SUN-052 - Igg4-Related Hypophysitis with High Igg4 Plasma Cell Infiltration and Atypical Histopathological Findings of Acute Inflammation and Fibrinopurulence

Background:
IgG4-related hypophysitis (IgG4-RH) is an increasingly recognized inflammatory pituitary disease that accounts for approximately 4% of all cases of hypopituitarism. Diagnosis requires histopathologic confirmation with >10 IgG4-positive plasma cells per high-power field (HPF) and >40% IgG4/IgG ratio. We present a case of IgG4-RH with an exceptionally high IgG4 plasma cell count (~100 cells/HPF, near-100% IgG4/IgG ratio) and an unusual clinical trajectory featuring spontaneous radiologic regression followed by explosive disease progression.

Clinical

Case:

A 60-year-old male presented with headaches, polyuria, and polydipsia. Water deprivation testing confirmed central diabetes insipidus (sodium 148 mmol/L, serum osmolarity 324 mosm/L). Initial MRI demonstrated a 9 mm cystic pituitary lesion with infundibular thickening. Serum IgG4 elevated at 196 mg/dL (upper normal 125 mg/dL). Remarkably, follow-up MRI one month later showed spontaneous regression to 2 mm without treatment. Despite radiologic improvement, the patient developed progressive panhypopituitarism requiring steroid, thyroid, and testosterone replacement.
Seven months later, the patient presented with severe headache and altered consciousness. Imaging revealed dramatic disease progression with a 32 x 17 x 16 mm mass demonstrating necrotic and cystic changes, sellar and suprasellar extension, and mass effect on the optic chiasm, hypothalamus, and third ventricle.
Transsphenoidal resection of lesion and open biopsy was performed, with initial pathology revealing anterior pituitary tissue with severe mixed acute and chronic inflammation, focal necrosis, and fibrinopurulent material. Initial tumor board consensus was lymphocytic hypophysitis. Subsequent IgG4 immunohistochemistry demonstrated 100 IgG4-positive plasma cells per HPF with an IgG4/IgG ratio of nearly 100%, establishing IgG4-RH.
High-dose glucocorticoid therapy (prednisone 120 mg daily) was initiated with gradual taper. Disease relapse occurred when prednisone was reduced to 7.5 mg daily, requiring dose escalation. Follow-up MRI demonstrated interval decreased enhancement about the infundibulum, hypothalamus, and floor of the third ventricle. The patient was ultimately initiated on inebilizumab after multiple relapses.

Conclusion:
This case of histopathologically confirmed IgG4-RH with unusually high IgG4 plasma cell infiltration demonstrates the unpredictable and rapidly changing clinical course, diagnostic challenges, and importance of immunohistochemistry in all cases of hypophysitis. The dramatic fluctuation from spontaneous regression to aggressive progression underscores the need for close monitoring and early consideration of immunosuppressive therapy in IgG4-RH.

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