C‐fiber function assessed by the laser doppler imager flare technique and acetylcholine iontophoresis

Vasodilation induced by both acetylcholine iontophoresis (ACh Ionto) and the laser Doppler flare technique (LDIFT) can be used to measure small-fiber function. The aim of this study was to compare the neurogenic nature of these methods. ACh Ionto and the LDIFT were performed on 10 controls, with and without local anesthetic cream. The local anesthetic cream blocks the axon reflex; thus, by determining the ratio of hyperemic area to stimulus area before and after anesthesia, the neurogenic nature of the hyperemia can be determined. The ratio of hyperemic area to stimulus area was significantly reduced by local anesthesia in the LDIFT [ratio (mean +/- SD): 2.33 +/- 0.67 with local anesthesia; 6.84 +/- 1.33 without local anesthesia; P < 0.0001], whereas this ratio was unaffected by local anesthesia in the ACh Ionto group (2.61 +/- 0.57 with local anesthesia; 2.67 +/- 1.27 without local anesthesia). This study confirms that the LDIFT measures small-fiber function. In contrast, hyperemia elicited by ACh Ionto outside the capsule is not blocked by local anesthesia and is thus non-neurogenic.