is responsible for a “figure-eight” shaped brain³⁰.

Cobblestone malformation

The diagnosis of Cobblestone malformation should be suspected when typical findings are present. An early diagnosis in the beginning of the second trimester can be suspected after 14 weeks with demonstration of a kinked brainstem and a cephalocele³⁵, as well as a hyperechogenic cortical rim. This diagnosis should also be considered in cases of early onset ventriculomegaly, irregular ventricular wall and a hyperechogenic cortical rim or echogenic band^{28, 29}. Additional features may include hydrocephalus, posterior pachygyria/agyria, anterior polymicrogyria-like appearance, abnormal operculization, and cerebellar dysplasia/hypoplasia and cysts. Myelination is a known feature, but would not be assessable on fetal MRI or sonography. Cobblestone lissencephaly in the dystroglycanopathies is also associated with fused colliculi, a small pons, a dysmorphic mesencephalon, a dorsal pontomedullary kink, vermian hypogenesis, and cerebellar hypoplasia. Microphthalmia and retinal dysplasia (small and dysplastic globe) are commonly seen as are posterior cephaloceles^{9, 16, 36}.

Lissencephaly due to Tubulinopathies

Fetal neuroimaging signs that can be associated with lissencephaly due to tubulinopathies include microcephaly, corpus callosum agenesis, hypoplastic brainstem and cerebellum, large germinal zones and ganglionic eminences and polygyria. The more severe mutations of TUBA1A and less often TUBB2B have 2-layered cortex³⁷, while the less severe mutations of TUBA1A or TUBB2B, or most mutations of TUBB and TUBB3 show distinct features intermediate between thick lissencephaly, ventricular asymmetry and polymicrogyria.

Heterotopias

Subcortical band heterotopia

SBH is generally associated with a normal or mildly simplified gyration pattern, broad gyri, and an increased cortical thickness. SBH is difficult to identify on prenatal ultrasound. 

Periventricular nodular heterotopia (PNH)

Irregular lateral ventricular walls are the clue to the sonographic diagnosis. On MR multiple small nodular subependymal foci of low signal intensity, isointense to the germinal matrix can be seen. Irregular ventricular walls due to heterotopias should not be confused with ventricular wall irregularity secondary to clastic events, such as CMV infection, ischemia or hemorrhage. Heterotopic nodules may be single, multiple, unilateral or bilateral (string-of-pearls appearance) and appear in various locations along the ventricular wall. Single nodules may be found following subependymal disruption as seen with hydrocephalus. “String-of-pearls” lesions along the lateral ventricular walls may be seen with FLNA mutations.  Multiple nodules can be associated with ACC and mega cisterna magna. A posterior or trigonal location is associated with hippocampal, cerebellar and brainstem anomalies³⁶.

Although the diagnosis of lissencephaly requires a dedicated multiplanar sonographic evaluation of the brain, initial suspicion can arise from the standard ultrasound examination through evaluation of the sylvian fissure. The maturation of the fetal Sylvian fissure during the second half of gestation is a major landmark of normal cortical development.

Typical patterns of the Sylvian fissure operculization in the standard axial trans-thalamic

plane have been described according to gestational weeks³⁸. These patterns are based on the degree of coverage of the insula by the temporal lobe and the angulation between the insula and the parietal and temporal opercular margins, normally demonstrating an acute angle after 24.5 gestational weeks. If a delay in maturation is suspected at this time, a dedicated neurosonogram should be carried out.

The available experience suggests that even expert ultrasound in pregnancies at risk will fail to diagnose fetal MCD in many cases. Multiplanar transvaginal high resolution brain imaging with adjunct MRI should be carried out in any case of suspected abnormal cortical development. A cortical malformation should be suspected in cases of earlier or later than expected sulci and gyri development, hemispheric or focal cortical asymmetry, irregularity or disruption along the cortical mantle, or an hyperechogenic cortical rim.

Following the suspicion of MCD by dedicated neurosonography and/or MRI, when appropriate and possible (depending on gestational age), the imaging diagnosis is supplemented by genetic

studies (CMA and whole exome sequencing of the fetus and both parents). When multiple PNH are seen in a female fetus, FLNA should be suspected and brain MR imaging of the mother should be completed.

In some instances, no further studies are required during pregnancy due to the clear and dire prognosis (i.e non development of cortical landmarks at an advanced gestational age, association of other CNS or systemic anomalies) and then the genetic evaluation can be deferred until after delivery or termination of pregnancy.

1.            Barkovich AJ, Kuzniecky RI, Jackson GD, Guerrini R, Dobyns WB. Classification system for malformations of cortical development: update 2001. Neurology. 2001;57(12):2168-78.

2.            Barkovich AJ, Kuzniecky RI, Jackson GD, Guerrini R, Dobyns WB. A developmental and genetic classification for malformations of cortical development. Neurology. 2005;65(12):1873-87.

3.            Barkovich AJ, Guerrini R, Kuzniecky RI, Jackson GD, Dobyns WB. A developmental and genetic classification for malformations of cortical development: update 2012. Brain. 2012;135(Pt 5):1348-69.

