various antidepressants especially selective serotonin reuptake inhibitors ( ssris ) are known to cause hyponatremia , especially in elderly and those with medical comorbidities or on polypharmacy . hyponatremia is also reported with other antidepressants , few antipsychotics and with mood stabilizers like carbamazepine and oxcarbazepine . however , among various antidepressants , there is limited literature to suggest association of mirtazapine and hyponatremia . in our review of literature ( done in early september 2013 by using pubmed ) , we could find only five cases of mirtazapine - induced hyponatremia . a review of these cases suggests that all these patients were elderly who had multiple physical comorbidities which by themselves predispose to hyponatremia . in this report , we present a case of mirtazapine - induced severe hyponatremia , which was seen in an adult patient , was related to the dose of mirtazapine and appeared within few hours of intake of increased dose of mirtazapine . a 46-year - old widow female , low socioeconomic status , with poor social support , who was premorbidly well adjusted , presented to psychiatric outpatient clinic with an acute onset illness , precipitated by family stress in the form of her son marrying against her wishes , of 4 months duration , progressive in nature , characterized by persistent sadness , bodyaches , lethargy , anhedonia , depressive cognition , anxiety spells , decreased sleep , and decreased appetite . her past history did not reveal any significant medical or psychiatric history and there was no family history of mental illness . her body weight was 54 kg , body mass index was 22.5 kg / m , and her blood pressure was in the normal range . she was diagnosed with moderate depressive episode without somatic syndrome ( icd-10 ) . because of marked sleep disturbance , mirtazapine was preferred and she was started on tab . mirtazapine 7.5 mg / day , which was increased to 15 mg / day after 4 days along with tab . routine investigations in the form of hemogram , serum electrolytes , renal function test , liver function test , fasting blood sugar levels , lipid profile were done , which did not reveal any abnormality . over the period of 4 weeks , her sleep and anxiety improved marginally , with overall 30% improvement in symptomatology ( score on hamilton depression rating decreased from 18 to 13 ) , following which tab . however , immediately after the increase in the dose of mirtazapine , her difficulty in falling asleep reappeared along with intermittent awakening . next morning , she was noticed to be restless , appeared confused and agitated . as the day progressed , she became disoriented to time , place and person , irritable , had labile affect , and her speech became incomprehensible . she was found to have disorientation to time , place and person , poor attention and concentration , impairment in short - term memory , labile affect , and incomprehensible speech . physical examination did not reveal any evidence of dehydration , hypervolumia , and her pulse rate and blood pressure were in the normal physiological limits . history did not reveal any evidence of head injury , taking over the counter medications or any overdose , restriction of diet , drug or alcohol abuse , and excessive intake of water . her mini - mental state scale score was 6 and her revised delirium rating scale ( drs-98 ) score was 21 . she was investigated including a computerised tomography of brain , x - ray chest , ultrasound abdomen , liver function test , serum urea and creatinine , serum electrolytes , and random blood sugar levels . no abnormality was noted except for evidence of hyponatremia ( serum sodium level 123 meq / l ) . she was managed with water restriction and intravenous sodium replacement with isotonic saline slow intravenous drip . over the next 2 days , her mental state improved , along with normalization of serum sodium ( 138 meq / l ) . her mini mental state examination ( mmse ) improved to 30 and her drs - r-98 score reduced to 7 . after a weeks time , serum sodium levels were repeated , which was in the normal range ( 138 meq / l ) , following which she was started on tab . milnacipran 100 mg / day , with which her depression remitted and she has been maintaining well with milnacipran for the last 5 months . index case exemplifies an association of hyponatremia with mirtazapine . on the naranjo adverse drug reaction probability scale , the score of index case was 7 , suggesting a probable association of hyponatremia and mirtazapine . in contrast to the most of literature , which suggests association of mirtazapine and hyponatremia in the presence of risk factors like advance age , hypertension , diabetes mellitus , concomitant prescription of medications known to cause hyponatremia , in the index case hyponatremia was noted in an adult patient who had no medical comorbidity and was not on any concomitant medications . studies which have found association of hyponatremia and other antidepressants have also reported low body weight and renal dysfunction to be other risk factors for development of hyponatremia . another interesting observation noted in the patient was emergence of hyponatremia , immediately after increasing the dose of mirtazapine from 15 to 22.5 mg / day . studies in literature have not focused much on the relationship of dosage of psychotropics and development of hyponatremia . though a few case reports of relatively later onset of hyponatremia has also been documented in the literature , this is contrary to the usual notion that antidepressant - induced hyponatremia occurs within a month of initiation and is unrelated to the dose of medication . mirtazapine is a serotonin receptor blocker which also effects norepinephrine through blockade of alpha-2 adrenergic receptor . the hyponatremia noted in patients with mirtazapine may possibly be mediated by the same syndrome of inappropriate anti - diuretic hormone secretion ( siadh ) mechanism mediated by its effect on serotonin . effect of antidepressants on 5-ht and 5-ht1c receptors has been thought to responsible for relapse of anti - diuretic hormone ( adh ) . however , the exact mechanism which can explain mirtazapine - induced or ssri - induced hyponatremia is not known . the case suggests that there is merit in monitoring serum sodium levels while increasing the dose of mirtazapine in apparently physically healthy subjects .
though there is ample evidence for the association between selective serotonin reuptake inhibitors and hyponatremia , evidence for the relationship between mirtazapine and hyponatremia is scarce . 
 we present a case of mirtazapine - induced hyponatremia in an adult patient , which was dose related .