Allodynia : Causes, Symptoms, Diagnosis & Treatment

• General Information
• Why Does Allodynia Occur?
• Physiological Causes
• Fibromyalgia
• Polyneuropathies
• Neuralgia
• Causalgia
• Shingles
• Cephalgia
• Other Reasons
• Diagnostics
• Treatment
• Conservative Therapy
• Surgery

Allodynia is the appearance of pain when exposed to stimuli that normally do not cause pain. It is noted in causalgia, neuropathies and polyneuropathies, herpes zoster, multiple sclerosis, migraine, condition after spinal cord injuries, strokes, spinal operations. To establish the cause of allodynia, data from a survey, general and neurological examination, electrophysiological, neuroimaging and laboratory studies are used. Treatment - analgesics, blockade with anesthetics, vitamins, antidepressants, physiotherapy techniques. Surgery is sometimes recommended.

general information

Allodynia is pain that occurs when exposed to painless stimuli. It differs from hyperalgesia, in which the pain reaction appears in response to the appropriate stimulus, but is excessive. It can be provoked by touch, wind, non-intense thermal effects, etc. It is one of the variants of neuropathic pain, observed in 15-50% of people with this syndrome.

In 83-94% of cases it is caused by damage to the peripheral nerves, in 6-17% it becomes the result of pathological processes in the central nervous system. There are such types of allodynia as:

• Mechanical. The most common. It is noted under tactile influences. Can be static or dynamic. The first option is observed with single touches (for example, an attempt to take the hand), the second - with repeated weak stimulation (for example, repeated movements of a brush or cotton pad over the skin when applying or removing cosmetics).
• Thermal. It develops under the influence of cold and hot stimuli, which normally do not cause discomfort. Allodynia can be observed when taking a bath or shower, touching warm or cool objects, leaving the room to the street.
• Muscular. Unlike the two previous options, it is not associated with exposure to the skin, but with movements. Patients complain of pain in the depths of soft tissues (in the projection of muscles, joints, ligaments, tendons) in the absence of organic changes in the listed anatomical structures.

Why does allodynia occur?

Physiological causes

In healthy people, allodynia provokes excessive irritation of a particular zone. Most often the head suffers. In women, the symptom becomes a consequence of the use of a hair dryer, the abuse of thermal styling, sleeping with hair curled on hard curlers. The cause of discomfort in people of both sexes is the long-term wearing of a tight headgear, sometimes pediculosis (primarily in children).

Short-term allodynia in the distal lower extremities is potentiated by tight or uncomfortable shoes. In the summer, soreness with non-painful effects develops against the background of light sunburn. In people who have undergone trauma and surgery, allodynia in the projection of the scar and adjacent tissues is often observed in the period after the removal of sutures, disappears as the final healing, restoration of small nerves and nerve endings.

fibromyalgia

It is more common in middle-aged women, develops against the background of a hereditary predisposition, is accompanied by constant diffuse pains in the body, often in combination with numbness, tingling and other sensory phenomena, mainly expressed in the extremities. Allodynia and spontaneous pain sensations cause fatigue, sleep disturbances, and depression. The provoking factors of fibromyalgia are:

• Condition after injuries: burns, bruises, fractures, damage to peripheral nerve trunks.
• Infectious diseases: Q fever, infectious mononucleosis, borreliosis.
• Endocrine disorders: hypothyroidism.
• Distress: psycho-emotional exhaustion due to acute and chronic stressful situations.
• Medical effects: taking certain drugs, vaccination.

Polyneuropathies

The clinical picture of polyneuropathy consists of sensory, motor (paresis) and vegetative (dry skin, disorders of vascular regulation) symptoms. Allodynia and other sensory disorders (paresthesia, hyperesthesia) are caused by damage to thin myelinated fibers, occur at the initial stage, and are later replaced by hypesthesia.

Most cases of allodynia are associated with the development of peripheral diabetic polyneuropathy. Pathology is found in more than 10% of patients with diabetes mellitus. The symptom is also determined with the following polyneuropathies:

• Toxic: alcohol, drug addiction, poisoning, occupational exposure.
• Infectious: with measles, mumps, diphtheria, HIV infection.
• Autoimmune: paraneoplastic, paraproteinemic, with Miller-Fischer syndrome.
• Metabolic: in renal and hepatic insufficiency.
• Hereditary: with Roussy-Levi syndrome, Refsum disease and Charcot-Marie-Tooth.
• Others: medicinal, which arose after chemotherapy.

neuralgia

The most common pathology from this group is trigeminal neuralgia. There are repeated attacks of extremely intense prosopalgia, which extends to half of the face and resembles an electric shock. When shaving, contact with cold water, going outside, exposure to wind, allodynia develops, turning into another paroxysm. Other possible causes of the symptom are the following neuralgia:

• Ear node. Throbbing or burning pain occurs in the ear and parotid region. It can be potentiated by hypothermia of the affected area, ingestion of hot food, experiences, emotional stress.
• Submandibular and sublingual nodes. Allodynia develops when eating cold or hot food. It transforms into a pain attack in the submandibular and sublingual regions, anterior parts of the tongue.
• Intercostal. Paroxysms are caused by movements of the chest (coughing, laughing, deep breaths, sharp turns of the body), less often by tactile contacts. The pain has a girdle character, spreads along the intercostal space.

Allodynia

Causalgia

Occurs after traumatic injuries (fractures, gunshot wounds). At the initial stage, causalgia is manifested by burning or baking pain that does not correspond to the severity of the injury. Painful sensations are provoked by any stimuli (movement, touch), decrease when the affected segment is immersed in water or a wet bandage is applied. Allodynia is complemented by hyperpathy, hyperalgesia, edema, and trophic disorders. Over time, paresis and contractures form.

