Severe hyponatremia following low-dose cyclophosphamide in a patient with breast cancer: a case report

Cyclophosphamide is a commonly used alkylating chemotherapy agent in the treatment of neoadjuvant, adjuvant, and metastatic breast cancer. Sudden onset, symptomatic, and severe hyponatremia (defined as serum sodium < 125 mmol/L) is a rare but potentially life-threatening complication associated with cyclophosphamide use. We report a case of severe acute hyponatremia in a 47-year-old female patient diagnosed with invasive ductal breast carcinoma. The patient received low-dose cyclophosphamide as part of adjuvant chemotherapy and experienced generalized tonic-clonic seizures within the first 24 h. Laboratory investigations revealed severe hyponatremia with a serum sodium level of 111 mmol/L. Neurological imaging and endocrine evaluations ruled out alternative causes. Cyclophosphamide-induced SIADH was considered the most likely cause of the patient's severe hyponatremia. Hypertonic saline therapy was initiated, leading to rapid normalization of serum sodium levels and complete resolution of neurological symptoms. Following discontinuation of thiazide-containing antihypertensive therapy, subsequent chemotherapy courses were completed without recurrence of hyponatremia. This case highlights the potential for life-threatening hyponatremia even with low-dose cyclophosphamide use and underscores the critical importance of early electrolyte monitoring, particularly in patients receiving concomitant diuretics.