Prenatal Screening for Neural Tube Defects and Aneuploidy ALM Sutton and JR Biggio, University of Alabama, Birmingham, AL, USA ã 2014 Elsevier Inc. All rights reserved.
Screening for and Diagnosis of Neural Tube Defects Prenatal Screening for Aneuploidy Maternal Serum Screening Ultrasound Screening for Aneuploidy
1 2 2 2
Neural tube defects (NTDs) and aneuploidies are the major causes of perinatal death and childhood morbidity. Routine screening for these anomalies has become a standard part of prenatal care. Improvements in screening tests for aneuploidy have led to a dramatic shift in the practice patterns and a decline in the number of diagnostic tests. A combination of folate supplementation for primary and secondary prevention and screening with maternal serum alpha-fetoprotein (MSAFP) followed by comprehensive ultrasound evaluation for screen-positive women has reduced the birth incidence of NTDs. Recent advances in combined first- and second-trimester screening strategies, utilizing both biochemical and ultrasound evaluation, have increased the detection rates of chromosomal abnormalities to such levels that many women elect to forego prenatal diagnostic procedures resulting in reduction of rates of these procedures by more than 50%.
Screening for and Diagnosis of Neural Tube Defects Worldwide, the incidence of NTDs is approximately 1–10 per 1000 births, with some geographic variation. Adequate maternal folate levels are essential for normal fetal neural tube development. Widespread dietary supplementation and fortification of food staples with folate in the United States produced a 23% reduction in spina bifida in less than a decade. Approximately 90–95% of NTDs occur in women with no family history of the condition. MSAFP levels are elevated in most of the pregnancies affected by NTDs. Traditionally, women with elevated MSAFP were offered genetic amniocentesis with measurement of AFP and acetylcholinesterase in the amniotic fluid. Advances in ultrasound imaging have dramatically changed the evaluation of screen-positive gestations. AFP is a major protein in the fetal circulation with concentrations 106-fold higher in the fetal circulation compared to the maternal circulation and accumulates in the amniotic fluid with advancing gestation. AFP reaches the maternal serum by a combination of transplacental and transamniotic diffusion. The serum concentration of AFP is gestational age dependent and peaks in the third trimester; therefore, an accurate estimation of gestational age is essential for interpretation of the concentration. Concentrations are usually expressed as multiples of the median (MoM) of unaffected pregnancies at the specific gestational age and comparisons are then made using MoMs. Defects in fetal skin, including open NTDs and ventral wall abnormalities, cause increased amniotic fluid AFP concentrations by a direct transudative process. Fetal renal disease, intrauterine demise, and contamination of the amniotic fluid with fetal blood also result in elevated AFP concentrations. Although there is considerable overlap between affected and unaffected pregnancies, a cutoff of 2.5 MoM results in a detection rate of 95% for anencephaly and 80–85% for spina bifida with a 3% false-positive rate. Detection is enhanced when pregnancy dating criteria are based on a biparietal diameter measurement. Serum screening for NTDs using MSAFP is not typically effective until after 14 weeks of gestation and is typically performed at 15–20 weeks in the window typically utilized in screening programs for aneuploidy. Serum marker levels can be affected by multiple factors including maternal weight, ethnicity, and the presence of insulin-dependent diabetes. Multiple gestations pose a diagnostic dilemma in serum screening for NTDs. In general, serum biomarkers, including AFP, in twin pregnancies are approximately twice those of singletons. Detection rates of NTDs are significantly lower in twins than in singletons. Using a cutoff of 4.0 MoM provides detection rates around 90–95% for anencephaly and 65–80% for open spina bifida with a 7–10% false-positive rate. Ultrasound screening for major fetal anomalies in the second trimester has become a routine part of prenatal care in many countries. Anencephaly results from failed closure of the cephalic portion of the neural tube and disrupted cranial development resulting in exencephaly, the precursor of anencephaly. Ultrasound findings include decreased crown rump length (CRL), absent calvaria, extruding lobulated cerebral tissue (exencephaly) or absent neural tissue, and abnormal head shape with the eyes delineating the upper portion of the fetal face. Open spina bifida is detected by both cranial and spinal abnormalities on ultrasound. The two major cranial anomalies, the so-called ‘lemon’ and ‘banana’ signs, are recognized in more than 90% of fetuses with open NTDs. The lemon sign describes the flattening or concavity of the frontal bones of the skull visible in the transverse plane. The banana sign signifies the caudal displacement of the cerebellum with alignment of the cerebellar hemispheres as part of type II Arnold–Chiari malformation. Sonographic identification of the area of spinal dysraphism is often more difficult than detecting cranial abnormalities in spina bifida. Traditionally, women with elevated MSAFP levels were further evaluated by amniocentesis so that amniotic fluid levels of AFP and acetylcholinesterase were measured to confirm the diagnosis of a NTD. Elevated amniotic fluid acetylcholinesterase, defined as
Reference Module in Biomedical Research
Prenatal Screening for Neural Tube Defects and Aneuploidy
an amniotic fluid AFP concentration greater than 2.0 MoM for the gestational age, detects essentially 100% of anencephaly and spina bifida cases with a false-positive rate of less than 1%. Ultrasound technology has advanced significantly since the introduction of MSAFP screening and carries detection rates for NTDs approaching that of amniocentesis.