Shingles

Herpes zoster manifests with nonspecific prodromal signs. Then itching and pain appear along the course of the peripheral nerve (in the vast majority of cases, the intercostal nerve). After a few days, painful pink spots form in the affected area, and a little later - vesicles with serous contents. After recovery, about 10% of patients have allodynia and other manifestations of neuropathic pain for several years.

cephalgia

Periodic cutaneous allodynia in the temple area is observed in 65% of patients with migraine. In 20% of cases, it causes significant discomfort, has an additional negative impact on the quality of life. Patients complain of periodic cephalgia, which cover half of the head, are complemented by nausea, fear of bright lights and loud sounds. Allodynia is also found in paroxysmal hemicrania, combined with pain and autonomic disorders.

Other reasons

Allodynia, caused by organic changes in the central nervous system, can be observed in such pathological processes as:

• syringomyelia;
• condition after a stroke;
• multiple sclerosis (in the event of acute transverse myelitis);
• condition after vertebral and spinal cord injuries and surgical interventions on the spine.

In addition, allodynia is found in combined pain syndromes that develop against the background of a violation of nociceptive mechanisms with the participation of psychogenic and neuropathic components. A common example is chronic pain in radiculopathy due to degenerative and inflammatory diseases of the spine. Less common is allodynia in the germination of nerves by malignant tumors.

Diagnostics

The cause of allodynia is established by a neurologist. With the development of neuropathic pain against the background of other pathologies, consultations with an endocrinologist, therapist, narcologist, etc. can be shown. As part of the survey, the doctor finds out the time of onset of the symptom, asks about other manifestations, and monitors the dynamics of the disease. Collects an anamnesis of life to identify possible provoking factors. The additional examination program includes the following procedures:

• Neurological examination. The neurologist determines the localization and type of allodynia using special tests (by normal touch, using a brush, warm and cold objects). Assesses various types of sensitivity, reveals sensory disorders. Examines muscle strength and reflexes.
• Electrophysiological methods. EMG and ENG are performed to assess the functional state of the nerve, to determine the severity of the process, the level and degree of damage to the nerve trunks, and to control recovery during treatment. Recommended for neuralgia, polyneuropathy, allodynia of central origin.
• Neuroimaging. With neuralgia, MRI of the brain and CT scan of the skull may be prescribed to clarify the cause of nerve damage (compression in a narrow bone canal, tumor). If vascular compression is suspected, MR angiography is performed. In conditions after injuries and operations on the spine, MRI of the spinal cord and CT of the spinal column are indicated. Neuroimaging methods are also used to diagnose CNS diseases accompanied by allodynia.
• Laboratory tests. With fibromyalgia, a decrease in the level of serotonin and L-tryptophan in the blood is determined. In metabolic polyneuropathies, according to blood biochemistry, hyperglycemia, signs of impaired liver or kidney function are detected, and in autoimmune - the corresponding antibodies. With an infectious etiology of allodynia, PCR, ELISA, and microbiological studies are required.

Neurological examination

Treatment

Conservative therapy

Treatment tactics are determined taking into account the etiology of allodynia:

• Fibromyalgia. For acute pain, central analgesics are used. Anticonvulsants are prescribed to reduce the severity of chronic pain syndrome, antidepressants are prescribed to eliminate depression and improve sleep. It is possible to introduce local anesthetics into trigger points, irrigation of pain zones. As part of non-drug treatment, psychotherapy, exercise therapy, hydrotherapy, acupuncture, biofeedback therapy are recommended.
• Polyneuropathy. With intense pain, tricyclic antidepressants are indicated. In diabetes, the insulin therapy regimen is corrected, in uremia, hemodialysis is performed, in case of intoxication, detoxification therapy is performed, in inflammatory polyneuropathy, membrane plasmapheresis is performed, and human immunoglobulin is administered. Symptomatic treatment includes B vitamins, neurotrophic agents.
• Trigeminal neuralgia. To eliminate hyperexcitation, anticonvulsants are used. To increase the effectiveness of the main drug, drugs with antihistamine action and microcirculation correctors are used. Painful paroxysms are stopped with the help of antispasmodics. Produce therapeutic blockade with local anesthetics and glucocorticoids. The analgesic and anti-inflammatory effect is provided by physiotherapeutic procedures: diadynamic currents, ultraphonophoresis with hydrocortisone, galvanization with novocaine.
• Other neuralgias. The list of medicines used includes adrenoblockers, ganglioblockers, blockers of cholinergic systems, ATP, vitamins C, PP, B. To normalize the psycho-emotional state and reduce pain, antidepressants, tranquilizers, and sedatives are indicated. During the period of convalescence, biogenic agents, reflexology, inductothermy, DDT, galvanization, SMT, ultraphonophoresis are recommended.
• Migraine. With paroxysms of moderate and low intensity, combined and simple analgesics, codeine-containing agents are prescribed. Sometimes therapeutic blockades are performed. Severe attacks are an indication for oral treatment or subcutaneous triptans.

Surgery

If the sublingual, submandibular nodes are affected, ENT operations (frontotomy, maxillary sinusectomy, tonsillectomy) may be required to eliminate the source of infection. Patients with involvement of the ear node are sometimes shown sanitizing surgery on the middle ear. With trigeminal neuralgia, microsurgical decompression or percutaneous radiofrequency destruction is performed. In some cases, the sensory root is destroyed using the methods of stereotaxic surgery.