There are five types of mental retardation caused by genetic mutations or abnormalities before birth, including Down syndrome, cranial abnormalities, cretinism, phenylketonuria (PKU), and nonsyndromic mental retardation. Down syndrome is caused by an extra chromosome and has physical characteristics such as a flat face and slanted eyes. Early intervention can help people with Down syndrome live more productive lives. Other types of mental retardation can be caused by head size at birth, thyroid disorders, or a lack of a critical enzyme. Nonsyndromic mental retardation is the most common and not genetic.
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 retardation as measured by IQ tests. Often, people with mental retardation also experience developmental delays, which affect their ability to perform basic life skills.
Down syndrome is 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 a nose that appears too wide. The eyes are typically slanted and the tongue looks larger than normal. Other physical characteristics include a short neck and hands with stumpy fingers. The little fingers of both hands may be curved.
There is no cure for Down syndrome, but early intervention could 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 of up to 55 years. The only known risk associated with this genetic disorder is the age of the mother when she becomes pregnant. Women over the age of 45 are more likely to bear a child with Down syndrome.
Some types of mental retardation result from the size of the head at or shortly after birth. Babies born with macrocephaly typically develop abnormally large heads and brains. Microcephaly means that the brain and head do not grow normally and could be caused by an infection or a woman’s exposure to radiation during pregnancy. Babies born with microcephaly typically develop cone-shaped heads, along with a receding forehead and chin. Hydrocephalus describes a rare condition when too much cerebrospinal fluid builds up in the brain.
Cretinism is one of the types of mental retardation linked to a thyroid disorder. It could lead to endocrine deficiency if the fetus does not absorb enough iodine. Excessive birth bleeding and certain diseases can also lead to cretinism, including measles, diphtheria, and whooping cough.
PKU occurs in a baby born without a critical enzyme that breaks down proteins. Without this chemical, amino acids build up in the blood, which could lead to brain damage. Symptoms typically appear between six and 12 months after birth as mental retardation. The disorder results from a malfunction of the recessive gene inherited from both parents. A PKU test soon after birth can identify the disorder, which can be treated through diet to reduce 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 malfunctions. Usually, there are no physical signs of this disorder, which is not genetic. The mutation typically occurs in a child’s brain as it develops.
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