When a child is mentally challenged, delays in normal development are experienced. The level of impairment can be mild, with the child having an IQ between 52 and 69. In more severe cases, the IQ can be as low as 20 to 35, according to the "Merck Manual Home Edition." If a child has profound mental disability, in which the IQ is 19 or below, medical attention is required. The National Institutes of Health (NIH) notes that 1 to 3 percent of the population is mentally challenged, though the cause is only known in 25 percent of cases.
Downs Syndrome
The World Health Organization (WHO) states that Downs syndrome is the most common cause of developmental delay, affecting 1 in every 800 newborns. Downs syndrome is not an inherited disorder, but an error in genes that can be traced back to conception. Normally, a child receives 23 chromosomes from each parent, totaling 46; a patient with Downs syndrome has an extra chromosome---47. That extra chromosome interferes in normal brain development, causing mental problems. A child with Downs syndrome will also have up-slanting eyes and a nose with a flat bridge. Other chromosomal disorders, such as cri du chat syndrome and fragile X syndrome, can also result in the child being mentally challenged.
Metabolic Disorders
Abnormal genes that affect metabolism can result in delayed brain growth. The NIH notes that many of these metabolic disorders are inherited, such as tuberous sclerosis, adrenoleukodystrophy, Tay-Sachs disease, galactosemia, Sanfilippo syndrome, Hunter syndrome, Rett syndrome, Hurler syndrome, Lesch-Nyhan syndrome and phenylketonuria. The WHO adds that phenylketonuria can be treated with a special diet soon after birth; treating phenylketonuria early can prevent brain damage.
Brain Infections
Acquiring a severe brain infection during infancy or childhood can result in mental problems. Examples include meningitis, listenosis, HIV infection and encephalitis. Intracranial hemorrhage, severe head injury or a lack of oxygen soon after birth can also result in brain damage that leaves the child mentally challenged. The NIH notes that infections that cause developmental delays can also occur while the child is still in utero. These infections include congenital cytomegalovirus (CMV), congenital toxoplasmosis and congenital rubella.
