There are numerous possible causes of mental retardation. Identification of the precise cause of mental retardation occurs in about fifty percent of those individuals affected. Experts also suggest that more than one cause is to blame in as many as fifty percent of cases. Between thirty and forty percent of individuals with severe symptoms and about seventy-five percent with mild symptoms, no specific cause can be found. Possible causes include genetic disorders, prenatal factors, environmental influences, toxins, and trauma.
Chromosomal abnormalities are the most common known cause of mental retardation. Chromosomal abnormalities occur when there is an error in the number of chromosomes or in the placement of genes on the chromosomes as well as any other defect in the chromosomes. Gene abnormalities and inherited metabolic disorders can also lead to mental retardation. Scientists have linked more than five hundred genetic disorders to mental retardation. There are generally three types of genetic disorders: single gene, chromosomal, and multifactoral.
Single gene defects include Fragile X syndrome, also known as Martin-Bell syndrome. This results in an abnormality of the X chromosome. Fragile X syndrome affects about one in one thousand females and on in one thousand five hundred males. The best-known and most researched chromosomal abnormality is Down syndrome. In most cases, when people think of the mentally retarded, Down Syndrome is synonymous with this disability. Down syndrome occurs in approximately one in one thousand births. Usually, only two of ten fetuses with Down syndrome are born alive. Down syndrome accounts for five to six percent of all cases of mental retardations. Most individuals with Down syndrome fall into the moderate to severe range of mental retardation with an IQ less than fifty. Other chromosomal disorders leading to mental retardation included Rett syndrome, Prader-Willi syndrome, Angelman syndrome, Cri du Chat syndrome, and Cornelia de Lange syndrome.
Metabolic disorders involve defective genes that are incapable to produce certain enzymes or proteins that some cells require to function. Phenylketonuria, Galactosemia, and Tay-Sachs disease are just a few of the three hundred genetic metabolic disorders scientists identified that can lead to mental retardation. Non-inherited metabolic disorders such as Reye's syndrome, hypoglycemia, and hypernatremic dehydration can also lead to mental retardation.
Infections, such as rubella, meningitis, encephalitis, and HIV, as well as exposure to toxins that include lead or mercury, alcohol, cocaine, and other drugs can lead to mental retardation. When certain infectious diseases have complications, such as whooping cough, chicken pox, and measles, mental retardation is a possible outcome. Scientists report that ten to twenty percent of mental retardations cases with an IQ in the fifty to eighty ranges are a result of Fetal Alcohol Syndrome.
Malnutrition, poverty, and low socioeconomic status are all been linked to mental retardation. Lack of mental stimulation during infancy may also contribute to the development of mental retardation. Nutritional problems may lead to mental retardation. Some of these problems include general malnutrition, Marasmus, and Kwashiorkor.
Another category linked to mental retardation is brain injury. This includes hypoxia, cerebral hemorrhage, and other head injuries. Trauma or neglect is the causative factor in an estimated fifteen percent of children with mild mental retardation. Head trauma is the biggest factor in most of these cases. In children, adolescents, and teenagers, falls bicycle accidents, and motor vehicle accidents account for the majority of head injuries. Studies show that boys are twice as likely as girls to suffer significant head trauma, consequently are more likely to develop trauma-induced mental retardation.