Archive for the ‘epidemiology’ Category

Asthma Prevalence, Health Care Use, and Mortality in the United States, 2001–2010

Two major asthma epidemiology reports today from CDC.

The National Center for Health Statistics released the latest data brief analyzing trends in asthma prevalence, health care utilization and mortality in the US from 2001-2010.

Most of the news is not good:

  • Prevalence rates of asthma have reached 8.4 percent, the highest ever recorded in the US.
  • Rates of emergency department visits and hospitalizations remained stable.

but mortality – which was highest among those 65 and older – did decline over the period.

The CDC’s Air Pollution and Respiratory Health Branch has also posted a new graphical overview of “Asthma’s Impact on the Nation”

“The information in this release is a stark reminder that asthma continues to be major public health concern with a large financial impact on families, the nation and our health care system,” Christopher Portier, director of CDC’s National Center for Environmental Health and the Agency for Toxic Substances and Disease Registry, said in an agency news release.

Portier also highlighted the lack of asthma action plans among the majority of children and adults with asthma, and encouraged those with asthma to work with their doctors to take control of this disease.

CDC releases National Asthma Control Program state profiles

CDC has set up a new page collecting short (two-page) burden of disease profiles from the 36 National Asthma Control Program grantee states across the US.

These summaries highlight key statistical data, such as prevalence and health care use in adults and children, as well as data on patient education and medication use from the Asthma Call-Back Survey.

Great to have these standardized briefs collected in one place. The obvious next step: Make the data underlying these PDFs readily available.

CDC – Asthma – National Asthma Control Program State Profiles

Parents misperceive asthma control in kids

The rise of asthma control and impairment as the main indicators of management has renewed interest in a longstanding challenge: Variability in the perception and experience of asthma symptoms. Parents and children have been shown to differ in their assessments of the existence of asthma, let alone the presence or severity of specific symptoms. And the meaning of symptoms, and the ties to medication taking, are other matters entirely.

A new report from a large interview study suggests that worldwide, few children and adolescents achieve control of their asthma and experience frequent symptoms. A significant portion (11 percent) reported mild asthma attacks at least weekly, while 35 percent required oral corticosteroids or hospitalization at least annually.

The team interviewed 1,284 parents of children with asthma in six countries (Canada, Greece, Hungary, the Netherlands, South Africa and the UK) and 943 of the children themselves. The results highlight the impact of frequent morbidity on daily life: Asthma restricted the child’s activities in 39 percent of families and caused 70 percent to change their lifestyle. The article was published in the European Respiratory Journal.

One reason for the significant morbidity may be parental misperception of asthma control. Parents in the study tended to underestimate the severity of their child’s asthma while overestimating the level of control. While 73 percent of parents described their child’s asthma as mild or intermittent, 40 percent of children/adolescents had C-ACT scores ≤19, indicating inadequate control. In addition, even fewer (14.7%) achieved complete control as defined by the more stringent Global Initiative for Asthma (GINA) guidelines.

Parent misperception of control in childhood/adolescent asthma: the Room to Breathe survey

W.D. Carroll, J Wildhaber and PLP Brand

Race, socioeconomic status and lung function

For the last two years, John Mullahy and Sheryl Magzamen and I have been working on an analysis of the apparent racial differences in normal lung function and the contribution of socioeconomic status to those patterns.

Our goal was to investigate whether alternative statistical methods (quantile regression) might better illustrate the effects of educational achievement (as a proxy for SES) across the entire distribution of lung function in a population, and to understand variability across racial/ethnic groups. In addition, we wanted to understand how sample selection criteria used to generate reference equations for normal lung function might alter estimates of the effect of socioeconomic status.

The resulting paper, Understanding Socioeconomic and Racial Differences in Adult Lung Function, has just been published in the current (September 2011) issue of the American Journal of Respiratory and Critical Care Medicine, along with an editorial (FEV1 in the Suburbs) authored by Peter Wagner of the Univ of California San Diego. The Univ of Wisconsin School of Medicine and Public Health has also issued a press release, with the following great quote from John about sample selection:

Seemingly subtle issues in how samples are constructed and data are analyzed ultimately have important implications for how we understand the roles of race and socioeconomic status as determinants of respiratory health.


Disease labels in national surveys – the case of COPD

The new CDC Framework for COPD Prevention, much better thought of as a well developed agenda for applied public health, estimates that half of the people with COPD in the US have not been diagnosed.

