The purpose of this research is to determine the effectiveness of two medications, tricyclic antidepressants (TCA) and opioids, compared to a placebo in the treatment of postherpetic neuralgia.
The specific aims are to compare the effects of opioids and TCA against placebo on pain, altered skin sensitivity, affective, and cognitive function. In addition, the study will determine if the presence of psychiatric co- morbidity, particularly depression, predicts the outcome of treatment with opiods, TCA and placebo. We have studied the relationships between ongoing pain, alterations in thermal sensibility, and allodynia to mechanical and thermal stimuli of patients with PHN to determine the role of peripheral and/or central mechanisms in PHN. Ongoing pain ratings were obtained using a verbal score (0-10). Sensory tests were performed within the affected site, and the corresponding, contralateral, normal site. The area of mechanical allodynia was mapped with a cotton swab and pain evoked by mechanical stimuli (soft hair brush, brass probe, von Frey hairs) was rated on a verbal scale of 0-10. Thermal thresholds to warm, cold, heat pain, and cold pain were determined using a Peltier device and a modified Marstock technique. To date, thirty-two patients (19F, 13M) with PHN of 3-216 months duration (M=39m) have been studied. The average rating of ongoing pain was 7.1 plus or minus 2(M plus or minus SD). The majority of patients (23/32) had allodynia to dynamic (hair brush), statis ( brass probe) or punctate (von Frey) mechanical stimuli (Z equal to or greater than 3.82, p equal to or less than .0001). No significant correlaation was observed between the intensity of ongoing pain and mechanical allodynia. Most patients (22/32) had an increase in heat pain threshold, i.e. hypalgesia (3.4 plus or minus 2.8oC greater than control) despite the presence of mechanical allodynia. The severity of ongoing pain was positively correlated with deficits in heat pain thresholds (r=.39, p=.028). Hence, the role of primary afferent input in the mechanism of PHN is uncertain. The presence of allodynia/hyperalgesia to mechanical stimuli, but the absence of hyperalgesia to thermal stimuli in most patients in this group suggests that central sensitization in PHN is not generalized for all modalities of sensation. It has been observed that patients with chronic pain have high rates of psychiatric conditions. Controversy has existed as to whether these conditions are uniquely related to chronic pain or simply the result of ongoing suffering from a chronic physical symptom. The results demonstrate a high rate of depression in PHN patients consistent with the existing literature. PHN patients compared to patients with chronic and distressing but non-pain physical symptoms also reported higher numbers of depression symptoms as well as other medically unexplained physical symptoms.
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