Case Presentation: A 63-year-old male presents to the hospital with complaints of one-day history of weakness, dizziness. His outpatient blood work demonstrated a serum sodium level of 124 mEq/L, and a creatinine of 3.1 mg/dL. His medical history was significant for high-grade urothelial bladder carcinoma status post radical cysto-prostatectomy, neobladder urinary diversion, and MVAC (methotrexate, vinblastine, doxorubicin, and cisplatin) neoadjuvant chemotherapy for 6 months which he completed successfully in July 2020. The patient was hospitalized three times for hyponatremia and profound prerenal azotemia, hyperchloremic metabolic acidosis, and hyperkalemia. On all occasions, the lab abnormalities were reversible with intravenous isotonic fluids, and he was discharged home from his first three admissions with sodium bicarbonate tablets and a high salt diet. Despite adequate compliance and good oral intake, he continued to present with symptomatic hyponatremia and volume depletion. Although profoundly dehydrated on presentation, his urine sodium and osmolality were found to be elevated during these episodes. During the current admission, the patient presented with serum sodium of 124 mEq/L, BUN of 143 mg/dL, creatinine of 3.1 mg/dL, urine osmolality of 380 mOsm/kg and urine sodium of 81 mEq/L (Table 1). This scenario of increased loss of sodium in the setting of hypovolemia suggests renal salt wasting. The possibility of Cisplatin induced renal salt wasting was considered. The patient was started on Fludrocortisone 0.1mg twice daily, oral salt tablets 1g QID, sodium bicarbonate tablets 650mg TID and normal saline at 150ml/h. During his five-day hospital course, his renal parameters steadily improved, and his serum sodium improved from 124 mEq/L to 139 mEq/L. The patient was discharged on the aforementioned medications and with recommendations to consume electrolyte rich fluids, a high protein and high sodium diet and obtain weekly chemistries. Although he has had mild fluctuations in his serum chemistries based on compliance and his ability to hydrate, further hospitalizations have been avoided over a nine-month follow-up.

Discussion: Cisplatin is a chemotherapeutic agent used in the treatment of various cancers including carcinomas, germ cell tumors, lymphomas, and sarcomas. Cisplatin is known to be associated with a multitude of renal problems including electrolyte disturbances, renal tubular acidosis, acute kidney injury with reduced glomerular filtration, Fanconi syndrome, nephrogenic diabetes insipidus, and renal salt wasting syndrome (RSWS). In a recent literature review, Hamdi et al. analyzed 17 patients who were reported with cisplatin induced RSWS. Patients developed RSWS as soon as 12 hours and up to 4 months following cisplatin therapy (5). Literature on the management of RSWS is sparse. Rehydration and sodium chloride remain the standard of care.

Conclusions: The diagnosis of RSWS is difficult and is commonly misdiagnosed as SIADH as the conditions share similar laboratory values such as hypotonic hyponatremia and increased urine sodium. Volume assessment is the most critical step in differentiating the conditions as patients with SIADH are euvolemic and those with RSWS are hypovolemic (2) (table 2). Literature on the management of RSWS is sparse. Volume and sodium chloride remain the standard of care (3). Therefore, the diagnosis of RSWS should be considered in patients with hyponatremia and hypernatriuria, especially in patients with abnormal renal function.

IMAGE 1: Table 1: Lab values on admission and discharge

IMAGE 2: Table 2: Differentiation of RSW and SIADH