The report – developed by a group of experts during a workshop in 2010 – proposes first among its four goals that the US improve the collection, analysis, dissemination, and reporting of COPD-related public health data. In particular, it highlights the need to develop and initiate new data collection within existing surveys, an obviously efficient and valuable objective. It recommends:

Refining the definition of COPD in existing surveillance systems by adding the terms ‘COPD,’ and ‘chronic obstructive pulmonary disease’ to the currently used ‘emphysema’ and ‘chronic bronchitis’ terms.

This sentence brought me to a stop, frustrated that this change still needs to be made.

Our public health agencies must soon recognize some basic limitations of our surveillance systems: If they attempt to assess prevalence by asking people to report a physician diagnosis of some disease, they have no choice but to coevolve with diagnostic and popular nomenclature.

In this case, COPD has now been the dominant label for several years. Changes in awareness and labeling have been driven by many forces, including an improved understanding of the pathophysiology and natural history of the disease. But also by:

  • the rise of national organizations with names like the COPD Foundation, which was founded way back in 2004,
  • the marketing and education efforts of our own federal agencies (see the NHLBI page for example, or, well, the title of the report, which one downloads from the COPD page at CDC),
  • an increase in direct to consumer marketing of pharmaceuticals for the disease (such as this page for a branded formulation of tiotropium).

If we expect accurate and reliable prevalence estimates, we need to require that national surveys such as BRFSS, NHANES, and NHIS match the terminology in circulation. It is no longer acceptable to field surveys characterized by diverging epidemiological measures and popular labels.

A quick look at search frequency for COPD and emphysema (via Google Trends) shows that, in the US, search for COPD is more than twice as frequent compared to that for emphysema, and has been since about 2007. The dramatic, transient increases in search frequency for emphysema, which followed highly-publicized celebrity deaths attributed to emphysema (Johnny Carson in January of 2004) and mis-attributed to emphysema (Amy Winehouse in 2008), underscore the dynamic popular landscape of disease labels but also show the resilience of the longer-term trend.

Moreover, there are not large regional variations here; search for COPD (in blue) dominates that for emphysema (in red) across all regions of the country.

Inertia in the methods of our national surveys undermines the utility and value of their resulting estimates and, while it may make for more stable measures, assures that they are steadily providing information about an increasingly inappropriate category.

Public health, population health, and mHealth

The stabilizing prevalence of asthma and the origins of the disease

Statistics Canada recently reported that the prevalence of asthma among 2-7 year old children had declined to its lowest level in more than 10 years, from 13.2 percent in 2000-01 to 9.8 percent in 2008-09, the most recent year for which data are available.

The findings, drawn from the National Longitudinal Survey of Children and Youth, echo many others that have previously pointed to a plateau and decline in asthma prevalence in the last decade in many high-income countries. Reports in respiratory journals began to appear as early as 2000 suggesting that the number of cases of asthma appeared to be stabilizing in some countries and even subsiding in others. These data imply that in Canada, exposure to the cause(s) of the epidemic is no longer increasing, and could even be declining.

The changing trends in asthma prevalence highlight fundamental shortcomings in our current epidemiological theories. Just when it appeared that we finally had the evidence and theoretical framework to explain the increasing prevalence of asthma worldwide, we now rather suddenly have to account for an inexplicable and unanticipated decline.

The hygiene hypothesis, for example, emerged as a predominant explanation for the epidemic of asthma. It suggested that lifestyle changes accompanying westernization and modernization had reduced exposure to microbes that had previously played a valuable role in training the immune system during childhood. It proved to be a useful explanation that made sense of important findings of lower rates of asthma among children raised on farms, for example. But the hygiene hypotheses could never really adequately account for rising rates of asthma in inner cities, and now, of course, it lacks the power to explain the decline in asthma.

Official reports announcing the declining prevalence of asthma in Canada pin it on reductions in the rates of tobacco smoking and improvements in other environmental exposures. In the report, Eleanor Thomas from Statistics Canada writes:

“A number of environmental factors may be related to the recent declines in childhood…asthma: changes in the population structure; changes in diagnostic practices; decreases in the prevalence of respiratory allergies; improvements in air quality; changes in hygiene practices (particularly, in child care settings); and reductions in children’s exposure to cigarette smoke at home….[R]educed exposure to tobacco smoke may be contributing to the decreased prevalence of…asthma among young children.”

