Allodynia

Excerpt

The official IASP definition of allodynia at the time of this article is "pain due to a stimulus that does not normally provoke pain." An example would be a light feather touch (that should only produce sensation) causing pain. Allodynia is different from hyperalgesia, which is an exaggerated response from a normally painful stimulus, although both can and often do co-exist. Both are types of neuropathic pain.

An example of the difference between allodynia and hyperalgesia on the physical exam would be softly rubbing a cotton-tipped swab against a patient’s skin. Lightly brushing a swab against the skin would cause a low-level stimulus, but should not elicit a pain response. A patient who experiences pain with a stimulus that should only cause sensation may have allodynia. If the clinician significantly increases the degree of pressure, some pain would be part of a normal response. A patient who feels an excessive amount of pain would be noted to have hyperalgesia. Thus, on physical exam, allodynia presents as a lowering of the pain threshold, while hyperalgesia presents as a heightening of response. While this often means that allodynia and hyperalgesia seem to exist along the same continuum of stimuli on physical exam, there is still a clear difference in modalities. With allodynia, the response to the stimulus differs from those who have normal sensation, while in hyperalgesia, the response to the stimulus is the same as those who have normal sensation, but it is an exaggerated response.

Allodynia can be due to an underlying disease such as diabetes-induced neuropathic tactile allodynia or can be the primary disease process itself, such as in postherpetic neuralgia. It is often further classified by the type of stimulus causing the nociception, such as tactile, thermal, dynamic or static allodynia, or by the principal site of nociception, such as cutaneous allodynia.