[The infrared laser in the diagnosis of normal and disturbed pain pathways.]

Cerebral potentials evoked by cutaneous heat stimuli from an infrared laser (LEP) enable overall controls of thin fibre function and anterolateral tract projection, which is of special meaning in the diagnosis of normal and disturbed pain pathways. Owing to the long-wave radiation, the laser energy is completely absorbed within the most superficial skin layers only a few 100 mum in depth and activates only the most superficial afferents, i.e. the thermo- and nociceptive A, delta and C fibres. According to the particular fibre spectrum activated, a single laser stimulus elicits a typical double pain sensation: the first pain appears with a mean reaction time of approximately 400 ms and is described as a sharp and stinging, well-localizable pinprick sensation; this pain is induced by A delta fibre activity with a mean conduction velocity of 14 m/s. It is followed by a second, more diffuse burning component, with a mean reaction time of 1300 ms, ascribed to selective C-fibre transmission (less than 1 m/s). These two pain sensations elicited by one laser-stimulus are accompanied by typical late and ultralate cerebral brain potentials. Both sensations and both evoked potentials can emerge to very different degrees in healthy subjects and in patients with neurological diseases. The diagnostic practicability of LEP is individuals illustrated with reference to patients with syringomyelia suffering from a dissociated sensory loss in cutaneous sensibility. In contrast to conventional electrical nerve stimuli, the laser stimuli allow monitoring of disturbances in the protopathic system projected to the anterolateral columns.