- Fusion failure Days 21-26 Gestation: Disorder of primary neurulation begins at approximately day 21 of gestation and finishes 26 days post conception (21).
- Endogenous factors: Currently research is focused on factors that contribute to the defect in closure and the resultant abnormal architecture of the placode. Folate deficiency has been found to be associated with upwards of 70% of myelomeningocele but other factors also can trigger the failure in fusion.
- Exogenous factors: Maternal heath and illnesses, environmental toxins and certain drugs have been linked to an increased risk for having a child with a myelomeningocele.
- Hydrocephalus: While the incidence of hydrocephalus appears to have fallen over the past several decades with more stringent criteria for place a shunt it still develops in roughly 80% of children (52).
- Hydrocephalus usually obstructive: Hydrocephalus in Chiari 2 Malformation may occur from aqueductal stenosis, fourth ventricular outlet obstruction or obliteration of subarachnoid space by crowding of the posterior fossa or narrowing of tentorial hiatus.
- Chiari 2 malformation: It is thought to occur secondary to abnormal CSF flow toward the myelomeningococele.
- Increased risk in relatives: Genetic basis is not well understood but there is a higher risk in relatives of children with myelomeningocele (13, 14). It may be associated with Trisomy 13 and 18.
- Genotypic and phenotypic studies ongoing
Elements of any germinal layer may be present: Squamous epithelium or fibrous connective tissue covers the translucent dome of the myelomeningocele, which contains the neural placode. Meninges, blood vessels, and hyalinized connective tissue are found in the myelomeningocele, as well. On rare occasion, striated muscle, islands of cartilage, or epidermal cysts are included. Leptomeninges beneath the dermis typically have an alveolar rather than a membranous appearance (21).