How should we measure pain in herpes zoster?

Pain occurs during the acute phase of herpes zoster and as postherpetic neuralgia (PHN) for months or years after the acute illness has healed. The acute pain results from viral replication leading to the death of neurons together with cutaneous inflammation. However, the exact mechanism of PHN is still conjectural. Historically, PHN has been defined as any pain that follows disappearance of the rash of herpes zoster, but a number of other definitions (eg, pain present for more than 1 or 2 months after rash onset) have also been used. Since pain is purely subjective and is usually felt as a continuum, any definition is entirely arbitrary and sheds no light upon the pathophysiology of the prolonged pain. An arbitrary division of pain poses problems for the measurement of the effect of acute therapy upon the duration of PHN. Use of a definition of PHN that involves the time of rash healing also leads to considerable difficulties in the assessment of therapies that affect the duration of the rash. The median times for resolution of both acute pain and PHN are likely to be biased in favor of therapy that heals the rash more rapidly, even if the continuum of pain is not affected. For these reasons, the term "zoster-associated pain," encompassing both the acute and chronic pain associated with herpes zoster, has evolved as a more meaningful way of measuring pain both for the individual patient and also for the comparison of two potential therapies.