MRI Insights into Unilateral Sacroiliitis in a German Shepherd

MRI Findings in a Dog with Unilateral Sacroiliitis

Laura López-Cubillo, Daniel Sánchez-Masián, Felipe de Vicente, Raquel Salgüero

Background
Sacroiliitis, or inflammation of the sacroiliac joint (SIJ), is an uncommon diagnosis in veterinary medicine but has been increasingly recognized in large-breed dogs. Clinical signs include lumbosacral pain and hindlimb lameness, which overlap with other conditions such as disc herniation or degenerative lumbosacral stenosis. This case report describes the MRI findings in a German Shepherd dog with unilateral sacroiliitis and highlights the role of advanced imaging in diagnosis.

Methods
A 6-year-old neutered male German Shepherd presented with chronic right hindlimb pain and reluctance to jump or lie on its right side. Neurologic and orthopedic examinations localized pain to the right hip and caudal lumbar spine. Radiographs and ultrasound of the abdomen and thorax were unremarkable. MRI of the lumbosacral spine and pelvis was performed using a 1.5-T scanner, with STIR, T1-, and T2-weighted sequences pre- and post-contrast. Cytology and histopathology of the affected SIJ were obtained via ultrasound-guided fine-needle aspiration and surgical biopsy.

Results
MRI identified a polyostotic lesion centered in the right SIJ, affecting the sacral wing and ipsilateral ilium. The lesion was hyperintense on STIR and hypointense on T1W and T2W, with moderate contrast enhancement. Soft tissue thickening and muscle atrophy were noted. Cytology revealed a mixed inflammatory response with lymphocytes, plasma cells, and neutrophils. Histopathology confirmed mild neutrophilic and lymphoplasmacytic osteomyelitis, ruling out neoplasia or infection. Symptomatic treatment with NSAIDs and strict rest led to clinical improvement within six weeks. However, four months later, the dog developed metastatic lymphadenopathy from an undiagnosed primary carcinoma or adenocarcinoma, leading to euthanasia.

Limitations
This is a single-case report, limiting broader conclusions about sacroiliitis in dogs. The underlying cause of sacroiliitis remains uncertain, and no definitive link to the subsequent neoplastic process was established. The absence of comprehensive neoplastic screening at initial presentation may have missed an early malignancy.

Conclusions
MRI is the gold standard for diagnosing sacroiliitis in dogs, as radiographs may not detect early changes. Sacroiliitis should be included in differential diagnoses for lumbosacral pain and hindlimb lameness, particularly in large-breed dogs. This case also raises awareness of potential paraneoplastic associations with sacroiliitis, although further research is needed.

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