Allodynia: types, causes and related disorders

Occasionally, lesions and neurological alterations affect the functioning of nociceptors, which are activated by non-painful stimuli; This phenomenon is known as allodynia.

There are different types of allodynia but all have similar causes and they appear as a consequence of the same disorders. Let's see in what contexts this alteration normally occurs and what characteristics define it.

• Related article: "The 13 types of pain: classification and characteristics"

What is allodynia?

The word "allodynia" comes from the Greek; it can be translated as "another pain" or "different pain". It is used to refer to a disorder that consists of the perception of pain or other annoying sensations , like burning or tingling, from stimuli that should not provoke them.

Thus, people with allodynia may feel pain or discomfort when passing a brush through their hair, shaving their face or legs, touching moderately hot objects and even receiving caresses on the skin.

In allodynia the painful response can occur anywhere in the body and does not remit with the use of analgesic drugs , as normal pain sensations do. It is usually attributed to injuries and alterations in the nerve endings, that is, in sensory pain receptors or nociceptors.

Unpleasant sensations may be initiated or sustained after the stimulation has ceased. Likewise, the pain and the event that causes it do not necessarily belong to the same sensory modality. It can also happen that the discomfort appears in an area other than the one that has received the stimulation.

This phenomenon differs from hyperalgesia in that in the case of allodynia the stimulus is not objectively painful, whereas in hyperalgesia there is a disproportionate pain reaction . On the other hand, the two disorders are similar at the functional and pathophysiological level, and often occur in the same people.

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Types of allodynia

Allodynia experiences are classified depending on the type of stimulation that causes the pain , that is, the affected sensory modality; thus, we can divide this disorder into mechanical or tactile allodynia, thermal and motor.

The different types of allodynia can occur in the same person: the separation refers to the symptoms themselves and not to the existence of differentiated subtypes of the disorder. However, theoretically it is also possible that there are cases in which only pain is felt 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.

exist two subtypes of mechanical allodynia : the static and the dynamics . We speak of static tactile allodynia when pain occurs in response to a single stimulus, such as pressure with a finger, and dynamic allodynia if the cause is a light and repeated stimulation, such as a caress or the friction of a towel.

2. Thermal

Thermal allodynia may occur as a result of hot or cold stimuli that normally would not generate pain sensations. The appearance of discomfort in the skin when coming into contact with shower water, for example, would be classified within this type of allodynia.

3. Motor

In this case the pain is caused by normal movements of muscles or joints . People with motor allodynia may notice symptoms when they sit down, when they walk, when they put on clothes or in any other situation that involves carrying out movements.

Causes of this phenomenon

Allodynia is attributed to the sensitization of the organism, which can occur as much as a normal reaction to intense stimulation as by a more lasting process related to the central nervous system.

The skin of any person can be sensitized transiently if certain conditions are met. For example, it is common that after being exposed to the sun the nociceptors are activated when receiving a gentle pressure or rubbing, such as when taking a shower or putting on a shirt.

The central sensitization is more pathological . It occurs at the level of the brainstem and the spinal cord; An increase in the receptivity of nociceptors causes them to send pain signals to the central nervous system without inhibiting them from higher structures, or they interpret non-painful sensations as if they were.

Different risk factors have been found that increase the likelihood of suffering from allodynia.These include obesity, tobacco consumption and increased levels of female sex hormones, which reduce the pain threshold. Also the presence of chronic or fibromyalgia migraine It favors that allodynia is also given.

Normally, allodynia disappears when its causes are corrected; however, the best way to treat this condition is to prevent the initial symptoms of central sensitization from progressing. In particular, the throbbing and throbbing sensations seem to precede the appearance of painful allodynia.

Related disorders

Several alterations involving the nervous system (both 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 injuries.

1. Chronic migraine

The people who suffer Intense and frequent migraine attacks they are more likely to suffer a central sensitization process, especially if they are migraines with aura; this can lead to the appearance of allodynia. Tension headaches also cause allodynia at times, but to a lesser extent than migraines.

• Related article: "The 7 types of migraine (characteristics and causes)"

2. Fibromyalgia

Fibromyalgia is diagnosed in cases of chronic muscle pain whose origin can not be identified. Like the headaches, fibromyalgia is included in the group known as "Central sensitization syndromes" , which also includes chronic fatigue syndrome, restless legs and some disorders of the temporomandibular joint.

3. Complex regional pain syndrome

This disorder, also known as "reflex sympathetic dystrophy syndrome," is a type of chronic pain derived from a nerve dysfunction, which can be localized at the central or peripheral level. Symptoms of this syndrome include increased sensitivity of the skin, changes in temperature or joint pain, all classifiable as allodynia.

4. Peripheral neuropathy

We talk about peripheral neuropathy when there is an injury or a malfunction of the nerves that connect the spinal cord with the brain and other parts of the body. Frequently it occurs as consequence of autoimmune diseases , infections, hypothyroidism, tumors or chronic alcoholism, among other causes.

5. Postherpetic neuralgia

Postherpetic neuralgia is a possible result of herpes zoster , a viral disease that affects the skin. The lesions it causes can increase sensitivity to touch, potentially causing allodynia.