Low Sodium After Surgery: Symptoms, Causes, and Complications

Sodium is essential for maintaining fluid balance, nerve function, and muscle contractions. After surgery, some patients develop low sodium levels (hyponatremia), which can lead to serious health issues if not properly managed. Recognizing the signs early can help prevent complications.

Prevalence In Surgical Context

Hyponatremia is a common postoperative electrolyte imbalance, with incidence rates varying based on surgery type, patient demographics, and perioperative management. Mild cases (serum sodium 130–135 mmol/L) affect up to 30% of hospitalized surgical patients, while severe cases (<125 mmol/L) occur in 1–4%. The condition is particularly prevalent in surgeries involving significant fluid shifts, such as major abdominal, orthopedic, and neurosurgical procedures, where aggressive intravenous fluid administration and hormonal changes contribute to sodium dysregulation. Elderly patients and those undergoing prolonged procedures are at greater risk. A study in The Journal of Clinical Endocrinology & Metabolism found postoperative hyponatremia was significantly more common in patients over 65, likely due to reduced renal sodium conservation and increased sensitivity to antidiuretic hormone (ADH). Cardiac surgery patients frequently experience hyponatremia due to the hemodilution effects of cardiopulmonary bypass, leading to excessive free water retention. In orthopedic procedures like hip fracture repair, the condition is often linked to preexisting frailty and the use of hypotonic intravenous fluids. Hyponatremia can develop within 24 hours postoperatively or emerge days later due to cumulative fluid imbalances. A study in Anesthesia & Analgesia found early cases were more common in patients receiving large volumes of hypotonic fluids, while delayed cases were often linked to prolonged ADH secretion from surgical stress. Delayed onset is particularly concerning, as symptoms may not appear until the condition becomes severe, increasing the risk of neurological complications.

Common Risk Factors

Postoperative hyponatremia arises from a combination of physiological, pharmacological, and procedural factors. One primary cause is excessive intravenous administration of hypotonic fluids, which dilute serum sodium concentrations. While isotonic fluids like normal saline are preferred, some protocols still use hypotonic solutions such as 0.45% saline or dextrose-containing fluids, increasing the risk of dilutional hyponatremia. A study in JAMA Surgery found patients receiving large volumes of hypotonic fluids had significantly higher rates of sodium imbalances than those given isotonic alternatives.

Surgical stress triggers hormonal changes that impact sodium regulation. Elevated ADH levels postoperatively promote water retention, diluting sodium levels—a condition known as syndrome of inappropriate antidiuretic hormone secretion (SIADH). This is particularly common in neurosurgery, thoracic, and abdominal procedures. Research in The American Journal of Medicine found prolonged ADH elevation post-surgery can lead to sustained hyponatremia, especially when paired with excessive fluid intake. Patients with preexisting conditions like pulmonary disease or malignancies are more prone to SIADH-related sodium disturbances.

Certain medications also contribute to sodium imbalances. Opioids, commonly prescribed for postoperative pain, increase ADH secretion, exacerbating water retention. Diuretics like thiazides promote excessive sodium loss, particularly in the elderly or those with renal insufficiency. A study in Nephrology Dialysis Transplantation found thiazide-associated hyponatremia was more common in patients over 70 due to age-related declines in sodium reabsorption. Additionally, selective serotonin reuptake inhibitors (SSRIs), often continued postoperatively, can induce hyponatremia by influencing ADH regulation.

Older adults are particularly vulnerable due to diminished renal function and impaired sodium and water homeostasis. Malnourished individuals or those with preexisting electrolyte disturbances are also at higher risk, as their baseline sodium reserves are often lower. Patients with chronic conditions like heart failure, liver disease, or kidney dysfunction are more susceptible to fluid retention, compounding sodium dilution during the perioperative period. A review in Clinical Endocrinology found individuals with cirrhosis had a significantly increased risk of postoperative hyponatremia due to impaired sodium handling and increased ADH activity.

Mechanisms Underlying Hyponatremia

Sodium balance is regulated by renal filtration, hormonal control, and fluid intake, all of which can be disrupted postoperatively. One primary mechanism is excessive free water retention due to heightened ADH release, which promotes water reabsorption in the kidneys and dilutes serum sodium levels. Surgical stress, pain, nausea, and anesthetic agents can all heighten ADH response, creating an environment where even normal fluid intake can cause hyponatremia.

