Holoprosencephaly is a "disorder caused by the failure of the prosencephalon (the forebrain of the embryo) to divide to form bilateral cerebral hemispheres, causing defects in the development of the face and in brain structure and function."
Okay, okay...That was the technical scientific definition. Let me tell you what it is in every day terms. The brain, while developing between the fourth and sixth week of gestation, does not divide correctly into the left and right brain. Because the brain did not develop properly, there are defects in the brain function and the development of the facial features.
There are three classes of holoprosencephaly that depends on how much the brain was able to divide while it was developing; alobar, semilobar, and lobar.
Alobar holoprosencephaly is when the brain did not divide and is considered the most severe of the three. The baby may not have a recognizable face. It may have one eye or no eyes. It may have a missing nose or a proboscis (a tubular-shaped nose) located above the eye.
Semilobar holoprosencephaly is the brain has somewhat divided. Some of the characteristics would be various degrees of mental retardation, hypotelorism (closely spaced eyes), or have small eyes.
Lobar holoprosencephaly is the mildest case. The brain has divided but there may be a range from mild retardation to normal brain function and have mild or absent facial malformations.
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