Sometimes injuries and neurological disorders affect the functioning of 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 appear as a consequence of the same disorders. Let’s see in which contexts this disorder normally occurs and what characteristics define it.

What is allodynia?

The word “allodynia” comes from the Greek; it can be translated as “other pain” or “different pain”. It is used to describe a disorder consisting 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, shaving their face or legs, touching moderately hot objects, and even being stroked.

In allodynia, the painful response can occur anywhere in the body and does not resolve with the consumption of analgesic medication , as normal pain sensations do. It is generally attributed to injury and alterations in the nerve endings, i.e. the sensory pain receptors or nociceptors.

The unpleasant sensations may begin or continue after stimulation has stopped. Also, 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 similar at a functional and pathophysiological level, and often occur in the same people.

Types of allodynia

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

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, it is also theoretically possible that there are cases where pain is only felt in one of these sensory modalities.

1. Mechanical or tactile

Mechanical allodynia is the most classic manifestation of this disorder. In these cases, unpleasant sensations appear in response to tactile stimuli, such as pressure or vibration.

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

2. Thermal

Thermal allodynia can occur as a result of hot or cold stimuli that would not normally generate pain sensations. The onset of skin discomfort upon contact with shower water, for example, would be classified under this type of allodynia.

3. Motorcycle

In this case the pain is caused by normal movements of the muscles or joints . People with motor allodynia may notice symptoms when sitting, walking, putting on clothes or in any other situation involving movement.

Causes of this phenomenon

Allodynia is attributed to sensitization of the body, which can occur either as a normal reaction to intense stimulation or by a longer-term process related to the central nervous system.

Anyone’s skin can become temporarily sensitive if certain conditions are present. For example, it is common for nociceptors to be activated after exposure to the sun by gentle pressure or rubbing, such as when taking a shower or wearing a T-shirt.

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

Several risk factors have been found to increase the likelihood of allodynia. These include obesity, tobacco use and increased levels of female sex hormones, which lower the pain threshold. The presence of chronic migraine or fibromyalgia also favours the development of allodynia.

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

Related disorders

Various disorders involving the nervous system (both central and peripheral) and/or related to pain perception 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 are more likely to undergo a central sensitisation process, especially in the case of migraines with aura; this can lead to the development of allodynia. Tension headaches also sometimes cause allodynia, but to a lesser extent than migraines.

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 resulting from nerve dysfunction, which can be located centrally or peripherally. Symptoms of this syndrome include increased skin sensitivity, changes in skin temperature or joint pain, all of which can be classified 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 to the brain and other parts of the body. It often 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 may increase sensitivity to touch, potentially causing allodynia.