But the situation in asthma epidemiology is actually quite a bit more complex. Statistics Canada has it mostly wrong about the environmental exposures it scrutinizes. And they have it wrong in a way that productively highlights an important difference between public health and population health.

Primary and secondary risk factors in asthma epidemiology

The key difference hinges on the distinction between primary and secondary risk factors. Statistics Canada, and much of asthma related public health at the moment, is focused on tracking secondary risk factors for asthma. Secondary risk factors are those inputs that influence which individuals in a population will develop asthma. By contrast, primary risk factors are those that determine the overall level of asthma in the population.

Tobacco smoke exposure offers a useful example. Over the years, research has repeatedly established that tobacco smoke exposure increases the odds that a given child will develop asthma. However, we also know that tobacco smoke exposure cannot, in and of itself, be responsible for the increase in asthma cases in the latter half of the twentieth century. The asthma epidemic occurred in populations at a time when rates of tobacco smoking were declining. Tobacco smoke exposure and asthma prevalence, in other words, demonstrate counter trends at the population level.

Statistics Canada’s own evidence on tobacco use bears this out. Here is a figure showing the prevalence of cigarette smoking between 1985 and 2001.

Cigarette smoking (above) declined from 35.1% in 1985 to 21.7% in 2001 while asthma prevalence (below) rose from approximately 3% in 1984 to more than 13% in 2000-01.

In addition to tobacco smoking, Statistics Canada highlighted improving environmental factors — improvements in air quality specifically — as potential contributors to the recent decline in asthma rates. But again we have evidence that atmospheric pollutants cannot be responsible for the epidemic of asthma because exposure to these risk factors has been reduced at the population level over the same time period. The following figure illustrates changes in the mean concentration of particulate matter (2.5 microns) as measured by Canadian National Air Pollution Surveillance network between 1990 and 2001.

The story is the same whether you look at mean annual concentrations of particulate matter (PM2.5 concentration in 2001 was 27% lower than in 1990, while the PM10 level was 34% lower), carbon monoxide (34% lower in 2001) sulfur dioxide (32% lower in 2001), or nitrogen oxides (NO in 2001 21% lower, while NO2 concentration15% lower). [For the full report see Environmental Protection Series National Air Pollution Surveillance (NAPS) Network. Air Quality in Canada: 2001 Summary and 1990-2001 Trend Analysis. Report EPS 7/AP/36 May 2004.]

In short, air quality in Canada improved in many places during the time period when there was a rather uniformly increasing trend in the prevalence of asthma, and continued to do so when the rates of asthma stabilized and began to decline. One cannot attribute only the recent decline in asthma prevalence to improving air quality.

What we see in these explanations is that many clinicians, researchers and epidemiologists and public health agencies tend to focus on associations and explanations involving secondary risk factors, such as tobacco smoke or air pollution, and to develop and field interventions that address them. There are certainly valuable reasons for doing so. These kinds of exposures are more ascertainable in clinical encounters, they are often modifiable by patients, and their control and mitigation may have an important positive effect on the individual and the day-to-day manifestation of their asthma. Indeed, the evidence from Statistics Canada suggests exactly that:

“A key finding is that the percentage of children with asthma who reported an asthma attack in the past 12 months fell steadily from 53 per cent in the mid-1990s to 36 per cent last year.”

In short, children with asthma in Canada are being less frequently exposed to asthma triggers and, as a result, reporting fewer asthma attacks. Good news. Reduced tobacco smoke exposure certainly deserves credit for contributing to this decline.

It should be evident, then, that there are clear, important risk factors for asthma that play a significant role in the natural history of the disease in a given patient, but which, all the same, cannot be responsible for the epidemic.

These secondary risk factors are not (necessarily) the same things that we would choose to focus on if we were trying to change the overall rate of asthma in the population. As Jeroen Douwes and Neil Pearce, two international asthma experts, wrote in a 2006 editorial:

“Thus, the ‘established’ risk factors for asthma do not appear to explain the global prevalence patterns and time trends. These risk factors were ‘discovered’ primarily on the basis of clinical studies and case reports of exacerbations in asthma patients. It is natural for physicians and patients to assume that the factors involved in secondary causation may also be important for primary causation. However, for most of the ‘established’ risk factors, the evidence of primary causation is relatively weak, and risk factors such as allergen exposure do not appear to explain the prevalence patterns and time trends.”

