In allodynia, touch or temperature stimuli are felt as unusually painful. The cause may be in the peripheral or central nervous system or in the patient’s psyche. Treatment is based on the primary cause.
What is allodynia?
Allodynia is associated with the expression of neuropathic pain. In the human skin and mucous membranes are located so-called sensory cells, which are considered the first place of any perception. Among them are the nociceptors, which are accessible to painful stimuli. Nociceptors are free nerve endings of the sensitive spinal cord neurons and are found in all pain-sensing tissues of the body. Pain receptors report surface pain, internal organ pain, and deep pain in the sense of muscle and joint pain to the central nervous system. Above a certain stimulus intensity in their receptive field, nociceptors form an action potential that travels via the spinal cord to the brain in the form of neuronal excitation, where it reaches consciousness. The stimulus threshold for the formation of the action potential differs from person to person. This is the origin of the statement that everyone has a different pain threshold. A moderately low pain threshold is therefore not necessarily associated with disease value. However, if the nociceptors already generate action potentials in response to pleasant touch stimuli and thus report pain, there is talk of disease value. This phenomenon corresponds to allodynia and refers to pain caused by innocuous stimuli that are generally tolerated. In addition, hyperalgesia is related to allodynia.
Causes
The cause of allodynia usually lies in diseased nerves and the areas of skin they supply, which are often pre-damaged. In cases of nerve damage, there is talk of physical causes of allodynia. Often in this context, patients have suffered polyneuropathies in the past that have activated spontaneous C-nociceptor activity. Chronic loss of nerve fibers in the context of polyneuropathy is related to this phenomenon. In this sense, neuropathic pain syndrome is also characterized by allodynia. In addition, allodynia may be associated with segmental tissue loss in the posterior horn of the spinal cord. It may also be caused by trigeminal neuralgia, in which unprotected nerve fibers are in close proximity and lower the threshold for stimulation via ephaptic neurotransmission. However, psychological factors can also play a role in the processes that trigger pain. For example, anxiety and somatization tendencies favor any hypersensitivity. Depending on its location in the nervous system, allodynia is classified as peripherally (primary) or centrally mediated (secondary). Centrally mediated allodynia is often preceded by a stroke or a disease such as multiple sclerosis. Patients with allodynia experience painless touch stimuli and temperature stimuli as painful. Depending on the symptoms, allodynia is divided into subgroups:
- We speak of mechanical dynamic allodynia when a light touch stimulus on the skin triggers pain, such as the stimulus of a cotton swab. The pain is characterized as sharp or burning and may spread beyond the point of contact. Mechanical static allodynia results in the sensation of pain when light pressure is applied to a specific area of the skin. For example, a light finger pressure triggers dull pain.
- Mechanical pinprick allodynia is a hyperalgesia. Patients with this form of allodynia experience mild stabbing touch stimuli on the skin as excessive spreading pain, such as the touch of a toothpick.
- Cold allodynia is also hyperalgesia and amplifies mildly painful cold stimuli into a more severe skin pain.
- The opposite phenomenon is heat allodynias, in which there is hypersensitivity to heat pain, leading to burning temperature insensations.
Diseases with this symptom
- Polyneuropathy
- Pain syndrome
- Trigeminal neuralgia
- Stroke
- CRPS
- Sudeck’s disease
- Multiple sclerosis
- Shingles
- Postdisectomy syndrome
Diagnosis and course
Allodynia is detected within the neurological diagnosis. The provocation test is used to establish the diagnosis. The examiner applies various types of stimuli to the patient’s skin. For this purpose, he uses tools such as the toothpick, a cold and a warm metal roller or his own finger. The patient is asked to report and describe the pain sensation. The identification of allodynia is followed by a detailed diagnosis of the underlying condition. If imaging of the central and peripheral nervous systems remains inconclusive, the allodynia likely has an underlying psychological cause. The prognosis of patients depends on the primary cause. Centrally mediated allodynia has the least favorable prognosis. Psychological causes are usually the best to resolve.
Complications
Various psychological complications may occur in the setting of allodynia. There may also be a pronounced avoidance behavior of the affected person. Thus, it is quite understandable that all situations that trigger pain are avoided. However, this behavior is not sensible, as it can ultimately lead to isolation of the affected person. For example, activities that are normally enjoyed can no longer be performed because they are associated with pain. Furthermore, some psychological side effects of allodynia are possible. Allodynia can lead to the development of chronic pain. The patient is then permanently tormented by a pain that is triggered by stimuli that do not normally lead to a sensation of pain. This circumstance, if prolonged, can trigger psychological reactions, such as a depressive episode. The avoidance behavior already mentioned, in combination with the chronic pain, can lead to a worsening of the psychological symptomatology. In addition to depression, anxiety can also occur. It is therefore clear to see that allodynia can lead to a whole range of other symptomatology, some of which reinforce each other. An important part of the therapy of allodynia is therefore the prevention of the development of chronic pain and the prevention of psychological consequences. If this has already occurred, therapy through a combination of analgesia and psychotherapy is advised.
