Spondyloarthritis (SpA) refers to a group of closely related disorders including ankylosing spondylitis (AS), psoriatic arthritis, reactive arthritis, enteropathic arthritis and undifferentiated spondyloarthritis. These diseases share some of the same clinical features, exhibit familial aggregation, and are associated with HLA-B27 positivity. The clinical manifestations of SpA are diverse and include inflammatory back pain (IBP), oligoarthritis occurring primarily in the lower extremities, phalangitis, heel or other sites of attachment, and extra-articular manifestations, such as uveitis, inflammatory bowel disease, and psoriasis. SpA can also be classified according to the site of joint involvement: patients with mid-axis SpA (axSpA) present primarily with mid-axis involvement, with inflammation of the sacroiliac joint (SIJ), the spine, or both the sacroiliac joint and the spine; patients with peripheral SpA present primarily with peripheral joint manifestations, including peripheral arthritis, attachment point inflammation, and finger/toe inflammation. The advantage of this classification is that it better describes the disease presentation and facilitates the treatment of the disease, as the treatment strategies for mesial SpA and peripheral SpA are different. the vast majority of the patient population with SpA is axSpA and AS, however, the diagnosis of axSpA and AS remains a challenge to date, and the ability to diagnose early and intervene promptly is essential to slow disease progression, reduce disease load, and avoid Early diagnosis and timely intervention are essential to delay disease progression, reduce disease burden, and avoid unnecessary treatment. Evolution of classification criteria for SpA Currently, the most widely used criteria for the classification of AS are the modified New York (mNY) criteria. According to the mNY criteria, a definitive diagnosis is made in patients who meet at least one of the clinical criteria (including inflammatory back pain, limited spinal mobility, and limited degree of chest expansion) and meet the radiological requirements (bilateral sacroiliac arthritis of at least grade 2, or unilateral sacroiliac arthritis of at least grade 3). The mNY criteria have serious limitations in clinical practice: they focus only on medial features and ignore other clinically relevant disease features. Moreover, spinal mobility and limitation of expansion often occur late in the course of the disease, and these symptoms do not represent active inflammation, but rather the result of inflammation. Because of the slow progression of damage on radiographs, the time between the first presentation of AS symptoms and the diagnosis of AS is often more than 10 years. Therefore, the mNY criteria can be used to identify definite cases of AS at the time of classification, but cannot be used in daily clinical practice to identify patients with early axSpA. In the early 1990s, two sets of classification criteria were developed: the European Spondyloarthropathy Study Group (ESSG) criteria and the Amor criteria. These criteria are applicable to early and mild SpA and cover the entire spectrum of SpA disease rather than focusing only on medial or peripheral SpA. These criteria take into account the presence of sacroiliac arthritis, but sacroiliac arthritis is not a condition that must be met. Patients should meet at least one SpA-related feature, i.e., meet the ESSG classification criteria, while meeting the inclusion criteria for IBP or peripheral synovitis. The Amor criteria include 12 items, but none of them is mandatory, and each item is scored with a different weight (score 1-3); a total score of 6 is considered to meet the Amor criteria. Compared with the ESSG criteria, the Amor criteria consider more items, such as the efficacy of NSAIDs and genetic predisposition (HLA-B27 positivity). In contrast, the ESSG criteria are easier to use as far as feasibility is concerned. However, both sets of criteria showed significantly lower performance when used in early, mild or suspected SpA cases and could not distinguish axSpA from peripheral SpA, indicating their limited application as diagnostic tools. Because of the presence of a period of no radiographic presentation in early AS, there is an urgent need to develop new criteria for identifying early-stage patients and differentiating between axSpA and peripheral SpA. experts from the International Society for the Assessment of Spondyloarthritis (ASAS) have developed classification criteria for axSpA and peripheral SpA. In order to cover as many aspects of axSpA as possible, the ASAS has tried to ensure that the criteria integrate the typical features of the diversity of SpA, such as clinical presentation, family history, imaging and laboratory tests. axSpA criteria were published in 2009 and cover the entire spectrum of axial disease, irrespective of the presence or absence of radiological manifestations of sacroiliac arthritis. The axSpA criteria consist of two groups: the “imaging group” and the “clinical group”. Both groups required the presence of chronic back pain of ≥3 months’ duration with onset before the age of 45 years. The imaging panel criteria required the presence of sacroiliac arthritis on MRI or conventional radiographs and at least one SpA feature. The Clinical Group criteria required that the patient should test positive for HLA-B27 and have at least two other SpA-related features; the ASAS axSpA criteria did not mandate the presence of sacroiliac arthritis when the patient met the Clinical Group criteria. The ASAS axSpA criteria have similar sensitivity but improved specificity compared to the Amor criteria and ESSG criteria, and this set of criteria may be useful in the diagnosis of disease in high prevalence situations. axSpA criteria incorporate the evaluation of multiple SpA features, including MRI and HLA-B27 testing, which is a major advance in the identification of various axSpA phenotypes. Is axSpA without radiological changes an early lesion of AS? Other signs and symptoms of early axSpA tend to be subtle, initially presenting as insidious back pain that