Sample selection

Diagnostic criteria for classification of dementia

Two raters (G. L. and T. S.) jointly made diagnoses in accordance with DSM—IV (American Psychiatric Association, 1994) and ICD—10 (World Health Organization, 1992) criteria. Alzheimer-type dementia, vascular dementia, dementia in Parkinson's disease and other/unspecified dementias were diagnosed using these criteria.

In addition, diagnoses were made as follows:

1. (a) Alzheimer's disease: National Institute of Neurological and Communicative Diseases and Stroke and the Alzheimer's Disease and Related Disorders Association (NINCDS—ADRDA) criteria (Reference McKhann, Drachman and FolsteinMcKhann et al, 1984), which have been shown to have a high degree of diagnostic accuracy (Reference Lim, Tsuang and KukullLim et al, 1999).

2. (b) Vascular dementia: National Institute of Neurological Diseases and Stroke/Association Internationale pour la Recherche et l'Enseignement en Neuroscience (NINDS—AIREN) criteria (Reference Roman, Tabemichi and ErkinjuntiiRoman et al, 1993), shown to have high specificity although low sensitivity for vascular dementia (Reference Gold, Giannakopoulos and Montes-Paixo-JuniorGold et al, 1997).

3. (c) Dementia with Lewy bodies: the consensus criteria for the diagnosis of DLB (Reference McKeith, Galasko and WilcockMcKeith et al, 1995), shown to have high sensitivity and specificity (Reference McKeith, Ballard and PerryMcKeith et al, 2000).

4. (d) Frontal lobe dementias: the Gregory and Hodges criteria (Reference Gregory and HodgesGregory & Hodges, 1993) for dementia of frontal type and the consensus criteria for frontotemporal dementia proposed by Neary et al (Reference Neary, Snowden and Gustafson1998) that are based on the Lund and Manchester criteria (Reference Brun, England and GustafsonBrun et al, 1994).

The rater carrying out the clinical assessments (T. S.) made clinical diagnoses prior to the application of the above criteria. In particular, the diagnosis of DLB was made in the presence of progressive dementia with prominent well-formed visual hallucinations if one or both of the following features were present: spontaneous motor features of parkinsonism; pronounced fluctuation in cognitive performance. Evidence of cerebrovascular disease on history, physical examination or neuroimaging did not rule out the diagnosis unless there was a clear temporal relationship between a cerebrovascular accident and the onset of symptoms. Similarly, no restriction was imposed on the duration of Parkinsonian symptoms prior to the onset of dementia. These separate ‘clinical’ diagnostic criteria were influenced by a post-mortem study by Hohl et al (Reference Hohl, Tiraboschi and Hansen2000), who reported a clinical diagnostic accuracy of 50% for DLB and found fewer hallucinations in the false-positive clinical cases, suggesting that hallucinations are an important diagnostic marker. This clinical approach to the diagnosis of DLB therefore differed from that taken by the consensus criteria in requiring the presence of visual hallucinations.

Stage 1

A total of 1085 participants were interviewed (response rate 85.3%).

Stage 2

Hospital notes were obtained for 84 (78.5%) of the 107 people screened in stage 1 as having dementia and 64 (59.8%) subsequently were assessed in person; 10 people (9.3%) refused further evaluation, 16 (15%) had died and 17 (15.9%) were not traceable. Eight (80%) of the people who refused further evaluation were female. In three cases the people themselves refused, saying they did not want to answer any more questions. In all other cases a family member refused to allow their relative to be interviewed, the most common reason given being that the relative was unlikely to benefit from further assessment. The mean age was 82.3 years for people who had died, 81.6 years for those who were not contactable and 78.9 years for those who refused. Fourteen (87.5%) of the people who were dead at the time of attempted follow-up had lived at home when first seen, compared with two (12.5%) in residential care. Of those who refused, eight (80%) lived in rented or their own accommodation and one each in nursing and residential care.

Diagnoses

Sufficient information for clinical diagnoses to be made was obtained from the medical case notes, social services' reports, general practitioners and relatives of eight of the people who were not interviewed after the initial screening. Diagnoses thus were made for a total of 72 people (67.3% of the original 107 who scored above the screening cut-off for dementia).

Table 1 indicates the numbers and proportions of people fulfilling the different diagnostic criteria. ‘Not applicable’ is used where a diagnostic category is not included in a particular classification system. ‘Alzheimer's disease’ includes all formulations for ‘dementia of the Alzheimer's type’ in DSM—IV and ‘dementia in Alzheimer's disease’ in ICD—10. Similarly, ‘vascular dementia’ includes all sub-categories under this heading in DSM and ICD. ‘Mixed dementia’ includes cases fulfilling DSM—IV criteria for ‘dementia due to multiple aetiologies’ and ‘other/unspecified dementia’, cases of ‘dementia in other medical conditions specified elsewhere’ and ‘dementia not otherwise specified’ in DSM—IV.

