What is the cause of hyponatremia in a patient with pneumonia, given the patient's serum and urine electrolyte and osmolality levels?

From the Guidelines

The cause of hyponatremia in this patient is likely the syndrome of inappropriate antidiuresis (SIAD), given the patient's recent diagnosis of pneumonia, which is a known cause of SIAD, as nonosmotic stimuli of arginine vasopressin (AVP) release include pulmonary disorders, such as pneumonia 1.

Key Findings

• The patient's pneumonia is a common childhood condition that can lead to SIAD or SIAD-like states, resulting in water retention and subsequent hyponatremia 1.
• The administration of hypotonic maintenance IVFs in the setting of elevated AVP concentrations is a common cause of hospital-acquired hyponatremia, which can be fatal or lead to irreversible brain injury if inadequately treated 1.
• Children are at particularly high risk of developing symptomatic hyponatremia due to their larger brain/skull size ratio 1.

Management

• To manage this patient's hyponatremia, it is recommended to restrict fluid intake and monitor serum sodium levels closely.
• If the patient's sodium level is severely low or symptomatic, consider administering hypertonic saline with close monitoring of serum sodium levels.
• The underlying pneumonia should be treated with antibiotics and the patient's clinical status should be closely monitored.
• It is also important to review the patient's medication list to ensure that they are not taking any medications that could be contributing to the hyponatremia, such as thiazide diuretics or selective serotonin reuptake inhibitors (SSRIs) 1.

From the Research

Causes of Hyponatremia in Pneumonia Patients

• Hyponatremia in patients with pneumonia can be caused by the Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH) 2, 3.
• SIADH can occur secondary to pulmonary disease, among other causes, and is characterized by a clinical euvolaemic state with low serum sodium and osmolality, raised urine sodium and osmolality, and exclusion of pseudohyponatraemia and diuretic use 2.
• The diagnosis of SIADH should be considered if the five cardinal criteria are fulfilled: hypotonic hyponatraemia, natriuresis, urine osmolality in excess of plasma osmolality, absence of oedema and volume depletion, and normal renal and adrenal function 3.

Role of Serum and Urine Electrolyte and Osmolality Levels

• The diagnosis of SIADH is based on the presence of low serum sodium and osmolality, and raised urine sodium and osmolality 2, 3.
• The severity of hyponatraemia and the rate of its development can affect the clinical manifestations of SIADH, with more severe and rapid onset leading to more significant symptoms 3, 4.

Treatment and Management

• Fluid restriction is the mainstay of treatment for SIADH, with a goal of restricting fluid intake to 800-1200 mL/24 hours 2.
• Patients with severe hyponatraemia and symptoms of altered mental state or seizures should be admitted to hospital for monitoring of fluid restriction and consideration of hypertonic saline 2.
• Careful monitoring is essential to avoid rapid correction of hyponatraemia, which can precipitate osmotic demyelination 2, 4.