Position Effect on Antidiuretic Hormone

HYPONATREMIA occurs commonly in patients with severe intracranial disease or head injury who are maintained on a normal intake of water. Such patients typically have hyperosmotic urine and urine-to-serum osmolal ratios greater than one. These findings could result from dehydration and sodium loss. However, when they occur in hydrated patients, particularly those who have received an adequate intake of sodium, it is assumed that the blood level of antidiuretic hormone (ADH) is elevated; a dilute urine, therefore, cannot be elaborated and, if the water intake is maintained at the usual level, hyponatremia occurs. This series of events was termed the "syndrome of inappropriate antidiuretic hormone secretion" (SIADH) by Schwartz et al¹in 1957. Since that time, an impressive body of direct and indirect evidence has shown that the blood ADH level is elevated in these circumstances. It is generally believed that the lesion producing the intracranial disease