Mechanisms of pain
The instant pain that follows a burn injury is due to stimulation of skin nociceptors that respond to heat (thermoreceptors), mechanical distortion (mechanoreceptors) and a selection of stimuli including chemical stimuli – exogenous (e.g. hydrofluoric acid) and endogenous (e.g. inflammatory mediators, notably histamine, serotonin, bradykinin, leukotrienes and prostaglandins). Nerve endings that are entirely destroyed will not transmit pain, but those that remain undamaged and exposed will generate pain throughout the time and course of treatment. These immediate pain sensations are elicited by activity in the unmyelinated C- and thinly myelinated Aδ-primary afferent neurons that synapse with neurons in the dorsal horn of the spinal cord.
Subsequently, the clinical picture becomes complicated by the emergence of both primary and secondary hyperalgesia.
Primary hyperalgesia
A burn induces a potent inflammatory response, and the release of inflammatory mediators sensitizes the active nociceptors at the site of injury. This causes the wound and immediately adjacent skin to become sensitive to mechanical stimuli such as touch, rubbing or debridement, as well as chemical stimuli such as antiseptics or other topical applications.
Secondary hyperalgesia
Continuous or repeated peripheral stimulation of nociceptive afferent fibres leads to increased sensitivity in surrounding unburned areas. This is mediated by the spinal cord and exacerbated by the mechanical stimulation that occurs as a result of frequent dressings changes.