Case Study for Chapter

Water Intoxication

A 60-year-old woman with a long history of mental illness was institutionalized after a violent argument with her son. She experiences visual and auditory hallucinations and, on one occasion, ran naked through the ward screaming. She refuses to eat anything since admission, but maintains a good fluid intake. On the fifth hospital day, she complains of a slight headache and nausea and has three episodes of vomiting. Later in the day, she is found on the floor in a semiconscious state, confused and disoriented. She is pale and had cool extremities. Her pulse rate is 70/min and blood pressure is 150/100

mm Hg. She is transferred to a general hospital and, during transfer, has three grand mal seizures and arrives in a semiconscious, uncooperative state. A blood sample reveals a plasma [Na+] of 103 mEq/L. Urine osmolality is 362 mOsm/kg H2O and urine [Na+] is 57 mEq/L. She is given an intravenous infusion of hypertonic saline (1.8% NaCl) and placed on water restriction. Several days after she had improved, bronchoscopy is performed.

Questions

1. What is the likely cause of the severe hyponatremia?

2. How much of an increase in plasma [Na+] would an infusion of 1 L of 1.8% NaCl (308 mEq Na+/L) produce? Assume that her total body water is 25 L (50% of her body weight). Why is the total body water used as the volume of distribution of Na+, even though the administered Na+ is limited to the ECF compartment?

3. Why is the brain so profoundly affected by hypoosmolality? Why should the hypertonic saline be administered slowly?

4. Why was the bronchoscopy performed?

Answers to Case Study Questions for Chapter 24

1. The problem started with ingestion of excessive amounts of water. Compulsive water drinking is a common problem in psychotic patients. The increased water intake, combined with an impaired ability to dilute the urine (note the inappropriately high urine osmolality), led to severe hyponatremia and water intoxication.

2. Addition of 1 L of 308 mEq Na+/L to 25 L produces an increase in plasma [Na+] of 12 mEq/L. The total body water is used in this calculation because when hypertonic NaCl is added to the ECF, it causes the movement of water out of the cell compartment, diluting the extracellular Na+.

3. Because the brain is enclosed in a nondistensible cranium, when water moves into brain cells and causes them to swell, intracranial pressure can rise to very high values. This can damage nervous tissue directly or indirectly by impairing cerebral blood flow. The neurological symptoms seen in this patient (headache, semiconsciousness, grand mal seizures) are consequences of brain swelling. The increased blood pressure and cool and pale skin may be a consequence of sympathetic nervous system discharge resulting from increased intracranial pressure. Too rapid restoration of a normal plasma [Na+] can produce serious damage to the brain (central pontine myelinolysis).

4. The physicians wanted to exclude the presence of a bronchogenic tumor, which is the most common cause of SIADH. No abnormality was detected. Today, a computed tomography (CT) scan would be performed first.

References

Grainger DN. Rapid development of hyponatremic seizures in a psychotic patient. Psychol Med 1992;22:513-517.

Goldman MB, Luchins DJ, Robertson GL Mechanisms of altered water metabolism in psychotic patients with polydipsia and hyponatremia. N Engl J Med 1988;318:397-403.

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Responses

  • Rudolph
    Why is the brain so profoundly affected by hypoosmolality and why should the hypertonic saline be ad?
    15 days ago

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