A 78-year-old man is brought to the hospital after being found down by his daughter at his home this morning. He was functional at baseline and last seen normal yesterday. He is a life-time smoker with chronic obstructive pulmonary disease (COPD) and was recently diagnosed with lung cancer. On physical examination, he is lethargic, has unsteady gait, and is confused. His temperature is 37.4°C, blood pressure is 127/94 mm Hg, pulse rate is 74 beats/min, and respiratory rate is 11 breaths/min. On examination, he has a normal jugular venous pressure, but he has decreased air entry at the lung bases. A CT head obtained shows age-related atrophic changes. Laboratory studies obtained are as follows:
Which of the following is the MOST appropriate next step in management of this patient?A. Fluid restriction to 800 mL
Correct Answer: E
The patient’s history and laboratory studies are consistent with hypotonic euvolemic hyponatremia. The important differentials are SIADH, adrenocortical insufficiency, polydipsia, physiological stimulus antidiuretic hormone (ADH) release (nausea, pain, anxiety), and hypothyroidism. Based on presentation, the most likely etiology of hyponatremia in this patient is SIADH. Patients with lung cancer, particularly small cell lung cancer, have a reported incidence of up 18.9%. SIADH is caused by the secretion of ADH from the posterior pituitary gland or unregulated ectopic production by tumor cells. Elevated levels of ADH lead to hyponatremia and hypoosmolality by decreasing the renal excretion of free water. While not all cases of hyponatremia require correction with hypertonic saline, this patient has acute onset, severe hyponatremia and has moderate symptoms. Symptoms of hyponatremia include headache, nausea, vomiting, confusion, disorientation, and seizures. Though his sodium needs correction with hypertonic saline, the sodium should not be corrected by more than 9 mEq/L in 24 hours to avoid osmotic demyelination syndrome (ODS). It is important to note that neurological effects related to ODS can take up to 1 week to manifest, including dysarthria and dysphagia.