Allodynia and Alloknesis
Allodynia and alloknesis are abnormal sensory states where normally innocuous stimuli elicit unpleasant sensations or aversive behavioural responses.
Allodynia and Hyperalgesia
The IASP defines allodynia as "Pain due to a stimulus that does not normally provoke pain." It is a nociceptive sensation or aversive response elicited by a normally non-nociceptive stimulus. "allo" means other, and "dynia" means pain.
Examples of normally non-painful sensations that may be painful in allodynia are pain from light stroking of the skin with the gliding of clothing, or pain from the intrinsic heat generated by the body.
Clinical examples of allodynia are skin sensitivity following sunburn, tenderness of the skin during migraine, painful gut mobility with irritable bowel syndrome, painful muscles following tissue trauma or inflammation, skin sensitivity with various nervous system disorders.
Allodynia can be experimentally induced by injected capsaicin intradermally.
In contrast, hyperalgesia is defined by the IASP as "Increased pain from a stimulus that normally provokes pain." An example is the prick of a needle, which is normally painful, but with hyperalgesia there is a greater than normal magnitude and/or duration of pain.
Alloknesis and Hyperknesis
The IASP do not provide a definition for alloknesis. It is the itch or pruriceptive sensation or scratching behaviour that is elicited by a stimulus that is not normally pruriceptive. "allo" means other, and "knesis" means itch. In the normal state, light stroking of the skin may elicit a ticklish sensation, but not itch. Following a mosquito bite or in an area of dermatitis, light stroking can evoke itch or exacerbation of an already present itch.
Hyperknesis on the other hand is increased pruriception from a stimulus that normally provokes pruritis. An example is a fine diameter hair, which normally elicits a slight prickle and itch sensation, is magnified in intensity and/or duration in the setting of hyperknesis.
Primary and Secondary
The above terms are also categorised as primary and secondary. Primary is changes in the area that was directly exposed to the stimulus. Secondary is when there is extension well beyond the area. An example is a first degree burn, which exhibits primary allodynia and hyperalgesia in the area of the burn, and secondary allodynia and hyperalgesia in the area surrounding the burn.
Resources
References
Robert LaMotte. Allodynia and Alloknesis. Encyclopedia of Pain 2013
