Holoprosencephaly (HPE) is a cephalic disorder in which the prosencephalon (the forebrain of the embryo) fails to develop into two hemispheres, typically occurring between the 18th and 28th day of gestation. Normally, the forebrain is formed and the face begins to develop in the fifth and sixth weeks of human pregnancy. The condition also occurs in other species.
Holoprosencephaly is estimated to occur in approximately 1 in every 250 conceptions and most cases are not compatible with life and result in fetal death in utero due to deformities to the skull and brain. However, holoprosencephaly is still estimated to occur in approximately 1 in every 8,000 live births.
When the embryo's forebrain does not divide to form bilateral cerebral hemispheres (the left and right halves of the brain), it causes defects in the development of the face and in brain structure and function.
The severity of holoprosencephaly is highly variable. In less severe cases, babies are born with normal or near-normal brain development and facial deformities that may affect the eyes, nose, and upper lip.
Signs and symptoms
Symptoms of holoprosencephaly range from mild (no facial/organ defects, anosmia, or only a single central incisor) to moderate to severe (cyclopia). The symptoms are dependent upon the classification type.
There are four classifications of holoprosencephaly as well as a mild "microform".
Alobar
Most severe form includes formation of synophthalmia (a single central eye), proboscis, and severe impairment. Semilobar Can present with severely decreased distance between eyes, a flat nasal bridge, eye defects, cleft lip and palate, and severe impairment. Lobar Can present with decreased distance between eyes, a flat nasal bridge, closely spaced nostrils, mental and locomotion delays may be present. Syntelencephaly or middle interhemispheric variant of holoprosencephaly (MIHV) Mild phenotypic presentation which can present with flat nasal bridge, metopic prominence, shallow philtrum, and possible mental and locomotion delays. "Microform" Mild phenotypic presentation with reduced distance between eyes, sharp nasal bridge, single maxillary central incisor.
Diagnosis
Holoprosencephaly is typically diagnosed during fetal development when there are abnormalities found on fetal brain imaging, however it can also be diagnosed after birth. The protocol for diagnosis includes neuroimaging (Ultrasound or fetal MRI prior to birth or Ultrasound, MRI or CT post birth), syndrome evaluation, cytogenetics, molecular testing, and genetic counseling.
There are four classifications of holoprosencephaly as well as a “microform". These classifications can be distinguished by their anatomical differences.
Alobar holoprosencephaly
Small single forebrain ventricle
No interhemispheric division
Absence of olfactory bulbs and tracts
Absence of corpus callosum
Non separation of deep gray nuclei
Semilobar holoprosencephaly
Rudimentary cerebral lobes
Incomplete interhemispheric division
Absence or hypoplasia of olfactory bulbs and tracts
Absence of corpus callosum
Varying non separation of deep gray nuclei
Lobar holoprosencephaly
Fully-developed cerebral lobes
Distinct interhemispheric division
Midline continuous frontal neocortex
Absent, hypoplasic or normal corpus callosum
Separation of deep gray nuclei
Syntelencephaly, or middle interhemispheric variant of holoprosencephaly (MIHV) Failure of separation of the posterior frontal and parietal lobes Callosal genu and splenium normally formed Absence of corpus callosum Hypothalamus and lentiform nuclei normally separated Heterotopic gray matter Microform Subtle defects of corpus callosum Subtle midline brain defects
Causes
In holoprosencephaly, the neural tube fails to segment, resulting in incomplete separation of the prosencephalon at the fifth week of gestation. According to one hypothesis, the holoprosencephalic brain is due to an incomplete axial twisting. According to the axial twist theory, each side of the brain represents its opposite body side because the anterior part of the head, including the forebrain, is turned around by a twisting along the body axis during early development. Accordingly, holoprosencephaly is possiby an extreme form of Yakovlevian torque.
The exact cause(s) of HPE 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 holoprosencephaly. In other cases, it often seems that there is no specific cause at all.