Our results confirm that in a large cohort of patients with either DPN or PHN, factor analysis of the 11 baseline SF-MPQ sensory pain qualities reveals a 2-factor model of pain: (1) stabbing pain quality (utilizing stabbing, sharp, and shooting pain descriptors) and (2) heavy pain quality (utilizing heavy, gnawing, and aching pain descriptors). Interestingly, the quality of ‘throbbing’ did not discriminate between the 2 factors. As an independent and large sample, these results confirm our prior work that was performed in a much smaller and more heterogeneous group of patients with suspected NeP.9
Similar pain factors were described for patients with localized osteoarthritis (OA).34
“Stabbing” was used as a descriptor of pain by 18% of patients with localized OA who completed the MPQ, as opposed to 4% of patients with generalized OA or rheumatoid arthritis (RA). While heavy pain was not explicitly measured in this study, terms such as “cramping” and “pressing” were used by patients with localized OA more frequently than by patients with RA or generalized OA. The authors suggested that specific pain descriptors may be sufficiently sensitive to discriminate between related painful conditions, but their sample size was too small to substantiate this claim.34
The 2-factor pain quality structure is important because it may inform us about mechanisms of neuropathic pain in general. It is notable that the 2-factor structure coincides with distinctive pain sensations carried by Aδ and C fibers. In particular, Aδ fibers have been associated with the pain sensation described as acute or “first pain” akin to the stabbing pain factor in this study, whereas C fiber stimulation has been described as possessing pain qualities such as dull, slow, “second pain,” 5,26
similar to the heavy pain factor described in this report. While we did not attempt to characterize the mechanisms associated with the stabbing and heavy factors in this study, and any association is admittedly speculative, future research should characterize the pain factor quality–mechanism relationships.
Limitations of our study are predominately due to the restricted number of pain qualities represented in the SF-MPQ. The SF-MPQ uses 11 words to characterize the sensory qualities of pain: throbbing, shooting, stabbing, sharp, cramping, gnawing, hot-burning, aching, heavy, tender, and splitting. Additionally, the SF-MPQ uses 4 affective qualities: tiring-exhausting, sickening, fearful, and punishing-cruel. We did not evaluate the affective qualities in this study. Several instruments have been developed to specifically characterize pain qualities in NeP including, but not limited to, the, Neuropathic Pain Scale,15
Leeds Assessment of Neuropathic Symptoms and Signs,3
Neuropathic Pain Symptom Inventory,7
Neuropathic Pain Diagnostic Questionnaire,6
and Neuropathic Pain Questionnaire20
(and its Short Form2
). While useful, several of these instruments do not address the non-traditional pain qualities found in the heavy pain factor such as aching, dull, or gnawing. However, the additional pain descriptors in these instruments may enhance the content validity of pain scores estimated from our 2-factor model. Furthermore, the additional pain quality descriptors may allow for identification of distinctive factor structures for DPN and PHN. In this study, we did not find a different structure for the 2 NeP conditions—despite the clearly different initial conditions—that supported the generalizability of the proposed 2-dimensional measure of pain in independent samples. Finally, we note that the population size of this study may be large with regards to the inherent variability of some descriptors, which could lead to overfitting of some factors in the analysis. However, the sample size for this study needed to be large enough to adequately power the CFA for interpretable results.
The specific qualities of pain may be indicative of pain mechanisms and may eventually guide physicians toward the most effective therapies for particular conditions. We previously described a 2-factor structure for the qualities of general neuropathic pain in a small study.9
In this report, a unidimensional conceptualization of pain (ranging from low to high levels) has been rejected. Instead, our results confirm the two related dimensions of pain (heavy and stabbing) in a large cohort of patients with two distinct painful neuropathies (DPN and PHN). In clinical settings, one patient may present a high level of heavy pain accompanied by a low level of stabbing pain whereas another patient may present high levels of both heavy and stabbing pain. Future studies should investigate whether this two dimensional representation of pain qualities is associated with distinctive pain mechanisms and treatment responsiveness to therapeutic agents, as well as the generalizability of two dimensional representation of pain in non-neuropathic populations.