A 33-year-old woman is brought to the emergency department after an episode of generalized tonic-clonic seizures. She is confused and unable to provide further history. Review of the medical record indicates that she has schizophrenia but stopped taking all her psychiatric medications 3 weeks ago. Temperature is 36.7 C (98 F), blood pressure is 130/76 mm Hg, pulse is 80/min, and respirations are 14/min. Examination shows normal pupils. The patient's lungs are clear to auscultation and her heart sounds are normal. Her abdomen is soft. Extremities examination shows no edema. Laboratory results are as follows: Which of the following is the most likely cause of this patient's electrolyte abnormalities?

Question

A 33-year-old woman is brought to the emergency department after an episode of generalized tonic-clonic seizures.  She is confused and unable to provide further history.  Review of the medical record indicates that she has schizophrenia but stopped taking all her psychiatric medications 3 weeks ago.  Temperature is 36.7 C (98 F), blood pressure is 130/76 mm Hg, pulse is 80/min, and respirations are 14/min.  Examination shows normal pupils.  The patient's lungs are clear to auscultation and her heart sounds are normal.  Her abdomen is soft.  Extremities examination shows no edema.  Laboratory results are as follows:  Which of the following is the most likely cause of this patient's electrolyte abnormalities?

Quizplus · Accepted Answer

11ec25c8_d4f3_63b3_bc81_eb68c8c669b0_MD0003_00 This patient has had seizures due to severe hyponatremia. Hyponatremia is often due to excess antidiuretic hormone (ADH) secretion leading to impaired renal water excretion. However, hyponatremia can also be due to significantly increased water intake that overwhelms the kidney's ability to excrete water. One approach to evaluate hyponatremia is to measure the serum osmolality. A serum osmolality >290 mOsm/kg suggests either marked hyperglycemia or advanced renal failure as likely causes of the hyponatremia. The next step in evaluating patients with serum osmolality <290 mOsm/kg is to measure the urine osmolality. A urine osmolality <100 mOsm/kg, as in this patient, suggests either primary polydipsia or malnutrition (eg, beer potomania). Primary polydipsia is more common in patients with psychiatric conditions (eg, schizophrenia), possibly due to a central defect in thirst regulation. These patients continue to drink water despite a decreased serum osmolality that should normally inhibit the thirst reflex. The kidney increases water excretion, which dilutes the urine maximally to an osmolality <100 mOsm/kg. However, hyponatremia can develop if the water intake is higher than the kidney's ability to excrete water. Patients with significant hyponatremia can develop confusion, lethargy, psychosis, and seizures. (Choice A) Adrenal insufficiency typically presents with hypovolemia, which leads to increased ADH and hyponatremia. However, the increased ADH would lead to concentrated urine with urine osmolality greater than the serum osmolality. (Choices C and D) Diabetes insipidus can be central (decreased ADH release from the pituitary) or nephrogenic (normal ADH levels with renal ADH resistance). The decreased ADH action in both types leads to decreased renal water reabsorption, water loss with polyuria, and dilute urine. However, patients typically develop hypernatremia. (Choices B and F) Many drugs (eg, carbamazepine, cyclophosphamide, selective serotonin reuptake inhibitors such as fluoxetine) can stimulate hypothalamic ADH production and cause syndrome of inappropriate antidiuretic hormone secretion (SIADH). Lung cancer (eg, small cell cancer) can also cause SIADH due to ectopic ADH production by the tumor cells. The excess ADH in both cases leads to water retention in the kidney and hyponatremia. However, the urine is inappropriately concentrated (instead of dilute). As a result, the urine osmolality is higher than the serum osmolality. Educational objective: Causes of hyponatremia include syndrome of inappropriate antidiuretic hormone secretion and primary polydipsia. Primary polydipsia is more common in patients with psychiatric conditions (eg, schizophrenia), possibly due to a central defect in thirst regulation. Patients typically develop hyponatremia and dilute urine with urine osmolality <100 mOsm/kg. Those with significant hyponatremia can develop confusion, lethargy, psychosis, and seizures. References: Hyponatremia and hypernatremia: disorders of water balance. (http://www.ncbi.nlm.nih.gov/pubmed/19322975) Psychogenic polydipsia review: etiology, differential, and treatment. (http://www.ncbi.nlm.nih.gov/pubmed/17521521)

A 33-Year-Old Woman Is Brought to the Emergency Department After