Severe Hyponatremia Triggered by Immune Checkpoint Inhibitor Therapy in a Patient with Mulvihill-Smith Syndrome

Background/Objective Immune checkpoint inhibitors (ICI), including PD-1, PD-L1, and CTLA-4 inhibitors, upregulate T-cell responses against tumor cells and are becoming a cornerstone in the treatment of various advanced solid and hematological cancers. Mulvihill-Smith Syndrome (MSS) is a rare genetic syndrome that has been associated with metabolic abnormalities and early-onset tumors, including malignancies. We report the first known case of ICI-induced hyponatremia attributable to SIADH in a patient with MSS. Case Report A 23-year-old female patient with MSS and hepatocellular carcinoma (HCC) presented with recurrent hyponatremia. Assessment of fluid status and electrolytes revealed a euvolemic, hypotonic process consistent with SIADH shortly after initiating adjuvant therapy with atezolizumab, a PD-L1 inhibitor. Discussion Endocrine etiologies for euvolemic hypotonic hyponatremia, including adrenal insufficiency and hypothyroidism, were excluded. The diagnosis of SIADH was confirmed based on electrolyte and osmolality studies. Sodium levels normalized with fluid restriction. Given the onset of hyponatremia 30 days after atezolizumab initiation, we posit that atezolizumab triggered severe hyponatremia due to SIADH. Conclusion With the expanding utilization of ICIs, including in patients predisposed to malignancies such as MSS, vigilant monitoring for ICI-mediated electrolyte imbalances is crucial. Monitoring for hyponatremia and SIADH in the setting of ICI therapy is recommended.