The population health possibilities

In his 1985 article “Sick Individuals and Sick Populations,” Geoffrey Rose made the observation that small changes in the risk of a disease, when summed across a population, could result in dramatic shifts in the prevalence of a disease. Although he didn’t consider the concept of primary and secondary risk factors, his work demonstrated how risk factors insignificant to an individual could be meaningful to the population, and prompted a reevaluation of public health preventive strategies.

The growing evidence of a plateau and decline in asthma should have had similarly major implications for our view of the (partially understood) potential etiologic mechanisms underlying the asthma epidemic and the origins of asthma. So far, however, the signs of shift to a population health perspective have been minimal. In part, our ability to think about and understand the complex causes of the long increase in asthma and the plateau and decline has been hampered by the continuing orientation of epidemiology and public health to surveillance and analysis of secondary risk factors.

In the case of asthma, it remains a struggle to identify primary risk factors from available public health data. We are limited to retrospective and outdated data on only the most severe exacerbations (those that result in hospitalizations and, in some states, emergency room visits). Altogether, that amounts to just a tiny fraction of the daily burden of asthma morbidity.

Moreover, our surveillance system is designed to ignore potentially valuable information about where and when the attack began, whether that is at home, at school, at work, or out in the community. Typically the only geographic information available is the billing address. As a result, we cannot understand the small area variation in rates of asthma that we know exists.

New approaches to chronic disease surveillance and epidemiology are emerging that, if guided by a population health perspective, have enormous potential to advance our understanding of chronic diseases. It is already clear that these systems will be increasingly driven by participatory ethics (see the 2010 article by Freifeld et al. in PLoS Medicine) and will collect and rapidly make sense of valuable new streams of data, drawn from distributed networks of sensors and connected devices. They will bring growing numbers of people (see notes below) and their daily experiences of health, disease, medicine and the environment under increasing statistical scrutiny (cf. RWJF Project HealthDesign), with many potential benefits. It is likely that such approaches will generate new, useful clinical knowledge. A recent analysis of data volunteered by individuals using CureTogether, for example, has identified a new symptom marker that predicts negative response to a specific migraine treatment.

But the materialization of an epidemiological advance from these tools will be more complex and less certain. It will, in fact, turn on our ability to use them to marshal a new hunt for primary causes – that will complement the traditional clinical and public health focus on secondary risk factors that has dominated them so far.

Even then, there may still be very difficult challenges. For example, it is worth considering the possibility that it may be too late to search for the primary cause(s) of a number of chronic diseases in the urban areas of high-income countries. In these settings, exposure to the primary causes of a given disease may already be ubiquitous across the population. Without variability in exposure to the disease’s primary causes, all that would be revealed are the effects of secondary risk factors. Similarly, repeated investigations over short periods will only be informative if they capture a dramatic, and thus unusual, change in lifestyle or environmental exposures.

In other words, our odds of making fundamental discoveries are tied to our ability to develop tools and systems that help us collectively uncover a new list of primary risk factors and hypotheses. To do this might mean we engage communities that display a broad range of prevalence (urban vs. rural areas of low income countries), or populations that abruptly change their environment and lifestyle (immigrants, refugees, adoptees, etc.) Such efforts would have a much greater probability of identifying primary causes of the chronic diseases we’re targeting, especially if the confounding role of secondary causes is also accounted for.

Our new systems should help us explain the overall rates of asthma in populations, its global gradients and time trends, and do so with some evolutionary probability. We need to pause to reflect on the u-turn that the prevalence of asthma has done and re-double our efforts to piece together the rules that underlie the surge, pause and decline in the prevalence of asthma. It will also almost certainly mean evolving our approaches to better sample experience, understand context, lifestyle and environment, and to look for connections where there may not be evidence of an effect on individuals.

In a recent article in Science on open mHealth architecture, Deborah Estrin and Ida Sim noted that if only 1 out of every 250 patients in the US taking antidepressants participated in a collaborative study of their long term efficacy, more than 100,000 patients enrolled would exceed the total number of patients enrolled in all antidepressant studies conducted worldwide since 2005.

The limits of retrospective health survey questions

The Health Interview Survey, conducted by the National Center for Health Statistics, asks respondents:

“Now, I’d like to read you a short list of different kinds of pain. Please say for each one, on roughly how many days–if any–in the last 12 months you have had that type of pain…How many days in the last year have you had headaches?”

The question is then repeated for backaches, stomach pains, joint pains, muscle pains, and dental pains.

I don’t think I could answer that question. Seems to me we’re asking people for things that they can’t tell us.

