Original articleClinical usefulness of laser-evoked potentials
Section snippets
General considerations
The visual and auditory pathways are projecting one modality only, and can thus be easily tested using visual and auditory evoked potentials (VEP, AEP). Pain pathways are a part of the somatosensory system. Therefore, neurophysiological studies of pain pathways may be expected to be related to the recording of somatosensory evoked potentials (SEP). However, the somatosensory system includes more than one modality and cannot electrophysiologically be tested by only one type of evoked potential.
Stimulation
In normal skin, the sensation evoked by laser stimuli near pain threshold is comparable to a weak pinprick or pulling a single hair follicle. In order to obtain reproducible evoked potentials, it is necessary to use suprathreshold stimuli, which are usually perceived as slightly stinging and/or burning [13], [57], and are less uncomfortable than the standard electrical nerve stimuli used for SEP recording. Within the range of 7–50 ms stimulus duration and 3–9 mm beam diameter, pain threshold is
Pathological LEP changes
The anatomical specificity of the peripheral and central thermoreceptive and nociceptive pathways represents the major rationale of a clinical use of the LEP method. Many groups documented disturbances of pain and temperature sensitivity by abnormal LEP that are typically missed by the standard SEP method in a variety of pathologies (Table 2). Evidence strongly indicates, and will be reviewed below, that LEP usefully supplement standard electrical SEP when minus-signs of the pain and
Topodiagnosis of lesions using LEP
As shown in the previous section, LEP can document lesions anywhere along the nociceptive pathway (Fig. 3). Although all dermatomes are accessible, the absence of an LEP in a patient cannot necessarily provide the exact level of the lesion without additional clinical, electrophysiological or imaging data. The absence of an LEP may be due to a lesion anywhere between the cortex and the dermatomal level of the spinal cord where the stimulation is done, or even in the periphery. It is important to
Conclusions
Standard SEP are of limited value in patients, who present with a dissociated sensory loss of pain and temperature sensitivity and preserved tactile and proprioceptive sensitivity. The technique of LEP recording allows the assessment of the functional status of the nociceptive pathways within the somatosensory system. According to the anatomy of the nociceptive pathways, this type of a sensory deficit occurs predominantly with peripheral, spinal, or brainstem lesions. LEP abnormalities consist
Acknowledgments
Supported by the Deutsche Forschungsgemeinschaft (Tr 236/13-2) and NIH (NS 38493).
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