Sometimes injuries and neurological alterations 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 alteration normally occurs and what characteristics define it.
The word “allodynia” comes from Greek; It can be translated as “another pain” or “different pain.” It is used to name a disorder that consists of perception of pain or other uncomfortable sensationssuch as burning or tingling, from stimuli that should not cause them.
Thus, people with allodynia may feel pain or discomfort when running a brush through their hair, shaving their face or legs, touching moderately hot objects, and even being caressed on the skin.
In allodynia, the pain response can occur anywhere in the body and does not subside with the consumption of analgesic medicationsas normal pain sensations do. It is generally attributed to injuries and alterations in the nerve endings, that is, in the sensory pain receptors or nociceptors.
Unpleasant sensations may begin or persist after the stimulation has ceased. 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 other than the one that received the stimulation.
This phenomenon differs from hyperalgesia in that in the case of allodynia the stimulus is not objectively painful, while in hyperalgesia there is a disproportionate pain reaction. On the other hand, the two disorders are similar at a functional and pathophysiological level, and frequently occur in the same people.
Types of allodynia
Allodynia experiences are classified depending on the type of stimulation that causes the painthat is, of the affected sensory modality; 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.
There are two subtypes of mechanical allodynia: statics and dynamics. We speak of static tactile allodynia when the 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 on the skin when coming into contact with shower water, for example, would be classified within this type of allodynia.
In this case the pain is caused by normal muscle or joint movements. People with motor allodynia may notice symptoms when sitting, walking, putting 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 well as a longer lasting process related to the central nervous system.
Anyone’s skin can become temporarily sensitized if certain conditions exist. For example, it is common for nociceptors to activate after being exposed to the sun when receiving gentle pressure or touch, such as when taking a shower or putting on a t-shirt.
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 by higher structures, or non-painful sensations are interpreted as if they were.
Different risk factors have been found that increase the probability of suffering allodynia. These include obesity, tobacco consumption and increased levels of female sex hormones, which reduce the pain threshold. Likewise the presence of chronic migraine or fibromyalgia It also favors allodynia.
Normally allodynia 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, stabbing and throbbing sensations appear to precede the onset of painful allodynia.
Related disorders
Various alterations that involve the nervous system (both central and peripheral) and/or that are related to the perception of pain have been associated with allodynia. These include migraine and fibromyalgia, as well as some specific neurological injuries.
People who suffer intense and frequent migraine attacks They have a greater probability of suffering from a central sensitization process, especially if they are migraines with aura; This can lead to the appearance 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 is included in the group known as “central sensitization syndromes”which also includes chronic fatigue syndrome, restless legs 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. The symptoms of this syndrome include increased skin sensitivity, changes in temperature 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 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 diseasesinfections, hypothyroidism, tumors or chronic alcoholism, among other causes.
5. Postherpetic neuralgia
Postherpetic neuralgia is a possible result of shinglesa viral disease that affects the skin. The lesions it causes can increase sensitivity to touch, potentially causing allodynia.
