EDITORIAL

EPIDEMIOLOGY OF DEMENTIA

Dr.S. SHAJI

THE DEMOGRAPHY OF AGEING
Population ageing is a global phenomenon which is no longer restricted to developed countries. From 1980 to 2005 there will be a threefold increase in the size of the world population aged over 60. However the increase will be largest in the  developing world, four fold compared with only two fold in developed world. In India according to 1981 census, out of the total population of 682.2 million, 44 million (6.4%) people were elderly (aged over 60 years). according to 1991 Census, 55.3 million people (6.6%) were over 60 years. Their number increased to 60 million (7.5%) by 200 AD. It is the number of elderly that is formidable rather than the percentage. Along with this demographic transition that is occuring throughout the world there will be an increase in age associated morbidity. Dementia being an important cause of age associated morbidity is expected to rise steadily.
Prevalence of Dementia
Prevalence is the proportion of existing cases of the disease in a defined population at a given point over a brief period in time. It is estimated trough cross-sectional study (survey) design, examining all persons (or a random sample of persons) in a community. Prevalence represents the disease burden of a population and is important when planning services and allocating resources. Prevalence itself depends on two other measurements: 1) incidence rate, or the proportion of new cases occuring in a  defined population over a given period of time (eg. 2 year). 2) duration of disease is how long affected persons live with the disease. Thus, either with high incidence or long duration can lead to high prevalence, whereas the reverse is true of low prevalence.
Estimates of the prevalence of dementia have been reported from population surveys done in the U.S., several European countries and several Asian Nations. Among citizens aged 65 and older, most estimates of the prevalence of all causes of dementia of at least moderate severity have been in the range of 3-10%. In general, estimated rates from the Asian nations have been somewhat lower than from the U.S., England and Europe. In a valuable review of reported surveys of dementia, Jorm et al2 noted that although age specific prevalence rate of dementia (all causes) varied widely, a fairly uniform pattern emerged when they were viewed collectively. The variation in reported prevalence estimates among nations are mostly in a range that might be explained by methodological differences in case detection and classification. The differences in case detection and classification. The most consistent finding across nations among research studies was an increase in rates of dementia with advancing age, age specific rates doubling about every five years. A world wide age specific prevalence rates  curve can be approximated by a trajectory that begins at about 1% at the age of 60, this rises to 2% at 70, 4% at 75 and 8% at 80 and 16% at age 85 years. These rates are for dementia of at least moderate severity3.
Studies of Dementia in India
Shanker etal4 reported the first autopsy confirmed case of Alzheimer's disease in a man with disease onset at age 73 and no family history of dementia. Satishchandra et al5 reported a histologically confirmed familial case of Alzheimer's disease in a women with onset at age 47. Barodawala and Ghadi6 noted at typical Alzheimer's disease pathology was present but rare in an outopsy series of 100 patients aged 60+ from Bombay.
In a review of literature on dementing disorders in India, Wadia7 noted that there were few systematic studies of adequate size and representativeness to provide good estimates of prevalence in India. He observed that in the Zorostrain Community (primarily in Bombay) where average survival had reached the eighth decade of life, Alzheimer's disease had become quit prevalent.
Prevalence of various demending disorders have been well documented in developed countries. In India, a few epidemiological studies have been conducted during the last decade. Rajkumar et al8 conducted two studies in Tamilnadu, one in Madras city, and the other in a rural community. Using the multistage stratified random sampling technique, 1300 individuals aged 65 years and above were selected from the city of Madras. The selected elderly persons were assessed using the third edition of Geriatric Mental Status Schedule Test (GMS) by trained staff. The prevalence of dementia was found to be 2.7%. A study9 to estimate the prevalence of dementia in a rural population was conducted in Thiruporur, a community located in the outskirts of Madras city. 750 elderly persons aged 60 years of age and above were selected using the cluster sampling technique, The prevalence of dementia was found to be 3.5% in this community.
Shaji etal10 investigated the prevalence of various dementing disorders in a rural community in Kerala. A door to door survey was conducted to identity elderly persons aged 60 years and above. A total of 2067 elderly persons were screened with the vernacular adaptation of MMSE. All those scored at or below the cut off score of 23, had a detailed neuropsychological evaluation by CAMDEX -Section B, and the caregivers of the people with confirmed cognitive impairment were interviewed using CAMDEX- Section H, to confirm the history of deterioration or impairment in social or personal functioning. In the third phase the subjects with confirmed cognitive impairment were evaluated at their home by a psychiatric and diagnosis, was made as per DSM-11 R criteria. The prevalence rate was found to be 33.9 per thousand. 58% cases were diagnosed as vascular dementia in Alzheimer's disease (ICD-10). Shaji etal11 conducted a study in the city of Cochin using similar methodology and the prevalence rate was found to be 3.4% in elderly persons aged 65 and above. 53% of dementia cases were diagnosed as Alzheimer's disease, 40% satisfied this criteria for vascular dementia and 7% were due cases like infection, tumor and trauma.
