Malformations of cortical development (MCDs) are abnormalities in the formation of the cerebral cortex during fetal life. They are a major cause of refractory epilepsy and developmental disability. The classification reflects the developmental process disrupted: cell proliferation/apoptosis (microcephaly, megalencephaly, FCD type II, hemimegalencephaly, dysembryoplastic neuroepithelial tumor), cell migration (lissencephaly, heterotopia), and cortical organization (polymicrogyria, schizencephaly). Many MCDs are now linked to specific genetic mutations, often in mTOR, MCPH, or migration pathway genes. This page covers the major MCDs.

Classification Framework (Barkovich)

  1. Disorders of neuronal proliferation: microcephaly, megalencephaly, hemimegalencephaly, focal cortical dysplasia (some types).
  2. Disorders of neuronal migration: classical lissencephaly, subcortical band heterotopia, periventricular nodular heterotopia, cobblestone (type II) lissencephaly.
  3. Disorders of cortical organization: polymicrogyria, schizencephaly, focal cortical dysplasia (type I).
  4. Disorders of late development: deletions, vascular events.

Microcephaly

  • Head circumference < 3rd percentile (small brain).
  • Primary (genetic): MCPH genes (MCPH1, ASPM, WDR62, etc.); autosomal recessive; severe.
  • Secondary: TORCH infections (toxoplasmosis, rubella, CMV, herpes, syphilis), fetal alcohol syndrome, hypoxic-ischemic encephalopathy, Zika virus.
  • Cortical thickness usually normal; surface area reduced.
  • Often with simplified gyral pattern.

Megalencephaly / Hemimegalencephaly

  • Brain enlargement; cortical thickening.
  • Hemimegalencephaly: unilateral enlargement of cerebral hemisphere; often associated with mTOR pathway mutations (somatic AKT3, PIK3CA, MTOR mutations).
  • Associated with intractable epilepsy.
  • Hemispherectomy may be required.

Lissencephaly

Classical Lissencephaly (Type I)

  • Absent or reduced gyration with thickened cortex (~12-15 mm vs normal 3-4 mm).
  • Cortex has only 4 layers instead of normal 6.
  • Severe developmental delay, refractory epilepsy.
  • Genes: LIS1 (PAFAH1B1), DCX (doublecortin, X-linked), TUBA1A, RELN.
  • X-linked lissencephaly with abnormal genitalia: ARX gene.

Subcortical Band Heterotopia (Double Cortex)

  • Band of heterotopic gray matter beneath cortex.
  • DCX mutation in females (X-linked; males with DCX have lissencephaly).

Cobblestone Lissencephaly (Type II)

  • Coarse irregular surface from over-migration through breaks in pial limiting membrane.
  • Associated with congenital muscular dystrophies (Walker-Warburg, muscle-eye-brain, Fukuyama).
  • Often with eye and muscle abnormalities.

Polymicrogyria

  • Excessive number of small, abnormally fused gyri.
  • Cortex is thin but with simplified architecture (often 4 layers).
  • Bilateral perisylvian polymicrogyria most common — pseudobulbar palsy, epilepsy, cognitive impairment.
  • Causes: vascular events, congenital CMV, genetic (GPR56, others), 22q11 deletion.

Schizencephaly

  • Cleft extending from cortex to ventricle, lined by polymicrogyric gray matter.
  • Open lip (separated) or closed lip (apposed).
  • Often with neighboring polymicrogyria.
  • Etiology: vascular, infectious, genetic.
  • EMX2 mutations in some.
  • Seizures, hemiparesis variable.

Heterotopia

  • Periventricular nodular heterotopia: nodules of gray matter along ventricular walls; FLNA mutations (X-linked); seizures, sometimes asymptomatic.
  • Subcortical band heterotopia: band beneath cortex; DCX mutations in females.

Focal Cortical Dysplasia (FCD)

Localized abnormalities of cortical organization; common cause of refractory focal epilepsy.

FCD Type I (ILAE Classification)

  • Abnormal cortical lamination; no dysmorphic neurons or balloon cells.
  • Subtypes Ia, Ib, Ic by pattern.
  • Often subtle on imaging; may be missed.

FCD Type II

  • Type IIa: dysmorphic neurons (enlarged, abnormally oriented, accumulated neurofilaments).
  • Type IIb: dysmorphic neurons + balloon cells (large cells with eosinophilic cytoplasm, eccentric nuclei; pathognomonic for FCD IIb).
  • mTOR pathway somatic mutations: AKT3, PIK3CA, MTOR, TSC1, TSC2 — somatic mosaicism in many cases.
  • Often refractory epilepsy.
  • Resection can be curative.

FCD Type III

Associated with other principal pathology:

  • IIIa: with hippocampal sclerosis.
  • IIIb: with epilepsy-associated tumors (ganglioglioma, DNET).
  • IIIc: with vascular malformation.
  • IIId: with other lesion.

