Allodynia: Types, Causes, and Related Disorders

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

There are different types of allodynia, but they 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 uncomfortable sensations, such as burning or tingling, from stimuli that should not cause them.

Thus, people with allodynia may feel pain or discomfort when brushing their hair, when shaving the face or legs, touching moderately hot objects, and even being caressed the skin.

In allodynia, the painful response can occur anywhere in the body and

does not remit with the consumption of pain relievers, as do normal pain sensations. It is generally attributed to injuries and alterations in the nerve endings, that is, in the sensory receptors of pain or nociceptors.

Unpleasant sensations can start or continue after stimulation has stopped. Likewise, 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 a different area 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 functionally and pathophysiologically similar, and often occur in the same people.

• You may be interested: "Chronic pain: what it is and how it is treated from Psychology"

Types of allodynia

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

The different types of allodynia can occur in the same person: the separation refers to the symptoms themselves and not to the existence of distinct subtypes of the disorder. However, theoretically it is also possible that there are cases in which pain is only 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: statics and 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 can occur as a result of hot or cold stimuli that would not normally generate pain sensations. The appearance of discomfort in the skin when coming into contact with the shower water, for example, would be classified within this type of allodynia.

3. Motorboat

In this case the pain is caused by normal muscle or joint movements. People with motor allodynia may notice symptoms when they sit, when they walk, when they put on clothes, or in any other situation that involves performing movements.

Causes of this phenomenon

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

Anyone's skin can become transiently sensitized under certain conditions. For example, it is common for nociceptors to be activated after being exposed to the sun when they receive gentle pressure or friction, such as when taking a shower or putting on a T-shirt.

Central sensitization is more pathological. It occurs at the level of brainstem and of the spinal cord; an increase in the receptivity of nociceptors causes them to send pain signals to the central nervous system without that these are inhibited from higher structures, or non-painful sensations are interpreted as if it were were.

Different risk factors have been found that increase the probability of suffering from allodynia. These include obesity, tobacco use and increased levels of female sex hormones, which lower the pain threshold. Also the presence of chronic or fibromyalgia migraine favors allodynia as well.

Allodynia usually 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, throbbing and throbbing sensations seem to precede the onset of painful allodynia.

Related disorders

Various 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 lesions.

1. Chronic migraine

People who suffer frequent and severe migraine attacks they are more likely to suffer a central sensitization process, especially if it is migraines with aura; This can lead to the development 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 cannot be identified. Like headaches, fibromyalgia falls into the group known as "Central sensitization syndromes", which also includes chronic fatigue syndrome, restless leg syndrome, and some temporomandibular joint disorders.

3. Complex regional pain syndrome

This disorder, also known as "reflex sympathetic dystrophy syndrome", is a type of chronic pain derived from nerve dysfunction, which can be located centrally or peripherally. Symptoms of this syndrome include increased skin sensitivity, temperature changes, or joint pain, all of which can be classified as allodynia.

4. Peripheral neuropathy

We speak of 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. It frequently occurs as consequence of autoimmune diseases, infections, hypothyroidism, tumors or chronic alcoholism, among other causes.

5. Postherpetic neuralgia

Post-herpetic neuralgia is a possible result of shingles, a viral disease that affects the skin. The lesions it causes can increase sensitivity to touch, potentially causing allodynia.