Mikulicz Disease Revealing IgG4-Related Tubulointerstitial Nephritis: A Case Report and Literature Review

Background and Clinical Significance: IgG4-related disease (IgG4-RD) is a chronic fibroinflammatory, immune-mediated multisystem disorder that can mimic neoplastic, infectious, or autoimmune conditions. Among its head-and-neck manifestations, IgG4-related dacryoadenitis and sialadenitis, historically referred to as Mikulicz disease, should be distinguished from the classical Mikulicz syndrome, which describes secondary lacrimal and salivary gland enlargement due to other systemic disorders. Renal involvement, most commonly in the form of IgG4-related tubulointerstitial nephritis (IgG4-TIN), is less frequent but carries major prognostic implications because delayed diagnosis may lead to irreversible kidney damage. Case Presentation: A 49-year-old man with no relevant past medical history presented with a 2-year history of intermittent polyuria and foamy urine. Laboratory testing revealed advanced kidney dysfunction, with serum creatinine of 4.2 mg/dL, estimated glomerular filtration rate of 16 mL/min/1.73 m², and proteinuria of 2874 mg/day. Physical examination showed bilateral parotid enlargement, upper eyelid edema, lacrimal gland enlargement, and sicca symptoms, raising suspicion for IgG4-related dacryoadenitis and sialadenitis (Mikulicz disease). Further work-up demonstrated marked eosinophilia, polyclonal hypergammaglobulinemia, and significantly elevated serum IgG4 levels (3180 mg/dL), while infectious serologies and autoimmune studies were negative. Kidney biopsy revealed plasma cell-rich tubulointerstitial nephritis with lymphoplasmacytic and eosinophilic infiltrates, interstitial fibrosis, tubular atrophy, and more than 40 IgG4-positive plasma cells per high-power field, supporting the diagnosis of IgG4-related tubulointerstitial nephritis in the setting of systemic IgG4-RD. Treatment with prednisone followed by mycophenolate mofetil led to improvement in glandular manifestations and a partial reduction in proteinuria, but renal recovery remained incomplete. The patient subsequently developed a severe pulmonary infection complicated by sepsis and oligoanuric acute kidney injury superimposed on chronic kidney disease, and ultimately progressed to end-stage kidney disease requiring chronic maintenance hemodialysis. Conclusions: This case highlights that a Mikulicz disease phenotype may represent the initial manifestation of systemic IgG4-RD and should prompt evaluation for extraglandular involvement, particularly renal disease. In patients with glandular enlargement, eosinophilia, hypergammaglobulinemia, and unexplained renal dysfunction, IgG4-RD should be actively considered. Kidney biopsy remains essential for diagnostic confirmation and prognostic assessment, as delayed recognition may result in irreversible renal damage and progression to end-stage kidney disease.