Florianopolis, Brazil

Definition
Anencephaly is a neural tube defect characterized by absence of the cranial vault and cerebral hemispheres

Incidence
0.1/10,000. It is the most common central nervous system (CNS) malformation.

Diagnosis 
First trimester 
The prenatal diagnosis of anencephaly can be made during the first trimester[1], by the absence of skull with exposure of the cerebral cortex to the amniotic fluid. Coronal sections in the ultrasound examination will show both hemispheres separated, given the impression of a "Mickey Mouse"[2].

Figure 1. Sagital section of a 10 week embryo showing absence of the cranial vault.

Figure 2. Coronal section through the fetal head of a 10 week fetus with anencephaly. Note the resemblance of the image with "Mickey Mouse".
Second trimester
Absence of skull and absence of the cerebral hemispheres. A coronal section through the fetal face will show bulging eyes with absence of the frontal bones, given the appearance of a "frog". This aspect may be observed during the first trimester as well.

Figure 3. Sagital section of a second trimester fetus depicting the typical absence of the cranial vault above the orbits, characteristic of anencephaly.

Associated anomalies
Myelomeningocele, microcephaly and amniotic band syndrome. In amniotic band syndrome, the damage is usually asymmetrical and associated with other anomalies, mainly limb amputations.

Prognosis 
Anencephaly is a uniformly lethal anomaly.

Genetic counseling and recurrence risk 
Most cases of anencephaly are compatible with a multifactorial model. Genetic factors seem important because of familial incidence, whereas geographic variation suggests an environmental cause. Concordance and discordance have occurred in monozygous twins. An increased incidence of anencephaly and other neural tube defects occur in women who have diabetes during pregnancy. Also, women who take valproic acid for a seizure disorder are at increased risk for anencephaly if their medication has been consumed prior to conception or during the first trimester of pregnancy.
Causative factors act on the developing embryo between the 16^th and 26^th day after conception.
The risk of recurrence of anencephaly is 1:20 (5%) and if there is a second affected child the risk rises to 13%.

Prevention 
All women of reproductive age should consume at least 0.4 mg (400 mcg) of folic acid daily to prevent neural tube defects. For women who have previously had a fetus affected with anencephaly, the Centers for Disease Control and Prevention (CDC), recommends increasing the intake of folic acid to 4 mg (4000 mcg) per day beginning at least one month prior to conception (Committee on Genetics, 1999).

Treatment 
There is no available treatment for anencephaly.

Links

Anencephaly Support Foundation (ASF)
Online Mendelian Inheritance in Man — OMIM entry 206500

References: