Clinical, methodological and theoretical issues in the assessment of cognition after anaesthesia and surgery: a review.

As people live longer, the burden of cognitive impairment to elderly patients, their families and society becomes increasingly common and important. The loss of independence, a reduction in the quality of life and increased mortality are possible correlates to the mental disintegration. Cognitive dysfunction following major surgery on the elderly is a significant problem which adds to other cognitive impairments caused by neurodegeneration, cerebrovascular impairments and other causes. There are challenges in reviewing the literature because of many methodological concerns. There is no standard definition; the diagnosis is made only by the results of neuropsychological tests which are not standardised for this purpose; test results are analysed by different statistical methods (some of them inappropriate); controls are often absent or poorly matched; and pre-existing mild cognitive impairment, which affects 10 to 20% of people older than 65 years and is similar to the subtle cognitive impairment following surgery, is not sought for and recognised. Reviews of the subject have varied from descriptions such as 'a well recognised and significant problem' to 'a hypothetical phenomenon for which there is no International Statistical Classification of Disease (ICD-9) code, and no Diagnostic and Statistical Manual of Mental Disorders (DSM-IV) code'. This article examines both sides of the spectrum in a detailed review which explains the necessary psychological 'jargon', discusses the methods used and points to areas of future research.