Bioimpedance for pain monitoring during cutaneous photodynamic therapy: Preliminary study

Summary 

Background

Pain is a well-known problem associated with light exposure during topical photodynamic therapy (PDT). Different methods for dealing with the pain have been developed over the past years, ranging from cooling with air or water to nerve blocking. However, the mechanisms responsible for the pain induction have not yet been fully understood.

Aim

This study aims to evaluate bioimpedance in situ measurements of human skin as a method to shed light on pain-inducing real-time changes during light exposure during topical PDT.

Methods

Cream containing 20% aminolevulinic acid (ALA) was applied on forearms of ten healthy human volunteers. After 24h incubation, the cream was removed and the spots were exposed to red laser light (636nm, 300mW/cm²). During light exposure bioimpedance measurements with a 4-electrode set-up were taken at two frequencies (10Hz and 100kHz).

Results

A significant drop in skin impedance at high and low frequencies coincided with onset of pain during light exposure of spots treated with ALA. A similar drop was not observed for controls.

Conclusions

Bioimpedance spectroscopy can provide valuable data for real-time observation of changes in skin, and may contribute to an increased understanding of the mechanisms responsible for induction of pain during topical PDT. Future studies are needed.

Keywords: Topical photodynamic therapy (PDT), Bioimpedance spectroscopy, Fluorescence spectroscopy, Protoporphyrin IX (PpIX), Aminolevulinic acid (ALA), Pain