There is frontal fusion (which constitutes the holoprosencephaly) and occipital division (which makes the case: semilobar).
Holoprosencephaly
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OVERVIEW
- Holoprosencephaly occurs when the prosencephalon (the embryonic forebrain) fails to divide into the doubled lobes of the brain.
- This results in a single-lobed brain with severe skull and facial defects.
- Typically, the malformations are too severe for the fetus to even reach birth; however, the disorder is on a broad spectrum.
- 1/250 during early embryo development.
- 1/10,000 to 1/20,000 at term.
- Alobar -- no division, associated with severe facial defects.
- Semilobar, partial division (intermediate severity).
- Lobar, prominent, but incomplete separation of the lobes (the least severe form).
Notable craniofacial characteristics
- Median cleft lip (aka premaxillary agenesis) (the least severe finding)
- Cyclopia: a single eye located at what is otherwise the root of the nose (the most severe finding)
- Microcephaly
- Midface flattening
- Hypotelorism (closely spaced eyes)
- Flat nasal bridge
- Single maxillary central incisor
Genetics
- Less than 1/3 of cases are caused by gene mutation (eg, SHH (sonic hedgehog)); at least 9 genes mutations have been discovered.
- The remainder of cases are due to other genetic and/or environmental factors.
- Trisomy 13
- Fetal alcohol syndrome
References
- Geng, Xin, and Guillermo Oliver. “Pathogenesis of Holoprosencephaly.” The Journal of Clinical Investigation 119, no. 6 (June 1, 2009): 1403–13. https://doi.org/10.1172/JCI38937.
- Raam, Manu S, Benjamin D Solomon, and Maximilian Muenke. “Holoprosencephaly: A Guide to Diagnosis and Clinical Management.” Indian Pediatrics 48, no. 6 (June 2011): 457–66.
- Solomon, Benjamin D., Andrea Gropman, and Maximilian Muenke. “Holoprosencephaly Overview.” In GeneReviews®, edited by Margaret P. Adam, Holly H. Ardinger, Roberta A. Pagon, Stephanie E. Wallace, Lora JH Bean, Karen Stephens, and Anne Amemiya. Seattle (WA): University of Washington, Seattle, 1993. http://www.ncbi.nlm.nih.gov/books/NBK1530/.