Epidemiology of Musculoskeletal Disorders and Workplace Factors

Repetition

Force

Awkward posture

Vibration

Temperature

Contact stress

Unaccustomed activities

Lifting/forceful movement

Heavy physical work

Whole body vibration

Static work posture

Handling heavy loads over long periods

Frequently repeated manipulation of objects

Static muscular load

Muscular inactivity

Monotonous repetitive manipulations

Physical environment

Psychosocial

Combined or combinations of above

Table used with permission, Map Managers, Inc. http://www.ctdmap.com/

Table 10.2

Common list of possible individual risk factors

Individual

Gender

Genetics

Biopsychosocial

Nonwork activities

Table used with permission, Map Managers, Inc. http://www.ctdmap.com/

Although there have been concerns expressed regarding the inclusion criteria and methodology, an additional reference source is Musculoskeletal Disorders and Workplace Factors—A Critical Review of Epidemiologic Evidence for Work-Related Musculoskeletal Disorders of the Neck, Upper Extremity, and Low Back by the US Department of Health and Human Services, Public Health Service, Centers for Disease Control and Prevention, and National Institute for Occupational Safety and Health (NIOSH), July 1997 (public domain) at www.cdc.gov/niosh/docs/97-141/pdfs/97-141.pdf.

Linking the Key Elements

Why is this explanation important? The answer is that because most of the “science” we have regarding MSDs comes from epidemiological studies which often use the inclusion criteria of a “survey” to establish a “diagnosis” of MSD, which is then used in determining an association (risk) between a specific activity and the onset of the MSD in question. Therefore, occupational exposures and their association with, or causation of, injuries and illnesses are often debated. Because a determination for association or causation is required to determine eligibility for compensation and, therefore, financial responsibility for workers’ compensation or tort cases, debates and disputed legal cases often ensue (Melhorn & Ackerman, 2008). The significance of such disputes is underscored by the reported 1997 data listing direct health-care costs for the nation’s work forces of more than $418 billion and indirect costs of more than $837 billion (Brady et al., 1997).

An example of how the science may differ from public opinion would be helpful. Carpal tunnel syndrome was linked to keyboard activities. Because this proposed linkage is appealing and pervasive and seems to make sense, the lay press has advanced this association despite quality scientific investigations that found little or no relationship between carpal tunnel syndrome and occupation or hand use (Andersen et al., 2003; Brenner, Bal, & Brenner, 2007; Clarke Stevens, Witt, Smith, & Weaver, 2001; Egilman, Punnett, Hjelm, & Welch, 1996; Fisher & Gorsche, 2004; Garland et al., 1996; Hadler, 1999; Lo, Raskin, Lester, & Lester, 2002; Lozano Calderon, Anthony, & Ring, 2008; Melhorn, Martin, Brooks, & Seaman, 2008, 2011, Nathan, Keinston, & Meadows, 1993, Nathan & Keniston, 1993; Nathan, Keniston, Myers, & Meadows, 1992; Nathan, Meadows, & Istvan, 2002; Nordstrom, Vierkant, DeStefano, & Layde, 1997; Ring, 2007). Using Bradford Hill causation criteria, if an activity is the cause, removal or modification of the activity (the keyboard) should result in a reduction of the incidence. Two studies found that keyboard modification did not change the incidence of carpal tunnel syndrome (Lincoln et al., 2000; Rempel, Tittiranonda, Burastero, Hudes, & So, 1999). So how do we know what we know? This chapter will discuss what we know and how we know the Epidemiology of Musculoskeletal Disorders and Workplace Factors. The terms musculoskeletal disorders (MSDs), musculoskeletal condition, and musculoskeletal pain will be used interchangeably.

Introduction

The huge costs of work-related musculoskeletal pain and its associated disability are not new or unique to the population of the United States, but are a worldwide problem. Many historical manifestations of workplace pain have been related to innovation and changing technology. Some examples include miners’ nystagmus (change from candle to battery-powered head lamps), train dispatchers’ nystagmus (due to watching fast moving trains pass by the station), telegraphists’ cramp (1900s due to tapping on key), and watchmakers’ cramp (spasm of the finger) (Culpin, 1933). A list of other historical conditions is provided in Table 10.3 (Zeppieri & Melhorn, 2000).

