Determining the Definitive Time Criterion for Postherpetic Neuralgia Using Infrared Thermographic Imaging

The time criteria used in many studies of postherpetic neuralgia (PHN) are arbitrary and do not have supporting evidence. Therefore, this study sought to determine the definite time criterion for PHN by analyzing the skin temperature to estimate the time point when zoster-induced skin inflammatory reaction ends.

Infrared thermography was used to measure the difference in skin temperature between the affected and unaffected areas (ΔTemp) in the craniocervical and thoracic regions of patients with herpes zoster (HZ). Because the ΔTemp changes from a positive value to zero when the skin is no longer inflamed, a ΔTemp ≤ 0 was defined as the end of skin inflammation, and this time point was considered the starting point for PHN. This cutoff time point was estimated using receiver operating characteristic (ROC) curve analysis.

A total of 503 patients were included in this study. The ROC curve analysis showed that the time point when the ΔTemp was ≤ 0 occurred at 12 weeks after HZ onset (95% confidence interval 11-15 weeks, area under the ROC curve 0.901). Using this time point as the time criterion of PHN, the sensitivity, specificity, and classification accuracy were 0.807, 0.905, and 0.871, respectively.

The transition of skin temperature from warm to cold occurs 12 weeks after HZ onset, which implies the end of local inflammation. Therefore, PHN associated with pathophysiologic change may be defined as 12 weeks after the skin rash. This finding provides a theoretical basis for the timing definition of PHN.