Prenatal Screening for Aneuploidy The association between advanced maternal age and an increased risk of Down syndrome was first described in the 1930s. In the early years of prenatal screening, maternal age primarily was used to select pregnancies at high risk for aneuploidy. Diagnostic testing, by amniocentesis or chorionic villus sampling (CVS), was offered to all women over a specific age cutoff, typically age 35; however, evidence has emerged that maternal age is not adequate to screen for aneuploidy. The American College of Obstetricians and Gynecologists (ACOG) now recommends offering prenatal screening for aneuploidy to women of all ages. ACOG further recommends that all women should have the option to undergo invasive testing with either CVS or amniocentesis, irrespective of screening results, for chromosomal abnormalities.
Maternal Serum Screening Precise estimation of gestational age is a prerequisite for valid screening with serum markers and these analyte concentrations are expressed as MoMs. A likelihood ratio for serum markers may be calculated by dividing the height of the affected pregnancy frequency distribution curve at a given MoM value by the height of the distribution curve of unaffected pregnancies at the same MoM. This likelihood ratio can then be multiplied by the age-specific risk to estimate the adjusted risk. In women carrying a fetus with trisomy 21, the concentrations of MSAFP tend to be 30% and unconjugated estriol (uE3) 20% lower than the median, while human chorionic gonadotropin (hCG) and inhibin-A (InhA) concentrations are twofold higher. The combination of uE3, MSAFP, hCG (the so-called ‘triple screen’), and maternal age results in a detection rate of greater than 75% with a false-positive rate of 3–5% and addition of InhA increases the detection rate by 15–20%. The three analytes – AFP, hCG, and uE3 – are decreased in pregnancies affected by trisomy 18, achieving a detection rate of 58–80% at a false-positive rate of just 0.3–0.6%. Similar to the second trimester, hCG levels are elevated in the first trimester in pregnancies with Down syndrome, while concentrations of pregnancy-associated plasma protein-A (PAPP-A) are 50–60% reduced. In combination with maternal age, hCG and PAPP-A have a detection rate of approximately 67%. In affected twin pregnancies, however, analyte levels are not consistent. The overall detection rate for aneuploidy in twin gestations is only 50–55% with secondtrimester serum screening. A number of factors, including ethnicity and diabetes, affect serum marker concentrations. hCG levels are higher in black women as compared to Caucasian women, but uE3 levels are similar among the two groups. Furthermore, AFP levels are also higher in black women; thus, race-specific medians have been developed for Caucasian and African–American women. AFP levels are significantly reduced in women with preexisting diabetes as compared to healthy controls. Additionally, uE3 and PAPP-A levels are slightly lower, and InhA levels are higher in diabetic women. Most programs make adjustments in the standard risk calculations in diabetic women.
