- Michele Curatolo
Pain from multiple body areas: when arithmetics doesn't work
Pain from multiple body sites is frequently disabling
Why is pain from multiple body sites particularly bothersome and disabling? One obvious reasons is that different parts of the body have different functions, and pain limits those functions incrementally. For example, having painful arthritis of the neck and low back limits driving ability more than having either arthritis of the neck or low back: neck pain limits the ability to turn the head to check traffic, and low back pain may prevent patients from sitting for prolonged time. However, there is more.
Pain signals arising from multiple body sites potentiate each other in the central nervous system
Several human experiments have demonstrated that stimuli that do not cause pain when applied to small body areas can induce pain when applied to larger body areas. Similarly, painful stimuli that are applied to small areas are perceived as more painful when applied to larger areas. Physiologically, this phenomenon is called "spatial summation": when inputs transmitted by multiple nerves converge to the same neurons in the central nervous system, they induce potent firing, and consequently high levels of pain.
Spatial summation occurs not only with pain arising from the same body region, but also with stimuli from areas distant to each other, including right and left body sites. This phenomenon is relevant to the many patients who suffer from multiple pain syndromes, and can be explained by convergence of signals to regions of the brain that process pain.
The knowledge of spatial summation can help both patients and health care professionals
When evaluating patients with multi-site pain, health care professionals should not be surprised by the association of relatively modest injuries and high levels of pain and disability. Patient information on the potential causes of this discrepancy is mandatory and can reduce frustration and misconception on the nature of pain.
Several studies have demonstrated that widespread pain increases the likelihood to develop persistent pain after an injury or surgery. This may result from an excessive activity of brain areas involved in the pain experience, leading to persistent pain after an injury or surgery. An evaluation and treatment of spatial summation could lead to a reduction in the likelihood to develop persistent pain after surgery. Preventive strategies may include pharmacological and non-pharmacological treatments that attenuate hypersensitivity of central neural pathways (read more here).
While we do not have yet validated clinically-applicable tools for quantifying spatial summation, a surrogate could be the evaluation of the areas of pain using body map drawings, questionnaires, or more sophisticated electronic measures of pain areas on tablets. Wide areas of pain can raise the suspicion of clinically relevant spatial summation and direct the focus of the plan towards strategies that attenuate pain amplification processes within the central nervous system. Options are anticonvulsants and antidepressants (also in the absence of depression), psychological treatments, and mindfulness-based stress reduction.