SIADH in the Elderly

ABSTRACT & COMMENTARY

Synopsis: Limiting CT scans in favor of good history taking and simple diagnostic testing is a reasonable approach to elderly patients with SIADH.

Source: Hirshberg B, Ben-Yehuda A. Am J Med 1997;103:270-273.

The syndrome of inappropriate antidiuretic hormone secretion (SIADH) is one of the major causes of hyponatremia in hospitalized patients. Characterized clinically by normovolemia, SIADH is associated with numerous drugs, tumors, and central nervous system and pulmonary disorders. While the specific features in the elderly with SIADH are not well studied, they are more likely to be symptomatic, and mortality appears to be related to the underlying cause rather than the degree of hyponatremia. Thus, the authors undertook this study in the elderly with SIADH to determine the following: clinical features, outcomes, and the extent of evaluation that was warranted.

The records at a large primary and tertiary care hospital were reviewed retrospectively from 1980 to 1994. The inclusion criteria for the elderly with SIADH included: 1) serum sodium level less than 130 mEq/L; 2) age 65 years or older; 3) urine osmolarity greater than serum osmolarity; 4) urinary Na+ greater than 30 mEq/L; and 5) euvolemic state with normal renal function and absence of edema. Fifty patients fulfilled the criteria, and clinical data, treatment, and outcome information were collected.

The average age of the patients was 77 years, and the gender distribution was about equal. The five major reasons for referral to hospital were weakness, general deterioration, fever, confusion, and seizures. The initial neurological presentation was variable: 48% were fully alert, 42% were stuporous, and 10% had seizures. The neurological signs on admission were not predictive of the degree of hyponatremia, and serum sodium levels were 119 mEq/L in those with sensory impairment, compared with 117 mEq/L in those without sensory symptoms and signs. Most patients underwent a comprehensive work-up including head, chest, and abdominal CT scans, EEG, thyroid, and adrenal function tests. The 29 head CT scans done showed no new abnormalities, and only one of 27 patients who underwent a chest or abdominal CT scan showed a lung cancer. The diagnostic procedure with the highest yield was chest x-ray, which revealed pneumonia in nine patients (18%). Drugs accounted for 12% of SIADH in the elderly (4 secondary to carbamazepine, 1 each to chlorpropamide and fluoxetine). The majority of cases (60%) were considered idiopathic, as no specific cause for SIADH could be found.

Simple treatment strategies were useful. About two-thirds of the patients became normonatremic with fluid restriction, and only 10% of patients needed hypertonic saline. Twenty percent of patients required specific therapy for an underlying cause, such as pneumonia or hypothyroidism.

Patient outcomes were good, and there was no mortality due to hyponatremia. Two patients died of sepsis, after their serum sodium normalized. The average hospital length-of-stay was 12.8 days, and there was no correlation with the degree of hyponatremia.

COMMENT BY KAMALJIT SETHI, MD

There are several important facts of note in the elderly with SIADH in this study:

1. The benign course and absence of mortality related to hyponatremia.

2. The high prevalence of idiopathic causes (60%).

3. The most common treatable cause was pneumonia (18%).

4. The importance of drug history (12% had drug-related SIADH).

5. The paucity of diagnostic information from 56 CT scans in 50 patients that had any therapeutic implication (1 patient showed lung cancer), suggesting that a comprehensive work-up may not be warranted.

The high percentage of idiopathic cases merits some attention. Other studies have noted fewer idiopathic cases. (See Table.) The high percentage of idiopathic SIADH in the elderly may be related to three common factors: mild hypovolemia (which may be missed clinically), pain (commonly due to DJD), and orthostatic hypotension (occurs in up to one-third of patients > 65 years). All three factors are potent simulators of ADH secretion.

Table

SIADH in the elderly

Study No. of Patients Idiopathic (%)

Hochman1 13 39%

Miller2 27 26%

Hirshberg, Ben-Yehuda* 50 60%

* Present Study

Should the data from this study cause us to spurn expensive technology in favor of good old history taking and simple diagnostic testing such as chest x-ray in elderly patients with SIADH? The evidence suggests that limiting CT scans, at the very least, in such patients is a reasonable approach. It would also reduce the cost of care.

References

1. Hochman I, et al. Isr J Med Sci 1989;25:73-76.

2. Miller M, et al. JAGS 1996;44:404-408.

Gemfibrozil treatment of post-CABG patients with isolated low HDL has been associated with marked reduction in subsequent clinical events despite lack of any demonstrated angiographic benefit.

The syndrome of inappropriate antidiuretic hormone secretion in the elderly is most commonly caused by:

a. drug therapy.

b. infections.

c. idiopathic etiology.

d. none of the above.