What is Hydrolethalus Syndrome? Hydrolethalus syndrome is an inherited disease caused by harmful genetic changes (mutations) in the HYSF1 gene. The HYSF1 gene plays an important role in fetal development. Hydrolethalus syndrome causes severe brain abnormalities. These can include missing portions of the brain and extra fluid surrounding the brain (hydrocephalus). The opening at the base of the skull (foramen magnum) has an atypical key shape. Affected infants display differences in the face and skull (craniofacial malformations), including a small jaw (micrognathia) and a notched (cleft) lip and roof of mouth (palate). Other findings include extra digits of the fingers and toes (polydactyly), heart defects, narrowing of the airway, and malformation of the lungs (unilobular lungs). The condition may be detected by an ultrasound scan around the thirteenth to fifteenth week of pregnancy due to the malformations of the brain and other organs. Extra amniotic fluid (polyhydramnios) is typically present in the later parts of the pregnancy. How common is Hydrolethalus Syndrome? Hydrolethalus syndrome is most common in individuals of Finnish descent, with an estimated incidence of approximately 1 in 10,000. It is extremely rare in individuals of other ethnicities. How is Hydrolethalus Syndrome treated? At this time, there are no cures or treatment options for individuals with hydrolethalus syndrome. What is the prognosis for a person with Hydrolethalus Syndrome? Unfortunately, the prognosis for an infant with hydrolethalus syndrome is poor. Most individuals are either stillborn or die shortly after birth. There have been rare cases where infants with hydrolethalus syndrome have lived for several months. Other names forhydrolethalus syndrome HLS HLS1 Hydrolethalus syndrome Hydrolethalus syndrome 1 Salonen-Herva-Norio syndrome References Dammermann et al., 2009, Genes Dev, 23(17):2046–59, PMID: 19656802 Online Mendelian Inheritance in Man, OMIM , 2016, http://www.omim.org/entry/236680 Salonen et al., 1990, J Med Genet, 27(12):756-9. PMID: 2074561 Shotelersuk et al., 2001, Clin Dysmorphol, 10(1):51-5, PMID: 11152149 Valente et al., 2014, Nat Rev Neurol, 10(1):27-36, PMID: 24296655 Resources Orphanet Orphanet is a database of information on rare diseases, founded by the French Ministry of Health and the Institut National de la Santé et de la Recherche Médicale. It is now funded by the European Commission. Online Mendelian Inheritance In Man (OMIM) Online Mendelian Inheritance In Man (OMIM) is a database of human genes and genetic disorders, authored by staff at Johns Hopkins University and published by the National Institutes of Health. It contains fairly scientific language.