Hyponatremia is a common electrolyte imbalance defined by a lower-than-normal concentration of sodium in the blood, specifically below 135 milliequivalents per liter (mEq/L). The medical evaluation is a systematic process aimed at confirming the diagnosis, determining the underlying cause, and establishing a safe treatment plan.
The Initial Patient History and Physical Exam
The evaluation begins with a detailed patient history to identify potential causes of low sodium. The physician inquires about recent fluid intake, including excessive water consumption or strenuous activity leading to fluid loss. A comprehensive review of all current medications, particularly diuretics and certain antidepressants, is also performed, as these can disrupt the body’s sodium-water balance.
The physician inquires about symptoms, which range from mild (headache, nausea) to severe (confusion, lethargy, or seizures). The physical examination assesses the patient’s fluid status, looking for signs of dehydration or fluid overload. This involves checking skin turgor, mucous membranes, and looking for edema in the extremities.
The neurological exam is also a part of this initial assessment, especially if the patient is experiencing confusion or altered mental status. Signs of fluid volume depletion, such as low blood pressure upon standing, are noted to help classify the condition. These initial observations guide the subsequent laboratory workup, helping to narrow down the potential causes even before blood test results are available.
Essential Laboratory Testing for Diagnosis
Once hyponatremia is suspected, laboratory tests are ordered to confirm the diagnosis and investigate the cause. The first test is the serum sodium measurement. A serum osmolality test is then performed to determine if the low sodium level represents a true hypotonic state (dilute blood) or pseudo-hyponatremia, which can occur with very high levels of fat or protein in the blood.
If true hyponatremia is confirmed, the next step involves analyzing the urine to understand how the kidneys are handling water and sodium. Urine osmolality measures the concentration of solutes, indicating whether the kidneys are appropriately excreting or retaining water. A low urine osmolality (typically below 100 mOsm/kg) suggests the body is appropriately suppressing antidiuretic hormone (ADH) and attempting to excrete excess water.
The urine sodium concentration helps differentiate between underlying causes. A level less than 20 mEq/L is often seen when the body conserves salt, such as with volume depletion from non-kidney causes like vomiting or diarrhea. Conversely, a concentration greater than 40 mEq/L can indicate kidney-related salt loss or the Syndrome of Inappropriate Antidiuretic Hormone Secretion (SIADH). Additional blood tests, including serum glucose and kidney function markers like BUN and creatinine, are analyzed to rule out other common causes.
Determining Severity and Underlying Cause
Following the laboratory confirmation, the medical team analyzes the data to classify the hyponatremia, which directly influences the treatment strategy. One classification is based on the time of onset, differentiating between acute and chronic hyponatremia. Acute hyponatremia develops rapidly, within 48 hours, and carries a higher risk of brain swelling and neurological complications.
Chronic hyponatremia develops over more than 48 hours, allowing the brain time to adapt to the lower sodium concentration, making severe symptoms less common. The second classification is based on volume status, which divides patients into three categories: hypovolemic, euvolemic, and hypervolemic. Hypovolemic hyponatremia, or low fluid status, occurs when both water and sodium are lost, but sodium loss is proportionally greater.
Euvolemic hyponatremia, characterized by a near-normal fluid status, is often caused by conditions like SIADH, where the body retains water without significant sodium loss. Hypervolemic hyponatremia, or fluid overload, occurs when both water and sodium increase, but the water gain is much greater, commonly seen in severe heart failure or liver disease. The combination of the patient’s clinical signs of fluid status and the urine test results allows the physician to pinpoint the category, directing the subsequent therapeutic approach.
The Role of Ongoing Monitoring
After the initial diagnosis and treatment begins, continuous monitoring ensures the sodium level corrects safely. Frequent follow-up blood tests, sometimes every two to four hours in severe cases, track the serum sodium concentration. This close observation prevents overly rapid correction of the sodium level, which is a significant risk in chronic hyponatremia.
The target rate of increase for serum sodium is carefully controlled, generally not exceeding 8 mEq/L over any 24-hour period. Exceeding this rate can lead to Osmotic Demyelination Syndrome (ODS), a severe neurological complication. Monitoring also includes continuous assessment of the patient’s neurological status and symptoms, such as headache or confusion, to confirm that the treatment is improving the patient’s condition.