Hyponatremia

From Lewis Blevins, MD – Hyponatremia is the term used to describe low serum sodium levels. A low serum sodium may result from salt losses or else from an excess amount of water in the bloodstream. The latter is definitely the more common cause of hyponatremia.

Arginine vasopressin is produced by the posterior pituitary gland. It acts upon the kidneys to cause them to retain water. This is a very important mechanism to prevent dehydration. Too much vasopressin can cause the kidneys to hold onto too much water thus diluting out the sodium level in the bloodstream. Think of this as a situation of being “water–logged.”

Hyponatremia has been described after pituitary surgery. Some reports put the incidence as high as about 35% whereas others show it to be as low as 5 to 7% following pituitary operations. The huge variation in reported incidence probably relates to the fact that some institutions do not routinely check for this disorder nor do they educate their patients to look out for related symptoms. I suspect that hyponatremia is under reported and the disorder is actually more common than recognized. In my experience, this disorder is more likely to occur when surgery manipulates the posterior pituitary gland, after surgery for Rathke’s cleft cyst, and especially when either ethanol or hydrogen peroxide has been used in the pituitary fossa. It is believed that injury to the axons of the vasopressin-producing neurons located in the posterior pituitary ultimately causes the cell bodies of these neurons located in the hypothalamus is to die and then release vasopressin in an unregulated fashion. Water is retained. The sodium level falls. About 20% of patients with Cushing’s disease will experience hyponatremia after surgery. This may be related to the aforementioned events but also to relative cortisol insufficiency as the body adapts to having much lower cortisol levels after surgery.

The time course of development of postoperative hyponatremia after pituitary surgery varies between three and seven days. Some patients have a period of diabetes insipidus prior to developing hyponatremia. The disorder may last 3 to 10 days. Usually, it is followed by recovery of normal osmoregulation but some patients develop diabetes insipidus after the period of hyponatremia.

Symptoms and signs of acute hyponatremia after surgery include worsening headache, nausea, and flulike symptoms. The symptoms are largely attributed to cerebral edema, or swelling of the brain. Severe cases may be associated with confusion, excessive sleepiness, and even coma. This disorder is potentially fatal. Early recognition and prompt treatment are of paramount importance. It is our policy to notify patients of the potential for development of hyponatremia and the relevant symptoms prior to discharge from the hospital after pituitary surgery.

Most patients with postoperative hyponatremia should be hospitalized for observation and management. Historically, the treatment for this disorder has been fluid restriction and support until the bout of hyponatremia resolves. Unfortunately, however, fluid restriction must be severe in order to not only ameliorate the tide of fluid retention but also to enable insensible losses, and time for resolution of the excess vasopressin secretion, to cause a loss of retained water resulting in a normalized sodium. Fortunately, however, there are drugs available to block vasopressin’s action on the vasopressin receptor in the kidney. The most commonly employed drug is Tolvaptan. In most patients with postoperative hyponatremia after pituitary surgery, one to two oral doses of Tolvaptan normalizes the serum sodium concentration. Symptoms resolve quickly. The time of hospitalization in patients treated with Tolvaptan is shortened when compared to patients treated with observation and fluid restriction. The management of hyponatremia is actually quite complex and should be undertaken by physicians with considerable experience in medical decision-making regarding evaluation and treatment.

Photo by Jason Armstrong

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