Neurophysiology of pruritus: interaction of itch and pain.

The discovery of an itch-specific neuronal pathway, which is distinct from the pain-processing pathway, has clarified the neuronal basis for the itch sensation. Albeit being distinct, there are complex interactions between pain and itch. The inhibition of itch by pain is well known and can explain the antipruritic effect of scratching. However, the opposite effect also exists and has major clinical implications: inhibition of pain processing (eg, by spinal opioids) can generate itch. Conversely, blockade of spinal opioid receptors can be used as an antipruritic therapy. Moreover, the spinal processing of pain and itch can be modulated, resulting in a hypersensitivity or hyposensitivity to pain or itch: similar to chronic painful conditions, ongoing activity of pruriceptors can induce a spinal hypersensitivity for itch in patients with chronic pruritus. Therapeutic antipruritic approaches therefore should target both local inflammation and spinal sensitization of itch processing.