Hyponatremia is an electrolyte disorder defined by a serum sodium concentration falling below 135 milliequivalents per liter (mEq/L). Sodium is a primary determinant of fluid movement and distribution across cell membranes, playing a part in maintaining normal blood pressure, transmitting nerve impulses, and facilitating proper muscle function. This imbalance typically represents an excess of water relative to sodium, requiring prompt medical attention.
The First Step in Diagnosis
The initial detection of low sodium levels occurs in initial care settings. Patients may first present with non-specific symptoms, including a persistent headache, general fatigue, or mental confusion. These symptoms arise from the shifting of water into brain cells as the body attempts to balance the low sodium concentration, causing mild cerebral swelling.
To confirm the condition, the initial care provider will order a blood test, specifically a basic metabolic panel, which measures the concentration of serum sodium. A result below 135 mEq/L confirms the diagnosis. Further testing, such as urine sodium and osmolality checks, is necessary to determine the underlying cause and guide the specialist referral process.
Primary Specialized Management
The specialized management of hyponatremia is primarily handled by two types of physicians: the nephrologist and the endocrinologist.
Nephrologists are kidney specialists whose expertise lies in the body’s fluid and electrolyte balance. They manage cases where the body is retaining too much water due to kidney disease, heart failure, or the ineffective use of diuretics, focusing on protocols to restore the correct water-to-sodium ratio.
The endocrinologist specializes in hormonal disorders, which frequently cause hyponatremia. A common cause is the Syndrome of Inappropriate Antidiuretic Hormone (SIADH), where the pituitary gland releases too much vasopressin. This hormone, also known as antidiuretic hormone (ADH), causes the kidneys to retain water, leading to dilutional hyponatremia. The endocrinologist diagnoses these hormonal disruptions and prescribes treatments that target the hormone’s action to normalize the sodium level.
Consulting Specialists for Root Causes
Hyponatremia is frequently a symptom of a deeper, underlying medical condition, necessitating consultation with other specialists once the sodium level is stabilized.
Cardiologists
Cardiologists may be brought in if the patient’s hyponatremia is hypervolemic, meaning there is fluid overload, often due to congestive heart failure. In heart failure, decreased blood flow stimulates the release of ADH and the retention of water, leading to diluted sodium levels. Managing the heart failure is necessary for a long-term cure of the electrolyte imbalance.
Oncologists
Oncologists play a part when the hyponatremia is a paraneoplastic syndrome, most commonly seen with small cell lung cancer, which can secrete ADH-like substances causing SIADH. Treating the underlying malignancy is the definitive path to correcting the sodium level in these instances.
Neurologists
Neurologists may also be consulted when the hyponatremia is severe enough to cause neurological symptoms like seizures. Consultation is also needed if the underlying cause is related to brain injury or stroke, which can directly affect ADH regulation.
Methods for Correcting Sodium Imbalance
For mild, asymptomatic cases, the first-line treatment is often fluid restriction, limiting daily water intake to allow the kidneys to excrete excess fluid and concentrate the blood sodium. If the patient is hypovolemic, treatment involves administering isotonic saline intravenously to replace both water and sodium losses.
Pharmacological intervention is reserved for more severe or symptomatic cases, sometimes involving an intravenous infusion of hypertonic saline, a highly concentrated sodium solution. Critically, the rate of sodium correction must be slow and controlled to prevent Osmotic Demyelination Syndrome (ODS). Guidelines recommend increasing the serum sodium level by no more than 8 to 10 mEq/L during the first 24 hours of treatment. In some euvolemic and hypervolemic cases, medications called Vaptans, which block the action of vasopressin at the kidney, are used to promote water excretion without losing sodium.