Table 1 Diagnoses by clinical criteria (DSM non-demented cases included)¹

Diagnoses by criteria	Dementia, n(%; Cl)	Alzheimer's disease, n(%; Cl)	Vascular dementia, n(%; Cl)	Dementia with Lewy bodies, n(%; Cl)	Frontal lobe dementia n(%; Cl)	Parkinson's dementia, n(%; Cl)	Mixed dementia/multiple causes, n(%; Cl)	Other/unspecified dementia, n(%; Cl)
DSM-IV	64 (88.9; 80-94)	25 (34.7; 25-47)	19 (26.4; 17-37)			2 (2.8; 1-10)	5 (6.9; 3-15)	13 (18.1; 11-28)
ICD-10	60 (86.1; 76-92)	33 (45.8; 35-57)	16 (22.2; 14-33)			2 (2.8; 1-10)	0	11 (15.3; 9-25)
NINCDS
Possible and probable		37 (51.4; 40-62)
Probable		20 (27.8; 19-39)
AIREN
Possible and probable			24 (33.3; 23-44)
Probable			14 (19.4; 12-30)
DLB consensus
Possible and probable				22 (30.5; 22-42)
Probable				7 (9.7; 5-19)
Gregory and Hodges					4 (5.6; 2-14)
FTD consensus					6 (8.3; 4-17)
Clinical	67 (93.1; 85-97)	26 (36.1; 26-48)	20 (27.8; 19-39)	5 (6.9; 3-15)	4 (5.6; 2-14)	2 (2.8; 1-10)	5 (6.9; 3-15)	3 (4.2; 1-11)

The results reveal Alzheimer's disease to be the most common cause of dementia by both DSM—IV and ICD—10 criteria. In addition, the majority of the sample fulfilled NINCDS—ADRDA criteria for either possible or probable Alzheimer's disease. The second most common diagnosis, again by both DSM and ICD criteria, was vascular dementia, with one-third meeting the NINDS—AIREN criteria for possible or probable vascular dementia. The majority of people diagnosed with unspecified dementia were those with severe dementia on whom insufficient collateral information was available with regard to onset and course of the illness for a probable aetiology to be identified.

Almost 10% of the sample fulfilled the consensus criteria for probable DLB, 30.5% fulfilling criteria for either probable or possible DLB. Of the cases of probable DLB, four (57.1%) met the DSM—IV criteria for Alzheimer's disease and one (14.3%) each met the DSM criteria for vascular dementia, dementia due to Parkinson's disease and unspecified dementia. Identical results were seen when probable DLB cases were compared with their ICD—10 diagnoses. In contrast, a clinical diagnosis of DLB based on the criteria described above, namely with emphasis on visual hallucinations and more flexible assessment of the time of onset of symptoms and contribution of vascular factors, was made in five cases (6.9%). All five cases fulfilled the consensus criteria for probable DLB.

Four people (5.6%) met the Gregory and Hodges criteria for FLD, compared with six (8.3%) who fulfilled the consensus criteria for frontotemporal dementia. All four Gregory and Hodges cases also met the consensus criteria. A comparison of diagnoses made using these two sets of criteria with the DSM—IV diagnoses is shown in Table 2.

Table 2 Diagnoses of cases of dementia with Lewy bodies (DLB) and frontal lobe dementia (FLD) by DSM—IV (DSM non-demented cases included)

	No dementia, n(%)	Alzheimer's disease, n(%)	Vascular dementia, n(%)	Dementia due to Parkinson's disease, n(%)	Unspecified dementia, n(%)
DLB consensus criteria (probable DLB)	0	4 (57.1)	1 (14.3)	1 (14.3)	1 (14.3)
Gregory and Hodges DFT	1 (25)	2 (50)	1 (25)	0	0
Consensus criteria for FTD	1 (16.7)	3 (50)	1 (16.7)	0	1 (16.7)

Eight people (11.1%) did not meet the DSM—IV criteria for dementia. Two did not have memory impairment, one of whom had an FLD and one of whom had English as her second language. Three were depressed and three had cognitive dysfunction but no significant impairment in social or occupational function due to cognitive deficit. Table 3 shows the numbers and proportions of people fulfilling the different diagnostic criteria once those not meeting DSM dementia criteria have been excluded. No DLB consensus cases of probable or possible DLB have been excluded (i.e. all of these cases met the DSM criteria for dementia). The proportions of people with probable DLB and probable plus possible DLB are thus higher, namely 10.9% and 34.4%, respectively. As noted above, one person fulfilling both sets of criteria for FLD was excluded. Table 4 indicates the DSM diagnoses of the remaining Gregory and Hodges and FTD consensus criteria cases of DFT/FTD.

