There are many potential causes of mental retardation, though doctors are only able to pinpoint a specific cause in about 20 to 25 percent of cases. In general, the more severe the retardation, the better the chance the cause can be identified. In most cases where causes can be identified, genetic conditions, problems during pregnancy or problems at birth may be to blame. The causes of mental retardation can be broken down into a few categories.
Infection or disease present at or occurring after birth can cause mental retardation, including HIV, meningitis, listeriosis, encephalitis, congenital toxoplasmosis, congenital rubella and congenital CMV. Genetic conditions, such as chromosomal abnormalities or inherited metabolic disorders (e.g., chromosome deletions, translocations or defects, Hunter syndrome, Rett syndrome, adrenoleukodystrophy, galactosemia and Hurler syndrome), are linked to mental retardation, as are environmental or metabolic risk factors, such as deprivation syndrome, hypoglycemia and congenital hypothyroidism, are associated with mental retardation, as is childhood malnutrition. Toxins such as alcohol, cocaine, amphetamines and other drugs the mother consumes while pregnant, as well as lead poisoning and methylmercury poisoning, may lead to mental retardation. Trauma before or during the birth, including intracranial hemorrhage, a lack of oxygen to the brain and severe head injury, is also a potential cause of mental retardation.
Iodine deficiency is the single most preventable cause of mental retardation, as it effects on the developing brain can be profound; iodine deficiency can lead to hypothyroidism, a cause of mental retardation, and can impair brain function. An absence of the arcuate fasciculus, a neural pathway in the brain, has also been linked to mental retardation.