Many cases of LD could be prevented by the following measures:

• improved antenatal and obstetric care, such as rubella immunization for girls, treatment of rhesus incompatibility, and special care of low-birthweight babies

• screening of neonates, to detect such metabolic conditions as phenylketonuria and hypothyroidism

• improved infant welfare, including immunization programmes, and early detection of mild LD and impaired sight and hearing

• genetic counselling, which has become more important with recent advances in molecular genetics. For parents who have already produced a child with a gene or chromosome abnormality, it may be possible to calculate a precise risk of any future children being affected. In the case of recessive conditions, the carrier state in relatives can be determined. Genetic counselling should preferably be carried out as a planned procedure, rather than delayed until a new pregnancy is already in progress

• prenatal diagnosis: whereby many congenital abnormalities can now be detected at an early stage of pregnancy, allowing the option of aborting an affected foetus should the mother choose this course.

Techniques include the following:

- ultrasound: to confirm gestational age and detect major structural abnormalities.

- serum alpha-fetoprotein (AFP): offered to pregnant women: raised levels suggest a neural tube defect such as anencephaly or myelomeningocele. Low levels can suggest Down's syndrome.

- amniocentesis: offered if AFP is abnormal. Amniotic fluid is sampled by the transabdominal route. Foetal chromosomes are examined, and specific genetic defects excluded by enzyme estimations or gene probes. Amniocentesis carries a small risk of causing miscarriage, infection, or damage to the child.

- chorionic villus biopsy (CVB): offered at 6-8 weeks for high-risk pregnancies only. Chromosome and gene abnormalities can be detected. Chorion sampling is carried out by the transvaginal or transabdominal route, and carries a small risk of causing miscarriage.

Before such tests are carried out, the mother (and father) should be informed about their purpose, their accuracy, the implications of both positive and negative results, and any risks attached to them.

Some important specific causes of LD will be described in the text. The box gives a fuller list, but is not complete; new genetic syndromes are continually being identified.

Box 19.1 Some Specific Causes of LD

Autosomal chromosome abnormalities: Down's syndrome*

Trisomy D (trisomy 13; Patau's syndrome) Trisomy E (trisomy 18; Edwards' syndrome) 'Cri du chat' syndrome Sex chromosome abnormalities: Males:

Fragile X syndrome (Martin-Bell syndrome)* XYY syndrome*

XXY syndrome (Klinefelter's syndrome)* Females:

XO syndrome (Turner's syndrome)* XXX syndrome*

Autosomal dominant gene abnormalities:

Tuberous sclerosis* Autosomal recessive gene abnormalities Phenylketonuria* Homocystinuria - aminoaciduria Hartnup disease - aminoaciduria Maple syrup urine disease - aminoaciduria Galactosaemia* Tay-Sachs disease*

Gaucher's syndrome - lipid storage disorder Niemann-Pick disease - lipid storage disorder Hand-Schueller-Christian disease - lipid storage disorder Hurler's syndrome (gargoylism) - mucopolysaccharide storage disorder

Laurence-Moon-Biedl syndrome* Congenital hypothyroidism* Sex-linked recessive gene abnormalities Lesch-Nyhan syndrome*

Hunter's syndrome - mucopolysaccharide storage disorder Prenatal infection Cytomegalovirus* Toxoplasmosis* Rubella* Syphilis* Birth complications Birth injury* Rhesus incompatibility* Post-natal damage

Encephalitis, meningitis HIV infection Severe head injury Lead poisoning* Miscellaneous Hydrocephalus* Microcephaly*

*See following text.

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