Chandra etal12 performed a community survey of a cohort of 5126 individuals aged 55 years and older, 73.3% of whom were illiterates. Hindi cognitive and functional screening instruments developed for and validated in this population, were used to screen the cohort, a total of 536 subjects (10.5%) who met operational criteria for cognitive and functional impairment and a random sample of 270 unimpaired control subjects underwent standardized clinical  assessment for Dementia using DSM-IV diagnostic criteria the Clinical Dementia Rating Scale and National Institute of Neurological and Communicative Disorders and Stroke-Alzheimer's disease and related disease association (NINCDS - ADRDA) criteria for probable and possible Alzheimer's disease. An overall prevalence of 0.84% was observed in the population aged 55 years and older and an overall prevalence rate of 1.36% in the population aged 65 years and older.
A prevalence study13 of major neurological disorders in the far northern Indian State  of Kashmir found no subjects with Alzheimer's disease. However, 42% of the population of that region was younger than 14 years, and the Kashmir survey included only 31 subjects aged 60+ years.
Epidemiological studies made it clear that there are differences in prevalence rates of dementia across various regions. All three studies conducted in southern part of India reported higher total and age-specific prevalence rates than those of Ballabgarh study conducted in Haryana in Northern part.
Regional Differences
Alzheimer's disease is the most common form of dementia in Western Countries. For most European and American populations, prevalence rate of Alzheimer's disease have been approximately half the total dementia figures, while the rates for vascular dementia have varied between 0 and 30% of the total dementia prevalence values. In most surveys from Japan, China and Taiwan the relative frequencies are reserved, with the rates of vascular dementia being 30-60% of the values for total dementia and the rate of Alzheimer's disease being 20-40% of the total dementia prevalence rates. It appears that prevalence rate for AD in U.S and Europe are 2-3 times than in Japan while vascular dementia prevalence rate in Japan is 1.5-2 times than those for  the U.S and Europe. This apparent difference in prevalence rate between two populations indicate that the difference may be related to some biological, factors rather than to methods of case detection.
A lower incidence of Alzheimer's disease could be mediated by differences in environmental or life style determinants or a lower prevalence of familial predisposition of Alzheimer's disease. The differences between Asian and European ancestry populations in the Alzheimer's disease : Vascular dementia ratio is best documented for Japan. Similar findings have been reported from China, Taiwan and India. The relative proportion of Alzheimer's disease in studies reported from India ranged from 41-53% while the proportion of vascular dementia ranged from 25-58% 
Risk Factors
Epidemiological studies suggest that a variety of factors contribute to the occurrence of Alzheimer's Disease (AD), particularly late onset AD. Age is clearly the most important risk factor for AD14,15. Meticulous epidemiological studies have established that being a women is an independent risk factor for Alzheimer's disease16. The higher prevalence of AD in older women has not yet been explained. Possible explanations include unrecognized environmental influences, unspecified hormonal effects, the presence of one or more predisposing genes on the x-chromosome; and the higher incidence of the apo E4 allele in the women.
Family history of Alzheimer's disease is one of the most consistent risk factors, increasing disease risk by approximately fourfold at any age. Established genetic risk factors are strongest for early onset, familial AD (three separate genes known as APP, PSI and PS2)17.18, but another gene called APOE419.20 is a risk factor for late onset and no-familial cases as well.
Head Trauma21,22 has been reported as a risk factor for AD in several, but not all, studies considering its potential role in the disease. In dementia pugilistica, repeated head trauma leads to dementia with the accumulation of neuropathology abnormalities associated with AD, including many NFTs and diffuse Alzheimer Amyloid. Epidemiological studies have not identified an environmental toxin that contributes to the development of AD. other than Aluminium, and the data for this are contradictory. Vascular disease appears to be a risk factor for Alzheimer's disease23,24. Other risk factors reported include Down Syndrome, major depression, diabetes, heart disease and thyroid disease. Protective factors for Alzheimer's disease reported in the literature include higher education, the APOE2 gene, intake of antioxidant substances (eg. Vitamins E and C), use of Oestrogen supplements in women, use of anti-inflammatory drugs and cigarette smoking.
METHODOLOGICAL ISSUES IN EPIDEMIOLOGIC RESEARCH.
Age ascertainment of elderly population is an important issue especially in literate populations. Many elderly people do not know their exact ages and may give differing ages at different times. The identification and classification of cases of dementia in a community survey require the use of appropriate instruments, methods and  criteria, and extremely rigorous attention to standardization and quality control at every step. It is possible that many cases of dementia may be missed if the screening instruments and diagnostic methods are not sufficiently sensitive. Use of cognitive screening tests not appropriate for poorly educated or illiterate elderly may over estimate cognitive impairment based on test scores. At times it is very difficult to estimate the functional impairment as the life-styles of rural elderly persons do not require them to perform instrumental activities of daily living comparable to those in industrialized societies. It is possible that many signs and symptoms of mild dementia may be misattributed as a part of 'normal ageing'. In most of the published reports comparing autopsy findings with premorbid diagnosis of Alzheimer's disease or vascular dementia, there was agreement for 75-85% of cases, i.e., the diagnosis made during life were incorrect for 15-25% of cases. 
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