Hydrocephaly
Definition
Hydrocephaly is an enlargement of the head caused by an abnormal accumulation of cerebrospinal fluid (CSF) in the cranium due to an imbalance between the production and absorption of CSF. This forces the ventricles to enlarge, which in turn exerts pressure on the surrounding brain tissue to shrink and the Head to enlarge (Schrander-Stumpel 1998, Buyse 1990, Vintziloes 1983).
Congenital hydrocephaly is one of the most common central nervous system anomalies. This generally refers to a condition that exists parentally and excludes other neural tube defects; this condition usually develops by the twentieth week of gestation, and defect can occur either alone, in association with spina bifida, or as part of greater syndrome, such as Dandy-Walker syndrome.
There are various types of classifications of congenital hydrocephaly. Aqueductal stenosis is a type of hydrocephaly that results from narrowing of the aqueduct of Sylvius, an opening connecting the third and fourth ventricles in the brain. It is the most common form of hydrocephaly. Dandy-Walker syndrome Is a group of defects consisting of enlargement of the fourth ventricle of the brain, complete or partial absence of the cerebellar vermis (the middle area between the two cerebral hemispheres), cysts of the posterior fossa and hydrocephaly. The hydrocephaly associated with Dandy-Walker syndrome may not be present at birth but develop later. Dandy-walker syndrome accounts for 5-12% of the hydrocephaly cases (Vintzileos 1983).
Etiology
Congenital hydrocephaly is heterogeneous in origin. This defect can occur as a result of chromosomal abnormality, a parental infection, or a non-genetic structural defect. Mental infection with toxoplasmosis and cytomegalovirus (CMV) can both result in hydrocephaly. These infections can cause structural problems in the fetus, especially cranial lesions that may cause hydrocephaly. Unexpectedly, these lesions are less likely to form in children born to mothers that experienced a delay in treatment or were not treated at all. It is unclear if this is due to lack of exposure to pyrimethamine-sulfadiazine treatments.
The defect can be associated with chromosomal abnormalities (trisomy and other syndromes (Walker-Wardbug syndrome, Meckel syndrome, Smith-Lemli-Optiz Syndrome, chondrodystrophies, etc), about one-quarter of the infants with hydrocephaly also have spina bifida; conversely, approximately 80% of children with spina bifida also have hydrocephaly. Hydrocephaly can also be secondary to central nervous system anomalies. The majority of congenital hydrocephaly cases have other additional birth defects such as congenital heart disease and cleft lip and palate (Schrander-Stumpel 1998, Stoll 1992, Buyse 1990, Drugan 1989, Hudgins 1988, Vintzileos 1983, and Chervenak 1985).
One specific gene that has been associated with this defect is L1, which is located on the Xq28 region. Mutations to the L1 cell adhesion molecule can manifest as a variety abnormalities and developmental delays. Multiple syndromes are associated with these mutations, including MASA (mental retardation, aphasia, hypoplasia, retardation, adducted thumbs, spastic paraplegia, and hydrocephalus) syndrome. This indicates that mental retardation associated with hydrocephaly may not be related to the reduced size of the ventricles, but related to an underlying genetic disorder (Partington 2001).
Non-genetic structural defects can occur for a variety of reasons. Hydrocephaly is positively associated with spina bifida. This defect is most often caused by a lack of folate/folic acid in the maternal diet (Partington 2001).
Demographic and Reproductive Factors
No clear racial differences in congenital hydrocephaly risk are recognized (Stoll 1992, Wiswell 1990). One study completed in Booklyn , New York Sherman
A secular trend for congenital hydrocephaly has been reported by one study, where a decline in prevalence over time was found, paralleling the decline in neural tube defect prevalence (stone 1989). However, another investigation observed no trend (Wiswell 1990). There does not appear to be any seasonal variation in congenital hydrocephaly prevalence (Stoll 1992, Castilla 1990, Wiswell 1990).
Hydrocephaly (ventriculomegaly) can be detected prenatally by Ultrasonography (Fadel 1989, Vinzileos, 1987). Studies from various birth defects surveillance systems have found that, in regions where elective termination is allowed, prenatal diagnosis and elective termination reduce the birth prevalence of congenital hydrocephaly (Stoll 1992, stone 1989, drugan 1989, hudgins 1988, and chervenak 1985).
Low socioeconomics status is a risk factor for all nongenetic defects, including hydrocephaly (Vrijheid 2000). Maternal resisdence near a landfill or solid waste incinerator is not a risk factor for this defect (Cordier 2003, Harrison 2003).
Congenital hydrocephaly prevalence does not appear to be influenced by geographic location (Stoll 1992). However, one study found a reduction in hydrocephaly risk with higher altitudes (Castilla 1999).
Parental Age does not seem to affect risk of having a child with hydrocephaly (Stoll 1992). In a different study, young maternal age was associated with a higher risk of hydrocephaly. However the authors felt that this may be due to the same extend as older mothers. This may lead to younger mothers carrying an affected infant to term (Reefhuis 2004).
A woman who has had one child with congenital hydrocephaly has a recurrence risk of 1-5% of having another affected child. If the hydrocephaly is associated with an inherited disorder, the risk is higher (schrander-stumpel 1998, Buyse 1990). Hydrocephaly rates have been reported to be higher for consanguineous parents (Rajab 1998, Stoll 1992).
Infant sex does not appear to influence congenital hydrocephaly risk; males and females are equally affected (Stoll 1992, Buyse 1990). However, one study did report predominance among males (Wisswell 1990). Plurality has not been linked to congenital hydrocephaly risk (Stoll 1992).
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