Immunoglobulin G4-related Inflammatory Pseudotumor Involving the Oculomotor Nerve: A Case Report.

Inflammatory pseudotumor involving the cranial nerves is extremely rare, and delayed diagnosis may cause irreversible neurological deficits. To our knowledge, this represents the third reported case of an inflammatory pseudotumor involving the cavernous sinus with extension along the oculomotor nerve. A 56-year-old woman presented with a visual field defect in her left eye, and her condition was initially managed conservatively because of her mild clinical presentation. Initial magnetic resonance imaging revealed a homogeneously enhanced dumbbell-shaped mass lesion extending from the left cavernous sinus through the superior orbital fissure into the intraconal orbital space. At 3-year follow-up, the patient's condition worsened during routine outpatient visits, with left visual acuity decreasing to light perception. Surgical biopsy revealed a fibrous mass involving the cisternal portion of left oculomotor nerve and extending into the cavernous sinus. Histopathological examination and laboratory findings confirmed the diagnosis of an immunoglobulin G4-related inflammatory pseudotumor. Oral corticosteroid therapy caused marked radiological regression of the lesion; however, visual impairment was not alleviated. This case highlights that delayed histopathological confirmation may produce irreversible neurological deficits despite a favorable radiological response to corticosteroid therapy.