Misdiagnosis of sacroiliac joint gout as ankylosing spondylitis: Solving the diagnostic dilemma with dual-energy computed tomography

Introduction

Gout is a disease caused by a purine metabolism disorder, and ankylosing spondylitis (AS) is a chronic inflammatory arthritis. Gout arthritis is an acute inflammatory reaction caused by the crystallization of uric acid in the joints and tissues around the joints. ¹ Urate crystals can be deposited in any part of the body except the central nervous system, but usually occur in the small joints of the foot (especially the first metatarsophalangeal joint), ankles, knees, and joints of both hands. ¹ When joint swelling and pain occur in other rare joint sites, diagnostic confusion may arise. This case presents unusual gouty arthritis symptoms appearing in typical sites of AS, offering new insights for the differential diagnosis of arthritis.

Case presentation

A 30-year-old female patient was admitted with “recurrent swelling and pain in both knee joints accompanied by low back pain for 2 years, aggravated for 4 days.” Two years prior, she began experiencing redness, swelling, heat, and pain in the right knee joint, without any obvious triggers, characterized by episodic flares. Over the next year or 2, she experienced intermittent joint pain, and she took NSAIDs intermittently for 2–3 days at a time, which relieved the symptoms. During this period, the highest previously recorded uric acid level was 580 μmol/L, and it was previously treated as hyperuricemia. However, swelling and pain in both knees persisted intermittently. One month later, she presented to our outpatient clinic, where laboratory tests revealed a serum uric acid level of 600 μmol/L and C-reactive protein (CRP) of 38.06 mg/L. Pelvic radiography suggested right sacroiliitis and an uneven density of both femoral heads. At present, the diagnosis considers AS and hyperuricemia. For the elevated uric acid and inflammatory markers, methylprednisolone and febuxostat treatment were initiated initially, and further treatment will be planned once AS is more clearly diagnosed. However, the patient did not return regularly.

Over the following year, joint symptoms recurred frequently. During this period, the patient was followed up only occasionally in the outpatient department, and she presented with worsening swelling and pain in the right knee joint, accompanied by significant restriction of movement. She also reported lower back and sacral pain, without any nocturnal pain. Morning stiffness and pain after prolonged sitting did not worsen, and physical activity offered little relief. Based on previous X-ray findings and the medical history, intermittent febuxostat use failed to improve symptoms, AS was suspected, and the patient was admitted for further evaluation.

Past medical history: The patient denied using diuretics, ethambutol, pyrazinamide, aspirin, or niacin. She had no history of smoking. Her mother and brother had a history of gout.

Physical examination during hospitalization: The patient was alert, with no yellow discoloration of the skin or sclera, or palpable enlargement of the superficial lymph nodes. Cardiopulmonary and abdominal examinations were unremarkable. An examination of the right knee revealed swelling (+) and tenderness (+). The patient could not cooperate to complete the bilateral Patrick’s Faber test, and sacroiliac joint percussion was positive on both sides.

Laboratory investigations during hospitalization: Routine blood tests and biochemical indices were within normal ranges. Serum uric acid levels were normal, the erythrocyte sedimentation rate was 51 mm/h, and CRP was 14.15 mg/L. A 24-h urinary uric acid level was 1763 μmol/L. Tests for HLA-B27, anti-cyclic citrullinated peptide antibodies, and antinuclear antibodies were negative.

Imaging: An MRI scan of the sacroiliac joints revealed bilateral sacroiliitis with a bone island on the sacrum’s right side. Dual-energy computed tomography (DECT) imaging of the knees showed urate deposits in the bilateral intercondylar fossae, right lateral collateral ligament, and around the fibular head, along with degenerative changes in both knee joints and a small amount of effusion in the right knee joint. DECT of the sacroiliac joints indicated urate deposits in the soft tissue around the right sacroiliac joint (Figure 1).

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Figure 1.

DECT of sacroiliac joint. Urate deposits in the soft tissues surrounding the sacroiliac joint (green represents urate deposits).