Table 3 Diagnoses by clinical criteria (DSM non-demented cases excluded)¹

Diagnoses by criteria	Dementia, n(%; CI)	Alzheimer's disease, n(%; CI)	Vascular dementia, n(%; CI)	Dementia with Lewy bodies, n(%; CI)	Frontal lobe dementia, n(%; CI)	Parkinson's dementia, n(%; CI)	Mixed dementia/multiple causes, n(%; CI)	Other/unspecified dementia, n(%; CI)
DSM—IV	64 (100; 94-100)	25 (39.1; 28-51)	19 (29.7; 20-42)			2 (3.1; 0-9)	5 (7.8; 3-17)	13 (20.3; 12-31)
ICD—10	61 (95.3; 87-98)	32 (50.0; 38-62)	16 (25.0; 16-37)			2 (3.1; 0-9)	0	11 (17.2; 10-28)
NINCDS
Possible and probable		36 (56.3; 44-67)
Probable		20 (31.3; 21-43)
AIREN
Possible and probable			23 (35.9; 25-48)
Probable			14 (21.9; 14-34)
DLB consensus
Possible and probable				22 (34.4; 24-46)
Probable				7 (10.9; 5-21)
Gregory and Hodges					3 (4.7; 2-13)
FTD consensus					5 (7.8; 3-17)
Clinical	63 (98.4; 91-100)	26 (40.6; 30-53)	20 (31.3; 21-43)	5 (7.8; 3-17)	2 (3.1; 0-9)	2 (3.1; 0-9)	4 (6.3; 2-15)	3 (4.7; 2-13)

Table 4 Diagnoses of cases of dementia with Lewy bodies (DLB) and frontal lobe dementia (FLD) by DSM—IV (DSM non-demented cases excluded)

	Alzheimer's disease, n(%)	Vascular dementia, n(%)	Dementia due to Parkinson's disease, n(%)	Unspecified dementia, n(%)
DLB consensus criteria (probable DLB)	4 (57.1)	1 (14.3)	1 (14.3)	1 (14.3)
Gregory and Hodges DFT	2 (66.7)	1 (33.3)	0	0
Consensus criteria for FTD	3 (60)	1 (20)	0	1 (20)

Three people (4.7%) who met the DSM criteria for dementia did not fulfil the ICD—10 criteria because they had not been suffering clearly from the disorder for 6 months or more.

Figure 1 indicates the degree of diagnostic overlap for cases meeting the DSM criteria for dementia. For example, a person fulfilling the consensus criteria for probable DLB but also the NINCDS—ADRDA, DSM and/or ICD criteria for Alzheimer's disease would fall in the area of overlap between DLB and Alzheimer's disease. The category ‘unspecified dementia’ has been excluded because this does not represent a diagnostic entity. The single cases of ‘pure’ DLB and FLD (i.e. those not lying in an area of overlap) had been diagnosed as unspecified dementia by DSM and/or ICD. Cases of mixed dementia have been included in the area of overlap between Alzheimer's disease and vascular dementia, because these were judged to be the combined aetiologies in each instance.

Fig. 1 Overlap of diagnoses: AD, Alzheimer's disease; DLB, dementia with Lewy bodies; FLD, frontal lobe dementia; PD, dementia in Parkinson's disease; VD, vascular dementia.

Main findings

Ours is the first study in the Western world to report the distribution of DLB and FLD as subtypes of dementia in the community. The community prevalence of screen-positive dementia was 9.86%, and this figure fell to 6.7% when people living in residential or nursing homes were excluded. This prevalence rate is comparable to that of 6.4% reported by Lobo et al (Reference Lobo, Launer and Fratiglioni2000) in a recent comparison of European prevalence studies. If we exclude those cases not meeting the DSM—IV criteria for dementia and we use the best-validated diagnostic criteria for each subtype of dementia (NINCDS—ADRDA for Alzheimer's disease, NINDS—AIREN for vascular dementia, consensus criteria for DLB and consensus criteria for FTD), then the distribution of the subtypes is as follows: probable Alzheimer's disease, 31.3% (95% CI 21-43); probable vascular disease, 21.9% (95% CI 14-34); probable DLB, 10.9% (95% CI 5-21); and FLD, 7.8% (95% CI 3-17). The results indicate that although Alzheimer's disease remains most common form of dementia, there are several other types that occur sufficiently frequently to be seen in routine clinical practice. The distribution of DLB in our study contrasts markedly with the findings of the only other published study of this type, by Yamada et al (Reference Yamada, Hattori and Miura2001), in which the prevalence of DLB in a Japanese population was 0.1%. Given that the prevalence of dementia in Yamada et al's study was 3.8%, the distribution of DLB as a subtype may be calculated as 2.63%. Yamada et al furthermore identified no cases of FLD in their study population. The reasons for the differences in results are unclear.