Table 10.3

History of workplace diseases

BC	Greece	Pain in workplace
1473	Ellenbon	Pain with work of goldsmith
1567	Paracelsus	Pain with work of miners
1700	Ramazzini	Pain with work of potters
1830	UK	Writer’s cramp (50 % bilateral)
1880	UK	Occupational neurosis—steel nib
1882	UK	Telegraphists’ cramp
1960	Japan	Cervicobrachial disease
1960	Sweden	Tension headache
1960	Finland	Occupational headache
1962	Switzerland	Tension headache
1979	Scandinavia	Occupational cervicobrachial
1980	Australia	RSI (repetitive strain injury)

Musculoskeletal pain is often separated into two categories: occupational and nonoccupational. This distinction is often considered when reviewing the outcome of treatment but is commonly overlooked during treatment. This legal distinction is not required by the physician for treatment of the condition, but it has great importance for the patient. Injuries or illnesses can cause musculoskeletal pain in the workplace. An occupational injury by definition is one that results from a work-related event or from a single instantaneous exposure in the work environment. Injuries are reportable by the employer on the Occupational Safety and Health Administration (OSHA) 300 log if they result in lost work time, require medical treatment (other than first aid), or the worker experiences loss of consciousness, restriction of work activities or motion, or is transferred to another job (United States Bureau of Labor Statistics, 1997). An occupational illness is any abnormal condition or disorder (other than one resulting from an occupational injury) caused by exposure to a factor(s) associated with employment. Included in this category are acute and chronic illnesses or diseases that may be caused by inhalation, absorption, ingestion, or direct contact (United States Bureau of Labor Statistics, 1997). Musculoskeletal injuries are often defined as traditional traumatic injuries such as fractures, sprains, strains, dislocations, or lacerations, while musculoskeletal illnesses are commonly called cumulative trauma disorders (CTD), repetitive motion injuries (RMI), or musculoskeletal disorder (MSD).

Occupational medicine presents a number of challenges to the physician. Management of work-related musculoskeletal pain is often frustrating. Patients may have more complaints and longer recovery times, require longer and more frequent office visits, and may be accompanied by the employer or nurse case manager during the office visit (Black & Frost, 2011; Daniell, Fulton-Kehoe, Chiou, & Franklin, 2005). They frequently have more questions about work status, require more phone calls, and have more paper work requirements. Many have attorneys, and they commonly require a permanent physical impairment rating with subsequent depositions or mandatory court appearances. NCCI (National Council on Compensation Insurance, Inc.) data suggest that the average treatment duration is four times greater in workers’ compensation (WC) cases than in non-WC cases—206.6 versus 51.9 days, respectively (https://www.ncci.com/NCCIMain/Pages/Default.aspx). Treatment outcomes often shift from good to poor (Kasdan, Vender, Lewis, Stallings, & Melhorn, 1996). The negative shift in outcome indicates that WC involvement introduces additional factors that influence patients and complicate treatment efforts (Berecki-Gisolf, Clay, Collie, & McClure, 2012). Traditional Western medical education is heavily weighted in the scientific study of the biologic systems of health and disease, often to the exclusion of biopsychosocial factors (Zeppieri, 1999). Physicians who provide care to those with work-related injuries are often inadequately prepared to deal with the biosocial (also labeled as psychosocial or biopsychosocial) issues—including motivation, social factors, psychological overlays, economic incentives, and legal complications—that influence the outcomes of treatment (Marchand & Durand, 2011; Melhorn, 1998a). Those physicians who are adequately prepared are often faced with the difficult task of separating fact from fiction. Occasionally, the patient’s symptoms can be disproportional to the clinic examination. Because an occupationally related OSHA event requires only a complaint of pain, multiple subjective issues must be reviewed. This can make the clinical picture confusing and require more tests and studies to be used to arrive at the appropriate medical diagnosis, relative to a similar nonoccupational patient. Other factors impacting treatment costs might include somatization behavior among patients and medicalization among physicians (Barsky & Borus, 1995; Gross & Battie, 2005), cost shifting from commercial insurance to WC insurance (Butler, 1996), and removing disincentives for early return to work (National Practitioner Data Bank, 1994).

