According to ASAS criteria SpA is broadly classified into three major subtypes based on the predominant area of involvement (Fig. 1):

axSpA: This subtype primarily affects the spine and SI joints. It includes:

• r-axSpA (also known as AS), in which structural damage is visible on X-rays.
• nr-axSpA, in which imaging changes are seen only on MRI or may be absent despite

Patients with axSpA may also exhibit minor peripheral joint involvement.

pSpA: This subtype mainly affects the limb joints and entheses and may present with dactylitis. Some patients with pSpA may also experience mild spinal symptoms.

Mixed/overlap forms: Arthritis affects 10–20% of patients with IBD, presenting as peripheral oligoarthritis of large joints or axial disease with sacroiliitis or AS-like features. It is typically non-erosive, often asymmetric, RF-negative, and linked to gut activity. Management of the underlying IBD usually improves the arthritis, although targeted therapy may sometimes be required.

Fig. 1: Classification of SpA according to the manifestations

Source: Proft F, Poddubnyy D. Ankylosing spondylitis and axial spondyloarthritis: recent insights and impact of new classification criteria. Ther Adv Musculoskelet Dis. 2018 Jun;10(5-6):129-139.

REFERENCE
1. Denis Poddubnyy, Classification vs diagnostic criteria: the challenge of diagnosing axial spondyloarthritis, Rheumatology, Volume 59, Issue Supplement_4, October 2020, Pages iv6–iv17, https://doi.org/10.1093/rheumatology/keaa250
2. Proft F, Poddubnyy D. Ankylosing spondylitis and axial spondyloarthritis: recent insights and impact of new classification criteria. Ther Adv Musculoskelet Dis. 2018 Jun;10(5-6):129-139.
3. Orchard TR. Management of arthritis in patients with inflammatory bowel disease. Gastroenterol Hepatol (NY). 2012 May;8(5):327-9. PMID: 22933865; PMCID: PMC3424429.