How can latent trajectories of back pain be translated into defined subgroups?

Abstract

Background: Similar types of trajectory patterns have been identified by Latent Class Analyses (LCA) across multiple low back pain (LBP) cohorts, but these patterns are impractical to apply to new cohorts or individual patients. It would be useful to be able to identify trajectory subgroups from descriptive definitions, as a way to apply the same definitions of mutually exclusive subgroups across populations. In this study, we investigated if the course trajectories of two LBP cohorts fitted with previously suggested trajectory subgroup definitions, how distinctly different these subgroups were, and if the subgroup definitions matched with LCA-derived patterns.

Methods: Weekly measures of LBP intensity and frequency during 1 year were available from two clinical cohorts. We applied definitions of 16 possible trajectory subgroups to these observations and calculated the prevalence of the subgroups. The probability of belonging to each of eight LCA-derived patterns was determined within each subgroup. LBP intensity and frequency were described within subgroups and the subgroups of 'fluctuating' and 'episodic' LBP were compared on clinical characteristics.

Results: All of 1077 observed trajectories fitted with the defined subgroups. 'Severe episodic LBP' was the most frequent pattern in both cohorts and 'ongoing LBP' was almost non-existing. There was a clear relationship between the defined trajectory subgroups and LCA-derived trajectory patterns, as in most subgroups, all patients had high probabilities of belonging to only one or two of the LCA patterns. The characteristics of the six defined subgroups with minor LBP were very similar. 'Fluctuating LBP' subgroups were significantly more distressed, had more intense leg pain, higher levels of activity limitation, and more negative expectations about future LBP than 'episodic LBP' subgroups.

Conclusion: Previously suggested definitions of LBP trajectory subgroups could be readily applied to patients' observed data resulting in subgroups that matched well with LCA-derived trajectory patterns. We suggest that the number of trajectory subgroups can be reduced by merging some subgroups with minor LBP. Stable levels of LBP were almost not observed and we suggest that minor fluctuations in pain intensity might be conceptualised as 'ongoing LBP'. Lastly, we found clear support for distinguishing between fluctuating and episodic LBP.

Keywords: Classification; Low back pain; Subgroups; Trajectory.

Conflict of interest statement

Ethics approval and consent to participate

Under Danish law, ethical approval was not needed for additional analysis of existing observational cohort data [23]. Patients gave consent to participate in the cohort study.

Consent for publication

Not applicable.

Competing interests

AK and LH are members of the Editorial Board of BMC Musculoskeletal Disorders. The Nordic Institute of Chiropractic and Clinical Biomechanics and AK’s position at the University of Southern Denmark are financially supported by the Danish Chiropractors’ Foundation. The authors declare to have no competing interests.

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Figures

Fig. 1

Outline of the study design

Fig. 2

Mean LBP intensity in eight previously identified latent LBP trajectory patterns. In order to initially describe general patterns of LBP in relatively stable trajectory periods, the measures from the first 9 weeks after initiating treatment (grey area) were not included in this study. Bars indicate +½ standard deviation, which were used to enhance the readability by avoiding overlap of graphs

Fig. 3

Examples of observed LBP trajectories within each of 16 defined subgroups

Fig. 4

LBP intensity and frequency in the LBP trajectory subgroups in weeks when any LBP was reported