Mental Retardation 

Mental retardation (MR) or intellectual disability (ID) is a descriptive term for subaverage intelligence and impaired adaptive functioning arising in the developmental period (< 18 y). MR/ID and other neurodevelopmental disabilities are seen often in a general pediatric practice.

Terminology for MR/ID has been particularly challenging as the term mentally retarded carries significant social and emotional stigma. The American Association for Intellectual and Developmental Disability (AAIDD) has been particularly influential in terminology changes such that most professionals working in the field now refer to mental retardation as intellectual disability. The DSM-V is expected to adopt this new terminology.^[1]

Developmental delay is often used inappropriately as synonymous with MR/ID. Developmental delay is an overly inclusive term and should generally be used for infants and young children (< 5 y) in which the diagnosis is unclear, such as those too young for formal testing.^[1]

Approximately 10% of children have some learning impairment, while as many as 3% manifest some degree of MR/ID. The population prevalence of these combined disorders of learning rivals that of the common childhood disorder asthma.

MR/ID originates during the developmental period (ie, conception through age 18 years) and results in significantly subaverage general intellectual function with concurrent deficits in functional life skills. The diagnosis of MR/ID requires an intelligence deficit of at least 2 standard deviations (SDs) below the mean IQ. This generally translates into an intelligence quotient (IQ) score of 70-75, given a population mean of 100. Equivalent deficits in at least 2 areas of functional life skills or adaptive skills also must be present to meet the diagnostic criteria for MR/ID. Adaptive skills encompass functional life skills within the domains of communication, self-care, home living, social and interpersonal skills, use of community resources, self-direction, functional academic skills, work, leisure, health, and safety.

MR/ID is currently categorized broadly as follows.^[2]

Table 1. Intellectual disability categorization (Open Table in a new window)

The DSM-V is widely expected to change the definition of MR/ID, relying less on specific IQ levels.^[3] IQ scores can vary based on age, instrument, and practitioner. Furthermore, testing prior to school age does not correlate well with future performance. IQ scores can be measured using assessment instruments, such as the Stanford-Binet, Wechsler Adult Intelligence Scale (WAIS), or Wechsler Intelligence Scale for Children (WISC-IV). Nonverbal children can be tested with the Leiter International Performance Scale (Leiter-R).

The second component of diagnosis, adaptive skills, is usually measured with a self-reported or parent/caregiver-reported inventory, such as the Vineland Adaptive Behavior Scales, Second Edition (VABS-II). The DSM-V diagnosis is expected to require adaptive measurements of less than 2 SDs as compared to the population mean, with standard scores of 70 or less, in at least 2 of the following domains:^[3]

• Conceptual skills (communication, language, time, money, academic)
• Social skills (interpersonal skills, social responsibility, recreation, friendships)
• Practical skills (daily living skills, work, travel)

MR/ID also can be categorized as syndromic, if associated with dysmorphic features, or nonsyndromic, if not associated with dysmorphisms or malformations. The understanding of specific MR/ID syndromes is expanding with recent molecular genetic advances. More than 800 recognized syndromes listed in the Online Mendelian Inheritance in Man (OMIM) database are associated with MR/ID, reflecting clinical diagnostic advances in the field. The most common associated chromosomal abnormality is trisomy 21, or Down syndrome. The most common X-linked abnormality associated with MR/ID is fragile X syndrome. However, for most cases of MR/ID, no specific genetic abnormalities are found.

Some forms of MR/ID are due to nongenetic factors and may be identifiable by their associated dysmorphisms and clinical presentation. Examples include prenatal exposure to teratogens (eg, anticonvulsants, warfarin, alcohol) or prenatal thyroid dysfunction. Prenatal and postnatal exposure to lead and the associated decrement in IQ may increase an individual's chance of functioning in the MR/ID range.

MR/ID is the end result of many disorders of CNS function. Most individuals with significant intellectual impairment have no discernible structural abnormalities of the brain. CNS malformations, a visual correlate of the disorders, are diagnosed in 10-15% of cases; dysfunction is localized primarily to the cortical structures, including the hippocampus and the medial temporal cortex. The most common malformations consist of neural tube defects, hydranencephaly, and microcephaly. Less commonly, CNS malformations include disorders of migration (the lissencephalies) and agenesis of the corpus callosum.

Multiple congenital anomaly syndromes with malformations confined to nonneurologic organ systems may be present in 5% of all patients with MR/ID. Between 3% and 7% of cases may be associated with a wide array of inborn errors of metabolism complicated by multiorgan system disease. Alcohol exposure in utero may account for as many as 8% of those with mild MR/ID.

