Age and Ageing Advance Access published online on August 5, 2008
Age and Ageing, doi:10.1093/ageing/afn147
Copyright © The Author 2008. Published by Oxford University Press on behalf of the British Geriatrics Society.
Is it time to separate subjective cognitive complaints from the diagnosis of mild cognitive impairment?
Alex J. Mitchell
Leicester General Hospital, Leicester LE5 4PW and Honorary Senior Lecturer in Liaison Psychiatry, Department of Cancer & Molecular Medicine, Leicester Royal Infirmary LE1 5WW, UK
Address correspondence to: A.J. Mitchell. Tel: (+44) 0116 225 6218; Fax: (+44) 0116 2951951. Email: ajm80{at}le.ac.uk
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Abstract
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Subjective cognitive complaints (SCC) are currently considered
to be a core feature of mild cognitive impairment (MCI). Yet
the implications of including or excluding subjective complaints
has not been previously considered. The key questions are how
many health people complain of SCC compared to those with MCI?
How is the epidemiology of MCI affected by the requirement for
SCC? How is the prognosis of MCI influenced by SCC? and how
should SCC be defined and measured? Findings to date suggest
that subjective complaints are one of many variables that comprise
risk in individuals with MCI. Individuals who do not have subjective
complaints and might not qualify under current definitions of
MCI may still have a disorder that is of clinical significance.
Despite a close association, SCC may be neither necessary nor
sufficient for a diagnosis of either MCI or dementia.
Keywords: subjective memory complaints, mild cognitive impairment, dementia, elderly
Subjective cognitive complaints (SCC, also known as subjective
memory complaints) refer to everyday concerns cited by people
both with and without objective evidence of memory impairment.
Such complaints are very common. It was found that 30% of the
unimpaired elderly report that they have ‘trouble remembering
things that have happened recently’ and a similar number
have ‘trouble remembering where belongings are kept’
[
1]. From the clinical perspective, there is likely to be an
important difference between those who agree that they have
slight difficulty on direct questioning and those who actively
seek help for memory complaints.
Lately, the importance of SCC has been emphasised by its inclusion as a core feature of mild cognitive impairment (MCI) in recent consensus reports [2, 3]. Despite this apparent consensus, their inclusion remains controversial and many research groups have not used SCC when diagnosing MCI. One issue is that there is no single optimal method to elicit SCC; rather there are at least 20 competing subjective memory questionnaires, few of which have adequate validation. An even more fundamental issue is that whilst many studies document a relationship between subjective and objective memory complaints, many have failed to find such a relationship (for review see [4]). A further complication is that SCC can represent the concerns of a patient or a close family member. This might prove to be important as preliminary studies have found that the association between subjective ratings and future cognitive decline is stronger for informant rather than patient complaints [5]. Collectively, these issues have led to uncertainty about the clinical significance of patient-reported SCC and a question mark over the use of SCC in defining MCI. In order to clarify the significance of SCC in diagnosing MCI, four questions which may help disentangle this complex issue are suggested: How many health elderly people compared to those with MCI complain of SCC? How is the prevalence of MCI affected by the inclusion of SCC in the definition? How is the prognosis of MCI affected by inclusion of SCC? How should SCC best be defined?
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How many health elderly people complain of SCC compared to those with MCI?
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Several studies have examined the rate of memory difficulties
in selected community samples, but few have done so in comparison
to those with known MCI [
4]. A complication is that the rate
of SCC is not stable but varies significantly over time. A second
complication is that the rate of SCC is strongly influenced
by age such that the rate in those aged under 65 is about 20%,
but this quickly rises to about 90% in those over 85 [
6]. One
of the best estimates of SCC comes from Crooks and colleagues
(2001) who studied a community sample aged 65 and older using
the single question ‘Do you have
severe memory problems?’
[
7] It was found that 38.6% of those with dementia, 12.4% of
those with loosely defined MCI and 1% of non-cognitively impaired
controls reported
severe SCC. Clearly, if one required SCC as
part of the criteria for MCI then by definition all such individuals
would have SCC.
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How is the prevalence of MCI affected by SCC?
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Ganguli and colleagues examined how many cases with cognitive
complaints satisfied the criteria for MCI. Out of 1,248 individuals
in the Steel Valley study, 36.7% had SCC but only 3.2% met full
criteria for MCI [
8]. Yet without the requirement for SCC, 6.3%
met criteria for MCI. In the Iowa Established Populations for
Epidemiologic Studies of the Elderly consisting of 3,673 persons
aged 65 or over, Purser et al. (2006) found that 8.9% met strict
criteria for MCI compared to 14% who met criteria if SCC were
not required [
9]. Luck and colleagues (2007) recently examined
the frequency of MCI in a primary care sample [
10]. The prevalence
was 15.4% for strict criteria and 25.2% for modified criteria.
These studies suggest that the effect of requiring SCC is to
reduce the rate of MCI by 30–50%, so that the overall
prevalence is very close to that of dementia itself. Yet, this
does not necessarily mean that inclusion of SCC is redundant,
particularly if the prognosis of MCI is strongly influenced
by the presence of SCC.
