Allodynia

Allodynia refers to pain caused by what would normally be non-painful stimulation, for example brushing the skin.

The touch of clothing can trigger allodynia, for instance causing a painful burning sensation, or a slightly cool or warm temperature may be unbearably painful.

Types
There are three main forms of allodynia:
 * Mechanical / Tactile
 * Dynamic: caused by movement across the skin such as a cotton bud, or brushing a painter's brush against the skin; or
 * Static: where pain is caused by pressure or touch such as clothing touching the skin


 * Thermal, which is caused by heat or cold that is not extreme enough to cause damage to skin tissues.
 * Hot
 * Cold


 * Chemical, caused by chemicals such as capsaicin (chilli extract), menthol (cooling sensation), histamine (itch)

ME/CFS
Allodynia is not a diagnostic symptom of ME or CFS, although pain in general is, with multiple types of pain mentioned. Allodynia is not directly mentioned in the International Consensus Criteria primer for Myalgic Encephalomyelitis but would fulfill the significant pain diagnostic criteria, which includes a number of types of pain including hyperalgesia and widespread pain that may meet the fibromyalgia criteria.

Fibromyalgia
Allodynia is a characteristic symptom of fibromyalgia, which is a chronic condition of widespread pain

A study of over 3,000 patients with fibromyalgia found that allodynia was "surprisingly common."

Possible causes
Allodynia is caused by altered pain processing, but it is not yet clear exactly how this happens. Physical injuries including amputations and spinal cord injuries can cause allodynia, even if injuries have fully healed, but allodynia can also occur without a any injury. Central pain sensitization (increased response of neurons) has been proposed as a possible cause, but this term has conflicting definitions and scientific evidence is unclear.

Allodynia compared to hyperalgesia
Allodynia is when pain is caused by a sensation that should not be painful, but hyperalgesia is significantly lowered pain threshold.

People with hyperalgesia feel a greater intensity of pain, but do not find everyday sensations painful.

Notable studies

 * 2005, Nerve damage and its relationship to neuropathic pain - (Chapter)
 * 2006, Contribution of glia to pain processing in health and disease - (Chapter)
 * 2008, Chronic widespread musculoskeletal pain—a comparison of those who meet criteria for fibromyalgia and those who do not - (Full text)
 * 2009, Models and Mechanisms of Hyperalgesia and Allodynia - (Full text)


 * 2010, A cross-sectional survey of 3035 patients with fibromyalgia: subgroups of patients with typical comorbidities and sensory symptom profiles - (Full text)


 * 2014, Allodynia and hyperalgesia in neuropathic pain: clinical manifestations and mechanisms - (Full text)

Learn more

 * Pain terminology - International Association for the Study of Pain