Referred pain makes acupuncture points sensitized and tender.

Referred pain is the pain felt in somewhere in the body other than in the site of origin. Pain referral has a neural basis. Specific pathways and neural connections in the brain are thought to lead to the possibility of pain referral.

One group of nerve fibres conduct information about touch and another group conduct information about tissue damage or noxious stimulation via different sensory nerves. Many sensory fibres from different parts of the different area can terminate on the same set of second order neurons. The second order neurones are part of the pathway that sends sensory information to higher centres for perception. However, since there is so much convergence of sensory information from different body parts onto the same second order neurones, these second order neurones may provide ambiguous information as to the exact location of the noxious stimulus. This neural mechanism is thought to be one way whereby the higher centres of the brain can become "confused" as to the exact location of the noxious stimulus.

Another explanation of pain referral is the activating of silent or latent synaptic connections. When there is prolonged and/or intense noxious stimulation, some of these ineffective synapses may become effective connections. The information is transmitted from other parts unrelated to the source of the pain. The brain therefore can become confused as to the correct location of the pain.

Recent research has investigated correlation between referred pain distribution and acupoint sensitization in patients with intestinal diseases. In clinical research, 443 patients from 8 hospitals were recruited, including the outpatients and inpatients of Crohn's disease, ulcerative colitis, chronic appendicitis and other intestinal diseases. The site with tenderness on the body surface and the morphological changes of local skin were observed and recorded in the patients. Using a sensory tenderness instrument, the pain threshold at the sensitization point was measured in 60 patients with ulcerative colitis. The referred pain on the body surface in the patients with intestinal diseases was mainly located in the lower abdomen, the lumbar region and the lower legs. The diameter of tenderness region was 1.5 to 2.5 cm. Compared with the region without sensitization, the pain threshold of the sensitization point in the patients with ulcerative colitis was reduced significantly. The referred pain on the body surface in the patients with appendicitis was located in the right lower abdomen, the waist and back and the right lower limbs on the medial side. The tenderness region was 1 to 2 cm in diameter and was irregular in form. Intestinal diseases induce referred pain on the body surface where is the same as or adjacent to the location of the spinal segment corresponding to the affected intestinal section. These sensitization regions are related to the locations of acupoints.

References
Cui X et al Zhongguo Zhen Jiu 2019 Nov 12;39(11):1193-8. doi: 10.13703/j.0255-2930.2019.11.016.