Understanding allodynia: Pain when there should be none

By Naveed Saleh, MD, MS | Medically reviewed by Kristen Fuller, MD
Published October 27, 2022

Key Takeaways

  • Allodynia is defined as pain caused by something that normally doesn’t bring on pain.

  • As it’s a type of neuropathic pain, allodynia can result from any disease—such as diabetes, post herpetic neuralgia, or chemotherapy—that causes neuropathic pain. Research suggests allodynia could be caused by an error in neuronal conduction.

  • Since allodynia is not a disease, but rather a symptom, treatment for the condition focuses on treating the underlying disease. Tricyclic antidepressants, serotonin-noradrenaline reuptake inhibitors, pregabalin, and gabapentin are recommended as first-line treatments.

Patients often struggle with pain that has clear causes. Allodynia, however, is a unique symptom defined as pain from stimuli that normally don't provoke pain, according to an article published by StatPearls.[]

A multidisciplinary approach is key to management of allodynia. Treatment is focused on addressing the underlying disorder causing this symptom.

Allodynia vs hyperalgesia

Allodynia and hyperalgesia are both types of neuropathic pain and although they can coexist in the same patient, they have different modalities, according to the StatPearls article.

Allodynia is demonstrated by a decreased pain threshold in physical exams.

An example of allodynia is a light brushing of a cotton swab on a patient’s skin that elicits a pain response.

With hyperalgesia, on the other hand, there is a higher pain response to something that is actually painful, such as increased pressure to the skin applied by the clinician.

“With allodynia, the response to the stimulus differs from those who have normal sensation, while in hyperalgesia, the response to the stimulus is the same as those who have normal sensation, but it is an exaggerated response,” wrote the StatPearls authors.

Potential causes of allodynia

Allodynia can be attributed to a known medical disorder such as diabetes (ie, diabetes-induced neuropathic tactile allodynia), trigeminal neuralgia, fibromyalgia, migraine, past injury or trauma, or idiopathic in nature.

It can be categorized by the form of stimulus that causes it, such as thermal, tactile, static, or dynamic, as well as the location of nociception, including cutaneous allodynia, as per StatPearls.

Surgery can also cause chronic neuropathic pain, with up to 68% of surgical patients having experienced this phenomenon based on the type of surgery performed, as documented in a study published in Molecular Pain.[]

Preventive analgesia may be useful in these patients, with treatments such as the anticonvulsants gabapentin and carbamazepine preventing chronification.

The specific mechanisms of allodynia are not yet fully understood, but evidence cited in StatPearls suggests the condition could be due to an error in neuronal conduction. Pain pathways could exhibit errors in long-term potentiation. Alternatively, superficial sensory components could play a role, as well as mental states impacting perception.

Allodynia can progress over time, which may complicate efforts to study the phenomenon due to neuronal confusion at various locations in the body, according to StatPearls.

Typically, a non-painful stimulus should activate only low-threshold A-beta nerve fibers.

With cutaneous allodynia, however, A-beta nerve fibers relay messages to pain pathways via various sodium channels and moderate the dorsal ganglia. The crisscrossing of fibers, however, can occur at levels as high as the cerebellum (eg, post-thalamic stroke pain).

Crossed nerves in allodynia are common and numerous.

Myelinated type A nerve fibers are further classified into alpha fibers (ie, proprioception); beta fibers (ie, light touch); and delta fibers (ie, pain/temperature). Unmyelinated type C nerve fibers can cause aching pain, impair temperature sensation, and actuate pruritus, as per the StatPearls article.

Evaluation for allodynia includes tests for light touch on physical exam, which is commonly elicited using the sharp tip of a broken cotton swab. Diagnostic tests such as EMG, biopsies, quantitative sensory testing, and metabolic panels may be useful.

Treating allodynia

Effective allodynia therapy focuses on the underlying disease. The underlying condition must be slowed, stopped, or reversed.

Nevertheless, allodynia can perpetuate despite addressing the underlying condition, as reported in StatPearls.

According to the results of a systematic review on neuropathic pain conducted by The Neuropathic Pain Special Interest Group of the International Association for the Study of Pain (IASP) published in Lancet Neurology, “Limited efficacy, large placebo responses, inadequate diagnostic criteria, and poor phenotypic profiling probably account for modest trial outcomes and should be taken into account in future studies.”[]

For treatment of general allodynia and other general neuropathies, the IASP recommends tricyclic antidepressants, serotonin-noradrenaline reuptake inhibitors, pregabalin, and gabapentin as first-line, with second-line options including lidocaine patches, capsaicin high-concentration patches, and tramadol.

Third-line options include strong opioids and botulinum toxin A. Only peripheral neuropathies are treated with topical agents and botulinum toxin A.

Other strategies reflect the multidisciplinary nature of general allodynia treatment, including physical therapy, psychotherapy, complementary medicine, and interventions (eg, nerve block).

For short-term relief of chronic pain, repetitive transcranial stimulation (rTMS) may be helpful. According to the authors of a review published in Frontiers in Neurology, “rTMS is not associated with serious complications and appears to be beneficial for treating [neuropathic pain] of various origins, including central pain and pain from peripheral nerve disorders, [fibromyalgia] and migraine.”[]

What this means for you

Allodynia is difficult to diagnose and treat and affects up to half of all people who experience neuropathic pain. Treatment of allodynia can be both medical and non-medical and addresses the etiology of the pain. Pain specialists and multidisciplinary teams can provide robust care.

Read Next: How to treat pain without medication
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