The types of mental retardation usually fall into five categories caused by genetic mutations or abnormalities that develop before birth. They include cranial abnormalities, cretinism, phenylketonuria (PKU), Down syndrome, and nonsydromic mental retardation. All types of mental retardation typically cause mental delays measured by intelligence quotient tests. Often, people with mental retardation also suffer developmental delays, affecting their ability to perform basic life skills.
Down syndrome represents one of the types of mental retardation caused by an extra chromosome at birth. Sometimes called mongoloids, children born with this defect usually develop a flat face and nose that appear too broad. The eyes typically are slanted and the tongue seems larger than normal. Other physical characteristics include a short neck and hands with stubby fingers. The little fingers on both hands might be curved.
There is no cure for Down syndrome, but early intervention might help people with the condition live more productive lives and cope with disabilities. Many patients with the disorder live partially independent lives as adults, with a life expectancy up to 55 years. The only known risk associated for this genetic disorder is the age of the mother when she becomes pregnant. Women over 45 years old face a greater chance of delivering a child with Down syndrome.
Some types of mental retardation stem from head size at birth or shortly after. Children born with macrocephaly typically develop abnormally large heads and brains. Microcephaly means the brain and head fail to grow normally, and might be caused by infection or a woman’s exposure to radiation during pregnancy. Babies born with microcephaly typically develop cone-shaped heads, along with receding foreheads and chins. Hydrocephaly describes a rare condition when too much cerebrospinal fluid accumulates in the brain.
Cretinism is one of the types of mental retardation linked to a thyroid disorder. It might lead to an endocrine deficiency if sufficient iodine is not absorbed by a fetus. Excessive bleeding at birth, and some diseases, might also lead to cretinism, including measles, diphtheria, and whooping cough.
PKU occurs in a child born without a critical enzyme that breaks down protein. Without this chemical, amino acids build up in the blood, which might lead to brain damage. Symptoms typically appear between six and 12 months after birth as mental retardation. The disorders stems from a recessive gene malfunction inherited from both parents. A PKU test immediately after birth can identify the disorder, which can be treated through diet to lessen the severity of brain damage.
Nonsyndromic mental retardation represents the most common of all types of mental retardation. It occurs when a gene responsible for memory and learning fails to function normally. Usually, there are no physical signs of this disorder, which is not genetic. The mutation typically occurs in the brain of a child as he or she develops.