PET Detects Pain Process in Tennis Elbow

Positron emission tomography with a tracer allows clinicians to visualize the physiologic process associated with the pain of tennis elbow, according to a small study published in PLOS ONE.

Researchers from Uppsala University in Sweden sought to investigate the amounts of neurokinin 1 (NK1) receptors among 10 patients with unilateral chronic tennis elbow, before and after treatment.

The patients underwent two sessions of PET imaging with the NK1 specific radioligand [11C]GR205171, once before and once after treatment with graded exercise.

For three months, the five men and five women (mean age 59 years) performed a daily exercise regime at home, with gradually increasing load on the extensor muscles of the affected forearm. They rated their pain levels on a 100 mm visual analogue scale (VAS). Three days prior to scanning, the subjects did not take anti-inflammatory medication; one day prior they did not take analgesics; 12 hours prior, the subjects refrained from tobacco, alcohol, and caffeine, and they did not eat for three hours before the examination.

The results showed elevated levels of NK1 in the images of the painful area compared with the non-painful arm. Following tissue damage, there is an up-regulation of the neuropeptide substance P and its receptor NK1. This occurs not only in the dorsal horn of the spinal cord, but also in the peripheral painful tissue. This up-regulation is part of an interaction between peripheral nerves, immune cells, and the tissue itself that seems to help guide the body's own repair process. In chronic tennis elbow, this up-regulation of the substance P-NK1 system lingers on.

Following the treatment, eight subjects reported lower pain ratings but signal intensity decreased in five and increased in three.

The researchers concluded that the increased NK1 receptor availability is interpreted as part of ongoing neurogenic inflammation and may have correlation to the pathogenesis of chronic tennis elbow. They acknowledge that PET imaging is an expensive and complicated examination, but they hope that findings such as these will aid in developing less expensive markers that can be used in everyday practice.