There is agreement that in high income countries, severe disability has being postponed but  there is scarce information for the very old age group, those beyond 85 years of age. Disability  trends are generally decreasing but less so in countries where disability had been already low,  such as the Netherlands and Sweden [1] where severe disability had already decreased between  1980 and 1990[1].  

Historical events have shaped social and economic conditions influencing exposures to chronic  disease risk factors, which in turn have changed the risk of chronic diseases throughout the life  course of individuals. A life course perspective is needed to interpret disability survey results. It  also allows to take into account the conditions of life of specific birth cohorts [2].

Using a life course perspective, the aim of this text is to review the current debate on population  disability trends in high income countries. We propose, first, that the concepts and methods of  life course epidemiology can increase our understanding of population health and disability  trends in a world undergoing rapid demographic, social and economic transformation.

The concepts and methods of  life course epidemiology can increase our understanding of population health and disability  trends in a world undergoing rapid demographic, social and economic transformation

Second,  we use the available evidence from three studies in Spain to argue that country level factors,  beyond individual health factors, may underline the observed disability trends by modifying the average risk of disability in the population. Last, we examine the opportunities for dementia  prevention and how mass preventive intervention can alter the magnitude of the announced  epidemic of dementia.

There is real potential to change the relationship between chronological age and the incidence of chronic disease and disability by reducing exposure to poverty and violence from childhood through old age, increasing healthy behaviors across the life course, and ensuring access and coverage to good chronic disease prevention and management in old age

We conclude that there is a real potential to change the relationship  between chronological age and incidence of chronic diseases and disability by reducing poverty  and violence exposures from childhood to old age, increasing healthy behaviours along the life  course and warranting access and coverage to good prevention and management of chronic  diseases in old age.  

Current disability burden.

The recently published Global Burden Disease 2013 report illustrates the challenges that  increasing trends of multimorbidity and disability pose to the health systems of all countries,  including high income countries[3]. We are told that population growth and population aging  are the driving forces of the increases in prevalence of chronic diseases and particularly diabetes. The number of people with neurological, respiratory and musculoskeletal conditions is  increasing in high income countries and particularly in Europe.

The leading cause of years lived with disability in Spain is diabetes, followed by back and neck pain, major depressive disorders, falls, hearing impairment, migraines, other musculoskeletal disorders (osteoporosis and osteomyelitis and associated fractures), chronic obstructive pulmonary disease, and anxiety

We are also warned that the first  leading cause of years lived with disability in Spain is diabetes, followed by back and neck pain,  major depressive disorders, falls, hearing impairment, migraines, other musculoskeletal  disorders (osteoporosis and osteomyelitis and associated fractures), chronic obstructive  pulmonary disease and anxiety. Main killers, cardiovascular disease and cancer, are not listed  here.  

Due to improved treatments, mortality is decreasing faster or increasing slower than prevalence  of disease. For instance, for diabetes in Europe, age standardised prevalence rates increased by  43% globally while mortality increased only 9% in the period between 1990 and 2013. What  would be the comparable figures for Spain, where diabetes has become the leading cause of years with disability? In addition, back pain and neck pain are disputing the second and third  places of the list and competing with major depressive disorders. Respiratory chronic conditions,  such as COPD and asthma are located in the list just prior to anxiety. Thus, the Global Burden of  Disease report ranks disability causes in different European countries and emphasizes the  importance of diabetes, muscular-skeletal conditions, mental health and respiratory chronic  conditions. To complete the picture, hearing impairment is clearly listed as the sensory deficit  responsible for more disability years. 

Hearing impairment is clearly listed as the sensory deficit  responsible for more disability years

Examining disability by age groups in those high income countries of the world, we are told that  only 0,03% of those over 80 years of age are free of sequelae from the diseases considered in  the GBD; 10% had between 1 and 4 sequelae, 64.6% had between 5 and 9 sequelae and 25.1%  had 10 or more sequelae. Since prevalence of chronic conditions increases with age and the  population distribution of ages shifts to older ages, there is been an increase of 52% in the  number of people with 10 or more sequelae between 1990 and 2013. Women were more likely  to fall in that group.  

These results give a good summary of the burden of disability in Europe. We conclude that,  compared with findings from previous GBD reports, we have an increasingly older population  with selected chronic conditions (diabetes, musculoskeletal conditions, chronic respiratory  disease, mental diseases and hearing impairment) leading to multiple sequelae and disability. More women than men are affected by multiple sources of disability and this gender difference  has been attributed to sex and gender differences.

We have an increasingly ageing population with certain chronic diseases (diabetes, musculoskeletal conditions, chronic respiratory diseases, mental illness and hearing problems) that result in multiple sequelae and disability

How does a life course perspective contribute to understanding these trends?  

The picture we have drawn corresponds to certain birth cohorts, who were born and have lived  through the twentieth century and who have been exposed to the living conditions prevailing in  Europe between the first and second world war (or the Spanish Civil War), the post-war period with the development of the Welfare State, the establishment of universal old age pensions, the  improvements in health coverage through National Health Systems, the recent advance of  neoliberalism, the successive economic crises of the last quarter of the twentieth century, the  2008 Great Recession and austerity measures, curtailing the Welfare State and imposed to face  the last and more profound economic and social crises of 2008-2014. These experiences have  shaped the health and function of the population of older adults at different stages of their life  course.  

Epidemiologic studies of older adults, conducted mostly in Europe and in North America, have  repeatedly shown the strong effects of childhood socioeconomic status and early life social  adversity on physical function and disability in old age. In addition, adulthood and old age  adversity have been shown to have cumulative effects on physical function in old age[4-6]. 