Most individuals with mild MR/ID and other learning disorders do not have other neurologic complications, CNS malformations, or dysmorphisms. They are more likely, however, to be born into families of low socioeconomic status, low IQ, and little education. The etiologic contribution of poverty to their poor cognitive function remains unclear. Clearly, however, poor cognitive functioning and MR are correlated positively with a life of poverty.

Frequency

United States

The frequency of MR/ID of all degrees ranges from 1.6-3% of the population. The statistical definition of subaverage intelligence (2 SDs below the mean) would indicate a predicted prevalence of 2.5%.

International

A study with excellent ascertainment conducted in Aberdeen, Scotland, yielded a prevalence of 1 in 300 for severe MR and 1 in 77 for mild MR. Among those with severe MR were more boys than girls (male-to-female ratio 1.2:1), and among those with IQ >70, in the mild range of deficiency, boys exceeded girls by a ratio of 2.2:1.^[4]

Although prevalence rates vary from country to country, the variance in prevalence may be attributed to ascertainment bias, the standardization methods employed from study to study, and a generalized upward drift in IQ scores over time. Even so, the greatest variance in statistics of prevalence is most likely to fall within the category of mild MR, a group for which the ascertainment bias is large.

Mortality/Morbidity

• MR/ID itself is not necessarily associated with an increased premature death rate. However, individuals with severe to profound MR/ID experience a decreased life expectancy related to the underlying etiology or additional complicating neurologic disorders, such as epilepsy. Neurologic dysfunction resulting in immobility, significant oral motor incoordination, dysphagia, and aspiration confers a greater risk of premature death than MR/ID itself. When significant neurologic dysfunction is associated with other organ system anomalies, an individual's life expectancy is shortened further.
• Respiratory disease is the most prevalent cause of death among individuals with profound MR/ID. In particular, respiratory infections were the leading cause of death among a Finnish cohort of children with MR/ID.^[5] For those affected by mild MR/ID, life expectancy does not differ from that of the general population.
• Comorbid psychiatric conditions are diagnosed more frequently in those with intellectual disabilities than in the general population. Even so, psychiatric disorders probably are underappreciated in this population.

  • Attention deficit/hyperactivity disorder (ADHD) is diagnosed in 8-15% of children and 17-52% of adults with MR.
  • Self-injurious behaviors require treatment in 3-15%, particularly in the severe range of MR/ID.
  • Major depression, bipolar disorders, anxiety disorders, and other mood disorders are the most common psychiatric diagnoses in adults with MR/ID. Obsessive-compulsive disorder, conduct disorder, tic disorders, and other stereotypic behaviors are also diagnosed more commonly in those with MR/ID. Schizophrenia may have a prevalence of 3% in individuals with MR/ID, compared to 0.8% in the general population.
• In the 1970 Isle of Wight study, as many as 30% of children with MR/ID exhibited an emotional or behavioral disorder, compared to 6% of children in the general population. MR compounded by epilepsy conferred a 56% risk of comorbid psychiatric disease in this study.^[6]
• Occult visual and auditory deficits occur in 50% of those with MR/ID, particularly when refractive errors are considered.
• The rates of transmittable diseases, including sexually transmitted diseases (STDs), hepatitis B, and Helicobacter pylori infection, are increased significantly among individuals with MR/ID.
• One in 5 individuals with MR/ID also has cerebral palsy (CP).
• As many as 20% of individuals with MR/ID have seizures.
• GI complications with MR/ID include feeding dysfunction, excess drooling, reflux esophagitis, and constipation.
• GU complications with MR/ID include urinary incontinence and poor menstrual hygiene.
• A profound social morbidity affects individuals with MR/ID and their families. This morbidity can be measured in lost wages, dependence on social services, impaired long-term relationships, and emotional suffering.

Race

Consistent racial differences in prevalence of MR/ID and associated mortality rates are not known to exist.

Sex

The gender ratios for mortality and morbidity do not differ from the gender ratio noted in the severe/profound ranges of intellectual disability (ie, male-to-female ratio, 1.2:1).

Age

MR/ID refers to intellectual impairment that develops prior to the age of 18 years.

Certain syndromes associated with MR/ID, such as Down syndrome, are associated with shorter life expectancy than the general population. In a comparison of those with MR/ID with and without Down syndrome from the California Department of Developmental Services cohort, excess mortality in the Down syndrome group tended to decrease with advancing age up to 35-39 years but increased thereafter. The increase in death rate from age 40 years was steeper in patients with Down syndrome than in those without Down syndrome.^[7]