Too Many Variables, Not Enough Equations.

To the Editor:

Thank you for the opportunity to respond to Jean Cromie and colleagues' Letter to the Editor (May 2000)[1] written in response to my Letter to the Editor in the November 1999 issue.[2]

Cromie et al[1] suggested that I "deny the existence of [occupational] injuries."[2] I did not state that, and I do not believe it. What I stated was that, in the absence of overt trauma, the term "occupational injury"[3] "is a misnomer that has not yet been defined medically and cannot currently be legally defended."[2(p1048)] This statement is true.

The question is not whether symptoms arise before, during, or after the performance of activities. Rather, I believe we should be asking: When symptoms occur, what caused the symptoms? Are the symptoms related to the performance of specific activities? Are the symptoms directly related to pathology? If there is a relationship between the performance of the activities and the development of pathology, who is responsible? We need reliable and valid measurements to quantify observations related to people who have symptoms at work before we can differentiate the variables that cause symptoms from the variables that do not cause symptoms. Possibly more important is the need to quantify the person's genetic predisposition for symptoms and the method of activity performance directly related to the person's personality and neuromusculoskeletal idiosyncrasies.

The crux of the matter is this: When a well-meaning clinician states, "The clinical reality for many therapists is that aspects of their work cause them pain,"[1] in the absence of overt trauma, there are no data to support such a statement. When clinicians make such claims without supporting data, they erode the scientific credibility of the profession and perpetuate misunderstanding. What we need, instead of unsupported hypotheses, are data. Until reliable and valid data concerning work-related symptoms can be generated, the credibility of our profession will suffer when unsupported statements are made.

Therapists are experts in observation. The observed phenomenon in this case is the report of a symptom. It is rare that we can confirm the cause of that symptom or why the person reported the symptom. If the symptom occurs at work, we cannot simply assume that work caused the symptom. We still have difficulty quantifying work (eg, force, distance, repetition, duration, posture) in a manner that will allow us to differentiate symptoms caused by performance of work, symptoms caused by leisure activity, symptoms caused by normal aging, and symptoms caused by personal factors. What makes the matter worse is that factors unrelated to work[4] influence the decision about whether to report a symptom; this in turn biases retrospective medical record or occupational data sets. Symptoms may develop over time, making differentiation of work-related symptoms from symptoms due to normal aging more difficult. Finally, if a symptom is ameliorated during absence from the work task that is suspected of causing the symptom, and if the symptom does not return following resumption of the same pre-symptom work task without modification, is it logical that the symptom was caused by performance of the work? What we have is an indeterminate problem: too many variables, not enough equations.

What we need are high-quality data related to the variables suspected of causing symptoms, first in a univariate manner, then using multivariate analyses. Discussion about the cause of symptoms should wait for reliable and valid measurements of work and symptoms.

Dennis L Hart, PT, PhD Director of Consulting and Research Focus On Therapeutic Outcomes Inc Great Falls, VA 22066

References

[1] Cromie JE, Roberston VJ, Best MO. Occupational injuries in PTs [letter to the editor]. Phys Ther. 2000;80:529-530.

[2] Hart DL. Occupational injury [letter to the editor]. Phys Ther. 1999;79:1084, 1086.

[3] Occupational Injuries and Illnesses in the United States by Industry, 1992. Washington, DC: US Dept of Labor, Bureau of Labor Statistics; 1992.

[4] Baldwin ML, Johnson WG, Butler RJ. The error of using returns-to-work to measure the outcomes of health care. Am J Ind Med. 1996;29:632-641.