According to a 2011 survey conducted for the Center for Disease Control (http://www.cdc.gov/Workplace/), 51.5 % of adults reported a chronic musculoskeletal condition in 2009, twice the rate of chronic heart or respiratory conditions. Musculoskeletal conditions are so ubiquitous that they have become the third most common reason that Americans seek medical attention. A US Department of Health study showed that, from 1996 to 2004, managing musculoskeletal conditions, including lost wages, costs an average $850 billion annually (compared to the 1997 data above), making it the largest WC expense (http://www.hhs.gov/news/). For employers paying WC claims, the economic strain has reached a breaking point. How significant is the category of musculoskeletal conditions? Consider the following data:

80 % of all claims under WC are musculoskeletal sprain/strain injuries, with lower back injury consuming more than 33 % of every WC dollar.
Back pain causes more than 314 million bed days and 187 million lost work days yearly (data from the US Department of Labor, 1998–2005).
Employers lose 5.9 h of productivity per week from those suffering from musculoskeletal pain who continue to be on the job (referred to as “presenteeism”).

It should be noted that the exact prevalence rates/figures for occupational injuries and illnesses are not available. The best data for the United States are provided by the Annual Survey of Occupational Injuries and Illnesses by the Bureau of Labor Statistics (BLS), US Department of Labor. The annual BLS data are obtained by having employers complete their data entry at http://www.bls.gov/respondents/iif/. The website states “Welcome to the Survey of Occupational Injuries and Illnesses respondent’s website. This website is your source for information that will help you to complete and submit your response to the Survey of Occupational Injuries and Illnesses. You have been selected to participate in this survey to help us to obtain a complete and accurate representation of work-related injuries and illnesses in America’s work places.”

In order to understand the data, it is important to know the definitions for injuries and illnesses. According to OSHA, an occupational injury is any injury such as a cut, fracture, sprain, or amputation that results from a work accident or from a single instantaneous exposure in the work environment. Minor injuries are defined as injuries requiring only first aid treatment (e.g., not involving medical treatment, loss of consciousness, restricted work, or transfer to another job) and are not recorded in the logs. An occupational illness is any abnormal condition or disorder, other than one resulting from an occupational injury, caused by exposure to environmental factors associated with employment. Occupational illnesses include acute and chronic illnesses or diseases that may be caused by inhalation, absorption, ingestion, or direct contact. All occupational illnesses are recordable. However, there are known limitations of the BLS data (Melhorn & Ackerman, 2008). The survey estimates of occupational injuries and illnesses are based on a selected probability sample, rather than a census of the entire population. Because the data are based on a sample survey, the injury and illness counts are helpful estimates but are not accrued values. Underreporting, along with selection bias, can occur. Additionally, the survey measures only the number of new work-related injury and illness cases that are recognized, diagnosed, and reported during the year.

In September 2010, the BLS completed a major revision to the Occupational Injury and Illness Classification System (OIICS). The OIICS is used in the Census of Fatal Occupational Injuries (CFOI) and the Survey of Occupational Injuries and Illnesses (SOII) to code various circumstances of the individual injury or illness reported. OIICS provides a structure to classify the nature of the injury and part of the body affected, source and secondary source of the injury, and event or exposure that precipitated the injury. Data for 2010 reported 3,063,400 cases involving days away from work. Sprains, strains, and tears were 370,130, back injuries were 185,270, and falls were 208,470. The total recordable cases of nonfatal occupational injury and illness incidence rates among private industry employers declined in 2010 to 3.5 cases per 100 workers, from 3.6 in 2009 (http://www.bls.gov/news.release/osh.toc.htm).

Interesting facts include:

Manufacturing was the sole private industry sector to experience an increase in the incidence rate of injuries and illnesses in 2010—rising to 4.4 cases per 100 full-time workers, from 4.3 cases the year earlier. The increased rate resulted from a larger decline in hours worked than the decline in the number of reported cases in the industry sector.
Health care and social assistance experienced an incidence rate of injuries and illnesses of 5.2 cases per 100 full-time workers—down from 5.4 cases in 2009—and was the lone industry sector in which both reported employment and hours worked increased in 2010.
National public sector estimates, covering more than 18.4 million state and local government workers, are available for the third consecutive year, with an incidence rate of 5.7 cases per 100 full-time workers in 2010; this was relatively unchanged from 2009 (Fig. 10.1).

