Mentally retarded individuals tend to develop additional mental illness diagnoses at three to four times the rate of the normal population. When more than one disorder is diagnosed at a time, the set of diagnoses are known as co-morbid disorders.
Mentally retarded individuals are prone to Attention Deficit Hyperactivity Disorder, or ADHD. While ADHD is diagnosed in 8 -15 percent of the general population of children, between 17 and 52 percent of adults with mental retardation may qualify for the diagnosis. The range provided here is large because studies disagree on the prevalence of ADHD within the mentally retarded population. ADHD is difficult to detect amidst the other prominent symptoms of mental retardation.
ADHD has behavioral characteristics that include inattention, impulsivity, and fidgeting. Individuals with ADHD have difficulty with organization, often lose things, and struggle to complete tasks, especially those that require concentration and sustained effort.
Individuals with mental retardation are prone to mood disorders, including major depression. The incidence of co-morbid depression in the mentally retarded population is difficult to determine with precision, as mentally retarded individuals have difficulty communicating their moods during evaluation. Depression is possible for mentally retarded individuals starting at any age of life. Children may become depressed upon realizing that they are different from their peers and unable to keep up. Depression may manifest itself in adolescents and adults through disturbances in sleep and eating routines, social withdrawal, and anxiousness. Bipolar disorder, another mood disorder, is between two and three times more frequent in the mentally retarded population than the regular population.
Mentally retarded individuals and individuals with Pervasive Development Disorders alike often develop Stereotypic Movement Disorders, which involve unnecessary repetitive movements that interfere with everyday functioning. The movements may take on a self-injurious form and restraints may be required at times to keep affected individuals from hurting themselves.
Dementia typically refers to the gradual degeneration of mental function that interferes with everyday functioning. Memory loss is significant and affected individuals often misplace objects, lose track of conversations, get lost while driving, or have difficulty completing tasks. Dementia may be a result of a medical condition or head trauma. Typically, individuals who have mental retardation do not have a heightened incidence of dementia later in life; however, Alzheimer's disease is common among middle-aged individuals with Down syndrome. Alzheimer's disease occurs with greater frequency and at a far earlier age than normal within the Down population. Down patients who develop Alzheimer's disease typically pass away within ten years of their diagnosis. Traumatic brain injury, such as may occur due to a fall or car accident, can lead to a diagnosis of mental retardation if the injury occurs prior to the age of eighteen or to a diagnosis of dementia if it occurs after age eighteen.
Schizophrenia is estimated to be present in about 3 percent of individuals with metal retardation, most of whom go undiagnosed. This is in comparison to a rate of 0.8 percent within the general population.
