Why do scars itch? The knee-jerk answer is easy; we don’t fully understand why scars itch. Over the years I have thought a lot about why wounds, burns and scars itch while I have helped patients understand and cope with their discomfort during the recovery process. The following information is my way of developing an understanding of scar itch and other discomforts while creating reliable expectations for the resolution of these discomforts.
Itchy Scars Change Over Time
The itch in a newly healing burn is a wound itch, as the early phase of burn healing involves an open wound. When wounds close, the microscopic environment changes quite dramatically. “Open wound” physiology switches to “closed wound” physiology. One of the most easily recognized tissue changes is the degree of inflammation and how rapidly the inflammation recedes. Swelling is classified in two ways. Edema is a soft and doughy sort of generalized swelling while Induration is a harder, inflamed, tender and more localized phenomenon. Induration commonly has a fairly clear border and is perhaps more chronic in nature. Both edema and induration will rapidly and steadily resolve once a wound is no longer open to the air. Edema and induration respond favorably to various treatments, even before the wound closes. Healing can be accelerated with the elevation of the wounded part, external compression, direct pressure, massage, diuretics, and anti-inflammatory medications.
Why Do Scars Itch?
Itch is a sensation caused by stimulation of a particular sensory nerve ending in the skin. Your skin then transmits a signal to the spinal cord and up to the brain, which we perceive as itch. However, with scar itch, damaged nerves might fire without a specific stimulus, signaling itch. Specialized nerve endings exposed to the inflammatory environment of an open wound or evolving scar may be triggered to signal “scar itch” to the brain. Specialized nerve endings that signal itch to appropriate stimuli are destroyed in deep second degree burns or 3rd degree burns. These specific nerve endings will mostly be replaced by nonspecific endings called a “cap” which is similar to the healed end of a pruned rose cane. These “caps” on the end of previously healthy, specialized nerve fibers, can be trapped within the evolving burn scar and generate random signals. These random signals stimulate the nerve fiber, thus activating the entire sensory apparatus which signals “scar itch” to the brain. This can happen even if no common “itch agent” is irritating the skin, such as a plant extract, a bug bite, etc. In fact, all the uncomfortable, abnormal sensations that are experienced in a healing burn and the subsequent scar, such as tingling, prickling, stabbing, zinging, itching, throbbing etc., are parts of what I like to call “neuropathic sensations, or dysesthesia”. Signals are sent up nerve fibers that once had very specialized nerve endings which only fired to very specific and appropriate stimuli. These nerve fibers, in essence, have all been “pruned” by the injury into a “cap”. When nerve fibers that have produced a “cap,” rather than regenerating the specialized nerve ending, generate a sensory impulse, the brain reads “itch”, or “tingle”, or “stab” when nothing happened at the level of the skin. Or even something completely unrelated, like a simple touch, or brush of clothing across the skin surface was the actual stimulus.
Your Brain Learns How to Ignore Scar Itch
Why does scar itch tend to fade over time? Who knows…? My guess is that two things happen. As general inflammation and healing finish, the nerve endings are stimulated less often. Our brains are fantastically sophisticated organs and may learn to “ignore” sensory inputs that are “inappropriate”. If one gets the same stabbing sensation from the same spot on their skin enough times and each time you look and nothing is poking your skin in that spot, your brain may begin to discount that input and regard it as something to be ignored. Perhaps some nerve endings heal with regeneration of a more appropriate, less over-sensitive to firing, nerve ending. Perhaps some simply die back to a deeper level in the body where there aren’t physical stimuli occurring that would result in a signal traveling up that nerve fiber.
What is Histamine?
Common sources of itch, like things we’re allergic to, stimulate nerve endings in ways we better understand. Some allergens produced by plants and animals, or found in our environment, stimulate the release of molecules called histamine from cells that live in our skin. Histamines are part of our warning system against noxious agents in our surroundings. Histamine produces a reproducible set of reactions in the skin; swelling, increased blood flow, and stimulation of itch fibers.
Histamine Causes Itch
Open wounds and healing scars are generally not environments where mast cells, the source of histamine, are active. Common anti-itch drugs, like Benadryl, which inhibit the release of histamine within the skin, have little effect on the itch of open wounds or burn scars. One of their side effects, however, is sleepiness. Sleepiness can be a very useful effect, as most individuals are more bothered by itch when they are attempting to fall asleep and other environmental stimuli are not available to distract one from the irritation of itch. Agents that reduce the reactivity of injured nerves, such as gabapentin (Neurontin) and pregabalin (Lyrica) appear to be more effective against all of the abnormal sensations generated within scar itch.
Scar Itch is a Common Response
So, in summary, it appears that the itch involved in wounds, burns and scars are about a common response to injury in the skin. Inflammation stimulates itch fibers, injury results in damage to nerves and the process of repair is imperfect. We all react to serious injury to our skin with scarring and remodeling of scar. These facts result in a host of abnormal sensations, which we often describe as scar itch. Perhaps you’ll notice that this sentence is the first place in this topic where I mention the word pain. We can commonly distinguish pain from itch. I will address the topic of pain in another place and time.