The role of cognitive assessment in
diagnosing ASD
(Appendix 6 of: Ministries of Health and Education. 2008. New Zealand Autism Spectrum
Disorder Guideline Wellington: Ministry of Health)
Process
A literature review was undertaken to address the role of cognitive
assessment in diagnosing ASD. This topic was broken down into specific
questions as follows:
1. Does performing a baseline cognitive assessment in individuals with ASD improve
outcome?
2. Does performing a baseline cognitive assessment in individuals diagnosed with
ASD enable more effective planning of an intervention programme and more
efficient use of resources?
A bibliography was prepared by The Clearing House for Health Outcomes and
Health Technology Assessment in March 2004 at the direction of Lester
Mundell, Chief Advisor, Disability Support Services, Ministry of Health. The
search strategy is outlined in the report submitted to the Ministry of Health. In
summary, a comprehensive number of online bibliographic databases, reviewand evidence-based databases, library catalogues and international
government health websites were searched using a variety of search terms
(together with related words) around the themes of ‘cognitive’, ‘diagnosis’ and
‘autism’.
Abstracts and/or titles (where abstracts were not available) from the identified
studies (n = 183) were analysed and given a preliminary grading on the basis
of the likely degree of relevance in terms of capacity to answer the above
questions. The selected articles are included in evidence tables with the
following relevance grading:
• Highly relevant (n=18)
• Probably relevant (n=36)
• Possibly relevant (n=23)
• Not relevant (n=106)
The 77 articles selected as being relevant were obtained through online and
available library sources. Not all were able to be obtained for the first analysis.
Of the above articles or publications, 15 of the Highly Relevant (83%), 19 of
the Probably Relevant (53%) and 11 of the Possibly Relevant (52%) were
obtained. The list and abstracts of those not obtained were peer-reviewed by
a subgroup of the Guideline Development Team, and a decision was made to
seek a further four articles. Eighty-one were finally subjected to analysis.
The analysed articles had a number of different study designs: reviews,
guidelines, cohort studies (of groups of higher functioning autistic children
who had cognitive tests at baseline and at follow-up) and studies of cognitive
differences and styles in autism versus controls (either normal peers or those
with intellectual or language difficulties). Relevant cohort and case control
studies were reviewed using an evidence template.
Conclusions
Grade
There are particular patterns of skills and weaknesses on formal tests of
cognition associated with autism (but none are so specific that they
significantly contribute to the clinical diagnosis).
A
Specific underlying cognitive deficits are postulated to be at the core of the
symptoms observed (theory of mind deficit, executive function deficit, weak
central coherence).
C
Achievement level in formal tests of non-verbal cognition (ie, IQ score) and
language are consistently identified as the best predictors of
outcome/prognosis.
A
Recommendation:
Grade
Formal baseline cognitive and/or developmental assessment is recommended
at diagnosis.
Answers to the research questions can be summarised as follows:
1. Does performing a baseline cognitive assessment in individuals with ASD
improve outcome?
There is no evidence for or against this, but there is good evidence it allows
better prediction of prognosis.
2. Does performing a baseline cognitive assessment in individuals diagnosed
with ASD enable more effective planning of an intervention programme and
more efficient use of resources?
There is no evidence for or against this, but extrapolation from studies
supports the conclusion that better definition of learning skills and
prognosis by cognitive assessment will assist planning optimal intervention.
Expert supporting opinion from the studies can be summarised as follows:
(a) Cognition is not part of the clinical criteria for autism but it is an important
variable that influences diagnosis, is related to associated medical disabilities
(such as epilepsy), and predicts outcome. Measures of nonverbal problem
solving in high functioning individuals with autism correlate with outcome
(whereas) the severity of autistic behaviours is a poor predictor of prognosis.425
(b) It is now recognised that assessment of cognitive functioning is crucial to the
differentiation of ASD from other disabilities and to the identification of
concomitant impairment in a child with an ASD. Cognitive ability also has an
important role in prognosis and intervention planning. An estimation of potential
is necessary for the following reasons:
• Functioning level, which includes cognitive and adaptive evaluation, is
important for differential diagnosis and intervention planning. A diagnosis of
ASD is appropriate when a child shows communicative, social or interest
deficits that are inconsistent with overall cognitive functioning. For example,
a child of 4 who is functioning at a 12-month developmental level would not
receive a diagnosis of ASD if he or she displayed communicative and play
behaviours similar to that of other 12-month-old children. It is also extremely
difficult to document significant social and communicative deficits below this
age level.
B
• Treatment research generally has supported the notion that response to
various treatment approaches has some relation to overall cognitive
functioning426. For example, certain intensive behavioural approaches have
been shown to be less successful with children at lower cognitive levels
who are unlikely to develop spoken language.
• Degree of cognitive functioning may indicate expected rates of progress.
This, of course, is dependent upon the relative degree of certainty with which
cognitive impairment can be established.33