Hyponatremia is a common, multifactorial clinical condition. Hyponatremia is usually defined as a plasma sodium level <135 mmol/L; however, some studies define it as a level <130 mmol/L. Hyponatremia results from the inability of the kidney to excrete a sufficient amount of fluid, or is due to excessive fluid intake. Increases in osmolality stimulate thirst and result in increased fluid intake. This increase in osmolality is recognized by the osmoreceptors located in the hypothalamus, which release antidiuretic hormone (ADH). Antidiuretic hormone works on the collecting ducts within the kidneys, triggering increased fluid reabsorption resulting in decreased fluid loss and a reduction in thirst.
The syndrome of inappropriate antidiuretic hormone (SIADH) occurs when there is persistent ADH stimulation resulting in hyponatremia. SIADH commonly presents as euvolemic hyponatremia. Common diagnostic criteria for SIADH are listed in Table 1.1
Medications are a major cause of SIADH, and psychotropics are a primary offender. Most of the data for drug-induced SIADH come from case reports and small case series, such as those described in Table 2.2-4 The extent to which each psychotropic class causes SIADH remains unknown. In this article, we focus on 3 classes of psychotropics, and their role in causing SIADH.
There is a fair amount of data associating antidepressants with SIADH. The incidence of SIADH with selective serotonin reuptake inhibitors (SSRIs) varies greatly among studies, from .06% to 40%.5-12 This wide variation is due to the way each study defined hyponatremia. A higher incidence was found when hyponatremia was defined as <135 mmol/L as opposed to <130 mmol/L. A large cohort study of SSRIs found that there was an increased risk with fluoxetine, escitalopram, and citalopram (.078% to .085%) vs paroxetine and sertraline (.033% to .053%).13 Studies comparing the incidence of SIADH with SSRIs and serotonin-norepinephrine reuptake inhibitors (SNRIs) found that the rates were equal or slightly higher with the SNRI venlafaxine.13 SNRIs as a group have an estimated incidence of .08% to 4%, based on studies that defined hyponatremia as <130 mmol/L.13,14 Tricyclic antidepressants have an estimated incidence of .005% to 16.7%, based on a retrospective study that reviewed 15 studies and 100 case reports.15 Mirtazapine and bupropion do not have enough evidence to obtain a true definition of incidence; case reports for these drugs suggest a causal link for hyponatremia. Table 37,9,12-15 provides an overview of the incidence rate of hyponatremia for select antidepressants. It is clear that a more stringent cutoff for hyponatremia (<130 mmol/L) reduces the incidence rates. More evidence is needed to identify the true incidence and prevalence of SIADH with these agents.
Compared with antidepressants, there’s less evidence linking SIADH with antipsychotics; this data come mainly from case reports and observational studies. Serrano et al16 reported on a cross-sectional study that included 88 patients receiving clozapine, 61 patients receiving other atypical antipsychotics, 23 patients receiving typical antipsychotics, and 11 patients receiving both typical and atypical antipsychotics. They reported incidence rates of 3.4% for clozapine, 4.9% for atypical antipsychotics, 26.1% for typical antipsychotics, and 9.1% for the group receiving both typical and atypical antipsychotics.16 The primary theory for the decreased incidence of SIADH with use of atypical antipsychotics is related to decreased rates of psychogenic polydipsia leading to lower incidence of hyponatremia.
Several studies have associated carbamazepine/oxcarbazepine, valproic acid, and lamotrigine with SIADH.17-23 Studies show incidence rates ranging from 4.8% to 41.5% for these medications. Carbamazepine appears to have the highest incidence of SIADH. A limitation of these studies is the small sample sizes, which ranged from 12 to 60 participants.
The kidneys are responsible for maintaining homeostasis between bodily fluids and serum sodium levels. ADH, which is produced by the hypothalamus, plays a significant role in fluid balance, thirst, and fluid retention. Inappropriate and continuous secretion of ADH, despite normal or high fluid status, results in hyposmolality and hyponatremia. The specific mechanisms by which psychotropic medications cause SIADH are listed in Table 4.24
Diagnosis of SIADH can be complex because there are many clinical reasons a patient may have hyponatremia. For example, SIADH and psychogenic polydipsia both result in hyponatremia, and sometimes the 2 conditions can be difficult to distinguish. Hyponatremia is typically discovered by routine blood testing if the patient is asymptomatic. Table 525 highlights the major laboratory markers that distinguish SIADH and psychogenic polydipsia.
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