Renal function plays a key role, but postoperative impairments can hinder the kidneys’ ability to excrete excess water. Reduced renal perfusion from hypotension or blood loss can activate compensatory mechanisms that further promote water retention. The renin-angiotensin-aldosterone system (RAAS) attempts to preserve circulatory volume by increasing sodium reabsorption, but persistent ADH secretion often counteracts this, leading to a net sodium dilution.

Fluid management strategies also influence sodium levels. Large volumes of hypotonic fluids, such as 5% dextrose in water (D5W) or half-normal saline, introduce free water that exceeds the body’s ability to excrete it. Even isotonic fluids can become effectively hypotonic in the presence of prolonged ADH secretion, as water retention shifts electrolyte balance. This effect is especially pronounced in major surgeries like cardiac or neurosurgical procedures, where systemic inflammatory responses and prolonged operative times amplify fluid shifts.

Signs And Symptoms

The severity and rate of sodium decline determine the clinical presentation of postoperative hyponatremia. Early signs are often subtle and nonspecific, making diagnosis difficult without laboratory testing. Patients may initially experience fatigue, headache, and nausea—symptoms frequently mistaken for normal postoperative recovery. As sodium levels drop further, neurological manifestations such as confusion, irritability, and difficulty concentrating may develop.

Severe hyponatremia can lead to cerebral edema, causing disorientation, slurred speech, or altered consciousness. In extreme cases, seizures occur due to disrupted ion gradients. A study in Neurology found patients with sodium levels below 120 mmol/L had a significantly higher risk of seizure activity, particularly when hyponatremia developed rapidly. If untreated, further deterioration can result in respiratory depression and coma.

Types Of Hyponatremia

Postoperative hyponatremia is classified based on volume status, which influences both the underlying cause and treatment approach. Proper differentiation is crucial to avoid complications.

Hypovolemic

Hypovolemic hyponatremia occurs when both sodium and water are lost, but sodium loss exceeds water loss. This often results from excessive blood loss, prolonged vomiting, or diuretic use. Postoperative drainage, particularly in major abdominal or thoracic surgeries, can also contribute. Patients typically present with dehydration signs like hypotension, tachycardia, and reduced urine output. Laboratory findings usually show low urine sodium unless diuretics have been used, in which case urinary sodium may be inappropriately high. Treatment involves restoring volume with isotonic saline while addressing the underlying cause.

Euvolemic

Euvolemic hyponatremia is characterized by normal total body sodium but increased free water retention, leading to dilutional hyponatremia. It is most commonly associated with SIADH, triggered by surgical stress, pain, nausea, or anesthetics. Patients often exhibit mild confusion or lethargy, making diagnosis challenging without lab tests. Urine osmolality is typically elevated due to persistent ADH activity, while serum osmolality remains low. Management includes fluid restriction to prevent further dilution, and in persistent cases, vasopressin receptor antagonists like tolvaptan may be used.

Hypervolemic

Hypervolemic hyponatremia occurs when both sodium and water are retained, but water accumulation is more pronounced, leading to dilutional hyponatremia. It is common in patients with heart failure, liver cirrhosis, or kidney dysfunction, where impaired fluid excretion results in volume overload. Postoperative patients receiving excessive intravenous fluids are at heightened risk. Symptoms include peripheral edema, pulmonary congestion, and jugular venous distension. Treatment focuses on fluid restriction and, in severe cases, diuretics to manage fluid overload while monitoring electrolyte balance.

Possible Complications

Failure to manage postoperative hyponatremia can lead to severe complications. One major concern is cerebral edema, which increases intracranial pressure and can cause lethargy, altered mental status, seizures, or coma. Younger patients and premenopausal women are particularly vulnerable due to lower tolerance for brain swelling.

Rapid sodium correction can result in osmotic demyelination syndrome (ODS), a serious neurological condition causing irreversible damage to the brainstem. This occurs when sodium levels rise too quickly, exceeding 8–10 mmol/L per day. Symptoms may progress to quadriplegia, dysarthria, and locked-in syndrome. To prevent this, sodium levels should be corrected gradually with close monitoring.

Hyponatremia is also linked to prolonged hospital stays, increased postoperative delirium, and higher mortality in critically ill patients. In elderly individuals, delirium can lead to functional decline and fall risk. Additionally, in surgical populations like orthopedic and cardiac patients, hyponatremia has been associated with impaired wound healing and increased infection risk. Early intervention and preventive strategies can significantly improve outcomes.