[Laser-evoked potentials: diagnostic approach to the dorsal root]

Functional diagnostics of the dorsal root are especially effective via nociceptive fibers on account of less intersegmental overlap compared to large-diameter fibers of the mechanoreceptive afferents. Laser-evoked potentials (LEP) are induced by short, painful heat stimuli. The aim of this work was to describe changes of the LEP in cases of dorsal root damage. The recorded LEP changes will be discussed with regard to their prognostic value as well as pathophysiologic aspects of dorsal root damage. Dorsal root function was tested in 21 patients suffering from clinically proven radiculopathy in one of the segments L4 to S1. Mechanosensibility and thermosensibility were clinically investigated. LEP were induced by slightly painful stimuli (80 on the affected and 80 on the contralateral unaffected dermatome). The LEP were evaluated by amplitude of the averaged electroencephalogram (EEG). In addition, a subjective pain rating was recorded after each stimulus. Investigation of dorsal root function by LEP in patients with radiculopathy yields two typical changes of the amplitude. On the one hand, there were significant reductions of amplitude, and on the other hand, a complete loss of LEP was found. LEP changes allow a graduation of patients who show no obvious differences in their clinical appearance. Which mechanisms are responsible as concerns deafferentiation and neuropathy of the dorsal root fibers are discussed. With regard to the LEP changes, a prognostic relevance in patients with dorsal root affections is likely. This question will be addressed in a prospective study soon.