Allodynia

Definition and generalities

In the medical field, when we talk about allodynia, we want to indicate a pain caused by a stimulus that, under normal conditions, would be harmless and unable to cause any kind of pain sensation.

• Superficial allodynia that occurs in the skin and mucous membranes; therefore, the perceived pain is, in fact, a pain of a superficial type. This type of allodynia can differentiate into:
  • Mechanical allodynia, caused by non-nociceptive mechanical stimuli, so that normally do not cause pain.

    In turn, this form of allodynia can be subdivided into static mechanical allodynia (when it is caused by a slight pressure exerted on the skin) or dynamic (when it is caused by a slight rubbing of the same skin).

  • Thermal allodynia, caused by thermal stimuli, hot or cold, which are also not nociceptive.
• Deep allodynia that is perceived, in fact, in the deeper tissues and visceral level.

When it comes to allodynia, it is essential that it is not confused with hyperalgesia . These two conditions, in fact, although sometimes they may occur in the same patient at the same time, are very different from one another.

Allodynia, as we have said, consists in the perception of a painful sensation following a harmless stimulus which, as a rule, should not cause any reaction. Hyperalgesia, on the other hand, consists in the excessive perception of pain following a painful stimulus; therefore, in the case of hyperalgesia we speak of a hypersensitivity to painful stimuli.

Causes of Allodynia

The mechanism underlying the onset of allodynia has not yet been precisely identified and is still the subject of studies and debates, although the fact that there is both nociceptor and mechanoceptors present throughout the organism, is now a certainty.

However, usually, allodynia is not a disease in itself, but falls within the symptomatic framework of a disease or other underlying pathological conditions.

Among the various pathologies and conditions whose symptoms may include allodynia, we recall:

• Fibromyalgia syndrome;
• Peripheral sensory neuropathy;
• Complex regional pain syndrome;
• Neuropathic pain;
• Multiple sclerosis;
• Post-herpetic neuralgia;
• Trigeminal neuralgia;
• Migraine and headache.

Treatment

In truth, there is no real cure aimed at counteracting allodynia as such. Generally - since allodynia is one of the basic symptoms of diseases or pathological conditions - we proceed with the treatment of the latter, in the hope that this symptom will also be resolved or that, at least, it will be controlled.

However, it has been noted that allodynia is a symptom that can be alleviated by the administration of drugs, such as:

• Opioid analgesics, such as, for example, tramadol;
• Local anesthetics, such as lidocaine;
• Antidepressants, such as clomipramine, venlafaxine and duloxetine;
• Antiepileptics, such as pregabalin and gabapentin.

Moreover, it seems that also naproxen (a common non-steroidal anti-inflammatory drug, or NSAID, if one wants to), could constitute a potential and valid aid in the symptomatic treatment of allodynia, in particular, with respect to forms of mechanical and thermal allodynia.