The terms we use, how we define them, are important things. Often times, we use similar terminology interchangeably. We're discussing nebulously defined disorders (autism and intellectual disability) that change over time and depending on the criteria being used.
What do I mean about interchangeability and criteria? The official umbrella in the DSM-IV is not autism spectrum disorders, but is instead pervasive developmental disorders. However, the NIH uses autism spectrum disorders, as do many people in the autism community. The clinical and research usages change faster than the official bible (and lets not forget some disorders, despite ample research, valid checklists, etc., aren't in the DSM-IV and are not slated to go into the DSM-V). High functioning and low functioning autism are used, both amongst the community and in some research, yet don't exist in the DSM-IV.
Many terms over the decades have been used to discuss individuals with what many now call intellectual disability (some textbooks still use mental retardation, as does the DSM, which has no intention of replacing the term, but the use of it within the specific field is on the decline). Mental retardation is in the DSM-IV:
"A. Significantly subaverage intellectual functioning: an IQ of approximately 70 or below on an individually administered IQ test (for infants, a clinical judgment of significantly subaverage intellectual functioning).
B. Concurrent deficits or impairments in present adaptive functioning (i.e.. the person's effectiveness in meeting the standards expected for his or her age by his or her cultural group) in at least two of the following areas: communication. self-care, home living, social/interpersonal skills, use of community resources, self-direction, functional academic skills, work, leisure, health, and safety.
C. The onset is before age 18 years.
Code based on degree of severity reflecting level of intellectual impairment"
The Merck Manuals Online Medical Library defines intellectual disability thusly:
"Intellectual disability (previously called mental retardation) is characterized by significantly subaverage intellectual functioning (often expressed as an intelligence quotient < 70 to 75) combined with limitations of > 2 of the following: communication, self-direction, social skills, self-care, use of community resources, and maintenance of personal safety. Management consists of education, family counseling, and social support."
There's a reason that there is considerable overlap between autism and ID: many of the functional life skills are impaired in an individual with autism, just as they are in individuals with an intellectual disability. There's a tendency, though, I think, in the public discourse about ID to ignore the functional domains and focus on the IQ score. If functional life skills are high, but academic ability low, as long as the basic functions of living independently can be carried out, individuals with mild to moderate ID have a good chance of independent lives, while autistic individuals with high IQs but impaired functional life skills may require lifelong care. Despite the commonality in some of the additional impairments necessary to diagnose ID (mental retardation in the DSM-IV) between ID and autism, there is no accompanying requirement that there be ID in order to be diagnosed with autistic disorder.
An interesting difference proposed between autistic individuals with low IQs and those with ID is offered by Anderson (2008): "In the case of autism it is argued that the low-IQ scores of people with autism are not likely to be due to a deficit in the cognitive process that is arguably the major cause of mental retardation—namely, speed of processing—but rather low IQ reflects the pervasive and cascading effects of the deficit in the information-processing module that causes autism."
Used interchangeably at times is cognitive impairment (often by me on this blog). I do so, in part, because we can look at specific areas of cognitive impairments that contribute to an overall diagnosis of intellectual disability. The other reason is because after my son's stroke, he was diagnosed with 294.0 Cognitive Disorder NOS. Cognitive impairment may be caused by diseases or accidents and are usually not present from birth.
The DSM-IV explains the Cognitive Disorder in this way (note that cognitive disorder/cognitive impairments are not mutually inclusive):
"This category is for disorders that are characterized by cognitive dysfunction presumed to be due to the direct physiological effect of a general medical condition that do not meet criteria for any of the specific deliriums, dementias, or amnestic disorders listed in this section and that are not better classified as Delirium Not Otherwise Specified, Dementia Not Otherwise Specified, or Amnestic Disorder Not Otherwise Specified. For cognitive dysfunction due to a specific or unknown substance, the specific Substance-Related Disorder Not Otherwise Specified category should be used."
In other words, the DSM is not always the best place to go, as the array of disorders and conditions can be dizzying and contradictory. The Cognitive Disorder works as a diagnosis in individuals with clear intellectual or cognitive impairments, but who may not meet the IQ score of 70 because of specific areas of impairment, rather than global impairments and who have these impairments as a result of disease or damage).
Cognitive impairment as a term is used across a wide array of diseases and over the lifespan and can refer to the early loss of cognitive skills evident in the lead up to Alzheimers (Winblad et al., 2004). Mild cognitive impairment (MCI) may not lead to Alzheimers, though. In this sense, this is a distinct usage separate from cognitive impairments present in childhood. Cognitive impairment is present in cases of multiple sclerosis, as well (Marrie et al., 2005). It signals the lack of specific areas of cognitive function but not necessarily a global deficit.
Wehmeyer and Obremski (well worth a careful read) look at intellectual disabilities in the broader context of cognitive disabilities. They write:
"Intellectual disability refers to limitations to intellectual functioning manifesting in activity limitations and participation restrictions across all life activity and human functioning domains.
In summary, then, intellectual disability refers to a disability manifesting as limitations in intellectual functioning (reasoning, planning, solving problems, thinking abstractly, comprehending complex ideas, learning quickly, and learning from experience) related to activity limitations, associated with participation restrictions and resulting from brain impairments or specific etiological factors."
Disability World provides some examples of various cognitive disabilities. The definition they offer is:
"Defining cognitive disability is not easy, and definitions of cognitive disability are usually broad. Persons with cognitive disabilities may have difficulty with various types of mental tasks."
Disability World includes Down Syndrome, ADHD, autism, and learning disabilities within its definition of cognitive disability. It is simply the widest umbrella term available. Any deficit in any cognitive ability fits under this broad term.
There is considerable overlap in the use of current terms because we are still drawing the lines of what are socially appropriate, sufficiently non-offensive terms to refer to disabilities and impairments so that individuals who have these issues are not stigmatized. There is also considerable overlap and interchangeability because the definitional lines are in flux. With future diagnostic sensitivity, it may be possible to diagnose specific cognitive impairments with more precise terminology, but the likelihood is that we will continue to use these terms interchangeably, as they convey the same general meaning: a difficulty with some level of cognitive processing, with ID indicated a more global impairment that was first manifested in childhood.
Marrie, R., Chelune, G., Miller, D., & Cohen, J. (2005). Subjective cognitive complaints relate to mild impairment of cognition in multiple sclerosis Multiple Sclerosis, 11 (1), 69-75 DOI: 10.1191/1352458505ms1110oa
Winblad B, Palmer K, Kivipelto M, Jelic V, Fratiglioni L, Wahlund LO, Nordberg A, Bäckman L, Albert M, Almkvist O, Arai H, Basun H, Blennow K, de Leon M, DeCarli C, Erkinjuntti T, Giacobini E, Graff C, Hardy J, Jack C, Jorm A, Ritchie K, van Duijn C, Visser P, & Petersen RC (2004). Mild cognitive impairment--beyond controversies, towards a consensus: report of the International Working Group on Mild Cognitive Impairment. Journal of internal medicine, 256 (3), 240-6 PMID: 15324367