Fig. 10.1

Nonfatal occupational injury and illness incidence rates by case type and ownership, 2010
Approximately 2.9 million (94.9 %) of the 3.1 million nonfatal occupational injuries and illnesses in 2010 were injuries. Of these, 2.2 million (75.8 %) occurred in service-providing industries, which employed 82.4 % of the private industry workforce covered by this survey. The remaining 0.7 million injuries (24.2 %) occurred in goods-producing industries, which accounted for 17.6 % of private industry employment in 2010, while workplace illnesses accounted for 5.1 % of the 3.1 million injury and illness cases in 2010. The rate of workplace illnesses in 2010 (18.1 per 10,000 full-time workers) was not statistically different from the 2009 incidence rate (18.3 cases).
Goods-producing industries, as a whole, accounted for 36.3 % of all occupational illness cases in 2010, resulting in an incident rate of 31.8 per 10,000 full-time workers—up from 29.1 cases in 2009. The manufacturing industry sector accounted for over 30 % of all private industry occupational illness cases, resulting in the highest illness incidence rate among all industry sectors of 41.9 cases per 10,000 full-time workers in 2010—an increase from 39.0 cases in 2009. Service-providing industries accounted for the remaining 63.7 % of private industry illness cases and experienced a rate of 14.6 cases per 10,000 full-time workers in 2010—statistically unchanged from the prior year. Among service-providing industry sectors, health care and social assistance contributed 24.2 % of all private industry illness cases and experienced an incidence rate of 30.2 cases per 10,000 full-time workers in 2010—down from 34.8 cases in 2009.
Review of injury case type and the employer type is suggesting and interesting pattern to nonfatal injury and illness. Efforts by private industry to reduce “risk factors” in the workplace appears to be having some impact, while state and local government efforts have been less successful.
Another source for data is the NCCI at www.ncci.com/. Their Workers Compensation Temporary Total Disability Indemnity Benefit Duration 2012 Update (https://www.ncci.com/nccimain/IndustryInformation/ResearchOutlook/Pages/WC-Temp-Benefit-2012-Upate.aspx) found that the average duration of temporary total disability (TTD) indemnity benefits began to increase at the onset of the recent recession and that the rate of increase had moderated for injuries occurring during the first 6 months of 2010. Using an additional 12 months of reported data, they find that this more moderate rate of increase continues for injuries occurring through the first 6 months of 2011.
NCCI estimated that the ultimate mean duration of TTD indemnity benefits rose from 130 days for Accident Year 2005 to 147 days for Accident Year 2009 and rose again to 149 days for claims in the first half of Accident Year 2011. The national unemployment rate deteriorated from 4.6 % in December 2007 to 8.9 % in December 2011.

Therefore, the statistics surrounding musculoskeletal conditions clearly define them as the primary threat to employers’ WC programs. The magnitude of this problem is related to the three principle issues related to the delivery of efficient and effective care: (1) The condition often lacks a reliable or precise diagnosis. (2) This can lead to the use of ineffective treatment methods. (3) And there has been limited application or emphasis on self-care and preventive strategies (http://www.ctdmap.com/downloadsinfo/1887.aspx). Thus, the occupational physician must recognize, understand, and address these multiple factors to achieve the more favorable outcomes to treatment that are seen in non-WC injuries and illnesses (Melhorn & Talmage, 2011).

Definitions

In order to provide a consistent approach to definitions and terms, this section has been provided with permission from the American Medical Association’s Press Guides to the Evaluation of Disease and Injury Causation (editors J. Mark Melhorn and William E. Ackerman, Chapter 1 Introduction) (Melhorn & Ackerman, 2008).

Evidence-Based Literature

Evidence-based medicine has become the standard for determining appropriate medical care. The most common definition was provided by Dr. David Sackett: “Evidence-based medicine is the conscientious, explicit, and judicious use of current best evidence in making decisions about the care of individual patients … [which] means integrating individual clinical expertise with the best available external clinical evidence from systematic research” (Sackett, Rosenberg, Gray, Haynes, & Richardson, 1996). Unfortunately, randomized controlled clinical studies are difficult to perform in the workplace and, hence, are uncommon. Therefore, most of the information available is from epidemiologic studies that can disprove, but not prove, an association (Hadler, 1999).

