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ACHS
8522
Clinical Information: Neural tube defects (NTDs) are a type of birth defect involving openings along
the brain and spine. They develop in the early embryonic period when the neural tube fails to completely
close. NTDs can vary widely in severity. Anencephaly represents the most severe end of the spectrum and
occurs when the cranial end fails to form, resulting in an absence of the forebrain, the area of the skull that
covers the brain, and the skin. Most infants with anencephaly are stillborn or die shortly after birth. NTDs
along the spine are referred to as spina bifida. Individuals with spina bifida may experience
hydrocephalus, urinary and bowel dysfunction, club foot, lower body weakness, and loss of feeling or
paralysis. Severity varies depending upon whether the NTD is covered by skin, whether herniation of the
meninges and spinal cord are present, and the location of the lesion. NTDs not covered by skin are
referred to as open NTDs and are typically more severe than closed NTDs. Likewise those presenting with
herniation and higher on the spinal column are typically more severe. Most NTDs occur as isolated birth
defects with an incidence of approximately 1 in 1,000 to 2 in 1,000 live births in the United States. Rates
vary by geographic region with lower rates being observed in the North and West than the South and East.
A fetus is at higher risk when the pregnancy is complicated by maternal diabetes, exposed to certain
anticonvulsants, or there is a family history of NTDs. Studies have shown a dramatic decrease in risk as a
result of maternal dietary supplementation with folic acid. The March of Dimes currently recommends
that all women of childbearing age take 400 micrograms of folic acid daily, increasing the amount to 600
mg/day during pregnancy. For women who have had a prior pregnancy affected by an NTD, the
recommended dose is at least 4,000 mg/day starting at least 1 month preconception and continuing
through the first trimester. When a NTD is suspected based upon maternal serum alpha-fetoprotein (AFP)
screening results or diagnosed via ultrasound, analysis of AFP and acetylcholinesterase (AChE) in
amniotic fluid are useful diagnostic tools. AChE is primarily active in the central nervous system with
small amounts of enzyme found in erythrocytes, skeletal muscle, and fetal serum. Normal amniotic fluid
does not contain AChE, unless contributed by the fetus as a result of an open NTD.
Useful For: Diagnosing open neural tube defects, and to a lesser degree, ventral wall defects
Interpretation: The presence of acetylcholinesterase in amniotic fluid is positive for a neural tube
defect if fetal hemoglobin contamination can be ruled out.
Reference Values:
Negative (reported as negative [normal] or positive [abnormal] for inhibitable acetylcholinesterase)
Reference values were established in conjunction with alpha-fetoprotein testing and include only
amniotic fluids from pregnancies between 14 and 21 weeks gestation.
Clinical References: 1. Muller F: Prenatal biochemical screening for neural tube defects. Childs
Nerv Syst 2003 Aug;19(7-8):433-435 Epub 2003 Jul 12 2. March of Dimes: Neural tube defects.
[Accessed 7/19/12] Available from URL:
http://www.marchofdimes.com/baby/birthdefects_neuraltube.html
Acetylcholinesterase, Erythrocytes
Clinical Information: Acetylcholinesterase (AChE) is anchored to the external surface of the RBC.
Its appearance in a lysate of red cells is diminished in paroxysmal nocturnal hemoglobinuria (PNH). The
use of red cell AChE for PNH has not gained widespread acceptance, and flow cytometry testing is most
often used for PNH (see #81156 "PI-Linked Antigen, Blood"). Red cell AChE is most often used to detect
past exposure to organophosphate insecticides with resultant inhibition of the enzyme. Both the
pseudocholinesterase activity in serum and red cell AChE are inhibited by these insecticides, but they are
dramatically different vis-a-vis the temporal aspect of the exposure. The half-life of the pseudo-enzyme in
serum is about 8 days, and the "true" cholinesterase (AChE) of red cells is over 3 months (determined by
erythropoietic activity). Recent exposure up to several weeks is determined by assay of the
pseudo-enzyme and months after exposure by measurement of the red cell enzyme. The effect of the
specific insecticides may be important to know prior to testing.
Useful For: Detecting effects of remote (months) past exposure to cholinesterase inhibitors
(organophosphate insecticide poisoning)
Interpretation: Activities less than normal are suspect for exposure to certain insecticides.
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