Rare Diseases Genetic Disorders Overview of Hydrolethalus Syndrome By Mary Kugler, RN Mary Kugler, RN Mary Kugler, RN, is a pediatric nurse whose specialty is caring for children with long-term or severe medical problems. Learn about our editorial process Updated on January 27, 2023 Medically reviewed by Daniel Combs, MD Medically reviewed by Daniel Combs, MD Daniel Combs, MD, is board-certified in sleep medicine and pediatrics. Learn about our Medical Expert Board Print Hydrolethalus syndrome is a fatal group of birth defects caused by a genetic disorder. Hydrolethalus syndrome was discovered when researchers were studying another disorder, called Meckel syndrome, in Finland. They found 56 cases of hydrolethalus syndrome in Finland, which means an incidence of at least 1 in 20,000 babies there. There have been at least 5 other reported cases of hydrolethalus syndrome published in the worldwide medical literature. Using the Finnish babies and their families, researchers discovered the gene mutation responsible for hydrolethalus syndrome in the Finnish population. The gene, called HYLS-1, is on chromosome 11. Research suggests that the genetic mutation is inherited in an autosomal recessive pattern. Andrew Brookes / Getty Images Symptoms of Hydrolethalus Syndrome Hydrolethalus syndrome consists of a group of distinctive birth defects, including: Severe hydrocephalus (excess fluid in the brain)Extremely small lower jaw (called micrognathia)Cleft lip or cleft palateA malformed respiratory systemCongenital heart defectsExtra fingers and toes (called polydactyly), especially a duplicated big toeMalformation of the brain, including missing structures Diagnosis of Hydrolethalus Syndrome Many babies with hydrolethalus syndrome are recognized before birth by prenatal ultrasound. The hydrocephalus and brain malformation suggest the diagnosis. Close examination of the fetus by ultrasound, or the baby at birth, is needed to rule out similar syndromes such as Meckel syndrome, Trisomy 13, or Smith-Lemli-Opitz syndrome. Outlook Often the baby with hydrolethalus syndrome is born prematurely. About 70% of babies with the syndrome are stillborn. Those born alive do not survive for very long. Sources Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy. Salonen, R., & R. Herva. "Hydrolethalus syndrome." Journal of Medical Genetics 27(1990): 756-759. Print. Tiller, George E.. "Hydrolethalus Syndrome 1." OMIM Database. 09 Jan 2007. NCBI. By Mary Kugler, RN Mary Kugler, RN, is a pediatric nurse whose specialty is caring for children with long-term or severe medical problems. See Our Editorial Process Meet Our Medical Expert Board Share Feedback Was this page helpful? Thanks for your feedback! What is your feedback? Other Helpful Report an Error Submit