Epidemiology

As noted earlier, epidemiology focuses on the distribution and determinants of disease in groups of individuals who happen to have some characteristics, exposures, or diseases in common. Viewed as the study of the distribution and societal determinants of the health status of populations, epidemiology is the basic science foundation of public health (Melhorn, 1999c). The goal of epidemiologic studies is to identify factors associated (positively or negatively) with the development or recurrence of adverse medical conditions. A search strategy of bibliographic databases was used to identify epidemiologic literature that addresses causation of specific medical conditions, as outlined in Guides to the Evaluation of Disease and Injury Causation (editors Melhorn and Ackerman, Chapter 4 Methodology) (Melhorn & Hegmann, 2008). Although the referenced Chapter 4 is copyrighted, Drs. Melhorn and Hegmann have decided to offer the materials therein as “in the public domain and may be freely copied or reprinted” if appropriate acknowledgment of the reference source is used.

Specific Definitions

Medical conditions are defined as an injury or illness that meets the standard criteria for an ICD-10 diagnosis (Melhorn & Ackerman, 2008).
Disability refers to an alteration of an individual’s capacity to meet personal, social, or occupational demands or statutory or regulatory requirements because of impairment. Disability is a relational outcome, contingent on the environmental conditions in which activities are performed (AMA, 2001).
Impairment refers to a loss, loss of use, or derangement of any body part, organ system, or organ function (AMA, 2001).
Occupational exposures and physical factors at work are defined as identifiable occupational exposures to possible exacerbating or aggravating agents. For the musculoskeletal system, physical factors are often described in terms of repetition, force, posture, vibration, temperature, contact stress, and unaccustomed activities (CtdMAP, 2006; Melhorn, 1998b). For hearing, sound levels are measured in decibels. Radiation exposure is measured in millirads, and chemical exposure in milligram per cubic meter or parts per million.
Nonoccupational exposures are defined as individual risk characteristics such as age, gender, hand preference, comorbid medical conditions such as diabetes, body mass index (BMI), depression, and hobbies.
Under paragraph 1904.5(b)(1), the Occupa-tional Safety and Health Act (OSHA) defines the work environment as the establishment and other locations where one or more employees are working or are present as a condition of their employment. The work environment includes not only physical locations but also the equipment or materials used by the employee during the course of his or her work (U.S. Department of Labor, 2006a).
Aggravation refers to a preexisting injury or illness that has been significantly aggravated, for purposes of OSHA injury and illness record keeping, when an event or exposure in the work environment results in any of the following:
- Death, provided that the preexisting injury or illness would likely not have resulted in death but for the occupational event or exposure
- Loss of consciousness, provided that the preexisting injury or illness would likely not have resulted in loss of consciousness but for the occupational event or exposure
- One or more days away from work or days of restricted work or days of job transfer that otherwise would not have occurred but for the occupational event or exposure
- Medical treatment in a case where no medical treatment was needed for the injury or illness before the workplace event or exposure or a change in medical treatment was necessitated by the workplace event or exposure (U.S. Department of Labor, 2006a)

The above are similar to aggravation as defined by the AMA Guides to the Evaluation of Permanent Impairment, fifth Edition: a factor(s) (e.g., physical, chemical, biological, or medical condition) that adversely alters the course or progression of the medical impairment or worsening of a preexisting medical condition or impairment (AMA, 2001).

Exacerbation is defined as a transient worsening of a prior condition by an injury or illness, with the expectation that the situation will eventually return to baseline or pre-worsening level (Talmage & Melhorn, 2005). Some take issue with this definition because the signs or symptoms of a preexisting injury or illness may be temporarily worsened by something (i.e., activity, exposure, weather, reinjury), but the “something” is not an injury or illness. For example, a set of tennis will temporarily worsen the symptoms of degenerative arthritis in the serving shoulder, but tennis is neither an injury nor illness. This concept is clarified by the following.

