Redefining the syndrome of cognitive impairment in DSM-5

DSM Digest

Introduction: what is changing in the DSM?

The Diagnostic and Statistical Manual of Mental Disorders (DSM)-5 Neurocognitive Disorders Work Group has proposed and implemented an entirely new label for cognitive disorders, one which will have major implications for clinicians dealing with dementia, mild cognitive impairment and related disorders (Ganguli et al., 2011). The number of people living with dementia is expected to double every 20 years, to 66 million in 2030. Under the new classification, dementia becomes ‘major neurocognitive disorder’ and mild cognitive impairment (MCI) becomes ‘minor neurocognitive disorder’. The DSM-5 working group proposed a fundamental re-examination of the criteria that determine when cognitive difficulties become clinically important. Major neurocognitive disorder requires only one cognitive domain to be impaired, but this must represent a change from baseline and be accompanied by evidence on objective testing of cognitive performance ‘typically’ > 2.0 SD below the mean (2.5th percentile). The authors suggest that impairment in two or more domains imply dementia but impairment in one domain alone implies amnestic disorder or another specific severe cognitive disorder yet to be labelled (Ganguli et al., 2011).

Why this is important

The new criteria require deficits decline from a previous level of performance and interfere with independence, such that the person requires assistance with instrumental activities of daily living. At first glance, this emphasis on deterioration is sensible in order to exclude those with pre-existing impairment, in particular developmental disorders. However, some studies suggest that as many as a third of those with dementia had no identified prodromal phase (Breitner, 2008). Excluding long-standing severe cognitive impairment may exclude some cases of brain injury which are not neurodegenerative. The DSM-5 emphasis on daily function is also sensible because decline in daily function strongly predicts onset and progression of dementia (Pérès et al., 2008). However, the proposed criterion of ‘loss of independence’ introduces a high threshold. Approximately 15% of patients with dementia maintain independence without significant assistance (Andersen et al., 2004). Conversely, about 15% of the general population who do not have dementia require daily assistance (Ganguli et al., 2011). In reality, decline in function is on a continuum and this categorical cut-point could pose problems. A better definition would be the presence of functional decline which is of clinical significance, leaving some degree of judgement to informed clinicians.

Likely impact on clinical practice

What of pre-dementia cognitive impairment? If cognitive deficits are not sufficient to interfere with independence, but yet there is still evidence on objective testing of impairment at least 1 SD (0.5 SD using serial testing) below the mean (16th percentile), then the category of minor neurocognitive disorder is invited. Minor neurocognitive disorder is an attempt to ratify MCI, cognitive impairment no dementia (CIND) and age-related cognitive decline (ARCD), but excludes decline purely related to aging. MCI has been a problem concept in psychiatry for over a decade. The issue is that the proponents of MCI have proposed restrictive criteria of subjective memory complaints and yet no significant functional impairment (Portet et al., 2006; Winblad et al., 2004). Only 25% of those people without dementia, but nevertheless impaired on neuropsychological testing, qualify for MCI (Pérès et al., 2008). Yet about a third of older adults with cognitive decline suffer more than ‘minimal’ functional impairment and only 40% of subjective memory complaints (Mitchell, 2008). Many older adults do not recognize or do not wish to see a clinician for memory complaints (Ramakers et al., 2009). Family members usually detect a change, but may be hesitant about seeking help. DSM-5 improves this restriction, by allowing evidence of decline to be inferred by the clinician but the same necessity for cognitive decline that exists for major neurocognitive disorder is also required here. If decline cannot be established (or inferred), then no such diagnosis is possible.

Likely impact on research

There are many unresolved questions about major and minor neurocognitive disorders. Both minor and major neurocognitive disorders may be further subclassified according to aetiology (e.g. Alzheimer type, vascular type, Lewy body type), although a mental cause (e.g. depression or schizophrenia) is not allowed. Criteria for these subtypes are very imprecise when based on clinical examination, and it is not clear how DSM-5 will improve this situation. A fundamental question is: Do these new labels help patients with cognitive difficulties? Labels that avoid the term ‘dementia’ will help reduce the tendency of clinicians to label any cognitive disorder as dementia and reduce the stigma of dementia but at the same time may restrict patients’ ability to access dementia-specific resources. The value of minor neurocognitive disorder is unclear but could lead to a more sophisticated description of pre-dementia stages and recognition that not all patients suffer progressive cognitive decline (Mitchell and Shiri-Feshki, 2009).