Ultrasound Screening for Aneuploidy Mid trimester thickening of the nuchal fold was first described as sonographic marker of Down syndrome. Using a cutoff of 6 mm or more for the nuchal fold has been demonstrated to allow detection of approximately 40% of fetuses with trisomy 21. The ratio of the femur or humerus length to the biparietal diameter is used to detect the relatively shortened femurs. The combination of a thickened nuchal fold and relatively short femur or humerus has a sensitivity of 75% and specificity of 98% for Down syndrome. A number of other ultrasound markers have been described, including choroid plexus cysts, renal pelvic dilation, echogenic intracardiac foci, sandal toe deformity, and a number of others. Multiple scoring systems have been developed to quantify the risk of aneuploidy with a combination of markers. By integrating maternal age into the scoring system, further refinement of the method can be used to modify the a priori risk of aneuploidy. More than one sonographic marker is associated with over a fivefold increase in the risk of Down syndrome, whereas a normal ultrasound decreases the chance of Down syndrome by approximately one-half. The nuchal translucency (NT) is a term used to describe a collection of fluid behind the fetal neck recognized during the first trimester. Fetuses with trisomy 21, as well as other chromosome abnormalities, have measurements that are two- to threefold larger than that of unaffected fetuses. An increased NT has also been identified in a multitude of single gene disorders and structural malformations. The NT is measured between 11 and 13 6/7 weeks at a corresponding CRL of 45–84 mm. The NT measurement may be expressed as the delta NT (the difference in millimeters between the normal median for the CRL and the measured NT) or in MoM by dividing the measured NT by the normal median at the gestational age allowing for an age-adjusted risk assessment, similar to biochemical screening. Using an increased NT as the sole criteria for fetal aneuploidy screening detects approximately 70–75% of fetuses with trisomy 21, and when combined with the maternal age-associated a priori risk, a detection rate in excess of 80% is achievable. Measurements are independent of maternal serum levels of hCG and PAPP-A; therefore, these markers can be
Prenatal Screening for Neural Tube Defects and Aneuploidy
combined into a single screening test. The combination of free b-hCG or hCG, PAPP-A, and NT with maternal age is able to detect approximately 80–85% of fetuses with trisomy 21 at a false-positive rate of 5%. Other ultrasound markers can also be incorporated into the algorithm in the first trimester including tricuspid regurgitation and absence of the nasal bone. By combining the ultrasound with biochemical analysis, high detection rates with low false-positive rates can be achieved. The First- and Second-Trimester Evaluation of Risk (FASTER) trial compared combinations of screening strategies in nearly 40,000 women, including sequential (first- and second-trimester results reported independently) vs. integrated (first- and second-trimester results reported as single risk assessment) screening. These combined screening strategies that incorporate both first- and secondtrimester components achieve the highest detection rates, up to 96% if the NT is included, at a false-positive rate of 5% or less.
Further Reading Adzick NS, Thom EA, Spong CY, Brock JW 3rd, Burrows PK, Johnson MP, et al. (2011) A randomized trial of prenatal versus postnatal repair of myelomeningocele. The New England Journal of Medicine 364(11): 993–1004. Cheschier N (2003) ACOG practice bulletin. Neural tube defects. Number 44, July 2003 (replaces committee opinion number 252, March 2001). International Journal of Gynaecology and Obstetrics 83(1): 123–133. Cicero S, Avgidou K, Rembouskos G, Kagan KO, and Nicolaides KH (2006) Nasal bone in first-trimester screening for trisomy 21. American Journal of Obstetrics and Gynecology 195 (1): 109–114. Ghi T, Pilu G, Falco P, Segata M, Carletti A, Cocchi G, et al. (2006) Prenatal diagnosis of open and closed spina bifida. Ultrasound in Obstetrics & Gynecology 28(7): 899–903. Lo YM, Tein MS, Lau TK, Haines CJ, Leung TN, Poon PM, et al. (1998) Quantitative analysis of fetal DNA in maternal plasma and serum: Implications for noninvasive prenatal diagnosis. The American Journal of Human Genetics 62(4): 768–775. Malone FD, Canick JA, Ball RH, Nyberg DA, Comstock CH, Bukowski R, et al. (2005) First-trimester or second-trimester screening, or both, for Down’s syndrome. The New England Journal of Medicine 353(19): 2001–2011. Nicolaides KH (2004) Nuchal translucency and other first-trimester sonographic markers of chromosomal abnormalities. American Journal of Obstetrics and Gynecology 191(1): 45–67. Nicolaides KH (2011) Screening for fetal aneuploidies at 11 to 13 Weeks. Prenatal Diagnosis 31(1): 7–15. Spencer K (2007) Aneuploidy screening in the first trimester. American Journal of Medical Genetics Part C: Seminars in Medical Genetics 145C(1): 18–32. Wald NJ, Kennard A, and Hackshaw AK (1995) First trimester serum screening for Down’s syndrome. Prenatal Diagnosis 15(13): 1227–1240. Wald NJ, Kennard A, Hackshaw A, and McGuire A (1998) Antenatal screening for Down’s syndrome. Health Technology Assessment 2(1): 1–112, i–iv.