Intracranial invasion of granulomatous sinusitis: report of a rare case of granulomatosis with polyangiitis.

The involvement of the central nervous system with granulomatosis with polyangiitis (GPA) is uncommon, and the formation of intracranial mass lesions is particularly rare. We describe the case of a Japanese woman in her thirties with GPA initially limited to the upper respiratory tract. Twelve years after the disease was onset, brain magnetic resonance imaging revealed a lobulated mass in the frontal lobe, and computed tomography findings suggested direct extension of granulomatous inflammation from the paranasal sinuses through the cribriform plate. Due to the risk of infection associated with cribriform plate destruction, surgical resection was performed for both diagnostic and preventive purposes. Histopathological examination of the resected intracranial lesion revealed necrotising granulomas without evidence of infection, consistent with GPA. Postoperatively, a moderate dose of prednisolone and rituximab was administered, resulting in clinical and serological remission. This case highlights a rare intracranial manifestation of GPA caused by direct contiguous spread from the paranasal sinuses, which can occur in the absence of systemic symptoms. Although sinonasal involvement is typically regarded as non-severe, the presence of bony destruction may signal a potentially organ-threatening course.