Impact of Low Back Pain Phenotypes on Function and Quality of Life in Ankylosing Spondylitis

This prospective, hospital-based study is designed to expand the phenotypic characterization of inflammatory low back pain in patients with ankylosing spondylitis and to investigate the relationship between pain phenotypes, their cognitive-emotional components, and their impact on functional status and quality of life. Patients diagnosed with ankylosing spondylitis who present with chronic low back pain persisting for at least three months will be consecutively recruited from the Physical Therapy and Rehabilitation outpatient clinics of Konya Beyhekim Training and Research Hospital. All participants will undergo a standardized, single-session evaluation conducted by trained clinicians.

Pain intensity and its cognitive-emotional dimensions will be assessed using the Visual Analog Scale (VAS) and the Pain Catastrophizing Scale (PCS). Functional status will be evaluated with the Roland-Morris Disability Questionnaire (RMDQ). Disease activity, functional impairment, and quality of life will be assessed using the Bath Ankylosing Spondylitis Disease Activity Index (BASDAI), Bath Ankylosing Spondylitis Functional Index (BASFI), and Ankylosing Spondylitis Quality of Life Questionnaire (ASQoL), respectively. In addition, detailed sociodemographic and clinical data, including age, sex, body mass index, education level, employment status, and income level, will be recorded through structured interviews.

Pain mechanisms will be classified into three categories based on validated tools and accepted clinical criteria. Neuropathic pain will be identified using the DN4 (Douleur Neuropathique 4 Questions) questionnaire, with scores above the established cutoff indicating neuropathic pain. Nociplastic pain will be determined according to current clinical criteria and grading approaches for musculoskeletal nociplastic pain. Patients who do not meet criteria for neuropathic or nociplastic pain will be classified as having nociceptive pain. This classification will enable a multidimensional evaluation of pain beyond traditional inflammatory definitions.

Due to the absence of directly comparable studies in the literature, an a priori power analysis could not be performed. Recruitment will continue until a target sample size of 200 participants is achieved. Following data collection, a post hoc power analysis will be conducted using appropriate statistical methods. If the calculated statistical power is below 80%, recruitment will be extended until adequate power is obtained. All participants will undergo detailed medical history taking and physical examination as part of the study protocol.

This study is expected to provide a more comprehensive understanding of pain heterogeneity in ankylosing spondylitis and to clarify the contribution of different pain mechanisms and cognitive-emotional factors to functional limitations and reduced quality of life.