Objectives: Cortical potentials evoked by carbon dioxide laser pulses have been applied in clinical practice to study nociceptive pathways for several years. In this study, we evaluate the properties of an infrared laser (thulium-YAG) with a penetration depth in the skin that matches the intracutaneous depth of nociceptors.
Methods: Temperature measurements and modelling showed that the thulium laser generates painful intracutaneous temperatures with less surface heating than the carbon dioxide laser and with no side effects (up to 600 mJ pulse energy). To develop clinical evaluation criteria, laser-evoked potentials (LEPs) were recorded from 3 midline positions (Fz, Cz, Pz) versus linked earlobes in 23 healthy subjects. Within a session, two skin areas were studied twice in a balanced sequence using randomized interstimulus intervals and two intensities in randomized order.
Results: After hand and foot stimulation with 540 mJ pulses, all subjects showed reproducible biphasic vertex potential, consisting of a negativity (hand: 210 ms, foot: 250 ms) and a positivity (hand: 330 ms, foot: 380 ms). Mean habituation of the vertex potential amplitude across runs was 25% (hand) or 16% (foot); due to the balanced sequence it did not affect the other comparisons. Following foot stimulation, peak latencies were significantly longer (by 40-50 ms) and amplitudes were significantly smaller than following hand stimulation (22.5+/-6.7 vs. 30.3+/-10.9 microV, mean+/-SD). Using 2. 5 standard deviations from the mean as a cut-off, absolute normative values were determined for peak latencies and amplitudes. In addition, relative normative values were determined for paired comparisons (hand-hand, foot-foot, hand-foot).
Conclusions: The thulium-YAG laser is a useful tool for assessment of impaired pain sensitivity. Representative case reports illustrate that unlike for early SEP components, the most frequent LEP abnormalities were amplitude differences.