Epidemiologic studies of older adults, conducted mostly in Europe and in North America, have  repeatedly shown the strong effects of childhood socioeconomic status and early life social  adversity on physical function and disability in old age

Disability trends need to take into account the strong influence of the social and economic  conditions during the life course on the patterns of chronic diseases and disability in old age. In  spite of this evidence, we are unaware of current disability predictive models that have  integrated a life course perspective. 

Disability trends in old age in Spain: three studies with different designs and some  contradictory findings. 

Three epidemiologic studies have analyzed the disability trends in the period around the turn of  the century. We will describe them briefly and we will try to draw conclusions in spite of  apparently diverging results.  

The first study compares prevalence estimates of severity of disability in the basic activities of  daily living (ADL) or in mobility using the Spanish National Disability, Impairments and handicap  surveys of disability conducted by the National Institute of Statistics in 1986 and 1999. Findings  show that age standardized ADL disability prevalence increased slightly for men (0.5%) and for  women (1.8%) but age standardized mobility disability declined from 28% to 15% in men and  from 37% to 25% in women. This very impressive decline in mobility disability would lead us to  believe in an optimistic future. Although these figures are based on surveys and do not give us  information on individual changes through time, they suggest that disability is being postponed  to advanced ages and the number of years lived with disability is decreasing at the individual  level[7]: the proportion of disability free life expectancy increased both in men and women over  65 years of age between 1986 and 1999. 

The second study is a longitudinal study of a representative sample of older adults living in  Leganes, a city in the Madrid Metropolitan Area. In the Aging in Leganes study, subjects were  followed for 6 years and a clear postponement of the onset of both ADL and lower functional  limitations related to mobility was observed when comparing birth cohorts. Thus, people who  were 70 years old in 1993 had the same prevalence of ADL disability than those 76 years old in  1999. Since institutionalization in the Leganes population in that period was negligible, these  results support a postponement of disability in successive cohorts. Postponement could be  related to the improvement in the conditions of life (socioeconomic, nutrition, education,  gender equality and work) of different cohorts and to the increased access and quality of health  services for chronic diseases management following the 1986 legislation to establish the  National Health System[8].  

The third publication is based on data from Barcelona Health Surveys of 1986, 2000 and 2006.  Results on the older population of this city do not show a decrease in the overall ADL disability  prevalence rates in the population over 65 between 1986 and 2006 but an increase (particularly  in older women) which is completely explained by population aging, that is the increase in the  number of very old women[9]. Age-adjusted prevalence of disability in men went from 28% to  31% in men and from 46% to 53% in women between 1986 and 2006 and these increases were  observed for almost all items in the ADL scale.  

Using a life course perspective, we advance an explanation for the disagreement between the  results of the Barcelona study and the previous studies based on national surveys or a local  cohort. The older population of the city of Barcelona has on average better socioeconomic  situations than the average of the national population or the Leganes cohort. Even if education  is just one dimension of socio-economic status, education levels may be used to support this  argument. In 1993, at baseline, 72.3% of men and 85.8% of women in the Leganes study sample  had less than primary education. These figures are much higher than the corresponding figure  for the sample in the Barcelona survey: 42.3% of men and 67.3% of women. In 1992, the  distribution of Leganes older adults was very close to that of the Spanish population of similar  age. While education is only one possible marker of socio-economic status, the maximum level 

of education attained is strongly determined by early childhood social and economic  environment. While older adults in the Leganes cohort had a rural and poor upbringing,  Barcelona older adults have more varied origins. Therefore, we propose that the improvement  of living conditions in Spain during the second half of the XXth century, and in particular for the  Leganes study participants, would have led to the observed decline in mobility disability (and  also in ADL disability, according to the Leganes cohort[8]) while for older adults living in  Barcelona[9], socioeconomic changes have been less marked both at the individual as at the  societal level, with little or no change in ADL disability. 

Opportunities for dementia prevention and mass prevention interventions through a life  course perspective.  

In recent years the ability to prevent dementia has been recognized. In fact, seven modifiable factors for dementia can explain more than half of dementia cases in the world population[10,11]. This list of 7 factors is not complete since it does not include other known factors related to  social networks[12] and nutrition[13]. Dementia prevention would require shifting of the population distribution of these known risk factors and would imply modification of social  conditions (improving education and social participation), behavioral factors (reducing physical  inactivity and smoking) or improving the clinical management of well-known chronic conditions  (hypertension, diabetes, depression and obesity). Several clinical trials are ongoing in Europe  and North America to assess the efficacy of modifying these factors to reduce dementia risk but  our knowledge on the etiology of dementia already supports preventive population  interventions from childhood to old age. Cognitive decline should be considered more as an age  –related condition that can be postponed than as a disease entity susceptible of therapeutic  intervention in old age[14]. Here again, the potential to change the relationship between  dementia and old age rests on our ability to modify risk factors prevalence along the life course. 

The list, although not complete, of the 7 factors that can prevent dementia would involve modifying social conditions (improving education and social participation), behavioral factors (reducing physical inactivity and smoking) or improving the clinical management of well-known chronic conditions (hypertension, diabetes, depression and obesity)

Conclusion 

Disability trends have been observed and predicted based on current and past patterns of  chronic diseases. However, the potential to change these trends lays on our ability to modify risk  factors along the life course, both for physical and cognitive decline and age-related disability  and dementia. Known risk factors include early child social and economic adversity, lifelong  education, income and social participation, healthy behaviours and good quality management of  chronic conditions in adulthood and old age. We have accumulated enough scientific  knowledge to pass to action and to assert that prevention of physical disability and dementia  requires a strong political will to reduce poverty and violence during the life course and to  promote good health behaviors since childhood to old age, to provide healthy environments in  old age and good quality health care.

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