A case of an immunoglobulin G4 related inflammatory pseudotumor involving the clivus and the nasopharynx: A challenging case of a perplexing entity with an exhaustive overview

Introduction: Immunoglobulin G4-Related Disease (IgG4-RD) is a systemic fibroinflammatory condition characterized by IgG4-positive plasma cell infiltration and fibrosis, capable of affecting nearly any organ. While neurological manifestations are recognized, skull base and clival involvement are rare, and the spectrum of neurological presentations continues to expand. Research question: This case report addresses the diagnostic and therapeutic challenges of an exceedingly rare presentation of IgG4-RD involving the clivus and nasopharynx, focusing on its unique neurological phenotype and response to treatment. Material and methods: We report the case of a previously healthy 42-year-old male presenting with a three-year history of progressively intractable headaches. Initial evaluation included contrast-enhanced CT, MRI of the brain, cavum, and clival region, and a whole-body 18F-FDG PET-CT. Diagnosis was established via endoscopic biopsy of the nasopharyngeal/clival lesion followed by histopathological and immunohistochemical analysis. The patient was managed with systemic corticosteroid monotherapy. Results: Imaging showed an aggressive, hypermetabolic mass with clival osteolysis1. Histopathology confirmed IgG4-RD (IgG4+/total IgG + ratio >40\%, >100 IgG4+ cells/HPF). The patient presented with Arnold's neuralgia-like symptoms-a previously unreported neurological finding in IgG4-RD. Corticosteroid monotherapy resulted in a rapid, near-complete resolution of his headaches and favorable clinical evolution. Discussion and conclusion: This case highlights a unique, previously undescribed Arnold's neuralgia-like presentation of IgG4-RD involving the clivus and nasopharynx. Our experience supports the efficacy of corticosteroids as first-line treatment, even with rare anatomical involvement. This report underscores considering IgG4-RD in the differential diagnosis of atypical head and neck pain, particularly with clival lesions.