المدينة للإستنساخ
OBSTETRICSPrenatal diagnosis of congenital and genetic diseases
Lecture د.نجمة
Prenatal screening and diagnostic tests
Screening tests: are tests performed for all women in order to identify those who at high risk for a disorder, they don’t confer any risk to the pregnancy & are performed for disorders with a relatively high prevalence and for which they are accurate prenatal diagnostic tests.
Diagnostic test: are test carried out on pregnancies that are has been identified at high risk by a prior screening test to detect abnormality of the fetus, they are usually invasive and a there is a small risk for miscarriage, there for the risk of being affected by the condition should be severe enough to warrant consideration for diagnostic tests, they are either:
Invasive diagnostic tests are: amniocentesis, chorionic villous sampling, and cordocentesis.
Non invasive: ultrasound.
Difference between prenatal screening tests and diagnostic test
Parameter
Screening tests
Diagnostic tests
Population tested
All women
Women at high risk
Purpose of testing
Select high risk group
To diagnose abnormality
Usual method of testing
Maternal History
Maternal biochemistry
Maternal virology
Ultrasound
Amniocentesis
Cordocentesis
Chorionic villus sampling
Prerequisite to test
Diagnostic test available.
Patient aware of potential risks.
Risk of test
Anxiety of screen positive result
Risk of abortion from invasive test.
Screening test:
Maternal history: which include:
Age.
Medical history.
Recent drug or alcohol use.
Exposure to radiation (X-ray).
Positive family history of specific genetic disorder (Down ’s syndrome, anencephaly), some genetic disorder in some ethnic groups like β-thalasemia (in Mediterranean basin) and hemophilia, sickle cell anaemia (in Africo-american, African, Africo-caribbean and Saudi Arabia), cystic fibrosis (in Ashkenazi Jews).
Ultrasound: performed in the 1st trimester at 11-14 weeks of gestation to detect nuchal translucency; if more than 2.5 mm indicates chromosomal anomaly (as Down’s syndrome), and also in the 2nd trimester at 18-22 weeks major anomaly scan for detection of major congenital abnormality as; neural tube defect (NTD), cardiovascular and skeletal abnormality, and in the 3rd trimester urinary tract abnormality as urethral valve.
Maternal Biochemistry and blood tests:
Maternal serum: α-fetoprotein, HCG, PCR, fluorescent in situ hyperdization.
Invasive diagnostic tests
Amniocentesis: a thin needle is passed transabdominaly under ultrasound guidance into the amniotic cavity, a small amniotic fluid is removed which contain fetal fibroblast, this test is usually performed at or after 15 week of gestation, its procedure’s related miscarriage rate is 1%, although technically possible to perform amniocentesis at earlier gestational age, this is generally avoided as it is associated with higher cell culture failure rate and high rate of miscarriage, neonatal and respiratory difficulties.
Complications:
Miscarriage.
Direct trauma.
Respiratory difficulties from oligohydromnios.
Difficult to perform in:
Anterior situated placenta.
Obese.
Oligohydromnios.
Multiple pregnancies.
Chorionic villus sampling:
A thin needle is passed transabdominaly or transcervically under ultrasound guidance into the placenta chorionic villi which are fetoplacental in origin, are either aspirated or biopsied through this needle, this test is usually performed at or after 10 weeks of gestation.
The risk of miscarriage is 1%.
Karyotype result within 1-2 days.
Culture results within 1-2 weeks.
Cordocentesis:
A thin needle is passed transabdominaly under ultrasound guidance into the umbilical cord to sample fetal blood.
This test is usually performed at or after 20 weeks gestation, its procedure related miscarriage rate 1% although it is technically possible to do this at earlier gestational age, this is generally avoided as it is associated with higher rate of miscarriage, the result within 1-2 days.
Laboratory analysis
Cytological analysis
Cells obtained from invasive prenatal diagnostic tests are cultured until enough cells in mitosis are available to more a cytogenetic diagnosis.
The more rapidly the tissue divide, the quicker the results are obtained or available, thence the time for diagnosis for amniocentesis, chorionic villus sampling and cordocentesis are 2-3 weeks, 1-2 weeks & 23-48 hours respectively.
DNA analysis:
Fetal DNA obtained from invasive tests can be used for DNA probe (sickle cell disease & cystic fibrosis), PCR (fragile X-syndrome, congenital toxoplasmosis and cytomegalovirus) or linkage analysis (fragile X-syndrome).
Biochemical and enzymatic analysis:
When DNA analysis is not possible, biochemical or enzymatic assays can be performed for specific diseases (congenital adrenal hyperplasia and mucopolysaccharidases).
Diagnosis of structural abnormality
Structural abnormalities constitute the majority if congenital abnormalities encountered in clinical practice, there are established screening programs for fetal neural tube defect and cardiac defects, these are discussed in detailed in this lecture, and other structural malformations occur less commonly and are sporadic in nature.
