Hyponatremia because of SIADH
In this article, the authors discuss an older adult with the Syndrome of Inappropriate Anti-Diuresis (also known as Syndrome of Inappropriate Anti Diuretic Hormone secretion). The patient had a serum sodium level of 128mmol/L with unsteadiness of gait and some degree of cognitive clouding. The authors remind us that while this syndrome can be idiopathic, it is important to look for diseases that trigger such inappropriate increases in ADH. The authors also stress that adrenal insufficiency and hypothyroidism are important differential diagnosis for those who have unexplained hyponatremia. The learning points from this article are:
Severe hyponatremia is defined by the authors as a serum sodium value less than 125mmol/L. However, other references (like UpToDate) define severe hyponatremia as a value below 120mmol/L.
Severe acute hyponatremia will have symptoms due to cerebral oedema (acute hyponatremia is defined as less than 48 hours duration). However, even mild (130 to 134mmol/L) and moderate (defined as 125 to 129mmol/L in this article) hyponatremia can cause cerebral symptoms when present for more than 2 days (chronic hyponatremia).
The presence of headache, drowsiness, unsteadiness of gait and cognitive changes, without an obvious explanation, should trigger the suspicion of hyponatremia, especially in older adults.
This syndrome is to be considered in all those who are euvolemic with hyponatremia. It is NOT the diagnosis to be considered when there is hyponatremia with dehydration or oedema. Tests of osmolality will show low osmolality of blood and high osmolality of urine in patients with this syndrome.
Pulmonary diseases (infections, malignancies), central nervous system disorders (infections, strokes, tumours) and medications like antidepressants (SSRI particularly) can cause this syndrome. As already mentioned, adrenal insufficiency and hypothyroidism should be excluded.
Fluid restriction (1500ml of water per day) is the first line treatment to increase serum sodium in those with only mild symptoms. But it may not be effective if the urine output is already less than 1.5 litres per day. Combining fluid restriction with salt tablets or normal saline infusions will be more effective.
Whenever SIAD is severe (associated with symptoms of cerebral oedema), initial treatment should be with hypertonic saline. 100ml of 3% saline solution, given as a bolus or as an infusion over one hour, may be enough to increase serum sodium by 4 to 6mmol/L within 6 hours. This small increase can reduce the severity of cerebral symptoms. If not, a repeat dose can be considered.
Increasing the serum sodium level by more than 8 to 10mmol/L within 24 hours can be dangerous because of risk of damage to neurons (central pontine myelinolysis) in the brainstem.
Drugs that help to promote free water excretion in urine (like frusemide, tolvaptan and empagliflozin) have been shown to be effective in treating this syndrome.