This is the most common major malformation of the CNS seen in the West, varying in incidence from 0.65 per 1000 live births in Japan to 3 per 1000 in the British Isles (Gabriel 1980).
There has been a remarkable decline in its incidence in the Netherlands and Australia (Danks and Halliday 1983; Romijn and Treffers 1983).
No cause for this reduction in neural tube defects of all types is evident. Evidence of the value of multivitamins, particularly folic acid, in a preventative role is conflicting (Smithells et al. 1981; Mills et al. 1989).
In anencepholy it is clear that genetic factors are important in all neural-tube defects (Carter 1976) and that sodium valproate (Robert and Guibaud 1982) and a number of other maternal drugs and illnesses are possible factors in causation (Lindhout and Schmidt 1986).
In anencephaly, the cephalic neural folds fail to fuse into a neural tube, with consequential degeneration of all forebrain germinal cells. The spinal cord, brainstem, and cerebellum are small, but the cord shows no descending tracts; above this level there are only a few glial and vascular tangles and remnants of midbrain.
The eyes are normal but the optic nerves are absent and the calvarium is rudimentary. Fifty per cent of anencephalic fetuses are aborted spontaneously, but if pregnancy goes to term the infants quickly succumb, showing only slow, stereotyped movements and frequent decerebrate posturing. Like spina bifida, anencephaly can be detected early in pregnancy by measuring α-fetoprotein (AFP) in the maternal serum or amniotic fluid (Brock and Sutcliffe 1972; Seller et al. 1973). After AFP estimation, the presence of anencephaly can be confirmed confidently by ultrasonic examination (Cuckle 1994).
Origin: Brain's Diseases of the Nervous System