Methemoglobin formation during laser induced photothermolysis of vascular skin lesions

Background and Objective Monitoring dynamic changes during laser induced photothermolysis of vascular skin lesions is essential for obtaining an optimal therapeutic result. Rapid photoinduced thermal damage occurs at a threshold temperature of about 70°C. It is therefore, relevant to identify markers to indicate if this threshold temperature has been reached. Methemoglobin, which is formed by a photo‐induced oxidation of hemoglobin, indicates that the temperature has reached this threshold value. This study presents a proof of concept of a method for monitoring the in vivo presence of methemoglobin immediately after laser exposure. Study Design/Materials and Methods The present study was designed to investigate the in vivo temperature dependence of hemoglobin absorption in the 450–800 nm spectrum range. In vivo diffuse reflectance measurements of port‐wine stain (PWS) and telangiectasia were performed prior to, and immediately after, laser treatment with a pulsed dye laser (PDL) at 585 nm wavelength. Results In vivo measurements following laser treatment of vascular skin lesions showed an immediate increase in the optical absorption of blood. This effect, caused by thermal stress, is a result of an increased dermal blood volume fraction and methemoglobin formation. The effect is light dose dependent, and reflectance spectra revealed methemoglobin formation in patients treated with fluences above 5 J/cm2 at 585 nm wavelength. Conclusions It was proved that methemoglobin can be measured in vivo by reflectance spectroscopy. Measurements of the average methemoglobin concentrations immediately after laser exposure may be a valuable diagnostic tool to verify that the blood temperature has been sufficiently high to induce thermal damage to the vessel wall. Lasers Surg. Med. 34:414–419, 2004. © 2004 Wiley‐Liss, Inc.