Exacerbation is defined in the AMA Guides to the Evaluation of Permanent Impairment, sixth Edition, as temporary worsening of a preexisting condition. Following a transient increase in symptoms, signs, disability, and/or impairment, the person recovers to his or her baseline status or what it would have been had the exacerbation never occurred. Given a condition whose natural history is one of progressive worsening, following a prolonged but still temporary worsening, return to pre-exacerbation status would not be expected, despite the absence of permanent residuals from the new cause (Oakley, 2011, p. 611).

Recurrence is defined as the reappearance of signs or symptoms of a prior injury or illness with minimal or no provocation and not necessarily related to work activities (Talmage & Melhorn, 2005).
Apportionment is defined as a distribution or allocation of causation among multiple factors that caused or significantly contributed to the injury or disease and resulting impairment. The factor could be a preexisting injury, illness, or impairment (AMA, 2001).

For purposes of this present chapter, the words diagnosis, disorder, condition, injury, or illness are essentially considered the same.

Why Epidemiology?

Epidemiology, the science, is used to determine association or causation between MSDs and risk factors (individual and workplace). Understanding association or causation allows for intervention and treatment (medical) and the determination of compensability (legal and financial responsibility). Medical and legal “causation” are not the same. This concept of medical and legal causation has been discussed in detail in the following publications: Chapter 1 Introduction, Chapter 2 Understanding Work-Relatedness, and Chapter 3 Causal Associations and Determination of Work-Relatedness in Guides to the Evaluation of Disease and Injury Causation, Second Edition, editors Melhorn, Ackerman, Talmage, and Hyman, AMA Press (2011) granted. Medical intervention and treatment can include prevention. Prevention comes in three forms (jmm, 1999). Primary prevention keeps disorders from occurring. It is focused on the universal application of safety and health and, when successful, reduces the risk and obviates the need for secondary or tertiary prevention. Once a disorder has emerged (become detectable), primary prevention is not possible; secondary prevention must be designed to keep the disorder from increasing in severity. The goal of secondary prevention is to arrest the growth of the disorder and, if possible, reverse or correct it. This process is the traditional health-care model. Tertiary prevention is designed for disorders that have reached advanced stages of development and threaten to produce significant side effects or complications. The goal of tertiary prevention is to keep the disorder from overwhelming the individual, leading to long-term disability. Unfortunately, prevention of musculoskeletal conditions has also been limited by legislated mandates such as ADA and Equal Employment Opportunity Commission (EEOC). ADA is the Americans with Disabilities Act of 1990, including changes made by the ADA Amendments Act of 2008 (Pub L 110-325), which became effective on January 1, 2009. The ADA was originally enacted in public law format and later rearranged and published in the US Code. The EEOC is the agency that watches for discriminatory practices that are prohibited under Title VII, the ADA, GINA, and the ADEA, for any aspect of employment, including testing, training and apprenticeship programs, and other terms and conditions of employment. GINA is the Genetic Information Nondiscrimination Act of 2008 (Pub L 110-233, 122 Stat. 881, enacted May 21, 2008). ADEA is the Age Discrimination in Employment Act of 1967. Although the goals of such legislation are socially appropriate, the application of the law has been detrimental to the prevention of musculoskeletal workplace injuries and illnesses. Additionally, the above legislation can be in conflict with the Occupational Safety and Health Act of 1970 (OSH Act) which requires the employer to provide a safe workplace. OSH Act is Pub L 91-596, 84 STAT. 1590, 91st Congress, S.2193, December 29, 1970, as amended through January 1, 2004, “To assure safe and healthful working conditions for working men and women; by authorizing enforcement of the standards developed under the Act; by assisting and encouraging the States in their efforts to assure safe and healthful working conditions; by providing for research, information, education, and training in the field of occupational safety and health; and for other purposes.” For example, if medical screening or testing has determined an increased risk for a musculoskeletal condition in an individual, an attempt by the employer to reduce the workplace risk could be interpreted as “a discriminatory action and therefore punishable” even though the effort on the employer’s part is to reduce risk of the musculoskeletal condition for the individual in compliance with the OSH Act. This obvious confusion has led many employers to elect not to precede with appropriate prevention programs to the detriment of the worker (Melhorn et al., 1999, Melhorn, Kennedy, & Wilkinson, 2002; Melhorn, Wilkinson, & Riggs, 2001; Melhorn, Wilkinson, & O’Malley, 2001; jmm, 1999).

