Neuropathic orofacial pain: a review and guidelines for diagnosis and management.

Abstract

Neuropathic pain is defined as "pain initiated or caused by a primary lesion or dysfunction in the nervous system". In contrast to physiological pain that warns of noxious stimuli likely to result in tissue damage, neuropathic pain serves no protective function. Examples of neuropathic pain states include postherpetic neuralgia (shingles) and phantom limb / stump pain. This pain state also exists in the orofacial region, with the possibility of several variants including atypical odontalgia and burning mouth syndrome. There is a paucity of information on the prevalence of neuropathic pain in the orofacial region. One study assessed patients following endodontic treatment and found that approximately 3 to 6percent of patients reported persistent pain. Patients predisposed to the condition atypical odontalgia (phantom tooth pain) include those suffering from recurrent cluster or migraine headaches. Biochemical and neurobiological processes leading to a neuropathic pain state are complex and involve peripheral sensitisation, and neuronal plasticity of the central and peripheral nervous systems. Subsequent associated pathophysiology includes regional muscle spasm, sympathetic hyperfunction, and centralisation of pain. The relevant clinical features of neuropathic pain are: (i) precipitating factors such as trauma or disease (infection), (ii) pain that is frequently described as having burning, paroxysmal, and lancinating or sharp qualities, and (iii) physical examination may indicate hyperalgesia, allodynia and sympathetic hyperfunction. The typical patient complains of persistent, severe pain, yet there are no clearly identifiable clinical or radiographic abnormalities. Often, due to the chronicity of the problem, afflicted patients exhibit significant distress and are poor pain historians, thus complicating the clinician's task of obtaining a detailed and relevant clinical and psychosocial history. An appropriate analgetic blockade test for intraoral sites of neuropathic pain is mucosal application of topical anaesthetics. Other, more specific, tests include placebo controlled lignocaine infusions for assessing neuropathic pain, and placebo controlled phentolamine infusions for sympathetically maintained pain. The treatment and management of neuropathic pain is multidisciplinary. Medication rationalisation utilises first-line antineuropathic drugs including tricyclic antidepressants, and possibly an anticonvulsant. Topical applications of capsaicin to the gingivae and oral mucosa are a simple and effective treatment. Neuropathic pain responds poorly to opioid medication. Psychological assessment is often crucial in developing strategies for pain management. Psychological variables include distress, depression, expectations of treatment, motivation to improve, and background environmental factors. To enable a greater understanding of neuropathic pain, thereby leading to improved treatments, high-performance liquid chromatography-mass spectrometry is one analytical technique that has the potential to contribute to our knowledge base. This technique allows drugs and endogenous substances to be assayed from one sample in a relatively short time. The technique can identify, confirm, and measure the concentrations of multiple analytes from a single sample.