Holoprosencephaly, also known as HPE, is a neurological disorder in which a developing fetus’ brain fails to divide into the two lobes characteristic of human and many animal brains. The condition is most studied amongst humans, but researchers also believe that can affect certain animals, too. In both humans and animals, the defect usually leads to miscarriage, which is often medically referred to as a “spontaneous abortion”: the problems are large enough and the development is impeded enough that the fetus dies in utero. At least in humans, brain division and growth tend to happen with the most intensity between five and eight weeks’ gestation. Most HPE-related miscarriages happen during this window. In rarer cases the pregnancies are viable, which means that the fetus will survive and will be born alive, but these children almost always have severe handicaps and limitations. Scholars have identified four stages or classifications of the disorder; while all result in a single-lobed brain structure, the effects of development and viability vary. In all cases, though, the condition causes skull and facial birth defects at full term, as well as problems with mental development and brain function more generally. How pronounced these are normally varies. Some children only live for a few weeks, while others survive to adolescence and, rarely, to adulthood.
Classifications
The medical community usually recognizes four classifications or levels of this disorder. Alobar is the most severe classification of HPE. It is characterized by a brain that has not divided at all. Typically, alobar comes with severe abnormalities in the skull and face formation, and most affected pregnancies end on their own.
Semi-lobar is a more moderate classification of HPE. This form of the disorder takes shape when the brain partially divides into two hemispheres in the rear of the brain, but not in the front. There are also moderate facial or skull abnormalities in this classification which make the condition more immediately apparent. It isn’t known how many affected fetuses survive till birth, but most likely die in utero.
Lobar is the third classification, and involves partial but delayed brain division. Most of these children do survive, though they will normally have some noticeable facial and skull abnormalities. The Middle Interhemispheric Variant (MIHV) occurs when the middle of the brain has not separated well or has separated incompletely. MIHV is usually the most compatible with life, and patients with this sort of expression are the most likely to survive for the longest, often leading full lives with engagement in most normal human activities.
Causes
The cause of the condition in any of its stages or classifications has yet to be determined, although risk factors include maternal diabetes, as well as infections or drug use during pregnancy. Miscarriages and bleeding in the first-trimester may also be indicators or symptoms of the problem. Abnormal chromosomes have been identified in some affected children suggesting that the condition can be inherited.
Incidence and Survival Rates
According to statistics gathered from hospitals and clinics, it’s estimated that the condition affects about 1 in 5,000 to 10,000 live births worldwide. Since only very small number of impacted fetuses survive to term — usually less than five percent — the frequency of raw occurence may be as high as 1 in 200 or 250 pregnancies. Depending on the classification and severity of the condition, those that do reach birth typically do not live for more than six months, although those with less severe abnormalities may go on to live a normal life span.
Characteristics of Surviving Children
Patients who are born with the condition typically display a number of signifying physical and mental characteristics. Most have proportionally small heads, for instance, and also also display a range of facial abnormalities. These facial abnormalities may include a flat nose that has only one nostril, eyes that are close together, a cleft lip or palate or just one upper middle tooth instead of two. The more severe deformities may include one eye that is centered on the face, a nose that is located on the forehead or an absence of some facial feature altogether.
Brain function is also normally impaired. Patients typically experience excessive fluid on the brain which can lead to mental retardation and developmental delays, and epilepsy and abnormalities of organ systems are common, too.
