HYPONATREMIA   49A

A 75-year-old man with small cell carcinoma of the lung presents to the emergency department with altered mental status. The patient’s wife states that over the past few days, he has become progressively more lethargic. His appetite has been poor, but he willingly ingests water, consuming 2 to 3 quarts per day. On examination, the patient is a cachectic man in mild respiratory distress. He is lethargic but arousable. He is oriented to person only. His temperature is 38°C, blood pressure is 110/60 mm Hg, heart rate is 88 beats/min, respiratory rate is 18 breaths/min, and oxygen saturation is 96% (on 3 L of O₂). His mucous membranes are moist. Breath sounds are decreased in the left lower posterior lung field with rales in the upper half. Extremities are without edema. Neurologic examination shows only bilateral positive Babinski reflexes and asterixis. Laboratory studies reveal a serum sodium level of 118 mEq/L.

What are the salient features of this patient’s problem? How do you think through his problem?

Salient features: Altered mental status; lethargy; increased free water intake; low serum sodium; an underlying diagnosis (lung cancer) associated with the syndrome of inappropriate antidiuretic hormone secretion (SIADH)

How to think through: In “true” hyponatremia (hypotonic hyponatremia), which hormone causes the problem? (ADH.) In normal physiology, how does the body regulate osmolarity? (By retaining water via ADH.) How does the body regulate intravascular volume? (By retaining sodium via the reninangiotensin–aldosterone axis, but ADH can be a powerful regulator of volume as well). What laboratory characteristic is shared by all patients with hyponatremia? (Urine osmolarity > serum osmolarity.) How do we classically break down the differential diagnosis of hyponatremia? (By volume status.) What are indications of volume status in this case? (Blood pressure, heart rate, mucous membranes, absence of edema.) What are the causes of euvolemic hyponatremia? (Water intoxication, SIADH.) What diseases are associated with SIADH? What is the appropriate initial treatment? (Free water restriction.)

HYPONATREMIA   49B

What are the essentials of diagnosis and general considerations regarding hyponatremia?

Essentials of Diagnosis

Serum Na⁺ is below 130 mEq/L (<130 mmol/L).

Hyponatremia usually reflects excess water retention relative to sodium rather than sodium deficiency.

The patient’s volume status and serum osmolality are essential to determine the cause.

Hypotonic fluids commonly cause hyponatremia in hospitalized patients.

General Considerations

It is the most common electrolyte abnormality observed in a general hospitalized population.

Most cases reflect water imbalance and abnormal water handling, not sodium imbalance.

ADH plays a primary role in the pathophysiology of hyponatremia.

A diagnostic algorithm using serum osmolality and volume status separates the causes of hyponatremia into therapeutically useful categories.

Stay updated, free articles. Join our Telegram channel

Join