Approach to the Patient: Hyponatremia and the Syndrome of Inappropriate Antidiuresis (SIAD)

Abstract

Hyponatremia is the most common electrolyte disturbance seen in clinical practice, affecting up to 30% of acute hospital admissions, and is associated with significant adverse clinical outcomes. Acute or severe symptomatic hyponatremia carries a high risk of neurological morbidity and mortality. In contrast, chronic hyponatremia is associated with significant morbidity including increased risk of falls, osteoporosis, fractures, gait instability, and cognitive decline; prolonged hospital admissions; and etiology-specific increase in mortality. In this Approach to the Patient, we review and compare the current recommendations, guidelines, and literature for diagnosis and treatment options for both acute and chronic hyponatremia, illustrated by 2 case studies. Particular focus is concentrated on the diagnosis and management of the syndrome of inappropriate antidiuresis. An understanding of the pathophysiology of hyponatremia, along with a synthesis of the duration of hyponatremia, biochemical severity, symptomatology, and blood volume status, forms the structure to guide the appropriate and timely management of hyponatremia. We present 2 illustrative cases that represent common presentations with hyponatremia and discuss the approach to management of these and other causes of hyponatremia.

Keywords: SIAD; fluid restriction; hypertonic saline; hyponatremia; tolvaptan.

Figure 1.

Water homeostasis. Abbreviations: AVP, arginine vasopressin; pOsm, plasma osmolality. Adapted from Hannon et al (8).

Figure 2.

Renal response to vasopressin. Abbreviations: AQ-2, aquaporin 2 channel; ATP, adenosine triphosphate; AVP, arginine vasopressin; cAMP, cyclic adenosine monophosphate; PKA, protein kinase A; V2R, vasopressin receptor type 2.

Figure 3.

Classification of the syndrome of inappropriate antidiuresis (types A-D) according to pattern of vasopressin (AVP) secretion (shaded area represents normal AVP response to rising plasma osmolality (POsm).

Figure 4.

Morbidity associated with both acute severe and chronic hyponatremia and its treatment. Abbreviation: ODS, osmotic demyelination syndrome. Adapted from Ellison et al, Adrogué et al, Verbalis et al, and Sterns et al (14, 29, 78, 115).

Figure 5.

Spectrum of symptoms associated with hyponatremia.