Use of dynamic light scattering for assessing acute pain

Currently there is no accurate objective measure for monitoring pain during the state of drug-induced unconsciousness (such as during surgical anesthesia). Moreover, the absence of an objective measure for detecting pain hampers the physician's ability to provide an optimal dose of analgesics. We have developed a novel method for detecting pain by quantifying skin blood flow dynamics using a miniaturized dynamic light scattering (mDLS) sensor placed on the skin. Healthy awake volunteers were studied with mDLS sensors placed on both index fingers while being subjected to a series of cutaneous painful stimuli (electric shock and heat), randomly applied in a range between the subjects' pain threshold and tolerance. Power spectrum analysis of the recorded signal was performed with a focus on two frequency bands, representing relative blood flow of non-pulsatile vessels and larger pulsatile arterioles. Relative blood flow of pulsatile vessels decreased while flow of non-pulsatile vessels increased in response to painful stimulation, with a high correlation between the responses obtained on the right and left index fingers. The changes in hemodynamics that occur during painful stimulation suggest a redistribution of blood flow between pulsatile and non-pulsatile vessels, probably related to central activation of the sympathetic system combined with local dynamic autoregulatory responses. Thus, optical parameters of skin blood flow can detect nociceptive stimuli and consequently can serve as objective biomarkers of pain.