Holoprosencephaly (HPE) results from an incomplete midline cleavage of the forebrain (prosencephalon). It includes a wide spectrum of intracranial and craniofacial midline defects and a myriad of clinical manifestations, consisting of neurologic impairment and dysmorphism of the brain and face. It is the most common malformation of forebrain development. Evidence suggests that HPE can be present either sporadically or can have a syndromic association. The defect associated with HPE occurs at approximately 3 to 4 weeks post-conception and is a disorder of gastrulation.
The HPE phenotype continuum has been divided into 3 categories:
Alobar: Most severe form that is characterized by a complete failure to partition the forebrain into left and right hemispheres. Results in a single, centrally-located ventricle.
Semilobar: Partial forebrain cleavage
Lobar: Almost complete forebrain cleavage
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