Temporal and Spatial Quantification of Pain- Related Small Fiber Functionality Assessed Using Laser Speckle Contrast Analysis.

BACKGROUND: This study aimed to establish a practical method for assessing pain symptomatology and develop criteria for quantifying small fiber functions using laser speckle contrast analysis (LASCA).

METHODS: Axon flare responses, which reflect small fiber functions, were induced either by histamine or capsaicin applied to the volar surface of the skin of the left forearm. To inhibit small fiber function, local anesthetic cream was applied to the skin of right forearm. The effects on pain symptomatology (severity/timing) and axon flare responses (temporal/spatial characteristics) were measured using LASCA. To highlight the clinical relevance, this method was applied to 3 different types of patients with small fiber dysfunction.

RESULTS: Forty-four forearms of 22 healthy volunteers were tested. Eutectic mixture local anesthetic cream reduced the severity and duration of pain, abolishing or prolonging the latency to reach 3-fold higher levels of basal skin microcirculation levels, and decreasing the area and intensity of flare responses. There were positive correlations between pain symptomatology and flare response characteristics in the control forearm. Similar results were confirmed in neuropathic pain patients.

CONCLUSIONS: Pain-related small fiber functions and symptomatology (two-in-one method) can be assessed via histamine- or capsaicin-evoked axon flare responses in as little as 15 minutes. The reduction of small fiber functions are characterized by decrease in flare size/intensity at 5 minutes after stimulation and prolongation/abolishment of the latency to reach 3-fold higher levels of baseline skin microcirculation. LASCA may be applied in the clinic to aid early diagnosis, monitor disease progression, and objectively assess treatment efficacy in patients with neuropathic pain.