What We Know and How We Know It

Epistemology (E·pis·te·mol·o·gy) [ih-pis-tuh-mol-uh-jee] is a branch of philosophy that investigates the origin, nature, methods, and limits of human knowledge. In other words “What We Know and How We Know It” or “What There Is to Know About Knowing” (Melhorn, 2008). Therefore, as is often the case, a decision on association or causation may be difficult because the determination is based on imperfect or inadequate information (the science). To understand epidemiology it is important to acknowledge these intrinsic limitations. Limiting causal conclusions to proven and established facts does not guarantee that future studies will not prove the current data wrong. Conversely, shunning everything unproven will result in rejection of many statements that are true but just not proven. The best illustration of this concept is in Table 10.4.

Table 10.4

Medical knowledge

J. Bernstein. Evidence-based medicine. J Am Acad Orthop Surg 12 (2):80–88, 2004. or 6319 (permission granted)

The Science

Health-care providers are often asked whether a condition is work-related or not (i.e., if it is causally related to a specific occupational injury or exposure). It is incumbent upon the clinician to give an opinion based on a careful review and analysis of the individual’s clinical findings and his or her workplace exposures and the literature linking (or not) the injury or exposure of concern and the condition in question (Melhorn & Ackerman, 2008). In contrast to a witnessed occupational injury causing immediate symptoms and corroborated by objective physical and diagnostic test findings, a cause-and-effect relationship between a disease (nontraumatic injuries are classified by OSHA as illnesses) and an agent or condition in the workplace may be unclear. Occupational diseases may develop slowly, with months or years between exposure and onset of symptoms and/or signs. Disease manifestations may be confused with changes due to normal aging. Information on past work exposure is often unavailable, inadequate, or incomplete. In addition, not all individuals react or respond in the same way to similar exposures to disease-producing agents. In some cases, there is a clearly identifiable single cause for the condition, whether work-related or nonoccupational. More often, causation is multifactorial, with one or more nonoccupational causes (e.g., age-related degeneration, smoking, or obesity), in addition to varying contribution from the workplace.

Causality determination may be difficult and result in contested claims. Honest differences of opinion are common when the facts are subject to different interpretations. Therefore, considerable judgment is necessary when data are lacking or incomplete. With occupational diseases, what appears obvious to some may nevertheless still be controversial, and it is important to assemble a complete database (history including occupational and nonoccupational exposures, physical and test findings, health-care records, etc.), be familiar with the relevant medical literature, and then review and analyze the data in a logical and unbiased manner to ensure a correct and equitable decision on causation. In 1976, the NIOSH created A Guide to the Work-Relatedness of Disease (Publication No. 79-116) to assist clinicians and, therein, provided a six-step method to assist in this decision-making process (Hegmann & Oostema, 2008; NIOSH, 1979). These six steps are listed in Table 10.5.

Table 10.5

NIOSH causation decision-making process

1. Consideration of evidence

2. Consideration of epidemiologic data

3. Consideration of evidence of exposure

4. Consideration of validity of testimony

5. Consideration of other relevant factors

6. Evaluation and conclusion

J. M. Melhorn. Epidemiological Methods for Determining Potential Occupational Health and Illness Issues. In: Handbook of Occupational Health and Wellness, edited by R. J. Gatchel and I. Z. Schultz, New York, NY:Springer, 2011—used with permission)

Consideration of Evidence

The first step in determining the probability of a cause-and-effect relationship, between an exposure in the workplace and the subject illness, is to establish that a disease does in fact exist and the disease and its manifestations appear to be the result of exposure to a specific harmful agent. Evidence elicited in the course of a medical evaluation should address these questions and specifically include the following:

Complete medical, personal, family, military, and occupational histories from the employee
A thorough physical examination and acquisition or review of appropriate radiographic, laboratory, or other diagnostic tests
Analysis and reporting of these clinical data

The occupational history should include, but is not limited to:

Job titles
Type of work performed (complete listing of actual duties)
Duration of each type of activity
Dates of employment and worker’s age for each job activity
Geographical and physical location of employment
Product or service produced
Condition of personal protective equipment used (if any) and frequency and duration of periods of use
Nature of agents or substances to which worker is, or has been, exposed, if known (including frequency and average duration of each exposure situation)

The resultant report should include a complete list of all diagnoses, with an opinion, whenever possible, as to which diagnoses are occupationally related and which are not.

