What are spondyloarthritis (SPA) and diffuse idiopathic skeletal hyperostosis (DISH)?

Spondyloarthritis (SpA) and Diffuse Idiopathic Skeletal Hyperostosis (DISH)

Spondyloarthritis (SpA)

Spondyloarthritis is a spectrum of chronic inflammatory diseases primarily affecting the axial skeleton (spine and sacroiliac joints), characterized by inflammatory back pain, sacroiliitis, enthesitis, and extra-musculoskeletal manifestations including uveitis, psoriasis, and inflammatory bowel disease. 1

Disease Classification

• SpA encompasses two main subtypes: ankylosing spondylitis (AS or radiographic axSpA) with definitive structural changes on sacroiliac joint radiographs, and non-radiographic axSpA without radiographic sacroiliitis 1
• Classification follows the Assessment of SpondyloArthritis international Society (ASAS) criteria, distinguishing between axial and peripheral patterns 1
• Peripheral manifestations include arthritis (affecting large joints in 30-50% of patients in an oligoarticular asymmetric pattern), enthesitis, and dactylitis 2

Clinical Presentation

• The hallmark symptom is chronic inflammatory back pain with morning stiffness predominating in the second half of the night that improves with exercise 1
• Progressive ankylosis leads to loss of spinal flexibility and the classic "bamboo spine" appearance from extensive ligamentous ossification and vertebral joint fusion 2, 3
• Uveitis is the most common extra-articular manifestation, occurring in a significant proportion of patients 2
• Inflammatory bowel disease is strongly associated, with the triad of erythema nodosum, Achilles tendon enthesitis, and sacroiliitis strongly suggesting IBD as underlying etiology 4

Radiographic Features

• Characteristic spine findings include: syndesmophytes (vertical bony bridges between vertebral bodies), vertebral body squaring from erosion and new bone formation at anterior corners, and "shiny corners" representing erosions at vertebral body corners 2, 3
• Sacroiliac joint involvement is universal in AS, forming the core of the modified New York classification criteria 3
• MRI demonstrates high specificity: three or more sites of inflammatory spondylitis have 94% specificity for AS, corner inflammatory lesions have 94% specificity, and fatty corner lesions have 98% specificity 3

Genetic Association

• HLA-B27 is present in 74-89% of AS patients, though only 1% of HLA-B27-positive individuals develop AS 2

Epidemiology and Impact

• Prevalence varies by region: 0.14% in Latin America, 0.20% in North America, and 0.25% in Europe for AS specifically 1
• Patients experience substantial deterioration in quality of life due to impaired function, work productivity, social interactions, and increased risk of depression and anxiety 1

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Diffuse Idiopathic Skeletal Hyperostosis (DISH)

DISH is a non-inflammatory metabolic condition characterized by calcification and ossification of entheses (tendon and ligament attachment sites to bone), affecting mainly the spine and peripheral sites, with flowing calcification along the anterior longitudinal ligament. 5, 6

Pathophysiology

• DISH is primarily considered a metabolic condition, though emerging evidence suggests local inflammation (either primary or secondary to metabolic derangements) may contribute to new bone formation 6
• The disease involves enthesitis and enthesopathies at both axial and peripheral sites, resembling to some extent the enthesitis seen in inflammatory SpA 6

Diagnostic Criteria

• Major criterion requires contiguous calcification of the anterior longitudinal ligament involving at least four contiguous vertebral bodies 7
• The criteria were designed to exclude radiographic signs of spondyloarthritis, though both conditions can coexist 7

Imaging Characteristics

• On conventional radiographs, SpA-like changes occur in DISH: 28% show such changes in the thoracic spine, though these are less common than in true SpA 8
• MRI reveals surprising inflammatory features: bone marrow edema is present in 37% of DISH patients in the thoracic spine and 35-42% in the lumbar spine, with fat lesions in 56% of thoracic spine 8, 9
• Three or more sites of MRI lesions consistent with bone marrow edema corners were detected in multiple DISH patients, a finding typically considered suggestive of axial SpA 9

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Critical Diagnostic Pitfalls

Overlapping Features Create Diagnostic Challenges

• Significant overlap exists between DDD, DISH, and axSpA for both inflammatory and degenerative imaging features, making differentiation difficult 8
• SpA-like spine radiographic features are found in approximately one-fourth of DISH patients, and MRI bone marrow edema in one-third 8
• Sacroiliitis by New York criteria was identified in 29% of DISH patients (compared to 76% in axSpA and 48% in degenerative disc disease), demonstrating substantial overlap 8

Key Distinguishing Approaches

• Age and symptom pattern matter: DISH typically affects older patients (median age 62 years) without the characteristic inflammatory back pain pattern of SpA 8
• Distribution patterns differ: DISH shows more thoracic involvement (28% with SpA-like changes), while axSpA shows more lumbar involvement (50% with SpA-like changes) 8
• Both conditions can coexist in the same patient, and overlap could result in undertreatment with negative prognostic impact 7
• Do not assume DISH is purely degenerative when inflammatory features are present on MRI—this may represent true inflammatory disease requiring different management 9