Anencephaly
DEFINITION:
A neural tube defect affecting the skull resulting in a large defect of the calvarium, meninges, and scalp which is incompatible with life.
EPIDEMIOLOGY:
*incidence: 1/1000 live births
*age of onset:
--newborn
*risk factors:
*geographic
--variable with incidence increased in Ireland and Wales and decreased in Asia, Africa, and South America
*genetic
--recurrence rate is 4% and increases to 10% if a couple has had 2 previous anencephalic infants
--whites > blacks (6:1)
--F > M
*environmental/nutritional
--low SEC, environmental and toxic factors
--nutritional and vitamin deficiences
--folic acid antagonists
--drinking water minerals
*perinatal
--maternal infections and/or hyperthermia
*associated anomalies:
--50% polyhydramnios (unable to swallow amniotic fluid)
--10-20% folded ears, cleft palate, congenital heart disease
PATHOGENESIS:
1. Background
*anencephaly arises from an embryonic defect occurring prior to day 26 which prevents the closure of the anterior neuropore - the frequency of anencephaly has decreased during past 2 decades - CNS findings:
--both cerebral hemispheres and thus the spinal cord pyramidal tracts are absent
--absence of the cerebellum
--residue of the brainstem usually present
--hypoplastic pituitary gland
--the rudimentary brain remaining consists of portions of con-nective tissue, vessels, and neuroglia
CLINICAL FEATURES:
1. Anencephaly
*distinctive appearance: a large defect in the vault of the skull (calvarium), meninges, and scalp exposes a soft angiomatous mass of neural tissue covered with a thin membrane continuous with the skin
*the cranial defect may extend inferiorly to the cervical region exposing a thin and flattened spinal cord (craniorachischisis)
*the optic globes may protrude due to inadequately-formed bony orbits
2. Neuroendocrine Defects
*hypoplastic pituitary
--adrenal insufficiency
--diabetes insipidus
INVESTIGATIONS:
1. Prenatal Diagnosis
*elevated maternal serum alpha-feto-protein (AFP)
*level II ultrasound
*amniocentesis - elevated AFP and acetylcholinesterase
MANAGEMENT:
1. No Treatment
*most infants are stillborn or die within several days of birth
*organ donation is controversial
2. Genetic Counselling
*amniocentesis, AFP, and/or an ultrasound between 14-16 weeks in couples with a history of an anencephalic infant
