A 75-year-old man with terminal small cell carcinoma of the lung presents to the emergency department with altered mental status. The patient’s wife, who cares for him at home, states that he is quite...

A 75-year-old man with terminal small cell carcinoma of the lung presents to the emergency department with altered mental status. The patient’s wife, who cares for him at home, states that he is quite weak at baseline, requiring assistance with all activities of daily living. Over the past few days, he has become progressively more lethargic. She has been careful to adequately hydrate him, waking him every 2 hours to give him water to drink. His appetite has been poor, but he willingly ingests the water, consuming 2–3 quarts per day. He is taking morphine for pain and dyspnea. On examination, the patient is a cachectic white man in mild respiratory distress. He is lethargic but arousable. He is oriented to person only. Vital signs reveal a temperature of 38 °C, blood pressure of 110/60 mm Hg, heart rate of 88 bpm, respiratory rate of 18/min, and oxygen saturation of 96% on 3 L of oxygen. On head-neck examination, pupils are 3 mm and reactive, scleras are anicteric, and conjunctivas are pink. Mucous membranes are moist. Neck is supple. There are decreased breath sounds in the left lower posterior lung field and rales in the upper half. Cardiac examination shows a regular heartbeat without murmur, gallop, or rub. Abdomen is benign without masses. Extremities are without edema, cyanosis, or clubbing. Neurologic examination shows only bilateral positive Babinski reflexes and asterixis. Laboratory studies reveal a serum sodium level of 118 mEq/L.

Questions

1. 
   
   A
   . What conditions are associated with SIADH? Which are present in this patient?


2. 
   
   B
   . What pathophysiologic mechanism produces SIADH?


3. 
   
   C
   . What is the cause of this patient’s lethargy, confusion, and asterixis?


4. 
   
   D
   . How would you treat this patient’s hyponatremia?