Should Causality Matter In The Edmonton Obesity Staging System?Thursday, August 18, 2011
One of the questions many readers and colleagues have asked, is whether or not the issue of ‘causality’ should matter in the the context of the Edmonton Obesity Staging System (EOSS).
In other words, should only conditions be counted that are ‘causally’ linked to obesity or is it enough that these conditional are merely more common in people with excess weight.
There are essentially two important but distinct aspects to this question that ultimately relate to how EOSS is to be used in clinical practice.
If the primary purpose of EOSS is to identify patients who would benefit from weight loss, then, yes, it matters whether or not the co-morbidities considered, are ‘causally’ related to obesity and can be reversed or ameliorated by reducing and sustaining a lower body weight.
However, if the primary purpose of EOSS is merely to identify obese patients, who are at high risk and need to be prioritized within the health care system in order to receive the appropriate care for their conditions (irrespective of whether or not this ‘care’ involves weight loss or just better management of their comorbidities), then the question of causality is really irrelevant.
Thus, in the first case, one would only count ‘comorbidities’ that are actually ‘causally’ related to excess weight – an example being sleep apnea. There is ample evidence that weight loss reduces symptoms of sleep apnea (while weight gain makes it worse) and so sleep apnea would count as an important comorbidity that can be addressed by obesity treatment.
In the second case, it does not actually matter if the comorbidity is in any way related to excess weight. All that really matters, is whether this comorbidity is present or not. An e.g. would be depression, which, while not caused by obesity and not likely to improve with weight loss (it may sometimes even get worse), may, when present, help identify obese patients, who do have a higher risk of premature death.
So while in the first example, EOSS would be used to decide who needs to lose weight, in the second example, EOSS simple serves to identify obese people, who are at highest risk of complications and death.
Apart from the second scenario being the real reason that EOSS was developed, it is also a far more practical approach to using EOSS, because for many comorbidities it may be impossible to answer the ‘chicken or egg’ question or even determine if these are simply two different chickens.
In clinical medicine this phenomenon is referred to as ‘phenocopy’, a term used to describe a case where two distinct and unrelated conditions, present clinically with the same symptoms or ‘phenotype’.
In obesity, this is particularly common, because, while many symptoms may be ‘causally’ related to obesity, these same symptoms may just happen to present in an obese individual but have nothing to do with that patient’s excess weight.
For EOSS, this question would not really matter – whether the knee pain is from an accident or from carrying around the excess weight makes no difference – the only thing that matters is that this is a patient with excess weight AND knee pain and therefore, this patient is an obese patient, who is at higher risk for mortality than an obese patient without knee pain.
Remember, bariatric care, as I define it, is not about losing weight but rather about medical care for the bariatric patient. As resources are limited, all I want EOSS to tell me is, who to see first.
Of course there are ‘normal-weight’ people with knee pain, but they are not my problem. There are also ‘normal-weight’ people with high blood pressure, sleep apnea, diabetes and depression – again, they are not my problem.
As someone, who works in an obesity clinic, my job is to assess obese patients and help them the best I can, whether their treatment requires weight loss or not. If EOSS can help me decide, who to see most urgently, then EOSS has done its job.
Of course, further research is needed to determine whether EOSS actually works well in clinical practice (e.g. outside of a speciality centre) and we may perhaps need to simplify and clarify the criteria. But the principle stands: it is simply not enough to look at BMI to decide who needs (urgent) medical attention and who does not.