Chronobiological characteristics of neuropathic pain: clinical predictors of diurnal pain rhythmicity.

OBJECTIVES: Neuropathic pain is often worse at night; however, little is known about pain rhythmicity during waking hours. We aimed to replicate previous observations of diurnal pain progression, evaluate associations between diurnal rhythmicity and clinical factors, and evaluate the impact of diurnal rhythmicity on treatment response.
METHODS: We performed exploratory analyses of 2 trials (The New England Journal of Medicine 2005; 352:1324; The Lancet 2009; 374:1252) in diabetic neuropathy and postherpetic neuralgia. Pain recorded at 8:00 AM, 4:00 PM, and 8:00 PM, at baseline and during drug treatment was examined.
RESULTS: Data from the latest trial replicated our previous observation of diurnal pain progression with a relative pain intensity difference of 33% between 8:00 AM and 8:00 PM. This pattern was preserved during treatment with gabapentin, nortriptyline, and their combination. Pooled multivariable analyses suggested that difference in pain intensity from 8:00 AM to 8:00 PM was more pronounced in females (vs. males) and in diabetic peripheral neuropathy (vs. postherpetic neuralgia). The baseline magnitude of pain intensity difference from 8:00 AM to 8:00 PM failed to predict treatment response. DISCUSSION: These observations suggest that neuropathic pain progressively increases throughout the day with clinically relevant morning-evening differences and further indicate that sex and underlying etiology may be important determinants of diurnal rhythmicity in neuropathic pain. Consideration of these patterns may guide improved therapeutic strategies and stimulate new directions of research that will improve our understanding and treatment of neuropathic pain.