Other Malformations

  • Hemispheric polymicrogyria + cleft.
  • Cortical tubers (tuberous sclerosis): well-circumscribed cortical lesions; covered in phakomatoses page.
  • Sturge-Weber malformation: leptomeningeal angioma.
  • Holoprosencephaly: failure of forebrain cleavage; alobar (severe), semilobar, lobar; midline facial defects.
  • Agenesis of corpus callosum: complete or partial; can be isolated or syndromic.
  • Dandy-Walker malformation: cerebellar vermis hypoplasia + cystic fourth ventricle + posterior fossa enlargement.
  • Chiari malformation I, II, III: posterior fossa anomalies.

Imaging

  • MRI is the standard.
  • FCD: subtle cortical thickening, blurring of gray-white junction, T2/FLAIR hyperintensity (“transmantle sign” in FCD IIb).
  • Lissencephaly: smooth cortex, simplified gyral pattern, thickened cortex.
  • Polymicrogyria: thin abnormally fused gyri.
  • Schizencephaly: cleft lined with gray matter.
  • Heterotopia: gray matter in abnormal location.

Clinical Features

  • Epilepsy: the most common clinical feature.
  • Developmental delay, intellectual disability.
  • Motor deficits.
  • Microcephaly or macrocephaly.

Treatment

  • Anti-epileptic drugs.
  • Epilepsy surgery for refractory cases with resectable lesions.
  • For mTOR-pathway disorders: mTOR inhibitors (everolimus, sirolimus) increasingly used.
  • Multidisciplinary developmental care.

🔍 Did You Know?

The recognition that focal cortical dysplasia type II is caused by somatic mutations in mTOR pathway genes has revolutionized our understanding of refractory pediatric epilepsy. Until recently, FCD was viewed as a developmental “accident” — a localized failure of cortical organization with unknown cause. The 2010s breakthrough was the discovery that most FCD type IIa and IIb cases harbor somatic mosaic mutations in genes of the mTOR pathway: AKT3, PIK3CA, MTOR, TSC1, TSC2, MTOR pathway adaptor genes. These mutations occur in a small subset of cells during cortical development, producing localized hyperactive mTOR signaling that drives the dysplastic features (cellular enlargement, abnormal lamination, balloon cells). Because the mutations are mosaic, they are usually not in blood and require analysis of the resected dysplastic tissue. The clinical implication is direct: mTOR inhibitors (everolimus, sirolimus) — already used for TSC-related SEGA and angiomyolipoma — are being investigated as treatment for FCD-related refractory epilepsy. The connection between FCD type II and TSC (also an mTOR pathway disease) is striking: FCD IIb shares histologic features with the cortical tubers of TSC (balloon cells, dysmorphic neurons, dysplastic cortex). FCD IIb is essentially “somatic mosaic TSC of one cortical region.” The lesson: epilepsy and developmental brain disorders increasingly reveal underlying molecular pathways that are targetable with existing drugs — and the implication is that more precise medicine for refractory epilepsy is emerging.

Pitfalls and Pearls

  • FCD type II (especially IIb with balloon cells): mTOR pathway somatic mutations; refractory epilepsy.
  • Transmantle sign: cortex-to-ventricle T2/FLAIR hyperintensity in FCD IIb.
  • Balloon cells: large cells with eccentric nuclei; characteristic of FCD IIb (and TSC tubers).
  • Lissencephaly type I: LIS1, DCX, TUBA1A; smooth cortex; 4-layer architecture.
  • X-linked DCX: lissencephaly in males; subcortical band heterotopia in females.
  • Cobblestone lissencephaly (type II): congenital muscular dystrophies; Walker-Warburg, MEB, Fukuyama.
  • Polymicrogyria: bilateral perisylvian common; congenital CMV cause.
  • Schizencephaly: cleft + polymicrogyria lining.
  • Periventricular nodular heterotopia: FLNA mutations; X-linked.
  • Hemimegalencephaly: somatic mTOR mutations.
  • Holoprosencephaly: alobar, semilobar, lobar; midline facial defects often.
  • Dandy-Walker malformation: vermis hypoplasia + cystic 4th ventricle.
  • Chiari malformations: I (cerebellar tonsils through foramen magnum), II (with myelomeningocele), III (occipital encephalocele).
  • mTOR inhibitors: emerging therapy for mTOR-pathway MCDs.
  • Epilepsy surgery: curative for many focal MCDs.

References

  1. 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(5):1348-1369.
  2. Blümcke I, Thom M, Aronica E, et al. The clinicopathologic spectrum of focal cortical dysplasias: a consensus classification proposed by an ad hoc Task Force of the ILAE Diagnostic Methods Commission. Epilepsia. 2011;52(1):158-174.
  3. Lim JS, Kim WI, Kang HC, et al. Brain somatic mutations in MTOR cause focal cortical dysplasia type II leading to intractable epilepsy. Nat Med. 2015;21(4):395-400.
  4. Marsh ED, Goodyear T, Reinl E. Genetics of cortical development malformations. Continuum (Minneap Minn). 2018;24(1):109-129.
  5. Mirzaa GM, Conway RL, Gripp KW, et al. Megalencephaly syndromes and activating mutations in the PI3K-AKT pathway. Am J Med Genet C Semin Med Genet. 2013;163C(2):122-130.