The New England Journal of Medicine. An estimated 12 of every 1,000 U.S. school children have mental retardation. Nearly twice as many males as females are affected. Mental retardation is a consequence of a disorder affecting the central nervous system, as well as a symptom of other disorders. Mental retardation is defined as below-average intellectual functioning with an impaired ability to adapt to the environment.
Mental retardation is characterized by a delayed pattern of development.
Affected children have problems with language and communication when compared to normal children of the same age. Children with mild to moderate retardation are identified later in childhood when delays in language and play become apparent. The normal brain development of a fetus or child requires a precise sequence of neurological developmental events which are proliferative, migratory, and maturational in nature. Malnutrition - the most common worldwide cause. Infection, such as congenital toxoplasmosis, congenital rubella, meningitis, and congenital cytomegalovirus encephalitis.