A patient reports severe pain in response to light touch on the facial skin. This phenomenon is termed:

Correct answer: B. Allodynia

• A. Paresthesia is an abnormal sensation that is not unpleasant — tingling, pins and needles, numbness.
• B. CORRECT. ALLODYNIA is pain provoked by a stimulus that is not normally painful — light touch, a breeze, shaving. It reflects central sensitisation and is a hallmark of neuropathic pain states including trigeminal neuralgia, post-herpetic neuralgia and complex regional pain syndrome.
• C. Dysesthesia is an unpleasant abnormal sensation, spontaneous or evoked — burning or electric — but it is not defined by the innocuousness of the trigger.
• D. Hyperalgesia is an EXAGGERATED response to a stimulus that IS normally painful. The distinction turns on whether the stimulus would ordinarily hurt.

Note: In trigeminal neuralgia the allodynic 'trigger zone' is diagnostically central: the patient can often point to a spot on the lip, nasolabial fold or gingiva where the lightest touch fires a paroxysm. Many stop shaving or brushing that side entirely. First-line therapy is carbamazepine.