Pain Phenotype and Response to Corticosteroid Injection in Patients With Subacromial Pain Syndrome (SAPS) (NAP SAPS CSI)

This study is part of a larger study investigating the prevalence of nociplastic pain phenotype according to the International Association for the Study of Pain (IASP) clinical criteria (The NAP SAPS Study). This prospective cohort component includes participants with SAPS who receive a corticosteroid shoulder injection as part of routine clinical care.

Musculoskeletal pain poses a significant burden on individuals and society. The IASP describes different pain mechanisms, including nociceptive, neuropathic and nociplastic pain. Identification of different pain mechanisms may be relevant for understanding pain presentation and variation in treatment response among patients with musculoskeletal pain.

Shoulder pain ranks among the most prevalent musculoskeletal complaints, with subacromial pain syndrome (SAPS) ranking highest, and serves as a common diagnosis for non-traumatic shoulder pain.

Exercise and other conservative treatments, including corticosteroid injection, are commonly used in the management of SAPS. However, a substantial proportion of patients experience limited or transient treatment effects and may develop persistent shoulder pain. Differences in underlying pain mechanisms may contribute to variation in treatment response among patients with SAPS. Corticosteroid injection is primarily used to target local inflammatory and nociceptive processes, and differences in pain phenotype may therefore be associated with variation in response to treatment. Emerging evidence suggests that altered pain processing may also be present in patients with shoulder pain, supporting the investigation of whether pain phenotype is associated with treatment response.

Clinical criteria have been proposed to identify different pain mechanisms in musculoskeletal pain conditions. The IASP clinical criteria for nociplastic pain provide a structured approach to classifying patients based on clinical features.

However, the possible association between pain response following corticosteroid injection and classification according to the IASP clinical criteria for nociplastic pain has, to our knowledge, not been sufficiently investigated among patients with SAPS.

The overall objective is to investigate the association between pain phenotype, classified according to the IASP clinical criteria for nociplastic pain, and subsequent change in pain intensity four weeks after corticosteroid injection.

Primary hypothesis: Patients classified as having a nociplastic pain phenotype will demonstrate a significantly smaller mean reduction in pain intensity (VAS) from baseline to four weeks after corticosteroid injection compared with patients classified as having a non-nociplastic pain phenotype.

Secondary hypothesis: A significantly lower proportion of patients classified as having a nociplastic pain phenotype will achieve a reduction of at least 2 points on the VAS four weeks after corticosteroid injection compared with patients classified as having a non-nociplastic pain phenotype.