The clinical findings of SIADH are primarily due to hyponatremia. In some cases, the patient will be asymptomatic. Patients could complain of fatigue, emesis, myalgias, and poor appetite. The extent of symptoms depends upon the rate of development of the hyponatremia and level of the serum sodium. As sodium levels fall below 100 mEq/L, patients can develop altered mental status, seizures, psychosis, lethargy, or coma.

CRITICAL DECISION

What are the treatment options for SIADH?

Treatment of SIADH depends on the severity of symptoms and the acuity of onset of the hyponatremia. Mild degrees of hyponatremia may not necessitate any immediate treatment. Mild fluid restriction until followup may be appropriate. The underlying malignancy should be treated. In cases unresponsive to fluid restriction, therapy with demeclocycline may be started to induce a reversible nephrogenic diabetes insipidus (DI) to counteract the influence of the excess vasopressin. If the SIADH is due to chemotherapeutic agents, the patient’s therapeutic regimen might need to be altered.

In those patients with more severe degrees of hyponatremia or those with significant CNS symptoms related to their hyponatremia, normal saline can be initiated, or for those with seizures and altered mental status, 3% hypertonic saline (300 to 500 mL at a time over 3 to 4 hours) may be administered followed by furosemide to control intravascular volume.¹⁷ It is desirable to control the rate of correction of serum sodium by no more than 0.5 to 1 mEq/L/hour in order to prevent CNS disorders such as central pontine myelinolysis. These patients will require admission to an ICU.

Tumor Lysis Syndrome

Tumor lysis syndrome is an oncologic emergency caused by a massive destruction of cancer cells with ensuing release of nucleic acids, potassium, and phosphate into the circulation. Breakdown of the nucleic acids into uric acid leads to hyperuricemia. The precipitation of uric acid into the renal tubules can lead to renal failure. Tumor lysis syndrome most commonly occurs in cancer types with a high proliferative rate, large tumor burden, or those particularly sensitive to cytotoxic therapy. These include acute lymphoblastic leukemia and Burkitt or other non-Hodgkin lymphomas, but other tumor types have been implicated.

Specific laboratory abnormalities have been proposed in 2004 to define tumor lysis syndrome.¹⁸ These are an elevated uric acid, above 8 mg/dL; a serum potassium of more than 6 mmol/L or a 25% increase from baseline; serum phosphate elevated above 6.5 mg/dL in children or above 4.5 mg/dL in adults; or a depressed serum calcium, lower than 7 mg/dL or a 25% decrease from baseline. Serum lactate dehydrogenase is typically elevated.¹⁹