Limitations

These figures are not immutable because the distribution of the different subtypes differs considerably, depending on the criteria used. Our results are restricted by the numbers of people identified at screening as having dementia on whom we could not make a diagnosis. The Short-CARE has not been validated as an instrument on people who have dementia without significant memory impairment and we do not know to what extent this may have influenced our findings, although we did detect some people with this syndrome. In addition, it is a limitation of the study that post-mortem diagnoses were not available.

A relatively high proportion (11.1%) of people identified by initial Short-CARE screening did not have dementia according to DSM—IV. For three of these eight people there was insufficient evidence of impairment in social or occupational functioning for dementia to be diagnosed, despite clear evidence of cognitive impairment in multiple domains. Because this was an epidemiological study, we employed a screening instrument and, subsequently, diagnostic criteria that produced categorical outcomes. Caution, however, clearly needs to be exercised in the use of these criteria in clinical practice in order to avoid excessively strict adherence to the inclusion and exclusion requirements at the expense of common sense.

Clinical implications

Previous studies of the frequency of DLB have concentrated on hospital in-patient or out-patient populations and, in particular, on the epidemiologically unrepresentative cohorts that come to post-mortem. It is highly likely that many of the figures from published studies on the distribution of other dementia subtypes are contaminated by DLB cases, particularly as neither of the two major classification systems currently in use, DSM—IV and ICD—10, include the diagnosis of DLB. Most cases of ‘probable’ DLB in this study were diagnosed as having Alzheimer's disease using DSM—IV and ICD—10. Almost one-third of the sample fulfilled DLB consensus criteria for either ‘probable’ or ‘possible’ DLB. This suggests that the consensus criteria are broad, at least with regard to the diagnosis of ‘possible’ DLB. The ‘clinical’ criteria used to diagnose DLB, which simultaneously were more restrictive in requiring the presence of visual hallucinations and more flexible in leaving unspecified the time relationship between the onset of parkinsonism and that of dementia, yielded a lower rate for this disorder.

As with DLB, neither DSM—IV nor ICD—10 permits the diagnosis of FLD, although both include Pick's disease as a possible aetiology. Because other aspects of cognition and activities of daily living are relatively preserved in early FLD, our rates may be conservative in that some cases may have been missed using a screening test that relies on orientation, memory and independence in activities of daily living (Reference Gregory and HodgesGregory & Hodges, 1996). There are suggested ways of overcoming this problem using additional bedside tests (Reference Gregory, Orrell and SahakianGregory et al, 1997). In our study, FLD was diagnosed more often using consensus criteria than the Gregory and Hodges criteria. This may be due to disagreement about the terminology used to describe these dementias: some groups prefer ‘frontotemporal dementia’ (Reference Brun, England and GustafsonBrunet al, 1994) whereas others regard this term as including disparate conditions and so prefer ‘dementia of frontal type’ (Reference Gregory, McKenna and HodgesGregory et al, 1998). Both sets of criteria used in our study are weighted towards a purely degenerative aetiology of FLD and exclude the possibility of a vascular contribution to symptoms.

Our results indicate that it is possible to determine a probable aetiology in most cases of dementia. The relatively small degree of overlap of the Alzheimer's disease and vascular dementia categories, as shown in Fig. 1, indicates that the diagnostic systems in current use lead to the same diagnostic conclusions for most individuals. This, however, is only true for Alzheimer's disease and vascular dementia.

The ability to distinguish particular subtypes of dementia is important for several reasons. It enables clinicians to identify associated risk factors, to implement specific treatment strategies, to inform patients and relatives more accurately of the prognosis of each one and to provide relevant services. This is particularly pertinent in the light of the development of treatments such as the cholinesterase-inhibiting drugs for Alzheimer's disease. The demand for these drugs, together with their high cost, requires accuracy of diagnosis and standardisation of diagnostic criteria. The inability of DSM and ICD to identify DLB and FLD, which are relatively common forms of dementia, means that they do not suffice to categorise these conditions. Both DLB and FLD should be incorporated in future editions of standard diagnostic criteria.

CLINICAL IMPLICATIONS

1. • Prevalence rates of dementia subtypes vary depending on the diagnostic criteria used.

2. • Dementia with Lewy bodies (DLB) and frontal lobe dementias (FLD) are relatively common and will be encountered frequently in clinical practice.

3. • Future editions of standard diagnostic systems such as DSM—IV and ICD—10 should include DLB and FLD as categories.

LIMITATIONS

1. • Not all people scoring above the screening cut-off point for dementia were demented on subsequent assessment.

2. • No diagnosis was made for almost one-third of people identified as demented at screening.

3. • Post-mortem confirmation of diagnoses was not available.