Syndrome of inappropriate antidiuretic hormone secretion (SIADH) is caused by various factors leading to the continuous secretion of endogenous ADH or ADH-like substances, resulting in impaired water excretion, which in turn causes hyponatremia, water retention, and related clinical manifestations.

bubble_chart Etiology

The most common cause is the synthesis and autonomous release of ADH by tumor tissue, accounting for about 80% of this condition, primarily including small cell lung cancer, pancreatic cancer, duodenal cancer, and lymphoma. Other conditions such as pneumonia, subcutaneous lung nodules, and neurological disorders affecting hypothalamic-pituitary function, such as trauma, inflammation, and tumors, can also lead to excessive ADH secretion.

bubble_chart Clinical Manifestations

In addition to the manifestations of the primary disease, this syndrome is mainly characterized by water retention and hyponatremia. Generally, serum sodium is below 130 mmol/L. When blood sodium is less than 120 mmol/L, symptoms such as weakness, loss of appetite, nausea, vomiting, drowsiness, dysphoria, and even mental disorders may occur. When blood sodium is less than 110 mmol/L, convulsions, unconsciousness, and even death may occur. Since water retention generally does not exceed 3 to 4 liters, and some water shifts into the cells, edema is usually absent.

bubble_chart Diagnosis

1. Hyponatremia, generally below 130mmol/L, but with increased urinary sodium, often greater than 20mmol/L.
2. Urine osmolality is higher than plasma osmolality.
3. No signs of hypovolemia.
4. Strict restriction of water intake can correct hyponatremia, low plasma osmolality, and high urinary sodium.
5. Manifestations of the primary disease can be identified.

bubble_chart Treatment Measures

In terms of treatment, for mild cases, simply restricting daily water intake to less than 100ml can lead to symptom improvement, weight loss, and subsequent increases in serum sodium and osmotic pressure, along with reduced urinary sodium excretion. For severe cases, a slow intravenous drip of 200-300ml of 5% sodium chloride solution should be administered to gradually increase blood sodium levels over several hours, and furosemide can be added to promote diuresis. Anti-ADH medications, such as demeclocycline, can be used to inhibit ADH-mediated water reabsorption in the renal tubules. Lithium salts have a similar effect but are more toxic. The most fundamental treatment for this condition is addressing the underlying cause; malignant tumors require surgical removal, supplemented with radiotherapy and chemotherapy.

bubble_chart Differentiation

This condition needs to be differentiated from water retention and/or hyponatremia caused by renal tubular lesions, adrenal cortical insufficiency, chronic heart failure, cirrhotic ascites, the use of hypertonic diuretics, and hypothyroidism.