Myofascial trigger points: relation to acupuncture and mechanisms of pain.

The gate control theory of pain describes the modulation of sensory nerve impulses by inhibitory mechanisms in the central nervous system. One of the oldest methods of pain relief is hyperstimulation analgesia produced by stimulating myofascial trigger points by dry needling, acupuncture, intense cold, intense heat, or chemical irritation of the skin. The moderate-to-intense sensory input of hyperstimulation analgesia is applied to sites over, or sometimes distant from, the pain. A brief painful stimulus may relieve chronic pain for long periods, sometimes permanently. Pain may be relieved by "closing the gate" by means of a central biasing mechanism possibly located in the brainstem reticular formation. Prolonged relief may require the disruption of reverberatory neural circuits responsible for the "memory" of pain. The termination of pain by either hyperstimulation, or by local injection of an anesthetic, normalizes function, which helps to prevent recurrence of abnormal neural activity. Thus, modulation of sensory inputs by use of many techniques may reduce pain more than by surgically interrupting the sensory input.