Allodynia

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Original Editors - Abbi Davis, Jessica Bradshaw, & Parker Hall as part of the Arkansas Colleges of Health Education School of Physical Therapy Musculoskeletal 1 Project
Top Contributors - Parker Hall, Melissa Coetsee, Abbi Davis, Jessica Bradshaw, Olajumoke Ogunleye and Kim Jackson

Overview

Allodynia is defined by the International Association for the Study of Pain (IASP) as

"Pain resulting from a stimulus that does not normally provoke/elicit pain".

For example, brushing a feather against the arm causes pain where it should only cause a sensation. It is a clinical term to describe this phenomenon of altered quality of a sensation, and does not imply a mechanism or specific diagnosis.It may however give insight into possible mechanisms at play.

Classification

Allodynia is categorised into three types:

1. Dynamic (or mechanical) when pain results from an object moving across the skin (stroking)
2. Thermal which results from mild temperature changes
3. Tactile (or static) which results from gentle touch or pressure.

Aetiology/Mechanism

Allodynia is considered a sign/symptom, not a disease. It may be a patient's chief complaint. Allodynia is the phenomenon of a non-painful stimulus producing a pain response, which implies an error in neuronal conduction. The mechanism behind this error is unclear. The strongest evidence that exists, thus far, suggests that sensory neuronal fibres may stimulate pain pathways, possibly due to an error in long-term potentiation. However, some studies suggest that superficial sensory components may also be involved. There is also evidence that different mental states can affect the perception of allodynia.

There is an analogy of "crisscrossed" fibres, where the actual location of the crisscrossing can vary and may be located almost anywhere along the peripheral to the central nervous system tract. Allodynia can involve both the peripheral nervous system and central nervous system via sensitisation, and the mechanism behind the inappropriate pain sensations can evolve. This might partially explain the existing contradictory studies - they may all be measuring allodynia with neuronal confusion at different locations. The expression of allodynia may be as elegantly simple as a consistent increase in sympathetic nervous system activity, which leads to a sensitisation of C-fibres. The increase in C-fibre sensitisation is also a reflection of disinhibition of secondary spinal networks. In other words, a loss of spinal segregation of touch and pain can be a key mechanism of allodynia.

Allodynia is often associated with conditions that involve sensitisation of the skin. Common examples include sunburn, inflammation or trauma.Allodynia is a normal protective response after tissue injury has occurred and will usually subside as healing progresses. It may however increase over time in certain conditions, such as neuropathic pain conditions.

Listed below are some of the most common diseases associated with allodynia:

• Fibromyalgia
• Trigeminal Neuralgia
• Diabetic Neuropathic Pain
• Migraine Associated Allodynia
• Neuropathic Pain
• Central Sensitisation
• Peripheral Sensitisation
• Complex Regional Pain Syndrome (CRPS)
• Chronic Low Back Pain
• Postherpetic Neuralgia
• Osteoarthritis

Differential Diagnosis

Allodynia vs. Hyperalgesia

Another clinical term that is often a differential diagnosis to allodynia is hyperalgesia. Where allodynia refers to changes in the quality of sensation, hyperalgesia refers to changes in the intensity of the sensation of pain.

Hyperalgesia is the condition of having increased sensitivity to pain or enhanced intensity of pain sensation. There is an exaggerated experience of pain from a stimulus that is normally painful - i.e. an increased response/pain on suprathreshold stimulation (see image) .

Treatment

There are multiple ways that healthcare providers can help treat allodynia. Providers can either treat the condition that is causing allodynia or try to reduce the pain.

• Medication: Anticonvulsants (used to treat seizures), triptans (used to treat migraines), and some antidepressants. Topical creams that can help manage allodynia will typically have lidocaine or capsaicin as the active ingredient.
• Psychosocial interventions: Counselling may be recommended if signs of depression are detected. Biofeedback, mindfulness training, and cognitive behavioural therapy can change the way a person responds to pain.
• Physiotherapy: The psychologically informed physiotherapist can include pain neuroscience education (PNE) and cognitive functional therapy. Physiotherapist can help manage allodynia with desensitization and/or mirror therapy.
• Other procedures: A nerve block injection may be recommended to reduce pain in a specific nerve or nerve group.

Prevention

Allodynia can not be directly prevented, however, one can lower the risk of acquiring a condition that causes allodynia. Risk management includes exercising regularly, monitoring health, managing mental health , maintaining a healthy weight, having a balanced diet, and having good sleep hygiene.