Sometimes neurological lesions and alterations affect the functioning of nociceptors, which activate in the face of painless stimuli; this phenomenon is known as allodynia.
There are different types of allodynia but they all have similar causes and appear as a result of the same disorders. Let’s see in what contexts this alteration usually occurs and what characteristics define it.
What is allodynia?
The word “allodynia” comes from the Greek; it can be translated as “another pain” or “a different pain”. It is used to name a disorder which consists of perception of pain or other bothersome sensations, Such as burning or stinging, due to stimuli that should not be causing them
So, people with allodynia may experience pain or discomfort when brushing their hair, shaving their face or legs, touching moderately hot objects, and even stroking them on the skin.
In allodynia, the painful response can occur anywhere in the body and does not recover with the consumption of painkillersJust like normal pain sensations. It is usually attributed to injuries and alterations in nerve endings, that is, pain sensory receptors or nociceptors.
Unpleasant sensations may begin or persist after the stimulation ends. Likewise, pain and the event that causes it do not necessarily belong to the same sensory modality. It may also happen that the discomfort appears in an area different from where you received the stimulation.
This phenomenon differs from hyperalgesia in that in the case of allodynia the stimulus is not objectively painful, whereas in hyperalgesia, a disproportionate pain reaction occurs. On the other hand, the two disorders are functionally and pathophysiologically similar and often occur in the same people.
Types of allodynia
The experiences of allodynia are classified depending on the type of stimulation that is causing the pain, That is, the sensory modality affected; thus, this disorder can be divided into mechanical or tactile allodynia, thermal and motor allodynia.
Different types of allodynia can occur in the same person: separation refers to the symptoms themselves and not to the existence of differentiated subtypes of the disorder. However, in theory, it’s also possible that there are instances where you only experience pain in one of these sensory modalities.
1. Mechanical or tactile
Mechanical allodynia is the most classic manifestation of this alteration. In these cases, unpleasant sensations appear in response to tactile stimuli, such as pressure or vibration.
they exist mechanical allodynia subtypes: Static and dynamic. We speak of static tactile allodynia when the pain occurs in response to a single stimulus, such as finger pressure, and dynamic allodynia if the cause is light and repeated stimulation, such as a caress or the rubbing of a towel.
Thermal allodynia can occur as a result of hot or cold stimuli that would not normally generate pain sensations. The appearance of discomfort in the skin on contact with shower water, for example, would be classified under this type of allodynia.
In this case, the pain is caused by normal movements of muscles or joints. People with motor allodynia may notice symptoms when sitting, walking, putting on clothes, or any other situation that involves movement.
The causes of this phenomenon
Allodynia is attributed to sensitization of the body, which can occur up to a normal reaction to intense stimulation as through a more lasting process linked to the central nervous system.
Anyone’s skin can become transiently sensitized if certain conditions are met. For example, after being exposed to the sun by nociceptors, it is common to activate a light pressure or rubbing on the reception, such as taking a shower or wearing a T-shirt.
Central sensitization is more pathological. It occurs in the brainstem and spinal cord; an increase in the receptivity of nociceptors causes them to send pain signals to the central nervous system without being inhibited by higher structures, or without interpreting painless sensations as if they were.
Different risk factors have been found that increase the likelihood of suffering from allodynia. These include obesity, smoking, and increased levels of female sex hormones, which lower the pain threshold. Also the presence of chronic migraine or fibromyalgia promotes allodynia.
Allodynia usually goes away when its causes are corrected; however, the best way to treat this disorder is to prevent the first symptoms of central sensitization from progressing. In particular, the tingling and pulsating sensations seem to precede the onset of painful allodynia.
Various alterations affecting the nervous system (central and peripheral) and / or related to the perception of pain have been associated with allodynia. These include migraine and fibromyalgia, as well as some specific neurological lesions.
1. Chronic migraine
People who suffer intense and frequent migraine attacks they are more likely to suffer from a central sensitization process, especially if they are migraines with aura; this can lead to the onset of allodynia. Tension headaches also sometimes cause allodynia, but to a lesser extent than migraines.
Fibromyalgia is diagnosed in cases of chronic muscle pain whose origin cannot be identified. Like headaches, fibromyalgia belongs to the group known as fibromyalgia “Central sensitization syndromes”, Which also includes chronic fatigue syndrome, restless leg syndrome and certain temporomandibular joint disorders.
3. Complex regional pain syndrome
This disorder, also called “reflex sympathetic dystrophy syndrome”, is a type of chronic pain resulting from nervous dysfunction, which can be localized centrally or peripherally. Symptoms of this syndrome include increased sensitivity of the skin, changes in temperature, or joint pain, all of which can be classified as allodynia.
4. Peripheral neuropathy
It is called peripheral neuropathy when there is injury or dysfunction of the nerves that connect the spinal cord to the brain and other parts of the body. This happens frequently as consequence of autoimmune diseases, Infections, hypothyroidism, tumors or chronic alcoholism, among other causes.
5. Post-herpetic neuralgia
Post-herpetic neuralgia is a possible result of shingles, A viral disease that affects the skin. The injuries it causes can increase sensitivity to touch, potentially causing allodynia.