is mild and nonspecific in the early stages of the disease. Although chronic back pain is the main symptom of axSpA, patients may also be pain-free for a long period of time. Early x-rays of axSpA may be normal and remain normal for many years after the onset of the disease. However, sacroiliac arthritis can be seen on MRI films early in the disease (“no radiographic change phase”). Similarly, inflammation of the spine can be seen on MRI films before structural damage occurs. Importantly, there is no difference between patients with axSpA without radiological changes and patients with confirmed AS in terms of disease severity, pain, quality of life, and efficacy of treatment. Several studies conducted in patients with axSpA without radiological changes have shown good efficacy of biological agents. In patients with early axSpA, the effectiveness of biological agents was similar or higher than in patients with confirmed AS. The ratio of axSpA patients without radiological changes to patients with confirmed AS in the axSpA population depends on the duration of back pain, the longer the duration, the more AS patients. The incidence of progression to AS in axSpA without radiological changes also correlates with the duration of disease, ranging from about 10% in 2-5 years to 25-60% in 5-10 years. Studies have shown that the risk factors for progression to AS in axSpA without radiological changes are mainly high levels of CRP (24% progress to AS within two years) and MRI inflammatory manifestations (30-87% will progress to (AS). In contrast to AS, axSpA without radiological changes is characterized by a lower rate of HLA-B27 positivity (42-75%); a higher incidence in women (60% vs 30%); a milder degree of inflammation; and a poorer response to TNF inhibitor therapy in some patients. Thus, axSpA without radiological changes is currently considered a subgroup of SpA, accounting for 20-80% of total SpA, of which 10-15% will stop progressing to AS, i.e., axSpA without radiological changes does not absolutely equate to early AS. Early diagnostic tools for axSpA without radiological changes IBP is the main clinical feature of axSpA, however, patients present with symptoms that must be distinguished from those caused by other rheumatic diseases, nonspecific pain syndromes, or mechanical causes. There are multiple sets of diagnostic criteria for IBP that perform similarly and can all be used in daily practice. IBP is seen in 70-80% of patients with axSpA, but it is also present in 20-25% of patients with mechanical back pain. IBP alone is not sufficient to diagnose axSpA; the presence of IBP only increases the probability of axSpA from 5% to 14-16%. The specificity and sensitivity of the HLA-B27 test is high compared with other diagnostic tests for SpA, although the performance of this test depends on the rate of HLA-B27 positivity in a given population. In the AS population, 75-95% of patients are positive for HLA-B27, whereas the rate of HLA-B27 positivity is significantly lower (42-75%) in undifferentiated and nonradiographic manifestations of axSpA. HLA-B27 negativity does not exclude the diagnosis of SpA and is associated with a longer delay in diagnosis. Since the early 1990s, SIJ and spine MRI have been increasingly used to evaluate patients with clinically suspected early axSpA who have normal or only ill-defined findings on pelvic radiographs.MRI has excellent spatial and contrast resolution to visualize the skeletal and soft tissue structures involved in the disease process of SpA. Because of this ability and the absence of ionizing radiation, MRI is the ideal imaging test for evaluating mesial skeletal inflammation, which is difficult to diagnose with clinical examination in these areas. Two MRI sequences are widely used to evaluate axSpA: fluid-sensitive short-time inversion recovery (STIR) sequences and fat-sensitive T1-weighted spin-echo (T1SE) sequences, both of which can provide complementary information on inflammation. Systematic studies of patients with axSpA have not found that enhanced MRI sequences using expensive contrast agents are superior to conventional STIR sequences in the assessment of SIJ and spinal inflammation. STIR and T1SE sequences are able to detect active and structural lesions in early axSpA, whereas post-inflammatory structural changes in advanced disease are where the limitations of radiographic imaging lie. Bone marrow edema is considered the primary abnormal manifestation suggestive of active inflammation in axSpA and is characterized by signal enhancement in STIR MRI slices, usually first seen in the cartilaginous area of the SIJ. Among the various structural lesions that can be clearly visualized by T1SE sequences, erosion of the SIJ is highly specific for the diagnosis of axSpA, and T1SE MRI images identify the presence of erosion based on disruption of cortical signal continuity and signal changes in the nearby bone marrow. In a joint ASAS-OMERACT expert consensus statement, it was proposed that two bone marrow edema lesions at the same SIJ level or one lesion in the same SIJ quadrant on at least two consecutive levels were suggestive of axSpA. later, a data-driven systematic approach proposed to include erosion in the definition of positive MRI, which has a high specificity for axSpA. Conclusions With the application of new imaging techniques, such as MRI, it is now possible to identify AS phenotypes without radiological manifestations, thus giving rise to the concept of axSpA and the emergence of the ASAS axSpA classification criteria, which facilitate the early diagnosis of SpA, but to avoid overdiagnosis, the diagnostic performance of this criterion needs to be further evaluated in the starting cohort of patients with suspected SpA. In addition, further studies are needed to confirm whether the ASAS axSpA classification criteria help to reduce the delayed diagnosis of AS. Although pelvic x-ray, MRI, and HLA-B27 examinations are important for disease diagnosis, negative results do not exclude the diagnosis of axSpA. The professional opinion of a rheumatologist remains the key to the diagnosis of the disease.