Persons with intellectual disabilities (ID, formerly mentally retardation) often have other psychiatric diagnoses. When more than one disorder is diagnosed at the same time, they are called co-morbid or co-occurring disorders.
Recall that there are many causes of intellectual disabilities. One set of causes are certain psychiatric conditions. The most common psychiatric condition associated with ID is the autism spectrum disorder (ASD). There are two primary symptoms associated with ASD. The first is severe and persistent communication problems. The second is restrictive or repetitive behavior. These unnecessary, repetitive behaviors often interfere with everyday life. These behaviors may involve self-injurious movements. Sometimes restraints and/or protective gear are necessary to prevent injuries.
Another psychiatric condition that commonly co-occurs with ID is Attention Deficit Hyperactivity Disorder, (ADHD). ADHD symptoms include inattention, impulsivity, and fidgeting. Individuals with ADHD have difficulty with organization. They often lose things and struggle to complete tasks. This is especially true with tasks requiring concentration and sustained effort. ADHD is diagnosed in 8% to15% of children with ID. However, 17% to 52% of adults with ID may qualify for ADHA diagnosis. This range is quite large because studies disagree on the prevalence of co-morbid ADHD and ID. ADHD symptoms are sometime difficult to distinguish from ID.
Individuals with intellectual disabilities are also prone to mood disorders. One mood disorder is major depression. It is difficult to determine the precise rate of co-morbidity. This is because many people with ID have difficulty communicating their moods. This makes diagnosis of mood disorders, such as depression, more difficult. Both adults and children with ID may experience depression. Children can get depressed when they realize they are different from their peers. For instance, they may notice they do not have same skills and abilities as their peers. Adolescents and adults with ID also get depressed. Disturbances in sleep and eating routines, social withdrawal, and anxiousness can indicate depression.
Bipolar disorder is another mood disorder. People with ID are two to three times more likely to be diagnosed with bipolar disorder than the general population.
A diagnostic criterion for ID is onset before age 18. If symptoms develop after age 18, the correct diagnosis is neurocognitive disorder (formerly dementia). However, a person with ID can receive both diagnoses if a further loss of functioning occurs after age 18. This may be due to a new brain injury (e.g., auto accident). Alternatively, a progressive brain disease or disorder may develop. Typically, people with ID are not at greater risk for neurocognitive disorders later in life. However, if the ID is caused by Down syndrome, early onset Alzheimer's disease is common. When people with Down syndrome develop Alzheimer's, they usually don't live more than 10 years after diagnosis.
About 3% of people with intellectual disabilities also have Schizophrenia. This is compared to 0.8% of the general population. This diagnosis is often missed.