Post herpetic neuralgia, schwann cell activation and vitamin D

While the underlying pathophysiology of herpes zoster infection has been well characterised, many of the mechanisms relating to the subsequent development of post herpetic neuralgia (PHN) remain uncertain. The dorsal horn atrophy and reduction in skin innervation seen in PHN patients does not adequately explain many clinical features or the efficacy of a number of topical treatments. In the central nervous system the glia, their receptors and their secreted signalling factors are now known to have a major influence on neural function. In the peripheral nervous system, schwann cell activation in response to infection and trauma releases a number of neuroexcitatory substances. Activation of the nervi nervorum in the peripheral nervous system also leads to the release of calcitonin gene related peptide, substance P and nitric oxide. Schwann cell and/or nervi nervorum activation could be an additional mechanism of pain generation in PHN. Such a paradigm shift would mean that drugs useful in the treatment of glial cell activation such as naloxone, naltrexone, minocycline, pentoxifyllline, propentofylline, AV411 (ibudilast) and interleukin 10 could be useful in PHN. These drugs could be used systemically or even topically. High dose topical vitamin D would appear to offer particular promise because vitamin D has the ability to both reduce glial inflammation and reduce nitric oxide production.