In<unk> , the neural tube fails to segment, resulting in incomplete separation of the prosencephalon at the fifth week of gestation. The exact cause(s) of<unk>  are yet to be determined. Mutations in the gene encoding the SHH protein, which is involved in the development of the central nervous system (CNS), can cause<unk> . In other cases, it often seems that there is no specific cause at all. Ultrasound scan of a fetal head at 14 weeks of pregnancy with partial absence of the midline Genetics Armand Marie Leroi describes the cause of cyclopia as a genetic malfunctioning during the process by which the embryonic brain is divided into two. Only later does the visual cortex take recognizable form, and at this point an individual with a single forebrain region will be likely to have a single, possibly rather large, eye (at such a time, individuals with separate cerebral hemispheres would form two eyes). Increases in expression of such genes as Pax-2, as well as inhibition of Pax-6, from the notochord have been implicated in normal differentiation of cephalic midline structures. Inappropriate expression of any of these genes may result in mild to severe forms of<unk> . Other candidate genes have been located, including the SHH<unk>  type 3 a.k.a.<unk> 3), TGIF, ZIC2, SIX3 and BOC genes. Although many children with<unk>  have normal chromosomes, specific chromosomal abnormalities have been identified in some patients (trisomy of chromosome 13, also known as Patau syndrome). There is evidence that in some families,<unk>  is inherited (autosomal dominant as well as autosomal or X-linked recessive inheritance). Features consistent with familial transmission of the disease (e.g., a single central maxillary incisor) should be carefully assessed in parents and family members. Non-genetic factors Numerous possible risk factors have been identified, including gestational diabetes, transplacental infections (the "TORCH complex"), first trimester bleeding, and a history of miscarriage. As well, the disorder is found twice as often in female babies. However, there appears to be no correlation between<unk>  and maternal age. There is evidence of a correlation between<unk>  and the use of various drugs classified as being potentially unsafe for pregnant and lactating mothers. These include insulin, birth control pills, aspirin, lithium, thorazine, retinoic acid, and anticonvulsants. There is also a correlation between alcohol consumption and<unk> , along with nicotine, the toxins in cigarettes and toxins in cigarette smoke when used during pregnancy.
Holoprosencephaly
Schizencephaly
Hydranencephaly
Hemihydranencephaly
Lissencephaly
Microlissencephaly
Microhydranencephaly
Hemimegalencephaly