INTRODUCTION

One of the most frequent electrolyte abnormalities in veterinary patients is hyponatremia. Most cases are temporary and without symptoms. One cause of hyponatremia that may be associated with symptoms and can be fatal is Syndrome of inappropriate antidiuretic hormone (SIADH).

Antidiuretic hormone (ADH) deficiency is relatively well known and is referred to as central diabetes insipidus. The antithesis of diabetes insipidus, an excess of ADH remains an obscure rarity in animals based on the frequency of case reports,^1-6 but its true incidence may be more common than diabetes insipidus. SIADH, also called Schwartz-Bartter syndrome, is characterized clinically in people by symptoms of depression and confusion. Affected animals may have central nervous system disease, pulmonary disease, or conditions requiring drugs that can cause SIADH.

The failure to recognize the true incidence of SIADH may be caused by lack of clinical suspicion, transient nature of some forms of SIADH, insufficient monitoring, rapid demise of the patient, or clinician distraction from investigating and managing concurrent diseases. Recognition of SIADH is important for many reasons, including causes that can be iatrogenic and remedied by drug withdrawal, or patient death that can be iatrogenic if SIADH is managed too aggressively.

CAUSES

ADH, also known as vasopressin, normally is secreted in response to an increase in serum osmolality (serum sodium concentration) or to maintain normal blood pressure and intravascular volume (see Chapter 177, Vasopressin). ADH actions are achieved by the promotion of free water resorption by the kidneys. Serum osmolality is monitored by theanterior portion of the hypothalamus. If blood pressure is normal or elevated, ADH secretion normally is inhibited by pressure receptors in the atria and great veins. A rise in serum osmolality is a more sensitive monitor (1% rise) and typical stimulus for ADH secretion than a decrease in blood pressure (9% decrease).^7,8 SIADH is defined as an excess of ADH without hypovolemia or hyperosmolality.

SIADH can be caused by cerebral disorders, pulmonary disease, or adverse effects of medications (Box 71-1).⁷ The cause in some cases remains idiopathic. Three cases of idiopathic SIADH have been reported in dogs.^2,3 Cerebral causes of SIADH in humans include hypothalamic tumors, head trauma, meningitis, encephalitis, cerebrovascular accidents, and hydrocephalus. Hypothalamic tumors, granulomatous meningoencephalitis, and probable distemper encephalitis have been reported to cause SIADH in dogs.^5,6 Intracranial disease may directly stimulate the supraoptic or paraventricular nuclei to secrete ADH or may alter the osmoreceptors to inappropriately stimulate ADH secretion. Other cerebral causes of SIADH are perception of nausea, pain, and psychologic stress.⁷

Box 71-1 Some Causes of the Syndrome of Inappropriate Secretion of ADH⁷

ADH, Antidiuretic hormone.