Heterogeneity

One of the most common questions I get from patients is "will this procedure work?"

My answer is always some variation of "I don't know for sure, but ..."

Some procedures work very well, examples including:
1. Transforaminal epidural steroid injections for radicular pain ("sciatica") associated with a herniated disk.
2. Carpal tunnel release for carpal tunnel syndrome
3. Knee replacements for knee osteoarthritis

For many other procedures, however, there is far greater ambiguity. One of the main reasons for this ambiguity is heterogeneity.

Heterogeneity refers to the patient population not having much in common with one another, as opposed to homogeneity, which refers to all of the patients being similar. So, one of the reason's carpal tunnel surgery works well is that, for the most part, almost all of the patients who get the surgery truly have carpal tunnel syndrome.

That is not always the case, however. In fact, it is rarely the case. Patients are human beings, and human beings are unique, and this makes it difficult to assume that just because one treatment works for one patient, that it will also mean that the same treatment approach will work the same on another, different patient.

For example, in one of my favorite studies by Lurie and Weinstein et al from the journal Spine in 2006, they investigated the diagnoses associated with different surgical procedures. What they found was that for hip replacements, there was a high degree of homogeneity in terms of why the diagnoses for which the surgery was performed (i.e., most patients had hip replacements because of osteoarthritis), but that there was a far, far higher level of heterogeneity for when a lumbar fusion was performed.

The consequence of this heterogeneity is that the outcomes for hip replacements are far better than they are for lumbar fusions. For hip replacements, if the patient has a hip replacement, there is a very high likelihood it was done for a legitimate reason. For lumbar fusions there is a mix of some patients who were good candidates for the fusion and other patients who probably were not good candidates. As result, the outcome data for fusions is not particularly good. If patients were more narrowly selected, perhaps the outcomes would be better.

This same phenomenon- heterogeneity leading to less than optimal outcomes, comes up quite frequently. Other examples:

1. Spine injections. One of the main scopes of my practice is spine injections, including epidural injections, zgyapophysial joint injection, sacro-iliac joint injections, etc. The outcome data is not particularly good overall, although there are some subsets of patients for which the data is good (e.g., the aforementioned use of transforaminal epidurals for radicular pain, or SI joint injections).
The main issue is, again, heterogeneity. Spine injectionists, as a group, do a horrible in terms of applying standard of care. There is a great paper by Janna Friedly in 2008 that showed that only 42% of spine injections are performed with use of fluoroscopy. Only 42%! That is a bare minimum standard for an appropriate injection, and that is not factoring in whether the appropriate pain generator was selected, whether the proper level was selected, and whether the appropriate approach was used. This means that the absolute upper limit of how many spinal injections are performed appropriately is 42%, but when you factor in these other factors, it's likely that only 20-30% of spine injections are being performed according to standard of care. It's not shocking, then, that we are not seeing great outcome data.

2. Fibromyalgia. Fibromyalgia is largely a waste-basket term that encompasses a huge number of different diagnoses. In other words, there is a high level of heterogeneity. Just this morning, I was discussiong a fibromyalgia patient I see along with one of my rheumatology colleagues, and she is far from the typical fibromyalgia patient. A large number of fibromyalgia patients may have other diagnoses that have not been fully explored, including rhuematologic disease, depression, bipolar II disorder, myopathy, cervical myelopathy, Lyme disease, Epstein Barr Virus, etc.

Which brings me back to my conversation with patients. This is why, given the real world heterogeneity of patients, so often my answer to there questions is "I don't know for sure, but ...."