> Mental Retardation/Developmental Delay

• Alternative Names
• Symptoms & Characteristics
• Treatment
• How Common Is It?
• Genetics & Inheritance
• Genetic Testing
• Support & More Information

Alternative Names

MR; DD

Symptoms & Characteristics

Mental retardation (MR) is defined as an intelligence quotient (IQ) score below 70 on a standardized IQ test. MR includes below-average general intellectual function and a lack of skills necessary for daily living. MR is typically diagnosed in a person before age 18 years.

Developmental delay (DD) is defined as a young child who does not achieve developmental milestones (such as grasping objects, crawling, rolling over, walking and talking) at the expected age(s). In some cases of DD, these children eventually develop normally over time. In other cases of DD, these children will eventually be diagnosed as having MR. 

Treatment

Treatment availability is dependent upon the cause of mental retardation/developmental delay.

How Common Is It?

The overall incidence of MR in the general population is 1-3%.

The overall incidence of DD is most likely higher than that of MR. However, an exact incidence is not known.

Genetics & Inheritance

Mental retardation and/or developmental delay can be acquired, multifactorial (caused by a combination of genetic and environmental factors) or caused by a inherited genetic condition. Sometimes, a cause cannot be identified. In fact, although there are many causes of MR and DD, a specific cause is only identified in approximately 25% of cases. Some causes of MR and DD include:

Acquired/Environmental/Multifactorial Causes

• Prenatal infections (such as congenital toxoplasmosis, congenital cytomegalovirus and congenital rubella)

• Prenatal teratogenic exposures (such as intrauterine exposure to alcohol and drugs)

• Infection (such as encephalitis and meningitis)

• Lead poisoning

• Injury/Trauma - before and after birth (such as delivery or surgical complications, severe head injury)

• Untreated maternal conditions during pregnancy (such as PKU)

Genetic Conditions (Inherited)

There are hundreds of genetic conditions that are associated with MR and/or DD. In these conditions, there are often additional, characteristic mental and/or physical abnormalities. Some of the more common genetic conditions associated with MR and/or DD include:

• Fragile X syndrome

• Chromosome abnormality (such as Down syndrome, Trisomy 18, Trisomy 13, Markers and Rings)

• Rett syndrome

• Angelman syndrome

• Prader-Willi syndrome

• Williams syndrome

• Smith-Magenis syndrome

• Smith-Lemli-Optiz syndrome

• Sotos syndrome

• Deletion 22q11.2 syndrome

• Many metabolic conditions

An evaluation by a medical geneticist may be helpful in establishing cause. This is not only important for the health and management of the affected person but also for an accurate estimation of recurrence risk within a family.

Genetic Testing

Clinical genetic testing for some of the genetic conditions associated with MR and/or DD may be available through an in person genetic consultation for people who are considered at risk. Testing options are based on an individual's clinical symptoms, but usually include (at minimum), a chromosome analysis (for the detection of a chromosome abnormality) and Fragile X syndrome. It is always recommended to see a genetic professional for a complete evaluation, accurate diagnosis and discussion of the benefits and limitations of testing and recurrence risk. Use our find a genetic professional directory to locate a trained genetic professional in your area.

• A medical geneticist can also be found at the American College of Medical Genetics website.

• A genetic counselor can also be found at the National Society of Genetic Counselors website.

Support & More Information

More information can be found at:

• MedlinePlus

Support can be found at:

• The Council on Mental Retardation, Inc.
• American Association on Intellectual and Developmental Disabilities