IgG4-related disease of the mastoid: a rare presentation of a novel diagnosis.

IgG-related disease (IgG-RD) is an inflammatory condition characterised by infiltration of tissue by IgG-positive plasma cells. This is the seventh reported case of IgG-RD affecting the mastoid and informs clinicians in diagnosing patients affected by this rare condition.A woman in her 20s presented with unilateral otalgia, hearing loss and vertigo. She deteriorated despite antibiotic therapy and cross-sectional imaging revealed a destructive extra-axial lesion of the mastoid cells. Biopsy confirmed a diagnosis of IgG-RD. She was successfully treated with prednisolone and azathioprine.Inflammatory conditions should be considered in patients with persistent middle ear symptoms after infection and malignancy are excluded. Delays in diagnosis can lead to irreversible mass effects and may occur as current diagnostic criteria exclude mastoid-specific features.IgG-RD remains a rare diagnosis. To avoid significant effects on a patients' quality of life, prompt multidisciplinary treatment is vital alongside development of diagnostic criteria specific to otolaryngology.