Ongoing efforts to develop mechanisms-based assessment and treatment of chronic pain have been hindered by the lack of assessment tools differentially sensitive to various phenomena underlying different mechanisms of pain. This study describes the development of an assessment instrument intended to measure neuropathic pain based on qualities of pain as they are inferred from pain descriptors. Subjects were 528 chronic pain patients from several clinics. Of these, 149 had strictly neuropathic pain, while 233 had non-neuropathic pain. Subjects completed a 32 item preliminary questionnaire, which asked them to rate their usual pain on multiple descriptors, as well as the degree to which their pain differed in response to various internal and external factors. This preliminary questionnaire was submitted to factor analysis, and this yielded 6 factors. Representatives of each of these factors were combined with additional items that demonstrated significant differences between neuropathic and non-neuropathic pain groups, to yield a 12 item Neuropathic Pain Questionnaire (NPQ). These items were able to differentiate neuropathic pain patients from non-neuropathic pain patients in a holdout sample with 66.6% sensitivity and 74.4% specificity. The newly developed instrument, NPQ, may be used for the initial screening of neuropathic pain patients. It also has the ability to provide a quantitative measure for the descriptors important in the diagnosis and assessment of neuropathic pain. Consequently, it can be used for monitoring of neuropathic pain treatments and as an outcome measure.
Neuropathic pain is the result of an injury or disease of the nervous system, and has been recognized for several centuries. 33 Maladaptive reparative mechanisms and the consequent dysfunction of the nervous system lead to abnormal neuropathic symptoms and signs. While there are many etiological causes of neuropathic pain, all neuropathic pain syndromes present with similar sets of pain symptoms, such as burning pain, hypersensitivity to external stimuli, sensory deficits, and various hyperalgesias. 12,35,44,49,50 Together with an abnormal neurologic examination, these verbal descriptors are used to diagnose neuropathic pain. Neuropathic pain is thus a categorical designation rather than a single entity.
The introduction of animal models of neuropathic pain has led to the elucidation of mechanisms that underlie a few of the phenomena that characterize neuropathic pain, such as mechanical dynamic hyperalgesia and expansion of receptive fields. The variability of human clinical symptoms is similar to the variability and evolution of different components of neuropathic pain in these models. 46–48 This in turn has given significant face validity to the symptoms and signs of neuropathic pain. 1,12,18
Initially, it was felt that no treatments could be offered for neuropathic pain, because it was viewed as resistant to “standard treatments.”37 Frequently, symptoms and signs of neuropathic pain were ascribed to “psychogenic causes”30 or plainly ignored. However, with advances in clinical practice, including the better utilization of old treatments, 9,43 and especially with the introduction of novel pharmaceuticals, 8 symptoms and signs are now considered important clinical and research features. 5,13,19,31,34,35
The symptoms and signs of neuropathic pain are unlike any other sensory experience: they are described in unusual terms such as electrocuting, and also in terms of a wide range of other descriptors. Clinical observation has indicated a great variability of neuropathic pain symptoms and signs between patients, and a wide spectrum of symptoms within each individual patient. 7,12,21,34,35,36 There is also great susceptibility to external factors and even internal emotional states. 7,44 In the same patient, symptoms, characteristics, and intensities of different components frequently vary from time to time.
Although pain descriptors used by patients with neuropathic pain are commonly recognized by clinicians, they have been studied formally in only one study, using a French version of the McGill Pain Questionnaire. Boureau and colleagues 3 found that they could differentiate neuropathic from non-neuropathic pain with 66% accuracy, based solely on patient descriptors. However, they did not attempt to develop an assessment instrument specifically for neuropathic pain.
Galer and Jensen 5 developed the Neuropathic Pain Scale (NPS) using items from their own clinical experience, to monitor the effects of therapy on different characteristics of neuropathic pain. They demonstrated that the NPS could discriminate between different categories of neuropathic pain and that NPS scores changed in response to treatments. Thus, the NPS has value as a general measure of neuropathic pain. However, the NPS was not developed to discriminate neuropathic pain from non-neuropathic pain, and its ability to do so remains untested.
Recently, Bennett 2 published the LANSS Pain Scale, an instrument intended to differentiate neuropathic from non-neuropathic pain. Its 4 pain descriptor groups assess tingling, hyperesthesia to touch, electric, and burning pain. The LANSS also asks patients to describe skin discoloration. Finally, the LANSS asks the physician to assess the presence or absence of allodynia and altered pin-prick threshold. The LANSS demonstrated sensitivity of 83% and specificity of 87% in its original validation group, with a slight decrease in accuracy to 85% sensitivity and 80% specificity in a cross-validation group.
This study was designed to investigate the factors which may underlie patients' descriptions of their own pain. Additionally, it sought to establish which of the frequently used descriptors of neuropathic pain are best able to discriminate between neuropathic and non-neuropathic pain. This information then formed the basis for the development of a new assessment instrument–the Neuropathic Pain Questionnaire, NPQ. This instrument is intended to provide a general assessment of neuropathic pain symptoms, as well as assisting in discrimination between neuropathic and non-neuropathic pain.