Table 3.

Pathological conditions that mimic axial spondyloarthritis.

Condition	Type	Characteristic features
Degenerative changes	Spine	Location–weight-bearing axis Other–old age, other degenerative findings, Modic classification
	Sacroiliac joint	Location–ligamentous portion, bone sclerosis of anterior and middle third Other–male, osteophytes, associated with pubic symphysis degeneration
Scheuermann disease	–	Location–thoracolumbar Other–adolescents, Schmorl nodes
Osteitis condensans illi	–	Location–iliac side at the ventro-caudal portion Other–bilateral, symmetric, women, middle-age, sclerotic area with triangular configuration, may demonstrate BME below arcuate line, no erosions
DISH and OPLL	DISH	Location–thoracic and lumbar segments, superior non-cartilaginous portion of the SIJ Other–old age, obesity, diabetes mellitus, occasional bridging, appendicular involvement
	OPLL	Location–cervical spine Associated with DISH
Fractures (sacrum/llium/vertebrae)	Acute	Insufficiency–more common at the sacral alae and bilateral, women
	Insufficiency	Stress–clinical history, unilateral and sacral side, no involvement of the subchondral bone, involvement of the pars interarticularis in the spine
	Stress response	Diastasis–clinical history of major pelvic trauma, may have backfill, asymmetry, posterior offset
	Post-trauma inflammatory-like	General–suggestive clinical history, absence of other findings to support axSpA
	SIJ diastasis/incongruence
Septic arthritis	Familial mediterranean fever/brucellosis	Pronounced edema and other inflammatory osseus and soft tissue changes
	Staphylococcus aureus
	Pyogenic spondylodiscitis
	Fungal
	Tuberculosis
Metabolic diseases	Idiopathic hypoparathyrodism	–
	Hyperparathyroidism	Other associated findings
	Alkaptonuria	–
	Hypophosphatemic osteomalacia	–
	Paget disease	Bone expansion, cortical thickening, coarsened trabecula
Crystal deposition arthropathy	Gouty sacroiliitis	Location–lumbar spine > rest of the spine or SIJ Other–middle-aged men, perimenopausal women, monoarthritis (mostly lower extremities), SIJ gout is non-specific
	Spinal/Sacro-iliac CPPD	Location–cervical > lumbar segments, atlanto-odontoid joint Other–peripheral arthritis more common, inflammatory flares at the intervertebral endplates
SAPHO syndrome/CRMO	–	Location–clavicles and sternum Other–Extra-musculoskeletal findings, progression from lytic to sclerotic and hypertrophic lesions
Charcot spine	–	Location–thoracolumbar segments Other–spinal cord injury, heterotopic ossification at the elbows and hip, paravertebral masses, bridging osteophytes, degeneration, bone erosions, pseudarthrosis, atrophic to hypertrophic forms
Behçet disease	–	Extra–articular findings, peripheral skeleton most involved, sacroiliitis controversial, atlanto-axial subluxation (anedoctal)
Rheumatoid arthritis	–	SIJ–bilateral and symmetric Other–femoroacetabular joints affected, clinical presentation
Hemoglobinopathies	–	Bone infarctions, bone marrow expansion and hyperplasia, growth disturbance, H-shaped vertebra, red marrow reconversion, extra-musculoskeletal findings
Sarcoidosis	–	Location–throacolumbar segments Other–women, extra-musculoskeletal findings, spinal involvement associated with CNS lesions, lytic, and/or sclerotic lesions in lacework pattern
Early axSpA		Location–dorso-caudal portion of the SIJ (posterior lower ilium)