A 50 year old chronic smoker with no other co-morbidities is admitted with us with altered behaviour for a month. He had undergone open cholecystectomy in a small private hospital for symptomatic gall stones 2 days before the present symptoms appeared. Following surgery the patient was extubated the same day but never regained his consciousness fully. He became listless, uncommunicative and incontinent. He was managed outside for three weeks and since there was no improvement he was brought to our center 1300 miles away. On evaluation he looked dehydrated, with BP of 110/70 mm Hg and disoriented but occasionally responsive. No focal neurological deficit was noted. A nephrology call was given for profound hyponatremia. His lab reports pertinent to hyponatremia are as follows :
Serum Na : 104 meq/l
Serum K : 3.1 meq/l
Serum osmolality : 236 mosm/l (calculated)
Urine osmolality : 117 mosm/l (calculated)
Urine Na : 12 meq/l
Urine K : 9 meq/l
Serum creat : 0.3 mg/dl
BUN : 5 mg/dl
Serum uric acid : 0.5 mg/dl
TSH and random cortisol : normal
Neuroimaging (plain CT and MRI) are essentially normal. HRCT chest revealed a right upper lobe cavitary lesion with significant mediastinal lymphadenopathy (2 cm) without central necrosis. BAL fluid is negative for malignant cells, AFB and FNAC report from the mediastinal lymph nodes is awaited.
Hyponatremia with a lung cavity with very dilute urine (though a little less than maximal) and low BUN and uric acid all suggest SIADH. But the very low urine Na and K suggest volume depletion. We treated the patient with hydration (NS) and hypertonic saline initially. The serum sodium was brought upto 125 over 3 days. Sensorium has improved very slightly. Presently he looks euvolemic and Tolvaptan has been started at 15mg OD today with fluid restriction.
My questions are :
1. Does the patient have both SIADH and volume depletion? Which one is predominant?
2. Are we justified in starting Tolvaptan considering the urine Na and K values?
3. Any other comments on the above scenario?