In this article we will discuss about the diagnosis of genetic diseases.
The most common and most successful test methods for prenatal diagnosis is the examination of amnion cells. Amniotic fluid is extracted by way of a suprapubic amniocentesis. This is done by the penetration of the amnion sac above the symphysis with a hollow needle. The process is almost painless.
The timing of the test is usually determined by the necessity; after all, the earlier the test, the earlier the condition of the embryo will be known. Some geneticists suggest that the twelfth and twentieth week of pregnancy is the best time. But some others are agreed that the period between the 14th and 16th week is the best time.
The amount of fluid which is to be extracted from the amnion generally lies between 5 ml. to 20 ml. Although, the amount of fluid extraction is actually dependent upon the availability of the amniotic fluid and the stage of pregnancy. The test involves no danger for the prospective mother and the risk of inducing a miscarriage seems to be less than 1%.
After extraction of the fluid, it is centrifuged and the precipitated cells are cultured. The culturing of cells takes time, sometimes up to a few weeks. Apart from the testing for chromosomal anomalies by using the cultured amniotic cells, supernatant amniotic fluid is biochemically assayed to determine the biochemical defect of the embryo, e.g., the determination of fetoprotein (specially α1 – fetoprotein) level in amniotic fluid. We are able to diagnose anencephaly and spina bifida. α1 – fetoprotein usually present in small traces in the amniotic fluid if the foetus is normal, but it shows a massive increase if the foetus is defected by anencephaly or spina bifida.
The amniocentesis allows us to diagnose different defects of the foetus prenatally which are originated due to chromosomal abnormalities, neural tube defect and biochemical disorders. By the use of amniocentesis we can also determine the sex of the foetus. The brief schematic representation of amniocentesis is as follows (Fig. 19.1).
During recent years sampling the amniotic fluid by amniocentesis is done in conjunction with solography for prenatal diagnosis of the genetic diseases. In some cases foetal blood sample is also taken to determine the diagnosis of certain diseases like thalassemia.
Remark:
Before going ahead with the test mentioned above, patients must be informed about the possibilities and the dangers of the test methods. They should know that the first test may be unsuccessful and amniocentesis may have to be repeated.
It must also be made clear that a negative test result does not guarantee a healthy child except for the one specific defect that has been excluded. Again, it is important to recognise that, in the presence of unsuspected twins, incomplete or misleading results may be obtained.
The following table gives some information regarding the genetic diseases for which prenatal diagnosis has already been successfully carried out:
A. Disorders of Lipid Metabolism:
1. Fabry disease
2. Gaucher disease
3. Gangliosidosis (generalized type and juvenile type)
4. Krabbe disease
5. Refsuim disease
6. Wolman disease
7. Hyperlipoproteinemia
B. Disorders of Carbohydrate Metabolism:
1. Galactosemia
2. Glycogen storage disease
3. G-6-P-D deficiency
4. Fucosidosis
C. Amino Acid and Related Disorders:
1. Cystinosis
2. Congenital Hyperammonemia
3. Histidinemia
4. Hurler’s disease
5. Hunter’s disease
6. I-cell disease
7. Adrenogenital syndrome
8. Lesch Nyhan syndrome
9. Xeroderma pigmentosum
10. Sickle cell anemia
11. Myotonic muscular dystrophy
12. Thalassemia
13. Anencephaly, spina bifida.