Neuropathic pain

Definition and epidemiology IASP previously defined neuropathic pain as “Pain initiated or caused by a primary lesion or dysfunction in the nervous system”. A new definition, proposed by Treede et al. (2008), which states “Pain arising as a direct consequence of a lesion or disease affecting the somatosensory system” was recently accepted by IASP with minor changes. Currently, neuropathic pain is defined as “Pain caused by a lesion or disease of the somatosensory nervous system” (IASP website, accessed June 2011). The information on neuropathic pain prevalence is not very accurate, mainly due to inconsistency in diagnostic criteria across the studies. However epidemiological data suggest that up to 6-8% of general population may suffer from neuropathic pain, or at least from pain with neuropathic features (Torrance et al. 2006; Bouhassira et al. 2008; Smith & Torrance 2011). Assessment Clinical examination is a crucial part of the diagnostic process of neuropathic pain, aiming at finding possible abnormalities relating to a lesion of the somatosensory system. Sensory testing is the most important part of this examination and includes testing of touch, vibration, pinprick, cold and warmth. Tactile testing may be assessed by a piece of cotton wool, vibration sense by a 128-Hz tuning fork, pin-prick sensibility by a wooden stick and thermal sense by warm and cold objects (e.g. metal thermorollers) (Cruccu et al. 2010; Haanpaa et al. 2011). Quantitative sensory testing can be used along with bedside testing, whenever possible, to provide independent verification of sensory signs. Importantly, no gold standard is available to label a specific pain as “neuropathic”. The best way to measure the intensity of neuropathic pain is by using a Visual Analog Scale (VAS) or the 0-10 Numerical Rating Scale (NRS). These are also recommended to assess the effect of treatments on neuropathic pain intensity in research and clinical practice (Haanpaa et al. 2011). Different questionnaires have been developed in order to further assess the neuropathic component of the pain, or monitor treatment outcomes of neuropathic pain. Several validated questionnaires exist and can be used (e.g. DN4, S-LANSS, PainDETECT, SF-MPQ, NPS and NPSI), however consensus is lacking on a single preferred tool. A 4-stage probability grading system for neuropathic pain has been proposed, that can be useful both for clinical and research purposes (Treede et al. 2008). The probability grading is based on i) neuroanatomical distribution of pain, ii) relevant medical history to indicate possibility of somatosensory nerve damage, iii) clinical examination to determine the presence of negative and positive sensory signs, and iv) further diagnostic tests (e.g. brain imaging or skin biopsy). The probability grading may assist in more accurate diagnosis of neuropathic pain, but its feasibility has not yet been tested in clinical trials. Clinical characteristics Neuropathic pain consists of a series of different diseases and conditions ranging from nerve compression as a consequence of neoplasms, neuropathies due to metabolic disorders such as diabetes to diseases of the CNS such as stroke and multiple sclerosis. In addition to a long list of different causes of neuropathic pain, these pains also differ in anatomical location and can be localized anywhere from the peripheral nociceptor to the highest centers in the brain (Jensen et al. 2009). Neuropathic pain is not one single disease but constitutes a heterogeneous group of diseases and lesions that produce a common syndrome characterized by pain within a territory that has lost its normal afferent input to the CNS. An essential element in neuropathic pain is the combination of sensory loss and the paradoxical presentation of hypersensitivity in the painful area (Jensen et al. 2009). In general, a neuropathic pain syndrome is, therefore, characterized by: (1) Pain in a neuroanatomical area with partial or complete sensory loss. (2) The presence of stimulus-independent ongoing types of pain. (3) The presence of stimulus-dependent evoked types of pain (allodynia, hyperalgesia). (4) Aftersensations (pain outlasting the period of stimulation). (5) Abnormal summation of pain (increased pain following repetitive stimulation). Management: Treatment of neuropathic pain can be challenging. Medical treatment include antidepressants (TCAs e.g. amitriptyline and nortriptyline and SNRIs e.g. duloxetine and venlafaxine), anticonvulsants (pregabalin, gabapentin, lamotrigine, carbamazepine, phenytoin, valproate and others), topical agents (local anesthetics or capsaicin), systemic local anesthetics (intravenous lidocaine and oral mexiletine), weak opioids (e.g. tramadol) or strong opioids (e.g. morphine, oxycodone, methadone). For evidence-based review on neuropathic pain management see other references (Dworkin et al. 2007; Finnerup et al. 2010). Non-pharmacological treatment modalities (e.g. acupuncture, TENS, physical therapy) have shown inconsistent results, and currently no method is recommended as first line therapy for neuropathic pain.