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How is the prognosis of MCI affected by inclusion of SCC?
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Regardless of any effect on the prevalence of MCI, most clinicians
want to know whether the risk of conversion to dementia is affected.
It has been suggested that SCC may have special significance
in that they may anticipate future decline above and beyond
baseline cognitive testing. Two adequately powered studies have
found little or no influence of SCC on progression in those
with no cognitive impairment at baseline [
9,
11]. In those with
memory complaints and objective evidence of decline, rates of
conversion to dementia are elevated even when the cause of cognitive
decline is unknown [
12]. More recent studies also appear to
show that SCC adversely influences future decline in those with
baseline MCI. For example, Fisk and colleagues examined the
outcome of MCI in the 5 year Canadian Study of Health and Aging
(CSHA) [
13]. There was a hierarchal risk of progression to dementia:
71.4% of those meeting all criteria for amnestic MCI converted
compared to 68% in those where deficits in function were allowed,
56% where no SCC were required and 56% where neither was required.
Indirect evidence also comes from other long-term studies where
SCC were specifically not required in the definition of MCI.
Here the progression rate to dementia tends to be much lower
than expected.
An important finding was recently reported from participants in the Kungsholmen project, interviewed 3 years before developing dementia [14]. One-third reported neither memory complaints nor objective cognitive deficits on the Mini-Mental State Examination (MMSE) 3 years before diagnosis. A further 16% had no complaints but evidence of decline on the MMSE. Thus, although there is a definite association with underlying cognitive disorders, subject complaints are neither necessary nor sufficient for a diagnosis of either MCI or dementia or the prediction of later dementia.
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How should SCC be defined?
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If SCC do have either diagnostic or prognostic significance,
what is the best way to elicit such complaints and are all complaints
of equal significance? In other words, should all possible complaints
be included under the rubric of MCI or only certain ‘high-risk’
complaints? Grut et al (1993) examined the significance of ‘slight’
versus ‘marked’ SCC [
15]. Marked deficits were more
discriminating of those with MCI versus without MCI occurring
in 19% versus 5% compared with 30 and 28%, for ‘slight’
deficits. Clarnette and colleagues (2001) compared 97 individuals
with and 38 without SCC (regardless of MCI status) [
16]. From
a small list of complaints, the most discriminating was word-finding
difficulty. This hints that not all types of cognitive complaints
are of equal significance and echoes the findings of neuropsychological
studies examining the significance of specific types of cognitive
test in diagnosing dementia and MCI.
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Conclusion
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From this data, it is clear that the relationship between subjective
and objective cognitive impairments is complex. Looked at categorically,
there are four subgroups of people depending on their subjective
and objective complaints (both, neither, subjective aloneand
objective alone). Lautenschlager and colleagues found that the
proportion in each of these categories was 10.6, 40.1, 46 and
3.4%, respectively [
17]. Risk of progression appears to be ranked
as follows: both> objective alone> subjective alone>
neither. Yet perceived forgetfulness is not always a sinister
finding. In the Maastricht Aging Study, 30% of those with memory
difficulties had little or no impairment in activities of daily
living and about 40% were not (or hardly) worried about their
forgetfulness. It seems likely that in the absence of any other
clinically concerning finding, isolated SCC are unlikely to
be clinically significant. In association with other features,
however, they do have added value (for prediction of later dementia)
but at a cost of reducing the proportion of people who can be
labelled with MCI. In statistical terms, they increase the specificity
and positive predictive value but reduce the sensitivity and
negative predictive value. There is also an implication for
screening for dementia where a combination of subjective and
objective tests could be more useful than either one used alone
[
18,
19]. SCC may be useful diagnostically because simple questions
concerning everyday cognitive abilities are more acceptable
to patients than lengthy objective testing. Regarding the diagnosis
of MCI, it might be useful to redefine the core criteria for
MCI on the basis of objective deficits alone and then to specify
the presence or absence of risk factors such as SCC, functional
impairment, vascular disease and biological markers. Indeed,
given adequate data a risk calculator might be possible akin
to that already used to calculate cardiovascular risk [
20].
There has been a great deal of useful research on SCC and MCI in the last 10 years, but no very large naturalistic studies that would allow accurate risk profiling, although such studies are underway. There have also been no studies examining how well SCC would differentiate those with MCI from those with depression, anxiety or other causes of cognitive complaints. Finally we have almost no information on the relative risk of different types of cognitive complaint, e.g. difficulties in memory compared with word-finding difficulty. Future studies on MCI should specify the degree and nature of both subjective and objective memory complaints.
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Key points
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- MCI is a condition of mixed aetiology which leads to dementia in about half of cases.
- Many but not all individuals with MCI report subjective cognitive difficulties.
- Otherwise healthy individuals with mild cognitive complaints are unlikely to be at high risk of future decline.
- A combination of subjective and objective deficits is a cause for concern.
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Received 14 February 2008; accepted in revised form 5 June 2008.
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How many health elderly...