Directed, cooperative exploration of health data

This is an update and expansion of a post I wrote for the Health 2.0 Developer Challenge earlier this summer. The original is here.

With the recent launch of the Community Health Data Initiative (CHDI) and the emergence of a growing number of health apps, the magnitude of health data and the variety and number of tools to explore and analyze that data is expanding quickly. But sitting in the audience on June 2 at the HHS/IOM CHDI forum, watching teams demonstrate search and visualization applications for community health data, I found myself thinking about an unrealized opportunity: The lack of means to pool our capabilities and organize cooperative action around health data and its exploration. 

What I envisioned while watching the health app demos that day was the inevitability of redundant searches by visitors to these sites. How many thousands of people would map air pollution against asthma prevalence, or the association between obesity and the distribution of parks or grocery stores. On the whole, such engagement is a sizable positive step, encouraging interest in the epidemiology of health, healthcare and the environment. But so far, health apps stop short of delivering tools to create with the data. 

As an epidemiologist, what I’m hoping to see emerge is a way to capture the knowledge created by the data exploration carried out by community members. In short, a way to distill scientific value from widespread interactions with health data – if only to help map out dark corners. In fact, there are many health problems for which the data are (vastly) more abundant than the time to analyze it. In most cases, scientists cannot explore all the plausible associations between a single disease and the hundreds – if not thousands – of potentially relevant variables and other diseases in even one dataset, such as NHANES. There is also a scarcity of perspectives in professional science; many discoveries are made as a result of the variability amateurs bring to the puzzles. The topics of searches themselves will reveal community interests and priorities, and the results of those searches – if captured and analyzed in realtime – could highlight important data gaps or suggest potential, previously unknown relationships worth investigating.

So, while opening up new horizons of health data and providing tools for people to interact with this information, we can also design parallel tools to encourage popular epidemiology. It is easy to envision a health app that stores graphs and visualizations created by individuals, ties them to their constituent datasets, and provides parallel community discussions. Something that aims to capture the emergent, collective analysis of newly available US health data. It should encourage communities, agencies, amateur or citizen scientists, professional scientists, media outlets, and healthcare providers, to cooperate on raising and answering specific questions by using our collective cognitive surplus (see Clay Shirky) to mine the new horizons of data and the possibilities that occur in the minds of individuals from every corner and background.

It could be productively modeled on distributed version control systems, like Git, which enable a diverse group of individuals to participate collaboratively in the creation of software. One can imagine a health app where individuals or groups could create repositories focused on a given health research question or topic. These repositories would contain annotated data, methods, tabular and graphical results, documentation, discussions and supporting code. Most importantly, they would have a trajectory that develops as material and knowledge in the repository accumulates. 

For example, a person or group with an interest in examining regional variability in tuberculosis incidence could check out a repository, carry out an analysis, and then write a short note summarizing and committing their results to the pool. Another individual could review or replicate the same work, see the same results, and vote to underscore its accuracy or importance. Subsequent visitors could review the latest information, reach the decision that a new variable, say air quality, should be included in the analysis. At this point, the person could fork the repository, import relevant air quality data and begin a new branch of research. 

Open data will be more valuable if we also capture and share open methods. One way to seed such analyses would be to encourage university epidemiology and health sciences faculty (as well as public health practitioners and agencies) to construct curriculum and lesson plans around community health data. In other words, to write and post exercises that teach some aspect of epidemiology by examining the relationship between variables in health data. If students carried out different versions of these exercises each time, the repository of methods and results and interpretations would develop. Similarly, a growing number of projects (such as Educate to Innovate, Teaching Opportunities for Partners in Science; and Retirees Enhancing Science Education through Experiments and Demonstrations; National Lab Day) recruit volunteer scientists and technology educators in an attempt to increase the performance of students in science, technology and engineering and to bring science, statistical and data literacy to local communities. We can start to assemble such a volunteer army for health data, and provide them with the tools to make them successful.

In fact, there is no requirement (other than simplicity) to stop at the analysis of existing data. While rather more complicated, we can imagine (and work towards) health apps that engage communities explicitly in the actual collection of data. Given the increasing difficulties involved in enrolling participants in national population-based surveys, such as NHANES, the time for exploring alternative methods of population-based surveys is in order. The academic health community has for some time been trying with mixed success to bring communities more into the research process. Providing tools to create value from health data – and the guidance of a community of volunteer experts – will complement and speed research and improvements to public health.