This blog is a summary for a wide readership of an article that I have written for the European Journal of Pain. See here.
Curatolo, M. (2023). Personalized medicine: Somatosensory phenotyping in musculoskeletal pain conditions. European Journal of Pain, 27, 1099–1106.
What is personalized medicine?
Treatments are considered to be efficacious or not based on clinical research. For instance, in a clinical trial on low back pain, a group of patients that receives a medication is compared to a group of patients that receives a placebo. If pain is less with the medication, we conclude that the medication is efficacious in low back pain.
However, the superiority of the medication, or any treatment, is based on the comparison of the average pain scores of the two groups. This by no means implies that the medication provides pain relief in all patients who receive it. In fact, in the best case, we need to treat three patients with an "efficacious" medication to have at least 50% pain relief in only one of the three patients. Therefore, giving the medication to all patients with low back pain will result in lack of pain relief and possible adverse effects in a substantial proportion of patients.
Personalized pain medicine takes into account differences among patients to predict which treatments are most likely to work for individual patients. This way, less patients will receive treatments that will not help them and may cause adverse events. Also, cost savings would result from avoiding unsuccessful treatment trials.
What are somatosensory tests?
Somatosensory tests consist in applying a stimulus to a patient and recording a response. For instance, we can apply pressure device to a painful area, and record the amount of pressure we need to apply to evoke a pain sensation (see device in the figure above) This measure is called "pain threshold". A low pressure threshold generally reflects a high sensitivity of pathways involved in transmission, elaboration, and perception of pain. Another example is brushing the skin gently and ask patient if this causes pain (allodynia) - indicating that pain likely is due to nerve damage. Many other somatosensory tests are available, but a review is outside the scope of this blog. Among health professionals, somatosensory tests are also known as quantitative sensory tests (QST).
Somatosensory test are potentially useful in personalized medicine. For instance, certain treatments could be particularly effective in reducing pressure pain, and therefore patients having a low pressure pain threshold may be more likely to benefit from these treatments.
Can somatosensory tests improve personalized medicine?
The short answer is "may be". Much research has associated test results with outcomes, for instance to predict persistent pain after surgery or predict efficacy of treatments. Significant associations have been repeatedly found. However, the first crucial question is whether using these tests has an effect on treatment decision. If so, the further essential question is whether the decision taken on the ground of somatosensory testing improves the patient's condition. Research has not addressed these two questions.
One could argue that having an association between a test and a treatment outcome should be enough to support the use of the test to determine whether a treatment should be offered or not. This is unfortunately not true. The main reason is that the efficacy of a pain treatment depends on many factors, and response to a test can cover only part of the factors that determine the effectiveness of a pain treatment. Because no test can make a perfect prediction, denying a treatment based on a negative result of somatosensory testing will prevent a significant proportion of patients from receiving a treatment that may help them. The same will happen for patients with a positive somatosensory test: some of them will still not respond to the treatment, although the test is positive.
How can we make somatosensory tests clinically useful?
This is discussed thoroughly in my article for the European Journal of Pain. Here a summary and a diagram, taken from the article.
Because of the complex and multifactorial nature of pain, somatosensory tests should not be studied in isolation, but embedded in a set of measures that, together, may inform us on whether a treatment is likely to work or not. Those measures can be in different domains, such as characteristics of the pain or elements of the physical examination, among others.
In order to be clinically applicable, we have to study tests that are performed within few minutes, easy to learn, and require minimal equipment. Many currently used tools do not fulfill these criteria and are therefore most suitable for research rather than clinic environments.
We need at least two independent studies, performed in two different patient groups and settings, that confirm an association between somatosensory measures and patient-relevant outcomes, such as effectiveness of a treatment.
Finally, and crucially, we need to study whether performing somatosensory measures influence medical decision, and eventually benefits patients. This should be done at best by randomized controlled studies that compare a group of patients who receive somatosensory tests with a group of patients who do not, and evaluate in the long term whether outcomes are better in the group that undergoes somatosensory testing.
This pathway, if validated, will provide clinicians with additional tools to improve personalized pain medicine.
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