What does local tenderness say about the origin of pain?
Updated: Feb 23, 2022
This blog has exactly the same title as a study we have published years ago. Learn why I am bringing this up again.
Pain evoked by local palpation is considered as a sign of an injury at that area
When we hurt somewhere, the first instinct is to touch and press the area of our pain. Physicians and physical therapists, in their examination, do the same: apply gentle pressure to different body parts of patients and ask them whether the pressure evokes pain. This applies to pain arising from muscle and joints, but not only. Another classical example is palpating the right lower quadrant of the abdomen when acute appendicitis is suspected. Whatever the condition, pain evoked by local palpation is commonly considered as a sign of an injury at that area.
Regrettably, it is not so easy.
In a study by my research group on neck pain, we wanted to the the hypothesis that applying pressure to painful areas would allow the identification of joints of the neck that generate the pain (facet joints). We first identified the painful joint by anesthesia of the nerves that supply the joints. Then, we applied pressure to that joint and other ones that were not causing any pain, namely the joints above and below, and those of the contralateral non-painful side. We used a device that accurately measures the pressure ("pressure algometer"), and made sure by ultrasound that we applied the pressure precisely.
Our hypothesis was that the joint from which the patient’s pain originates ("injured joint"), was the one with the most painful reaction to pressure. This hypothesis was rejected by the results. There was no difference in pain sensitivity to pressure between the “injured” joint and the “healthy” ones. There was not even a difference between the "injured" joint and the joints of the contralateral side, where patients did not report any neck pain.
The central nervous system is to be blamed for the confusion
It seems that tenderness to pressure at injured body areas is not necessarily more than tenderness at healthy areas. What is the explanation for this finding? Pain sensitivity is the result of complex processes at different sites of the pathways that are involved in the generation, transmission, elaboration, and perception of pain. When a painful signal arises from an injured area (in our example, a joint), it activates areas of the spinal cord and the brain that are connected not only with the joint, but also with other adjacent and contralateral body sites. Those areas become excessively sensitive. Therefore, pressure stimuli applied to body parts that are not injured arrive to the activated areas of the spinal cord and brain, resulting in a pain sensation. You can read more in other blogs on pain sensitivity and muscle pain.
But palpation of the abdomen is always diagnostic for acute appendicitis...
This is not because palpation is an excellent diagnostic method, but because of the very high prevalence of acute appendicitis in patients with acute pain at that area. If acute appendicitis is the cause of lower lateral abdominal pain in 90% of patients, any test will be able to guess the diagnosis with 90% chance (high "positive predictive value"). This is unfortunately not the case with most chronic pain conditions, such as neck pain in our example, as there are many potential causes. For neck pain arising from the facet joints, palpation has no diagnostic value. The same likely applies to many other musculoskeletal pain conditions, although rigorous research has not been conducted systematically.
We will continue to assess local tenderness in chronic pain. However, we should be aware and made patients aware that identifying a tender spots, per se, does not tell us whether that body part is injured and is causing the pain. At best, assessing local tenderness is one of the elements that contribute to the diagnosis.