The Neurophysiologic monitoring of pain: the laser evoked potential (LEP)
ESES therapy

Myelinated Ad and unmyelinated C fibers are the primary nociceptive afferents
of the skin. Melzack and Wall (1965) have suggested an inhibition of
nociceptive processing within the spinal dorsal horn and/or thalamus by
simultaneous activation of tactile afferents mediating touch (gate-control
theory). Based on this concept, patients with chronic pain are being invasively
treated by electrical stimulation of tactile Ab afferents within
peripheral/cranial nerves or the dorsal columns/roots of the spinal cord.
Furthermore, ESES therapy is being used as an afferent stimulation technique
for pain alleviation. Although these neuromodulatory methods have been applied
for decades, their clinical application and indication are controversial
because the supposed antinociceptive effect in humans has not been objectively
proven so far. A major reason for the limited progress on the knowledge on
nociception was the lack of a quantifiable method to selectively activate Ad
and C-fibers with no or low concurrent activation of other sensory modalities.
Infrared laser stimulation of the skin has been shown to selectively activate
Ad and C fibres, being followed by a late evoked potential at the vertex (Laser
Evoked Potential (LEP), which consists of a N2 and P2 component in a time
window between 175 and 500 ms), mainly due to Ad stimulation and an ultralate
evoked potential (>800 ms) caused by C fiber stimulation. Late LEPs are
considered to be closely related to the perception of pain and a strong
relationship between the amplitude of the LEP and subjective pain rating has
been found.
Therefore, determining late LEP characteristics may be an objective way to
evaluate the effects of invasive (spinal cord stimulation) pain alleviation
methods.

This study will evaluate several LEP characteristics before and during pain
alleviation methods.
Does pain treatment affect VAS scores, noted by the subject before and after a
set of applied stimuli.

Observational study of FBSS patients who recently received a SCS device.

Patients enrolling in this study are asked to participate during 60-90 minutes.
The test will be planned directly after a regularly set appointment at a pain
consultant. Patients will document their measurements of subjective pain
perception (VAS) in a pain diary, during 1 week before this test.
They will be connected to an EEG-recorder. Laser stimuli will be applied to the
dorsum of the foot, which may cause transient redness which will resolve within
days. Other risks associated with this study are negligible.
Different pain treatment modalities are being widely used, in spite of lack of
evidence that support clinical usefulness. Benefit is in expanding knowledge of
objective evaluation of pain treatment modalities.