View source for WBR0214

 {{WBRQuestion
|QuestionAuthor=Vendhan Ramanujam
|ExamType=USMLE Step 3
|MainCategory=Community Medical Health Center, Primary Care Office
|SubCategory=Endocrine, Electrolytes
|MainCategory=Community Medical Health Center, Primary Care Office
|SubCategory=Endocrine, Electrolytes
|MainCategory=Community Medical Health Center, Primary Care Office
|SubCategory=Endocrine, Electrolytes
|MainCategory=Community Medical Health Center, Primary Care Office
|MainCategory=Community Medical Health Center, Primary Care Office
|SubCategory=Endocrine, Electrolytes
|MainCategory=Community Medical Health Center, Primary Care Office
|SubCategory=Endocrine, Electrolytes
|MainCategory=Community Medical Health Center, Primary Care Office
|SubCategory=Endocrine, Electrolytes
|MainCategory=Community Medical Health Center, Primary Care Office
|SubCategory=Endocrine, Electrolytes
|MainCategory=Community Medical Health Center, Primary Care Office
|MainCategory=Community Medical Health Center, Primary Care Office
|SubCategory=Endocrine, Electrolytes
|Prompt=A 58 year old man who was diagnosed with schizophrenia in his early twenties is found wandering around his halfway house confused and disoriented.  He is rushed to a local emergency room, where his serum sodium concentration is found to be 123 meq/L (Normal 135 mEq/L to 145 mEq/L).  His urine sodium concentration is 5 meq/L (Normal >20 mEq/L).  The patient was being treated with risperidone 4 mg per day for the past 3 years.  He had been compliant with the medication and had good control over his symptoms.  His roommate reports that the patient makes many trips to the nearby water cooler and often complains of being thirsty.  What is the most likely cause of this patient’s metabolic disturbance?
|Explanation=Self-induced [[water intoxication]] should always be considered in the differential diagnosis of confusional states and seizures in [[schizophrenic]] patients.  As many as 20% of patients with a diagnosis of schizophrenia drink excessive amounts of water.  At least 4% of these patients suffer from chronic [[hyponatremia]] and recurrent acute water intoxication.  They typically have low plasma and urine [[osmolality]].  Medications that cause excessive water retention, such as [[lithium]] and [[carbamazepine]], can aggravate the symptom.  [[Phenothiazine]] and [[sarcoidosis]] infiltrating the thirst center are the other causes of primary or [[psychogenic polydipsia]]. 

'''Educational objective:'''
The following conditions present with polydipsia and polyuria,
Psychogenic polydipsia has a low serum osmolality (<285 mOsm/Kg of water), hyponatremia (<135 mEq/L) and diluted urine (<300 mOsm/Kg of water).
[[Diabetes insipidus]] has an elevated serum osmolality (>295 mOsm/Kg of water) and diluted urine (<300 mOsm/Kg of water).  
[[Diabetes mellitus]] has hyperglycemia with glucosuria.
[[SIADH]] does not present with polyuria and polydipsia.  It has a low serum osmolality (<285 mOsm/Kg of water), hyponatremia (<135 mEq/L) and high urine osmolality (>300 mOsm/Kg of water).

|AnswerA=Inappropriate ADH secretion 


|AnswerAExp='''Incorrect'''-Polyuria and polydipsia are not features of SIADH although it results in hyponatremia.  Besides low serum osmolality, they have inappropriately high urine osmolality.
|AnswerB=Medication side effect
|AnswerBExp='''Incorrect'''-The common side effects of [[risperidone]] are weight gain, anxiety, fatigue and extrapyramidal effects (sudden, often jerky, involuntary motions of the head, neck, arms, body, or eyes). 
|AnswerC=Osmotic diuresis 
|AnswerCExp='''Incorrect'''-[[Osmotic diuresis]] occurs in patients with hyperglycemia, glucosuria and mannitol administration.  Both serum and urine osmolality will be elevated and urine osmolality will be greater than serum osmolality.
|AnswerD=Psychogenic polydipsia
|AnswerDExp='''Correct'''-Self-induced water intoxication should always be considered in the differential diagnosis of confusional states and seizures in schizophrenic patients.  As many as 20% of patients with a diagnosis of schizophrenia drink excessive amounts of water.  At least 4% of these patients suffer from chronic hyponatremia and recurrent acute water intoxication.  They typically have low plasma and urine osmolality.  
|AnswerE=Central diabetes insipidus 
|AnswerEExp='''Incorrect'''-Diabetes insipidus will present with excretion of dilute urine and elevated serum osmolality.
|RightAnswer=D
|Approved=Yes
}}