Courtesy of MedicalExams.com -

A 25-year-old male patient complains of increased thirst and increased frequency of urination. He denies any trauma or recent surgery. A urine sample is taken and urine specific gravity is 1.001 and osmolality is 350. He is admitted to the hospital for observation and water restriction. After 6 hours, his urine specific gravity is 1.003 and his serum osmolality is 365. He is given intranasal desmopressin (DDAVP) and his urine specific gravity remains unchanged while his serum osmolality increases to 400. The patient becomes increasingly lethargic and irritable and eventually has a tonic clonic seizure. A stat metabolic panel is obtained and his plasma sodium comes back at 158 mEq/L.


What medication most likely caused his hypernatremia?

A) Aspirin

B) Furosemide

C) Gentamicin

D) Lithium

E) Hydrochlorothiazide

Answer before looking at the answers below......
{
{
{
{
{
{
{
{
{

The correct answer is choice D

This patient is suffering from nephrogenic diabetes insipidus which is a side effect of lithium. Diabetes insipidus is characterized by increased thirst (polydipsia) and increased urination (polyuria) with the inability to to concentrate urine due to a lack of ADH. Therefore, urine specific gravity will be low (<1.006) and serum osmolality will be >250. Nephrogenic diabetes insipidus (lack of response to ADH) can be distinguished from central (loss of ADH production) because nephrogenic DI will not respond to DDAVP which is a ADH analog.

Clinical manifestations of hypernatremia can be subtle, consisting of lethargy, weakness, irritability, and edema. With more severe elevations of the sodium level, seizures and coma may occur. Severe symptoms are usually due to acute elevation of the plasma sodium concentration to above 158 mEq/L (normal is typically about 135-145 mEq/L). The differential diagnosis of hypernatremia can be narrowed by assessing the patient’s volume status.

Hypovolemic
  • Inadequate intake of water, typically in elderly or otherwise disabled patients who are unable to take in water as their thirst dictates. This is the most common cause of hypernatremia.
  • Excessive losses of water from the urinary tract, which may be caused by glycosuria, or other osmotic diuretics.
  • Water losses associated with extreme sweating.
  • Severe watery diarrhea

Euvolemic
Excessive excretion of water from the kidneys caused by diabetes insipidus, which involves either inadequate production of the hormone,vasopressin, from the pituitary gland or impaired responsiveness of the kidneys to vasopressin.

Hypervolemic
  • Intake of a hypertonic fluid (a fluid with a higher concentration of solutes than the remainder of the body). This is relatively uncommon, though it can occur after a vigorous resuscitation where a patient receives a large volume of a concentrated sodium bicarbonate solution. Ingesting seawater also causes hypernatremia because seawater is hypertonic.
  • Mineralcorticoid excess due to a disease state such as Conn's syndrome or Cushing's Disease

Aspirin (choice A) can cause acute tubular necrosis (ATN) and acute interstitial nephritis thereby increasing serum creatinine but it would not cause an low urine specific gravity or a high serum osmolality. It would also not cause hypernatremia.

Furosemide (choice B) is a loop diuretic which would cause decreased urine specific gravity but the serum osmolality would not increase as much. It would also not cause hypernatremia.

Gentamicin (choice C) an cause acute tubular necrosis (ATN) thereby increasing serum creatinine but it would not cause an low urine specific gravity or a high serum osmolality. It would also not cause hypernatremia.

Hydrochlorothiazide (choice E) would cause hyponatremia by making the body excrete more sodium in urine.