Holoprosencephaly is a neurological disorder where the brain fails to divide into two lobes, affecting both humans and animals. It often leads to miscarriage, but in rare cases, the pregnancy is viable, resulting in severe handicaps and limitations. The disorder has four classifications, with varying effects on development and vitality. The cause is unknown, but risk factors include maternal diabetes and infections or drug use during pregnancy. The condition affects approximately 1 in 5,000-10,000 live births worldwide, with survival rates being low. Patients often have physical and mental abnormalities, including facial deformities and impaired brain function.
Holoprosencephaly, also known as HPE, is a neurological disorder in which the brain of a developing fetus fails to divide into the two lobes characteristic of the human and many animal brains. The condition is most studied among humans, but researchers also believe it can affect some animals as well. In both humans and animals, the defect usually leads to miscarriage, which is often medically referred to as a “miscarriage”: the problems are large enough and development is so impeded that the fetus dies in uterus. At least in humans, brain division and growth tend to be most intense between five and eight weeks of gestation. Most HPE-related aborts occur during this window. In rarer cases the pregnancies are viable, meaning the fetus will survive and be born alive, but these babies 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 vitality vary. In all cases, however, the condition causes birth defects of the skull and face at full term, as well as problems with mental development and brain function more generally. How these are usually pronounced varies. Some children live only a few weeks, while others survive into adolescence and, rarely, into adulthood.
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The medical community usually recognizes four classifications or levels of this disorder. Alobar is HPE’s strictest classification. It is characterized by a brain that has not split at all. Typically, the alobar presents with severe abnormalities in the formation of the skull and face, and the 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 back of the brain, but not in the front. There are also moderate facial or cranial abnormalities in this classification which make the condition more immediately apparent. It is not known how many affected fetuses survive to birth, but they most likely die in utero.
Lobar is the third classification and involves partial but delayed brain division. Most of these children survive, although they usually have some obvious facial and cranial abnormalities. The middle interhemispheric variant (MIHV) occurs when the center of the brain has not separated well or has separated incompletely. MIHV is usually the most life-compatible, and patients with this type of expression are more likely to survive longer, often leading full lives with engagement in most normal human activities.
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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 can also be indicators or symptoms of the problem. Abnormal chromosomes have been identified in some affected children, suggesting that the condition may be inherited.
Incidence and survival rates
According to statistics gathered from hospitals and clinics, the condition is estimated to affect approximately 1 in 5,000-10,000 live births worldwide. Because only a very small number of affected fetuses survive to term — usually less than five percent — the crude occurrence rate can be as high as 1 in 200 or 250 pregnancies. Depending on the classification and severity of the condition, those who reach birth typically do not live for more than six months, although those with less severe abnormalities may go on to live normal lives.
Characteristics of surviving children
Patients who are born with the condition typically display a number of significant physical and mental characteristics. Most have proportionally small heads, for example, and also display a variety of facial abnormalities. These facial abnormalities may include a flat nose that has only one nostril, close-set eyes, a cleft lip or palate, or only one upper middle tooth instead of two. More severe deformities may include an eye centered on the face, a nose that is set against the forehead, or the complete absence of certain facial features.
Brain function is also normally impaired. Patients typically experience excess fluid in the brain that can lead to mental retardation and developmental delays, and epilepsy and organ system abnormalities are also common.
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