Neural tube defects (NTD)
Occur due to defect in the formation of neural tube during embryogenesis, the etiology is multifactorial:
Environmental.
Genetic.
Pharmacological.
Geographical factor implicated.
Recurrence rate is 5-10% when a parent or previous sibling has had NTD.
Prenatal screening and diagnosis of NTD:
Mid trimester maternal serum α-fetoprotein levels are increase in pregnancies affected by open NTDs, these were once used as the established screening test for NTDs, and with screening positive women being referred amniocentesis.
The presence of acetylcholine esterase (a CNS neurotransmitter) in amniotic fluid was taken as being diagnostic of open NTDs, the need for 2 step screening diagnostic process was quickly superseded by the development of high resolution ultrasound.
Anencephaly and encephalocele are detectable on first trimester ultrasound if an adequate examination of the cranial vault is performed, spina bifida requires the systemic detailed examination of the fetal spine at the routine 20 weeks anomaly scan, the diagnosis may be suspected from visualization of the lemon shape of the skull and banana (absent cerebellum) sings in the fetal brain at this exam.
The sensitivity of the ultrasound for both open and closed NTDs is greater than 95%.
The structural abnormality of the CNS such as hydrocephaly can also be detected at the 20 weeks scan.
Prevention of NTDs:
Folate deficiency and drugs that interfere with folate metabolism (i.e. antiepileptic drugs) are implicated in about 10% of NTDs cases, periconceptional folate supplementation of the maternal diet reduces the risk of developing defect by one half, folic acid should be given for at least 3 months prior to conception and for the 1st trimester of pregnancy, the dosage of folic acid is 4 mg for women with history of NTDs, and 400 µg for primary prevention.
Congenital heart defect:
50% are either lethal or require major surgery and the remaining are asymptomatic, the etiology is heterogeneous and include genetic abnormalities, environmental factors and viral infection, gene mutation and chromosomal abnormalities are account for less than 5% of cases.
DM.
Viral infection.
Drugs (lithium).
Down’s syndrome.
Prenatal screening tests for CHD:
When a previous sibling or gather with CHD, the risk is 2%, when 2 sibling or the mother have CHD, the risk f recurrence is 10%, the 2nd major risk group are offspring for women with type l DM (the incidence is doubled).
Prenatal diagnosis:
Although 90% of major CHD is detected in specialist centers (antenatally), in most units performing at 20 week anomaly scan this figure is closed to 30% as specialist fetal echo can’t be performed in all pregnancies, the limiting factor in diagnosis is selection of cases for referral to these specialist units.
VSD, ASD, AV canal is detected by Doppler ultrasound.
Chromosomal abnormalities:
Aneuploidy, trisomies occur in majority of cases due to non-disjunction in meiosis.
This abnormality of gametogenesis is known to occur more frequently with advancing maternal age, rarely trisomies may occur due to unbalanced translocation (6%) or mosaisim (4%).
Although any chromosomal may be affected, the majority of trisomies result in 1st trimester miscarriage except for trisomies 13 (Patau’s), 18 (Edward’s syndrome) and 21 (Down’s syndrome), Down’s syndrome is associated with characteristically mental and physical feature that their diagnosis is usually suspected on antenatal ultrasound.
Since trisomies 13.18 have a very high intrauterine lethality 90-95%. Screening programs are directed mainly towards the antenatal detection of Down’s syndrome, which is the commonest chromosomal abnormality at birth.
In Down’s syndrome intrauterine lethality may reaches 40% at 12-14 weeks.
Clinical features of Down’s syndrome:
Mental retardation.
Macroglossia.
Intestinal atresia.
Premature aging.
Decrease immunity (leukemia).
Decrease life span.
Deafness.
Flat facies.
Cardiac septal defect.
Short sightedness.
Screening for Down’s syndrome:
History: increase with maternal age more than 35 years and with previous Down’s syndrome.
False positive result 5-10%, poor sensitivity 25%, but it is simple test.
Maternal serum biochemistry at 15-22 weeks
False positive results 5%, sensitivity (60-75%), cheap, operator independent.
Measures hormones:
α-fetoprotein decrease.
HCG increase.
Estriol decrease.
Serum inhibin give 76% sensitivity.
a,b,c are called triple test and give sensitivity 75%, after introduction of d the sensitivity rise to 76%.
Nuchal translucency (10-14 week)
Ultrasound scan measures translucent, the back of neck with 5% false positive result.
75-80% sensitivity, operator dependant, in early results surgical evacuation can be used.
Second trimester ultrasound scan for Down’s syndrome:
Double bubble sign.
Dilated renal pelvis.
Sex chromosome abnormality
Turner’s syndrome (X0)
Klinefilter syndrome (XXY) 47, male, tall stature, gynecomastia.
