The corpus callosum is the largest fiber tract in the central nervous system and the major interhemispheric fiber bundle in the brain. Formation of the corpus callosum begins as early as 6 weeks' gestation, with the first fibers crossing the midline at 11 to 12 weeks' gestation, and completion of the basic shape by age 18 to 20 weeks (Schell-Apacik et al., 2008). Agenesis of the corpus callosum (ACC) is one of the most frequent malformations in brain with a reported incidence ranging between 0.5 and 70 in 10,000 births. ACC is a clinically and genetically heterogeneous condition, which can be observed either as an isolated condition or as a manifestation in the context of a congenital syndrome (see MOLECULAR GENETICS and Dobyns, 1996). Also see mirror movements-1 and/or agenesis of the corpus callosum (MRMV1; 157600). Schell-Apacik et al. (2008) noted that there is confusion in the literature regarding radiologic terminology concerning partial absence of the corpus callosum, where various designations have been used, including hypogenesis, hypoplasia, partial agenesis, or dysgenesis.

Brunelli et al. (1996) described schizencephaly as an extremely rare congenital disorder characterized by a full-thickness cleft within the cerebral hemispheres. The clefts are lined with gray matter and most commonly involve the parasylvian regions (Wolpert and Barnes, 1992). Large portions of the cerebral hemispheres may be absent and replaced by cerebrospinal fluid. Two types of schizencephaly have been described, depending on the size of the area involved and the separation of the cleft lips (Wolpert and Barnes, 1992). Type I schizencephaly consists of a fused cleft. This fused pial-ependymal seam forms a furrow in the developing brain, and is lined by polymicrogyric gray matter. In type II schizencephaly, there is a large defect, a holohemispheric cleft in the cerebral cortex filled with fluid and lined by polymicrogyric gray matter. The clinical manifestations depend on the severity of the lesion. Patients with type I are often almost normal; they may have seizures and spasticity. In type II abnormalities, there is usually mental retardation, seizures, hypotonia, spasticity, inability to walk or speak, and blindness. Schizencephaly may be part of the larger phenotypic spectrum of holoprosencephaly (HPE; see 236100).

Atrophy of the cortex of the cerebrum.

Brunelli et al. (1996) described schizencephaly as an extremely rare congenital disorder characterized by a full-thickness cleft within the cerebral hemispheres. The clefts are lined with gray matter and most commonly involve the parasylvian regions (Wolpert and Barnes, 1992). Large portions of the cerebral hemispheres may be absent and replaced by cerebrospinal fluid. Two types of schizencephaly have been described, depending on the size of the area involved and the separation of the cleft lips (Wolpert and Barnes, 1992). Type I schizencephaly consists of a fused cleft. This fused pial-ependymal seam forms a furrow in the developing brain, and is lined by polymicrogyric gray matter. In type II schizencephaly, there is a large defect, a holohemispheric cleft in the cerebral cortex filled with fluid and lined by polymicrogyric gray matter. The clinical manifestations depend on the severity of the lesion. Patients with type I are often almost normal; they may have seizures and spasticity. In type II abnormalities, there is usually mental retardation, seizures, hypotonia, spasticity, inability to walk or speak, and blindness. Schizencephaly may be part of the larger phenotypic spectrum of holoprosencephaly (HPE; see 236100).

With the current widespread use of multigene panels and comprehensive genomic testing, it has become apparent that the phenotypic spectrum of EPG5-related disorder represents a continuum. At the most severe end of the spectrum is classic Vici syndrome (defined as a neurodevelopmental disorder with multisystem involvement characterized by the combination of agenesis of the corpus callosum, cataracts, hypopigmentation, cardiomyopathy, combined immunodeficiency, microcephaly, and failure to thrive); at the milder end of the spectrum are attenuated neurodevelopmental phenotypes with variable multisystem involvement. Median survival in classic Vici syndrome appears to be 24 months, with only 10% of children surviving longer than age five years; the most common causes of death are respiratory infections as a result of primary immunodeficiency and/or cardiac insufficiency resulting from progressive cardiac failure. No data are available on life span in individuals at the milder end of the spectrum.

In WDR62 primary microcephaly (WDR62-MCPH), microcephaly (occipitofrontal circumference [OFC] = -2 SD) is usually present at birth, but in some instances becomes evident later in the first year of life. Growth is otherwise normal. Except for brain malformations in most affected individuals, no other congenital malformations are observed. Central nervous system involvement can include delayed motor development, mild-to-severe intellectual disability (ID), behavior problems, epilepsy, spasticity, and ataxia.

Brain small vessel disease-2 is an autosomal dominant disorder characterized by variable neurologic impairment resulting from disturbed vascular supply that leads to cerebral degeneration. The disorder is often associated with 'porencephaly' on brain imaging. Affected individuals typically have hemiplegia, seizures, and intellectual disability, although the severity is variable (summary by Yoneda et al., 2012). For a discussion of genetic heterogeneity of brain small vessel disease, see BSVD1 (175780).

Osteogenesis imperfecta (OI) is a connective tissue disorder characterized by bone fragility and low bone mass. Due to considerable phenotypic variability, Sillence et al. (1979) developed a classification of OI subtypes based on clinical features and disease severity: OI type I, with blue sclerae (166200); perinatal lethal OI type II, also known as congenital OI (166210); OI type III, a progressively deforming form with normal sclerae (259420); and OI type IV, with normal sclerae (166220). Most forms of OI are autosomal dominant with mutations in one of the 2 genes that code for type I collagen alpha chains, COL1A1 (120150) and COL1A2 (120160). Keupp et al. (2013) and Pyott et al. (2013) described osteogenesis imperfecta type XV, an autosomal recessive form of the disorder characterized by early-onset recurrent fractures, bone deformity, significant reduction of bone density, short stature, and, in some patients, blue sclera. Tooth development and hearing are normal. Learning and developmental delays and brain anomalies have been observed in some patients.

The spectrum of COL4A1-related disorders includes: small-vessel brain disease of varying severity including porencephaly, variably associated with eye defects (retinal arterial tortuosity, Axenfeld-Rieger anomaly, cataract) and systemic findings (kidney involvement, muscle cramps, cerebral aneurysms, Raynaud phenomenon, cardiac arrhythmia, and hemolytic anemia). On imaging studies, small-vessel brain disease is manifest as diffuse periventricular leukoencephalopathy, lacunar infarcts, microhemorrhage, dilated perivascular spaces, and deep intracerebral hemorrhages. Clinically, small-vessel brain disease manifests as infantile hemiparesis, seizures, single or recurrent hemorrhagic stroke, ischemic stroke, and isolated migraine with aura. Porencephaly (fluid-filled cavities in the brain detected by CT or MRI) is typically manifest as infantile hemiparesis, seizures, and intellectual disability; however, on occasion it can be an incidental finding. HANAC (hereditary angiopathy with nephropathy, aneurysms, and muscle cramps) syndrome usually associates asymptomatic small-vessel brain disease, cerebral large vessel involvement (i.e., aneurysms), and systemic findings involving the kidney, muscle, and small vessels of the eye. Two additional phenotypes include isolated retinal artery tortuosity and nonsyndromic autosomal dominant congenital cataract.

Complex cortical dysplasia with other brain malformations-11 (CDCBM11) is an autosomal recessive disorder characterized by dilated ventricles and reduced white matter and associated with axonal developmental defects (Qian et al., 2022). For a discussion of genetic heterogeneity of CDCBM, see CDCBM1 (614039).