Laser-evoked potentials in post-herpetic neuralgia.
OBJECTIVE
We evaluated the reliability of laser-evoked potentials (LEPs) as a diagnostic tool in patients with post-herpetic neuralgia (PHN), i.e. a chronic painful condition that causes small-diameter fibre dysfunction. Furthermore, we sought information on pathophysiology of PHN pain.


METHODS
We recorded 'late' LEPs after stimulation of the supraorbital, upper cervical, lower cervical, upper thoracic, mid thoracic, and lower thoracic territories in 12 control subjects and 40 patients with PHN. We also determined the correlation of LEP data with age, duration of disease, and severity and quality of pain.


RESULTS
At all stimulation sites, laser pulses invariably evoked high-amplitude brain potentials related to small-myelinated (A-delta) fibre activation. The laser perceptive threshold and LEP latency correlated with the distance of the dermatome from the brain (P<0.001). In patients, the perceptive threshold was higher and the LEP amplitude was lower in the affected dermatome than on the contralateral side (P<0.001). We found no significant LEP-clinical correlation except for a correlation between LEP abnormality and age.


CONCLUSIONS
Being sensitive and reliable in assessing sensory function also in proximal dermatomes, LEPs are a promising diagnostic tool in radiculopathies. Although PHN severely impairs small myelinated fibres, the lack of a significant correlation between LEP abnormalities and pain suggests that pain in PHN does not chiefly arise from a dysfunction of small-myelinated afferents.
Truini
A
A
Department of Neurological Sciences, University La Sapienza, Viale Università 30, 00185, Rome, Italy.
Haanpää
M
M
Zucchi
R
R
Galeotti
F
F
Iannetti
G D
GD
Romaniello
A
A
Cruccu
G
G
D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D005073:Evoked Potentials, Somatosensory / Q000502:physiology*; D006562:Herpes Zoster / Q000150:complications* / Q000503:physiopathology*; D006801:Humans; D007834:Lasers; D008875:Middle Aged; D036421:Nerve Fibers, Unmyelinated / Q000502:physiology; D009437:Neuralgia / Q000175:diagnosis* / Q000503:physiopathology* / Q000821:virology; D010147:Pain Measurement; D017288:Pain Threshold; D011843:Radiculopathy / Q000175:diagnosis / Q000503:physiopathology / Q000821:virology; D011930:Reaction Time / Q000502:physiology; D012720:Severity of Illness Index