Irritation of a visceral organ frequently produces pain that is felt not at that site but in a somatic structure that may be some distance away. Such pain is said to be referred to the somatic structure (referred pain). Knowledge of the common sites of pain referral from each of the visceral organs is of importance to a clinician. One of the best-known examples is referral of cardiac pain to the inner aspect of the left arm. Other examples include pain in the tip of the shoulder caused by irritation of the central portion of the diaphragm and pain in the testicle due to distension of the ureter. Additional instances abound in the practices of medicine, surgery, and dentistry. How ever, sites of reference are not stereotyped, and unusual reference sites occur with considerable frequency. Cardiac pain, for instance, may be referred to the right arm, the abdominal region, or even the back, neck, or jaw.
When pain is referred, it is usually to a structure that developed from the same embryonic segment or dermatome as the structure in which the pain originates. For example, the heart and the arm have the same segmental origin, and the testicle migrated with its nerve supply from the primitive urogenital ridge from which the kidney and ureter also developed.
The basis for referred pain may be convergence of somatic and visceral pain fibers on the same second-order neurons in the dorsal horn that project to the thalamus and then to the somatosensory cortex (Figure 1). This is called the convergence-projection theory. Somatic and visceral neurons converge in the ipsilateral dorsal horn. The somatic nociceptive fibers normally do not activate the second-order neurons, but when the visceral stimulus is prolonged, facilitation of the somatic fiber endings occurs. They now stimulate the second order neurons, and of course the brain cannot determine whether the stimulus came from the viscera or from the area of referral.
Fig1. Schematic illustration of the convergence projection theory for referred pain and descending pathways involved in pain control. The basis for referred pain may be convergence of somatic and visceral pain fibers on the same second order neurons in the dorsal horn of the spinal cord that project higher brain regions. The periaqueductal gray is a part of a descending pathway that includes serotonergic neurons in the nucleus raphe magnus and catecholaminergic neurons in the rostral ventromedial medulla to modulate pain transmission by inhibition of primary afferent transmission in the dorsal horn.
