Overview of Hydrolethalus Syndrome

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.

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 palate
• A malformed respiratory system
• Congenital heart defects
• Extra fingers and toes (called polydactyly), especially a duplicated big toe
• Malformation of the brain, including missing structures

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.

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.