During this hospitalization, based on her medical history, physical examination, laboratory findings, and DECT results, the diagnosis was revised to primary gouty sacroiliitis, replacing the initial diagnosis of AS. She was treated with oral diclofenac sodium enteric-coated tablets (25 mg, three times daily) and benzbromarone capsules (50 mg, once daily). The patient experienced significant improvements in knee and lower back pain and was subsequently discharged. During months of follow-up, the patient benefited from consistent urate-lowering therapy, with no occurrence of joint swelling or pain, including both axial and peripheral joints. In the 6 months after the patient was discharged from the hospital, the uric acid level gradually decreased to 401 μmol/L (Supplemental Figure 1). She has been followed up for 1.5 years now and currently remains below 300 μmol/L.

Discussion

Although gout theoretically can affect any joint in the body, only a few cases of sacroiliac gout have been reported.^2–4 This case involves a young female who experienced recurrent pain in the lumbosacral region and large joints of the lower limbs. At the onset of her illness, she intermittently used medication for treatment, with uric acid levels repeatedly rising. Along with axial joint involvement, pain in the lower limb large joints, elevated inflammatory markers, imaging findings of bilateral sacroiliitis with a right sacral bone island, and no significant effect of normal uric acid levels at admission and intermittent uric acid-lowering treatment, initially raised suspicion of AS. AS typically begins in young adulthood and tends to improve after the age of 45 years. Although 90% of patients with AS are HLA-B27 positive, in clinical practice, HLA-B27 positivity can sometimes lead to the misdiagnosis of AS. ⁵

In this patient, the absence of HLA-B27 and the atypical characteristics of inflammatory low back pain, along with no history of psoriasis or family history of AS, warrant further differential diagnosis. The episodic presentation of red, swollen, hot, and painful joints, and a family history of gout with a history of persistently elevated serum uric acid levels, prompted us to perform DECT imaging of the knees and sacroiliac joints. The results confirmed urate crystal deposition in the sacroiliac joint, accompanied by bone erosion. Gout was diagnosed based on the 2015 American College of Rheumatology/European League Against Rheumatism gout classification criteria. The final diagnosis was revised to sacroiliac joint gout. The young female patient developed symptoms at 28 years of age and had no history of alcohol consumption or smoking. A family history of gout in her mother and brother suggested a possible familial genetic predisposition.

A case report published in 2020 described a middle-aged male patient misdiagnosed with AS secondary to sternal gout. He presented with pain and tightness in the sternocostal region and was unresponsive to NSAIDs and etanercept treatment for 6 months. DECT confirmed gout, and benzbromarone 50 mg daily improved his symptoms significantly. ⁶ Similarly, in 2018, a patient with psoriasis was initially diagnosed with psoriatic arthritis and treated unsuccessfully with TNF-α inhibitors. DECT revealed gouty tophi, and the patient benefited from gout-specific treatments. ² Cases of coexisting AS and gout have also been reported. A cohort study of over 23,000 participants revealed that gout is common among patients with AS and may be more prevalent in this population. ⁷ In such scenarios, enthesitis and precise imaging-based differential diagnoses are critical, particularly when the course of AS becomes acute or the gout process becomes chronic. ⁸ In gout with axial joint involvement, any part of the spine can be involved, with the lumbar spine being the most common.^9,10

Conclusions

This case underscores the pivotal role of DECT in resolving diagnostic dilemmas when atypical presentations of axial joint pain overlap with features of inflammatory arthritis. In this young female patient with a family history of gout, elevated uric acid levels, and ambiguous imaging findings, DECT provided definitive evidence of urate crystal deposition in the sacroiliac joint—a critical step that distinguished gout from seronegative spondyloarthropathies such as AS. Importantly, DECT bypassed the limitations of conventional imaging modalities (radiography and MRI), which often fail to differentiate inflammatory sacroiliitis from crystal-induced arthropathy. While axial involvement in gout remains rare, this case advocates for the targeted use of DECT in patients with unexplained axial/large joint symptoms and risk factors for hyperuricemia, even in the absence of classical gout manifestations. By enabling precise localization of monosodium urate deposits, DECT not only prevents diagnostic delays but also guides the timely initiation of urate-lowering therapy, ultimately improving patient outcomes.

Supplemental Material