When should one go to the doctor?
A physician should be consulted if there are signs of nerve irritation or hypersensitivity to pain, as well as temperature associated with more severe than normal pain. A mild or known sensation of pain is usually of little concern. This pain is characterized by the fact that it disappears quickly and no further impairment is present. In contrast, severe pain indicates that the single or multiple nerve fibers have been destroyed. Self-testing can be done quickly by applying pressure to the skin. Since an untreated nerve can lead to the death of additional nerve fibers, clarity should be obtained through a medical examination. Allodynia can have various causes. Not all of them are to be classified as serious. Nevertheless, only a clarification and determination of the origin can give more information about the severity and the chances of cure. The more time that passes before a visit to the doctor or the start of treatment, the further the damage can progress. Furthermore, the risk of irreparable damage increases. Since psychological accompanying symptoms such as anxiety or depression can occur in addition to allodynia, a visit to the doctor is essential if the intense pain sensation persists. Alternatively, the likelihood increases that the accompanying symptoms of allodynia can only be eliminated with lengthy therapies or that chronic consequences will develop.
Treatment and therapy
Physically altered pain sensation can only be treated to a limited extent. The cause determines the approach. Psychologically induced allodynia, for example, is worked through in psychological accompaniment. Psychological accompaniment can also be useful for other forms of allodynia to help patients cope better with their altered pain sensitivity. In the case of all organic causes such as injury to nerve tissue, the damage is usually considered irreparable. This is especially true for centrally mediated allodynia.Treatment of the primary cause can lead to an improvement in the symptoms, but often does not allow them to subside completely. Pain therapies can be useful in this context, for example the implantation of a pump with analgesic drugs. In some cases of organic causes, a re-increase to normalization of the pain threshold has been reported after patients have been exposed to actual strong pain stimuli. Even after regular exposure to mild but perceived severe pain, some were able to induce an adaptation or habituation effect and thus an improvement in allodynia.
Outlook and prognosis
In many cases, allodynia leads to severe restriction of the patient’s life. Intimate touch in particular is limited, which can lead to problems with a partner. A physician can usually perform a diagnosis, determining whether the allodynia is caused by physical and by psychological conditions. In case of psychological causes, the symptom is treated by a psychologist. The further course of the disease strongly depends on the psychological condition of the patient. Often the allodynia leads to a slightly aggressive behavior of the patient, which is why they withdraw and isolate themselves socially. In addition to the actual pain due to the disease, depression or anxiety often develops. A specific treatment at the doctor’s office is not possible, because the doctor cannot understand exactly how the pain arises. However, pain therapies can be helpful and greatly limit the symptom. Temporarily, painkillers can also be taken. However, long-term use should be avoided. In most patients, adaptation to pain also occurs. Thus, they no longer react to real pain as strongly as healthy people. This can become dangerous in some situations. To prevent the symptom, damage to the nervous system should be avoided. This includes, above all, the excessive consumption of alcohol and other drugs.
Prevention
Peripherally and centrally mediated allodynia can be prevented only to the extent that damage to the nervous system can be prevented. Psychologically mediated allodynia can be prevented by promptly bringing painful events and fears to resolution.
Here’s what you can do yourself
Primarily psychologically mediated allodynia can positively affect affected individuals in a variety of ways. Strategies and methods learned in appropriate therapies can be well integrated into everyday life. Body-oriented relaxation methods, hobbies, conversations, pleasure training, careful desensitization instead of avoidance, adapted exercise in the fresh air and sufficiently long regeneration phases help to counteract increased self-observation and focus on the pain event. In the case of physiologically based allodynia, self-help measures often remain ineffective. Participation in social life despite pain protects against loneliness and redirects the focus from one’s own body to the outside world. Those affected make it easier for their relatives and friends to deal with the often difficult-to-understand reactions to affection and physical contact by involving them in the psychoeducational process. In self-help groups, they can address concerns and distress while counteracting social withdrawal and isolation. Participating in discussions in online forums or documenting their own experiences in diaries or blogs also helps many pain patients cope with the disease. The goal of supportive self-therapy should be to learn healthy withdrawal and self-protective behavior instead of disease-maintaining avoidance strategies. Special attention should be paid to an adapted intake of painkillers, since an incorrect, too low, or too high dosage is counterproductive. Despite a lack of evidence, complementary methods can also be helpful in individual cases.
