Traditionally, the American Medical Association has classified four progressively advanced stages of mental retardation, diagnosed through traditional intelligence quotient tests. According to the Encyclopedia of Mental Disorders, mild mental retardation encompasses about 85 percent of all cases and IQs range from 50 to 70. Moderate mental retardation accounts for about 10 percent of all cases, with IQs between 35 and 55. Severe mental retardation exists in 3 to 4 percent of all cases, who have IQs between 20 and 40. Only one to two percent of all cases are classified as profound mental retardation, with IQs under 20 to 25.
Mentally challenged kindergarten students struggle with basic problem-solving skills, including abstract thinking. This shows up in day-to-day situations requiring generalization between settings. These children tend to think more concretely. Social skills such as listening to others and taking turns also prove to be a challenge. They also struggle with cause and effect situations, such as the basic concept that actions result in consequences.
Doctors identify some medical causes for mentally challenged children, such as Down's Syndrome or other genetic disorders, even before birth. Other medical conditions may coexist with mental retardation, which may be diagnosed at different phases in the child's life. Additional examples include fetal alcohol syndrome, rubella, hydrocephalus, toxoplasmosis and birth defects during pregnancy. Childhood diseases such as bacterial infections, hyperthyroidism, measles, whooping cough and chicken pox can also cause mental retardation.
Visible physical indicators can be the first warning signs of mental challenges, even prior to kindergarten. Indicators at birth can include facial features, head shape and size and smaller physical size. Delayed development in reaching significant milestones such as sitting, crawling, speech delays and walking can be further causes for concern.