Classic HPE encompasses a continuum of brain malformations including (in order of decreasing severity):
During the fifth and sixth weeks of gestation, Hox genes tell the brain to separate and the facial features to start forming. In rare cases, these Hox genes do not signal the brain to separate into two hemispheres. The result is a mild to severe form of Holoprosencephaly.
In the mildest case, the brain shows significant signs of separation, but facial anomalies or deformities may be present. Brain function may be only slightly impaired and infants can live a normal life.
Patua syndrome or trisomy 13 is a syndrome that is most often associated with holoprosencephaly.
The most severe form of Holoprosencephaly, called Alobar holoprosencephaly can result in severe facial trauma, miscarriage, stillbirth or death soon after birth. The condition Cyclocephaly is associated with Alobar holoprosencephaly. Cyclocephaly is a condition where one eye forms where the nose should be and the condition is fatal.
There is no known cure for Holoprosencephaly. The condition is not degenerative, so the damage present at birth will not grow worse with time.