Sodium disorders are commonly encountered in clinical practice and are frequently misunderstood. Abnormal serum sodium levels are due to an imbalance in free water. Hyponatremia and hypernatremia can be asymptomatic if mild and chronic; however, acute and severe changes in sodium levels may cause substantial symptoms. In acute mild hyponatremia, symptoms can include confusion, vomiting, and weakness. In severe cases, seizures can occur. Determining the patient’s volume status helps to determine the underlying etiology and appropriate treatment strategy. Euvolemic hyponatremia is the most common presentation due to the prevalence of syndrome of inappropriate antidiuretic hormone. Depending on severity, hyponatremia can be managed in the outpatient or inpatient setting. Management is directed at treating underlying causes and relieving severe symptoms while decreasing the risk of serious adverse effects from treatment. In patients with chronic hyponatremia without severe symptoms, rapid correction of serum sodium should be avoided to minimize the risk of osmotic demyelination syndrome. Hypernatremia is caused by a loss of free water, inadequate water intake, or salt overload. Patients with acute hypernatremia may present with fatigue, signs of dehydration, and weakness. Symptomatic acute and severe hypernatremia require inpatient admission and management of free water deficit with intravenous hypotonic fluids.
Olson A. Acid-Base and Electrolyte Disorders: Sodium Disorders. FP Essent. 2026;565:7-13.
Case 1. LT is a 63-year-old woman admitted to the hospital for acute shortness of breath. During the physical examination, you note bilateral inspiratory crackles at the lung bases and lower extremity edema. A chest radiograph shows pulmonary congestion. You receive a call from her nurse that the serum sodium is low at 125 mEq/L (125 mmol/L).
Physiology
Sodium disorders are commonly encountered in clinical practice and are frequently misunderstood. They differ from other electrolyte disorders in that abnormal serum sodium levels are not due to a sodium imbalance but to an imbalance of free water. Osmolality and tonicity are important concepts for understanding fluid balance, and sodium is the major contributor to both.
Plasma osmolality is a measure of solute-water balance and is expressed in milliosmoles per kilogram of plasma water (ie, mOsm/kg). Tonicity is the concentration of functional osmoles (ie, osmoles that do not freely cross cell membranes), but it is not easily measured, so osmolality is used as a surrogate measure.
Osmolality can be estimated by calculation or measured directly. A difference of more than 10 mOsm/kg (10 mmol/kg) between estimated and measured values indicates the presence of unmeasured osmoles.1 Normal serum sodium ranges from 135 to 145 mEq/L (135-145 mmol/L).
Tonicity is primarily regulated by vasopressin (ie, antidiuretic hormone), which is produced in the hypothalamus, stored in the pituitary gland, and secreted when hypertonicity triggers hypothalamic osmoreceptors. Vasopressin release is also stimulated by decreased perfusion of carotid baroreceptors due to hypotension, hypovolemia, or conditions with low effective arterial circulation (eg, heart failure, cirrhosis). Vasopressin increases renal reabsorption of water in the collecting ducts, thereby concentrating urine and diluting plasma. Factors such as pain, nausea, and certain malignancies can stimulate vasopressin release, leading to syndrome of inappropriate antidiuretic hormone (SIADH).1
Read the full edition
Get immediate access, anytime, anywhere.
Choose a single edition, 1-year or 2-year full-access subscription.
Earn 4 CME credits for this edition.
