Chronic Pain as a Continuum: Autoencoder and Unsupervised Learning Methods for Archetype Clustering and Identifying Co-existing Chronic Pain Mechanisms

Abstract

Chronic pain (CP) is a personal and economic burden that affects more than 30% of the world's population. While being the leading cause of disability, it is complicated to diagnose and manage. The optimal way to treat CP is to identify the pain mechanism or the underlying cause. The substantial overlap of the pain mechanisms (i.e., Nociceptive, Neuropathic, and Nociplastic) usually makes identification unreachable in a clinical setting where finding the dominant mechanism is complicated. Additionally, many specialists regard CP classification as a spectrum or continuum.

Despite the importance, a data-driven way to identify co-existing CP mechanisms and quantification is still absent. This work successfully identified the co-existing CP mechanisms within a patient using Unsupervised Learning while quantifying them without the help of diagnosis established by the clinicians. Two different datasets from different cohorts comprised of patient-reported history and questionnaires were used in this work. Unsupervised Learning (k-prototypes) revealed notable overlaps in the data. It was further emphasized by the outcomes of the Semi-supervised Learning algorithms when the same trend was observed with some diagnosis or class information. It became evident that the CP mechanisms overlap and cannot be classified as distinct conditions. Additionally, mixed pain mechanisms do not make an individual cluster or class, and CP should be considered as a continuum.

To reduce data dimension and extract hidden features, Autoencoder was used. Using an overlapping clustering technique, the pain mechanisms were identified. The pain mechanisms were also quantified while elucidating overlaps, and the dominant CP mechanism was successfully pointed out with explainable element. The hamming loss of 0.43 and average precision of 0.5 were achieved when considered as a multi-label classification problem.

This work is a data-driven validation that there are significant overlaps in CP conditions, and CP should be considered a continuum where all CP mechanisms may co-exist.