Ankylosing spondylitis (AS) is a chronic inflammatory rheumatic disease primarily affecting the axial skeleton, including the spine and sacroiliac joints. The most prominent clinical manifestations of AS are chronic back pain and progressive spinal stiffness; however, the disease may also involve peripheral joints and entheses.

The primary goals in the management of AS are to reduce pain and stiffness, preserve functional capacity, and decrease overall disease burden. Nonsteroidal anti-inflammatory drugs (NSAIDs) constitute the first-line treatment, while biologic agents and disease-modifying antirheumatic drugs (DMARDs) are used in appropriate patients.

Back pain in AS represents one of the hallmark symptoms of the disease and mainly arises from inflammation involving the spine and sacroiliac joints. This symptom is frequently associated with functional impairment and reduced quality of life.

Although contemporary treatment strategies can effectively suppress disease activity in many patients, a subset of individuals continue to experience persistent symptoms and disease burden despite treatment, a condition commonly referred to as "residual disease." Residual symptoms are frequently reported even in patients achieving clinical remission or low disease activity, and a considerable proportion of these patients continue to suffer from ongoing pain. Therefore, persistent back pain despite controlled disease activity has emerged as an important unmet clinical need, potentially associated not only with inflammatory mechanisms but also with accompanying mechanical factors and comorbid conditions.

The erector spinae plane (ESP) block is a regional analgesic technique performed under ultrasound guidance by advancing the needle tip into the fascial plane deep to the erector spinae muscle at the level of the transverse process, followed by injection of local anesthetic and steroid agents. Owing to the cranio-caudal spread of the injectate, the ESP block can provide analgesia across multiple dermatomes. In patients with persistent back pain, ESP block has been described as a complementary interventional option administered at thoracic or lumbar levels, either as a single or repeated procedure depending on the clinical scenario, with the aims of reducing pain severity, improving functional status, and decreasing analgesic requirements. However, the current evidence remains limited for many chronic pain indications, consisting predominantly of case reports, case series, and a limited number of clinical studies.

The present study aims to evaluate the effects of erector spinae plane block on pain severity and clinical outcomes in patients with ankylosing spondylitis whose disease activity is adequately controlled under medical treatment but who continue to experience persistent back pain.