Allodynia R20.-

In medicine, allodynia (gr. allos "different"; odyne "pain") is a pain sensation that is triggered by stimuli that normally do not cause pain.

Dynamic-mechanical allodynia: Slightly moving skin stimuli, e.g. cotton balls, trigger pain. Location: In the primary zone of the disease, spreading partly far into uninjured skin areas (secondary zone).

Punctiform allodynia: Normally slightly stinging but not painful stimuli (stiff von Frey hair) trigger pain. Location: In the primary zone of injury and spreading partly far into uninjured skin areas (secondary zone).

Cold allodynia: Light cold stimuli trigger pain. Typical in traumatic nerve lesion and some polyneuropathies.

Heat allodynia: Mild warmth stimuli trigger pain. Typical in traumatic nerve lesion and some polyneuropathies.

Allodynia by summation: Repeatedly applied identical noxic stimuli are perceived as a steadily increasing pain sensation.

The pathogenesis of allodynia has not yet been fully clarified. It is conceivable that the stimulus may be redirected via low-threshold, normally non-nociceptive Aß touch afferences. Two different mechanisms are conceivable here:

• Central changes in the functionally effective synaptic structures so that impulses from Aß fibres in the spinal cord are switched to overexcited secondary nociceptive neurons (central sensitisation),
• Anatomical connection of Aß-fibres in the spinal cord to secondary nociceptive neurons (anatomical reorganisation in the posterior horn)

The symptom of allodynia occurs in a variety of neurological diseases. A typical example is zoster infection. Other diseases:

• Neuropathies of other genesis
• Fibromyalgia
• postherpetic neuralgia
• Migraine

Allodynia must be distinguished from hyperalgesia, where an already painful stimulus triggers a more intense pain.

The pathogenetic analogue of allodynia is allocnesis/hyperknesis in the triggering of itching.