4.            Di Donato N, Chiari S, Mirzaa GM, Aldinger K, Parrini E, Olds C, Barkovich AJ, Guerrini R, Dobyns WB. Lissencephaly: Expanded imaging and clinical classification. Am J Med Genet A. 2017;173(6):1473-88.

5.            Forman MS, Squier W, Dobyns WB, Golden JA. Genotypically defined lissencephalies show distinct pathologies. J Neuropathol Exp Neurol. 2005;64(10):847-57.

6.            Dobyns WB, Truwit CL, Ross ME, Matsumoto N, Pilz DT, Ledbetter DH, Gleeson JG, Walsh CA, Barkovich AJ. Differences in the gyral pattern distinguish chromosome 17-linked and X-linked lissencephaly. Neurology. 1999;53(2):270-7.

7.            Dobyns WB, Stratton RF, Parke JT, Greenberg F, Nussbaum RL, Ledbetter DH. Miller-Dieker syndrome: lissencephaly and monosomy 17p. J Pediatr. 1983;102(4):552-8.

8.            Ramer JC, Lin AE, Dobyns WB, Winter R, Aymé S, Pallotta R, Ladda RL. Previously apparently undescribed syndrome: shallow orbits, ptosis, coloboma, trigonocephaly, gyral malformations, and mental and growth retardation. Am J Med Genet. 1995;57(3):403-9.

9.            Guerrini R, Dobyns WB. Malformations of cortical development: clinical features and genetic causes. Lancet Neurol. 2014;13(7):710-26.

10.          Kato M, Dobyns WB. Lissencephaly and the molecular basis of neuronal migration. Hum Mol Genet. 2003;12 Spec No 1:R89-96.

11.          Ackerman Wt, Kwiek JJ. Role of the placenta in adverse perinatal outcomes among HIV-1 seropositive women. J Nippon Med Sch. 2013;80(2):90-4.

12.          Pilz DT, Matsumoto N, Minnerath S, Mills P, Gleeson JG, Allen KM, Walsh CA, Barkovich AJ, Dobyns WB, Ledbetter DH, Ross ME. LIS1 and XLIS (DCX) mutations cause most classical lissencephaly, but different patterns of malformation. Hum Mol Genet. 1998;7(13):2029-37.

13.          Kerner B, Graham JM, Jr., Golden JA, Pepkowitz SH, Dobyns WB. Familial lissencephaly with cleft palate and severe cerebellar hypoplasia. Am J Med Genet. 1999;87(5):440-5.

14.          Bahi-Buisson N, Guerrini R. Chapter 68 - Diffuse malformations of cortical development. In: Dulac O, Lassonde M, Sarnat HB, editors. Handbook of Clinical Neurology. 111: Elsevier; 2013. p. 653-65.

15.          Vuillaumier-Barrot S, Bouchet-Séraphin C, Chelbi M, Devisme L, Quentin S, Gazal S, Laquerrière A, Fallet-Bianco C, Loget P, Odent S, Carles D, Bazin A, Aziza J, Clemenson A, Guimiot F, Bonnière M, Monnot S, Bole-Feysot C, Bernard JP, Loeuillet L, Gonzales M, Socha K, Grandchamp B, Attié-Bitach T, Encha-Razavi F, Seta N. Identification of mutations in TMEM5 and ISPD as a cause of severe cobblestone lissencephaly. Am J Hum Genet. 2012;91(6):1135-43.

16.          Andrade CS, Leite Cda C. Malformations of cortical development: current concepts and advanced neuroimaging review. Arq Neuropsiquiatr. 2011;69(1):130-8.

17.          Barkovich AJ. Subcortical heterotopia: a distinct clinicoradiologic entity. AJNR Am J Neuroradiol. 1996;17(7):1315-22.

18.          Golden JA, Harding BN. Developmental neuropathology. International Society of N, editor. Basel: International Society of Neuropathology; 2004.

19.          Watrin F, Manent JB, Cardoso C, Represa A. Causes and consequences of gray matter heterotopia. CNS Neurosci Ther. 2015;21(2):112-22.

20.          van Kogelenberg M, Ghedia S, McGillivray G, Bruno D, Leventer R, Macdermot K, Nelson J, Nagarajan L, Veltman JA, de Brouwer AP, McKinlay Gardner RJ, van Bokhoven H, Kirk EP, Robertson SP. Periventricular heterotopia in common microdeletion syndromes. Mol Syndromol. 2010;1(1):35-41.

21.          Pisano T, Barkovich AJ, Leventer RJ, Squier W, Scheffer IE, Parrini E, Blaser S, Marini C, Robertson S, Tortorella G, Rosenow F, Thomas P, McGillivray G, Andermann E, Andermann F, Berkovic SF, Dobyns WB, Guerrini R. Peritrigonal and temporo-occipital heterotopia with corpus callosum and cerebellar dysgenesis. Neurology. 2012;79(12):1244-51.

