Aberrant C nerve fibre function of the healthy scalp

Background The scalp is commonly associated with itch and burning sensations. Previous studies have shown the scalp is highly innervated compared with other body areas. However, limited data are available on sensory testing associated with C nerve fibres of the healthy scalp. Objective To examine the function of C nerve fibre-mediated sensations such as warmth, heat pain, itch and neurogenic inflammation in two areas of the scalp (crown and occiput) in comparison to forearm skin. Methods Twenty-two healthy subjects (11 women, 11 men) underwent quantitative computerized thermosensory testing to assess warmth and heat pain thresholds. Itch was induced using histamine iontophoresis and application of cowhage spicules, and the intensity of each itch was assessed. Skin blood flow was measured at baseline and after each itch induction to assess degree of neurogenic inflammation. Results A majority (64%) of the crown warmth threshold measurements exceeded 50 °C while all 22 forearm warmth thresholds measurements were significantly lower than 50 °C. Both scalp sites had significantly higher warmth and heat pain thresholds compared with the forearm (P < 0·001). Itch ratings of histamine and cowhage on both scalp sites were significantly lower than on the forearm (P ≤ 0·001). Histamine and cowhage induced an increase in skin blood flow from baseline on the forearm compared with the scalp (P < 0·001). Conclusions This study demonstrates a significant insensitivity of C nerve fibres of the scalp to warmth, heat pain, itch and neurogenic inflammation. These results suggest that the scalp has an aberrant response of C nerve fibres.