Can the Neuropathic Pain Scale Discriminate Between Non-neuropathic and Neuropathic Pain?

OBJECTIVES: 1) To determine if the neuropathic pain scale (NPS) can be used to classify chronic pain patients (CPPs) as having primarily neuropathic vs non-neuropathic pain, and furthermore; 2) to determine what, if any, cut-off score can be used to reliably make this determination. DESIGN: A total of 305 CPPs consecutive admissions to The Rosomoff Pain Center were administered the NPS and were assigned a diagnosis according to the physical examination and all available test results. CPPs with a diagnosis of chronic radiculopathy and spondylolysis/degenerative arthritis were segregated into two groups for the purposes of having a group representative of neuropathic pain (chronic radiculopathy) and non-neuropathic pain (spondylolysis/degenerative arthritis). Applying neuropathic pain criteria to each "of these two groups": a neuropathic pain "subtype" was identified within the chronic radiculopathy group; and, a non-neuropathic pain "subtype" was identified within the spondylolysis/degenerative arthritis group. This step was performed in order to assure that the CPPs selected for further analysis were truly representative of neuropathic and non-neuropathic pain. Discriminant function analysis was then employed to determine if NPS scoring could differentiate between these two "subtypes." Results from the discriminant function analysis model were utilized to derive an NPS cut-off score above which CPPs would be classified as having neuropathic pain. For the diagnoses of myofascial pain syndromes, spinal stenosis, epidural fibrosis, fibromyalgia, complex regional pain syndromes 1 and 2, and failed back surgery syndrome, a predicted NPS score was calculated and compared with the cut-off score. SETTING: Multidisciplinary pain facility. PATIENTS: Chronic pain patients. RESULTS: The NPS appeared to be able to separate CPPs into neuropathic pain vs non-neuropathic pain subtypes. The derived cut-off score from the model was 5.53. Myofascial pain syndrome and spinal stenosis had predictive scores lower than this cut-off score at 3.81 and 4.26, respectively. Epidural fibrosis, fibromyalgia, complex regional pain syndromes 1 and 2, and failed back surgery syndrome had predictive scores higher than the cut-off score at 6.15, 6.35, 6.87, 9.34, and 7.19, respectively. CONCLUSIONS: The NPS appears to be able to discriminate between neuropathic and non-neuropathic pain. A debate is currently raging as to whether diagnoses, such as fibromyalgia and complex regional pain syndrome 1, can be classified as neuropathic. Our NPS cut-off score results suggest that these diagnoses may have a neuropathic pain component. The reliability and validity of our NPS method will need to be tested further in other neuropathic pain models, such as diabetic peripheral neuropathic pain.