22.          de Rijk-van Andel JF, Arts WF, Hofman A, Staal A, Niermeijer MF. Epidemiology of lissencephaly type I. Neuroepidemiology. 1991;10(4):200-4.

23.          Ishigaki K, Ihara C, Nakamura H, Mori-Yoshimura M, Maruo K, Taniguchi-Ikeda M, Kimura E, Murakami T, Sato T, Toda T, Kaiya H, Osawa M. National registry of patients with Fukuyama congenital muscular dystrophy in Japan. Neuromuscul Disord. 2018;28(10):885-93.

24.          Fenton LZ. Imaging of Congenital Malformations of the Brain. Clin Perinatol. 2022;49(3):587-601.

25.          Severino M, Geraldo AF, Utz N, Tortora D, Pogledic I, Klonowski W, Triulzi F, Arrigoni F, Mankad K, Leventer RJ, Mancini GMS, Barkovich JA, Lequin MH, Rossi A, Malformations obotENoB. Definitions and classification of malformations of cortical development: practical guidelines. Brain. 2020;143(10):2874-94.

26.          Malinger G, Kidron D, Schreiber L, Ben-Sira L, Hoffmann C, Lev D, Lerman-Sagie T. Prenatal diagnosis of malformations of cortical development by dedicated neurosonography. Ultrasound Obstet Gynecol. 2007;29(2):178-91.

27.          Righini A, Parazzini C, Doneda C, Avagliano L, Arrigoni F, Rustico M, Consonni D, Re TJ, Bulfamante G, Triulzi F. Early formative stage of human focal cortical gyration anomalies: fetal MRI. AJR Am J Roentgenol. 2012;198(2):439-47.

28.          Haratz KK, Birnbaum R, Kidron D, Har-Toov J, Selemnick Y, Brusilov M, Malinger G. Malformation of cortical development with abnormal cortex: early ultrasound diagnosis between 14 and 24 weeks of gestation. Ultrasound Obstet Gynecol. 2022.

29.          Lacalm A, Nadaud B, Massoud M, Putoux A, Gaucherand P, Guibaud L. Prenatal diagnosis of cobblestone lissencephaly associated with Walker-Warburg syndrome based on a specific sonographic pattern. Ultrasound Obstet Gynecol. 2016;47(1):117-22.

30.          Fong KW, Ghai S, Toi A, Blaser S, Winsor EJ, Chitayat D. Prenatal ultrasound findings of lissencephaly associated with Miller-Dieker syndrome and comparison with pre- and postnatal magnetic resonance imaging. Ultrasound Obstet Gynecol. 2004;24(7):716-23.

31.          Montaguti E, Bellussi F, Rizzo R, Livi A, Salsi G, Toni F, Maffei M, Lenzi J, Bonasoni MP, Pilu G. Sylvian fossa sonographic measurements in 18 to 23 weeks fetuses with and without cerebral malformations. Am J Obstet Gynecol MFM. 2021;3(4):100357.

32.          Iruretagoyena JI, Shah D, Malinger G. Dedicated fetal neurosonographic evaluation improves patient care and maternal fetal medicine fellow training. J Matern Fetal Neonatal Med. 2016;29(3):482-6.

33.          Pugash D, Hendson G, Dunham CP, Dewar K, Money DM, Prayer D. Sonographic assessment of normal and abnormal patterns of fetal cerebral lamination. Ultrasound in Obstetrics & Gynecology. 2012;40(6):642-51.

34.          Lerman-Sagie T, Leibovitz Z. Malformations of Cortical Development: From Postnatal to Fetal Imaging. Can J Neurol Sci. 2016;43(5):611-8.

35.          Crowe C, Jassani M, Dickerman L. The prenatal diagnosis of the Walker-Warburg Syndrome. Prenat Diagn. 1986;6(3):177-85.

36.          Abdel Razek AA, Kandell AY, Elsorogy LG, Elmongy A, Basett AA. Disorders of cortical formation: MR imaging features. AJNR Am J Neuroradiol. 2009;30(1):4-11.

37.          Fallet-Bianco C, Laquerrière A, Poirier K, Razavi F, Guimiot F, Dias P, Loeuillet L, Lascelles K, Beldjord C, Carion N, Toussaint A, Revencu N, Addor MC, Lhermitte B, Gonzales M, Martinovich J, Bessieres B, Marcy-Bonnière M, Jossic F, Marcorelles P, Loget P, Chelly J, Bahi-Buisson N. Mutations in tubulin genes are frequent causes of various foetal malformations of cortical development including microlissencephaly. Acta Neuropathol Commun. 2014;2:69.

38.          Pooh RK, Machida M, Nakamura T, Uenishi K, Chiyo H, Itoh K, Yoshimatsu J, Ueda H, Ogo K, Chaemsaithong P, Poon LC. Increased Sylvian fissure angle as early sonographic sign of malformation of cortical development. Ultrasound in Obstetrics & Gynecology. 2019;54(2):199-206.

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