IQ and general cognition

What is IQ and how is it measured?

Intelligence quotient (IQ) is derived from standardised tests used to measure general cognitive functioning. IQ is most commonly measured using the Wechsler Adult Intelligence Scale (WAIS). The WAIS is designed to measure all aspects of cognitive functioning, and is divided into subtests measuring verbal IQ (verbal comprehension and working memory) and non-verbal IQ (perceptual organisation and processing speed). Other tests used to assess IQ include the Mini-Mental State Examination (MMSE), which assesses cognitive impairment; the National Adult Reading Test (NART), which assesses premorbid intelligence; the Wide Range Achievement Test (WRAT), which assesses both verbal and mathematic ability; and the Raven’s Progressive Matrices, which assesses general intelligence.

What is the evidence for IQ and general cognition in bipolar disorder?

Moderate to high quality evidence suggests a small, significant effect of poorer general cognitive functioning in people with bipolar disorder compared to controls prior to the onset of bipolar disorder when assessments were conducted retrospectively, but not prospectively. After illness onset, there was a significant, medium-sized effect of poorer general cognitive functioning in people with bipolar disorder compared to controls, with no changes over time (3-7 years). The effect was smaller in people with first-episode patients than in chronic patients.

Moderate quality evidence suggests a large impairment in global cognition in youth with bipolar disorder aged ~13 years who were matched to controls for age and IQ, but no differences in elderly people with bipolar disorder and controls matched for age and education.
High quality evidence suggests a small effect of poorer global cognition in people with bipolar disorder and a history of psychotic symptoms compared to people with bipolar disorder and no history of psychotic symptoms, and in people with bipolar I disorder compared to people with bipolar II disorder.

Moderate to high quality evidence suggests no differences in IQ between first-degree relatives of people with bipolar disorder and controls. However, when the analysis was contained to young relatives aged 10 to 25 years, high quality evidence shows a small effect of lower IQ compared to controls.

Moderate to high quality evidence suggests a small to medium-sized effect of higher IQ in relatives of people with bipolar disorder than in relatives of people with schizophrenia.

March 2019

Last updated at: 4:50 am, 31st March 2019
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