Allodynia is an abnormal pain resulting from a stimulus that does not usually produce pain. The term is derived from the Greek words “allo,” which means other, and “odyne,” which means pain.
Even though the exact cause is not known, allodynia is considered to be the result of a process called central sensitization, which is an increase in the excitability of neurons within the central nervous system. The central nervous system includes the brain and spinal cord. Peripheral nerves branch out to the rest of the body.
Usually the central sensitization occurs because of peripheral sensitization. In peripheral sensitization, the peripheral nerve endings keep sending the pain signals to the brain even in the absence of a pain stimulus. During central sensitization, a normally harmless stimulus, such as a light touch to the skin, activates neurons in the spinal cord and brain that are usually activated only in response to noxious stimuli (intense stimuli that may cause damage to the tissue), or the neurons may get activated even in the absence of any stimulus.
- Touch allodynia (cutaneous allodynia). Pain experienced from stimulus that does not normally produce pain. Examples of activities that may produce allodynia include combing or brushing hair, shaving, showering and wearing glasses.
- Location allodynia (allesthesia or allachesthesia). Pain that occurs in a location of the body other than the one stimulated. One example of location allodynia is when pain is experienced in the forearm when a hand is rubbed against a beard.
- Temperature allodynia (thermal allodynia). Abnormal pain resulting from exposure to heat or cold. One example occurs when a cold breeze produces a feeling of burning.
Additional symptoms that may accompany allodynia include soreness or tenderness, difficulty resting on the side of the body experiencing allodynia, hot or burning sensation and localized tenderness. Sometimes the person may not be able to tolerate even the touch of cloth to the area affected by allodynia.
Allodynia is often a response to a change in the nature of a tactile or thermal stimulus. For example, a person who had not been experiencing pain may suddenly feel pain when a fan is turned on.
Allodynia is different from hyperalgesia (extreme sensitivity to pain). This occurs when a stimulus that is normally painful causes an unusually exaggerated and prolonged amount of pain. It is also different from referred pain, paresthesia and psychosomatic pain.
Allodynia is often a component of neuropathic pain. This pain is the result of damage or disease to the peripheral or central nervous system, such as that caused by diabetes. Neuropathic pain is different from other types of pain, such as nociceptive pain, which is usually the response to noxious stimuli.
The exact cause of allodynia is difficult to identify. Research has shown that inflammation can cause certain substances that help nerve cells communication, such as NR2B proteins, to heighten awareness of discomfort.
- Migraines. A severe type of headache that recurs over time. Migraines may occur with symptoms such as nausea, aura, vomiting and sensitivity to light. Allodynia occurs frequently in people who experience migraines. They can experience allodynia even while not experiencing a migraine.
- Fibromyalgia. A chronic pain condition characterized by aches, stiffness, tenderness of soft tissues, fatigue and sleep disturbances. Allodynia frequently occurs along with hyperalgesia (extreme sensitivity to pain) in patients with fibromyalgia.
- Complex regional pain syndrome. A chronic condition that typically involves continuous, intense arm pain or leg pain. Allodynia is a characteristic symptom of complex regional pain syndrome, formerly known as reflex sympathetic dystrophy syndrome and causalgia.
- Central pain syndrome (CPS). A neurological condition caused by damage to the central nervous system. A number of conditions can cause central pain syndrome, including diabetes, multiple sclerosis and stroke, although some stroke patients may not experience CPS until months after the stroke. Spinal cord injury can also cause CPS. Allodynia occurs frequently in patients with CPS.
- Peripheral neuropathy. Damage to the peripheral nervous system. Causes include diabetes, alcoholism, cancer and trauma.
- Neuralgia, including postherpetic neuralgia and cranial neuralgias.
- Myelitis. Inflammation of the spinal cord.
- Residual limb (stump) pain and phantom limb pain. Conditions following an amputation.
In general, any nerve injury (neuropathy) can cause susceptibility to allodynia.
There have also been cases of allodynia caused by medications, including morphine and triptans.
To diagnose allodynia, a physician may review the patient’s medical history and list of medications and perform a physical exam. Diagnosis relies on the physical because there are no lab or imaging tests to detect allodynia.
Patients may be asked to complete a pain assessment form or describe the pain by providing information regarding:
- The location in the body experiencing pain
- How much pain is being experienced (e.g., on a numeric scale)
- The type of pain felt (e.g., sharp or burning)
- How long the pain has lasted
- What triggered or started the pain (e.g., any injury)
- Whether the pain is associated with a disease, such as diabetes or cancer
- Circumstances that relieve or worsen the pain
A physician may perform sensory testing, which are tests to determine how the body responds to certain stimulus. To conduct the test, a cotton swab or camel-hair brush may be used to lightly touch the skin.
- Migraines. Headache medications include triptans and nonsteroidal anti-inflammatory drugs (NSAIDs).
- Fibromyalgia. Options may include medications, exercise therapy, relaxation techniques, cognitive behavioral therapy and educational programs to better understand and manage the condition.
- Complex regional pain syndrome. Nerve blocks or sympathetic blocks (see Injection Therapy), other medications, physical therapy, biofeedback or electrical therapy may help.
- Central pain syndrome. Patients may be treated with analgesics, although often they provide little or no relief, or anticonvulsants.
- Diabetic neuropathy. The U.S. Food and Drug Administration has approved two drugs to relieve diabetic nerve pain: the antidepressant duloxetine (Cymbalta) and the anticonvulsant pregabalin (Lyrica). Other antidepressants and anticonvulsants, and opioids, may also help ease peripheral neuropathic pain, according to the American Society of Pain Educators.
- Cranial neuralgias and other neuralgias. Antidepressants, anticonvulsants and massage therapy (see Manipulation Therapy) may bring relief.
Symptomatic treatments of allodynia are available. Local medication such as a lidocaine anesthetic patch or capsaicin cream (made from pepper) can ease symptoms. Because of the neuropathic nature of the disease, anticonvulsants have brought significant improvement in some people.
Preparing questions in advance can help patients have more meaningful discussions with their physicians regarding their conditions. Patients may wish to ask their doctor the following questions about allodynia:
- Do I have allodynia, or do any of my conditions put me at risk of developing it?
- I get cold very easily. Does that mean I have allodynia?
- What other symptoms might I experience?
- How can a physician determine whether I have allodynia?
- Does having allodynia mean my nervous system is damaged?
- Which type of allodynia do I have?
- What is causing my allodynia?
- What are my treatments options for allodynia? Which do you recommend?
- Will my allodynia go away?
- Is there anything I can do to prevent allodynia?