Intellectual disability, formerly called mental retardation, involves impairments of general mental abilities that impact intellectual functioning (also known as IQ) and adaptive skills, including: communication, social and self-care skills. It can happen any time before the age of 18 years, even before birth. Intellectual disability is the most common developmental disability; approximately 1-3 percent of the worldwide population is affected by it. Symptoms may include lack of or slow development of motor skills, language skills, and self-help skills, failure to grow intellectually or continued infant-like behavior, lack of curiosity, inability to connect actions with consequences, difficulty with problem-solving and following social rules. Patients affected with MRT54 have been shown to have normal gross brain structure, but impaired cognition as well as hyperactivity.
Diagnosis of ID is based on abnormal Denver developmental screening test, development way below that of peers, adaptive behavior score below average, and intelligence quotient (IQ) score (between 70 and 55 or lower) on a standardized IQ test. About 25-50% of ID cases are due to germline defects in single genes. However, close to 60 different loci have been identified in such single gene disorders affecting ID.