Fragile X-syndrome detected by PCR
Unlike trisomies, the prevalence of sex chromosome abnormality doesn’t change with maternal age, the cumulative prevalence of turner’s, klinefelter’s and other sex chromosomal abnormalities is greater than that of Down’s syndrome, Turner syndrome individuals are infertile female of normal IQ and short stature.
Klinefelter’s syndrome individuals are infertile male with slightly reduced IQ, testicular dysgenesis and tall stature, as many of the characteristics of these conditions are mild, many affected individuals remain undiagnosed throughout their lifetime, routine screening for these conditions is not available and often the diagnosis made incidentally.
Fragile X-syndrome is the most common inherited cause of mental retardation, explaining the excess of males affected by non specific mental retardation in population.
The fragile X-gene (FMR1) becomes hypermethylated with inactivated multiple repeat (more than 200).
The estimated prevalence of the condition 1:4000 males, prenatal diagnosis is possible by using PCR & southern analysis, but only for male fetuses at present.
As screening of fragile X-syndrome is not feasible, prenatal testing is reserved for families in which one of parents is known to be carrier by virtue of a pervious affected pregnancy.
Genetic disorders:
Cystic fibrosis:Is an autosomal lethal genetic disease, recessive condition is Caucasians, the cystic fibrosis gene has been isolated from long arm of the chromosome 7 and there are more than 700 mutations identified to this region that are responsible for the disease.
The commonest of these mutations is tri F508 which is present 68% of cases, multiple gene mutations and the cast of DNA testing for population are the major reasons why prenatal screening has not been effective to date.
At present prenatal diagnosis is offered only to parents who are known carriers, usually because they already have affected child.
Haemoglobinopathies:
Sickle cell anaemia and thalasemia are both autosomal recessive conditions with considerable disease heterogenicity.
The carrier frequency may be as high as 20%, especially in African (sickle cell disease) and Mediterranean (thalasemia).
Screening of at risk population is possible by Hb electrophoresis.
Sickle cell mutation are limited in number and fairly well characterized, hence prenatal diagnosis is usually possible.
As there are numerous thalasemia mutations, prenatal studies are prerequisite to established whether a fetal diagnosis is possible, prenatal diagnosis is made on fetal DNA, which can be obtained by any of the invasive techniques.
As the risk of an affected pregnancy is high 25, or parent who are carrier, early testing through chorionic villus sampling is advocated.
Congenital and parasitic infection:
Fetal infection with rubella, paravirus, CMV and toxoplasmosis is known to have potentially serious deleterious effect.
Maternal viral infection in pregnancy os relatively infrequent, with the likelihood of trasnplacental transfer and fetal infection increasing with gestational age, however 95% of fetuses remain unaffected.
The risk of congenitally infected fetuses being affected is inversely proportional to the gestational age, hence although the chance of fetal infection is low in early pregnancy, if infected, the fetus is likely to be seriously affected and the pregnancy os doomed to miscarriage.
Therefore the most susceptible pregnancies are those infected at 12-18 weeks gestation, when infected fetuses are likely to be seriously affected and yet survive.
Screening for congenital viral and parasitic infection
Rubella (screening program for rubella is established), rubella susceptible women are advised:
To avoid antenatal exposure.
Vaccinated in the puerperium.
CMV & toxoplasmosis the screening is not advocated, because of the low incidence, high false positive results rates and the risk of miscarriage consequent on the invasive prenatal diagnostics test necessary to confirm fetal infection doesn’t necessarily indicate that the fetus has been affected.
Prenatal diagnosis
In case of confirmed maternal viral infection, regular fetal ultrasound to detect the characteristic features of congenital infection us advocated.
There is limited evidence that the treatment of toxoplasmosis infected mother with spiromycin may prevent congenital fetal infection.
Congenital paravirus infection may result in temporary fetal aplastic anemia and hydrops, supportive therapy with fetal intra uterine blood transfusion in these cases dramatically improve the prognosis.
Characteristics of congenital viral and parasitic infection
Item
Rubella
CMV
Toxoplasma
Para virus
Source
Infected persons
Infected person
Cat letter, undercooked meat
Infected children
Feature of congenital infection
Cataract
Heart defect
Growth restriction
Hepatomegaly
Thrombocytopenia
Mental retardation
Microcephaly
Ventriculomegaly
Cerebral calcification
Heart defect
Growth restriction
Hepatomegaly
Thrombocytopenia
Mental retardation
Microcepahly
Ventriculomegaly
Cerebral calcification
Heart defect
Growth restriction
Hepatomegaly
Thrombocytopenia
Mental retardation
Aplastic anemia
hydrops
New developments:
Fetal cells in maternal circulation.
Pre-implantation genetic diagnosis.
Three dimensional ultrasound.
Fetal magnetic resonance imaging.