Laser evoked potentials using the Nd:YAG laser

Pain-related cortical potentials were evoked by skin stimulation of the face and the limbs with 5-ns-duration laser pulses delivered by a Q-switched Nd:YAG laser. Such laser pulses, in the nanosecond range, were able to induce pinprick pain sensations and to evoke reproducible laser evoked potentials (LEPs) without visible skin lesions for an energy density of less than 18 mJ/mm(2). Low energy densities, around 10 mJ/mm(2), were sufficient to reach the pain threshold and to induce LEP. The mean conduction velocity of the stimulated afferent fibers was close to 20 m/s, consistent with the stimulation of Adelta fibers. The amplitude of LEP correlated with pain perception rather than with energy density. The differences, such as wavelength and stimulus duration, between the Q-switched Nd:YAG laser we used and the lasers that are currently used in LEP studies (i.e., CO(2), argon, or Tm:YAG lasers in the millisecond range) are discussed. Our study opens novel perspectives in the LEP field of research by using a new type of laser with a very short pulse duration.