Consideration of Epidemiological Data

The essential approach of epidemiology is the investigation of relative and absolute measures of frequency while comparing the characteristics of individuals with and without the condition. The most obvious measures of frequency are case counts and their variations, which are often referred to as numerator data. This number (the numerator) describes the frequency of the disorder, without reference to the underlying population at risk (the dominator data). The US Congress recognized that statistics on workplace injuries and diseases were essential to an effective national program of occupational disease prevention (Melhorn & Ackerman, 2008). Therefore, when the OSHA was passed in 1970, employers were required to maintain records on workplace injuries and illnesses (commonly labeled as OSHA 300 logs). The act delegated the responsibility for collecting statistics on these occupational injuries and illnesses to the BLS. To comply with the OSHA, the BLS conducts an annual survey of the occupational injuries and illnesses in the United States (U.S. Department of Labor, 2006b). The survey compiles the OSHA 300 logs from over 200,000 establishments, grouped together by industry codes established by BLS as the North American Industry Classification System (NAICS) (http://www.bls.gov/bls/naics.htm). The frequency of the particular disorder can also be expressed as a proportionate ratio (the number of cases of the particular disorder, compared to cases of all disorders, in the study population). By itself, numerator data cannot provide useful information regarding the risk or probability of acquiring the disorder. The case frequency has to be related to the underlying population that could have potentially developed the disorder (the denominator). Without the denominator (the number of people at risk), it is not possible to estimate the risk of a specific condition in the population or to test hypotheses regarding risk factors for a specific condition.

There are, though, known limitations of the BLS data. The survey estimates of occupational injuries and illnesses are based on a scientifically selected probability sample rather than a census of the entire population. Because the data are based on a sample survey, the injury and illness estimates probably differ from the figures that would be obtained from all units covered by the survey. Also, the survey measures the number of new work-related illness cases that are recognized, diagnosed, and reported during the year. Some conditions (e.g., long-term latent illnesses caused by exposure to carcinogens) often are difficult to relate to the workplace and are not adequately recognized and reported. These long-term latent illnesses are believed to be understated in the survey’s illness measures. In contrast, the overwhelming majority of the reported new illnesses are those that are easier to track (e.g., contact dermatitis) (Melhorn & Ackerman, 2008). Furthermore, employer bias in selecting which conditions to report may result in underreporting. Additionally, the OSHA definition for work-relatedness is more inclusive than most. Injuries and illnesses that occur at work may not have a clear connection to an occupational activity or substance peculiar to the work environment. For example, an employee may trip for no apparent reason while walking across a level factory floor, be sexually assaulted by a co-worker, or be injured accidentally as a result of an act of violence perpetrated by one co-worker against a third party. For this reason, rates are often used when the objective is to assess the risk of the disorder or determinants of disorders or their outcomes.

Rates

Rates describe the frequency of a disorder (or disorder per unit size of the population per unit time of observation). The most common rates are incidence and prevalence. The incidence rate is based on new cases of a disorder, whereas the prevalence rate reflects existing cases. Because they are based on new versus existing cases, incidence and prevalence rates have different uses and limitations. Therefore, the incidence rate is a rate of change, often described as the frequency with which people change from healthy to injured, sick, or disabled. Thus, the appropriate denominator is the population at risk of acquiring the disorder (i.e., those who are free of the disorder at the start of the time interval). The incidence rate may be quantified in a number of ways when the population is stable and the number of new events is counted each year. This is often expressed as the number of new events per 1,000 persons per year. Alternatively, incidence rate may be quantified as the number of new events per 1,000 person-years, as is done in prospective studies where a fixed population is followed until the end of the study. In practice, although the best denominator for incidence rates is the number of people free of the disorder at the start of the time interval, surveillance incidence rates (and prevalence rates) that are based on case reports often use the total population derived from estimates or census data.

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