Holoprosencephaly refers to a spectrum of malformations of the brain and face caused when the front portion of the brain fails to split into two hemispheres. The true incidence rate of holoprosencephaly is unknown, but it is estimated that it occurs in 1 in 8,000 second trimester pregnancies.
There are four subtypes of holoprosencephaly which vary in severity: (1) alobar holoprosencephaly, the most severe form, which is characterized by the absence of the midline structures of the brain and no division between the two hemispheres of the front brain; (2) semilobar holoprosencephaly, which refers to the partial but incomplete separation of the two frontal hemispheres of the brain; (3) lobar holoprosencephaly, which refers to a normal separation and the presence of anomalies in the midline structures of the brain, and (4) middle hemispheric holoprosencephaly, the rarest form, which occurs when the brain fuses, or fails to separate, in the middle rather than the front.
Holoprosencephaly can occur as an isolated diagnosis or in association with other anomalies. Survival is possible for isolated cases of holoprosencephaly. In cases with co-diagnoses, holoprosencephaly is highly lethal with an estimated 89% mortality rate in the perinatal period.
An estimated 40% of holoprosencephaly cases are aneuploidy, with trisomy 13 being the most common chromosomal abnormality. Holoprosencephaly is also associated with heart and kidney defects, fusion or webbing of fingers and toes, and facial malformations like absent nasal structures, cleft lip and cleft palate, closely placed eyes, a flat nose, a protrusion called a probiscus, and cyclopia, the condition of having a single eye or a fused double eye.
The recurrence risk for holoprosencephaly varies widely based on co-diagnoses. In cases of chromosomal abnormalities, the recurrence rate is around 1%. [30]