Sinus bradycardia as an atypical presentation of severe hypercalcemia: a case report and a literature review.

Introduction and importance: Severe hypercalcemia typically produces QT interval shortening or ventricular arrhythmias, whereas bradyarrhythmias are exceptionally rare. The coexistence of hypercalcemia with acute kidney injury (AKI) - a condition that usually causes hypocalcemia - further underscores diagnostic complexity. Case presentation: An 85-year-old man presented with AKI, confusion, and symptomatic sinus bradycardia (40 bpm) without atrioventricular block or QT prolongation. Laboratory studies showed markedly elevated corrected calcium, suppressed parathyroid hormone, and increased parathyroid hormone-related peptide. Chest CT revealed a 7 × 8 mm right-lung nodule. These findings are suggestive of humoral hypercalcemia of malignancy, although the etiology remains unconfirmed in the absence of tissue diagnosis. Treatment with intravenous hydration and zoledronic acid led to normalization of calcium, restoration of sinus rhythm (61 bpm), and improvement of renal function, obviating the need for pacing. Discussion: This case is distinctive for demonstrating reversible sinus bradycardia in the setting of severe hypercalcemia and acute kidney injury - an atypical and physiologically unexpected combination. The biochemical profile raises suspicion for a paraneoplastic process, though this remains unconfirmed. The case underscores the importance of recognizing hypercalcemia as a reversible cause of bradyarrhythmia and considering malignancy in the appropriate clinical context when PTH is suppressed. Conclusion: Hypercalcemia-induced bradycardia, though rare, is fully reversible with prompt metabolic correction. Awareness of this atypical presentation can prevent misdiagnosis, unnecessary pacemaker implantation, and delayed recognition of paraneoplastic disease.