✅The correct answer is: E.
🔺This patient's acute nausea is likely due to hyponatremia induced by recent use of intranasal desmopressin. Desmopressin is an analogue of antidiuretic hormone (ADH) commonly used to treat central diabetes insipidus; it also promotes the release of von Willebrand factor, and can treat mild to moderately heavy menstrual bleeding associated with von Willebrand disease. However, since desmopressin induces physiologic effects of ADH (eg, renal water reabsorption, concentration of urine), it can cause hyponatremia and excess ADH stimulation, manifesting as syndrome of inappropriate ADH secretion (SIADH). Therefore, serum electrolytes should be obtained first in this patient to check for hyponatremia and rule in the diagnosis of SIADH.
🔺Expected laboratory findings in patients with SIADH include hypotonic hyponatremia (due to ADH- mediated water retention and reabsorption), high urine osmolality (ie, concentrated urine), and high urine sodium (due to increased secretion of natriuretic peptides).
🔺As seen in this patient, SIADH is also associated with euvolemia (eg, absence of edema or jugular venous distension) on physical examination. At first, ADH-mediated water reabsorption slightly increases extracellular volume; however, increased natriuretic peptide secretion leads to sodium and water excretion, ultimately normalizing extracellular volume.
❎ (Choice A) AIthough hypoxemia (eg, from pulmonary embolism) may manifest with nausea, elevated heart rate, and dizziness, this patient has no risk factors for pulmonary disease or symptoms of respiratory distress (eg, shortness of breath, chest pain). Therefore, serum electrolytes are a better initial test.
❎ (Choice B) Although acute liver failure may cause excessive bleeding and present with nausea and dizziness, this patient lacks associated findings of jaundice and abdominal pain and has no risk factors (eg, infection, excess alcohol intake).
❎ (Choice C) MRI of the brain can be used to assess for CNS pathology (eg, bleeding, acute stroke) that could manifest with headache or dizziness. However, this patient lacks focal neurologic signs; therefore, a metabolic workup with serum electrolytes is a more appropriate initial test.
❎(Choice D) Plasma desmopressin is not routinely measured. It may be high in this patient, given her recent medication use with the onset of menses, but serum electrolytes are a more rapid initial step for confirming hyponatremia and SIADH.
💡 Educational objective:
▪️The administration of desmopressin, an analogue of antidiuretic hormone (ADH), can induce the syndrome of inappropriate ADH secretion.
▪️Urinary water excretion is impaired, leading to hypotonic hyponatremia.
▪️Laboratory studies, including serum electrolytes, urine osmolality, and urine sodium, are the first step in establishing the diagnosis.
✳️Syndrome of inappropriate antidiuretic hormone:
🔺Etiology
CNS disturbance (eg, stroke, hemorrhage, trauma)
Medications (eg, carbamazepine, SSRIs, NSAIDs)
Lung disease (eg, pneumonia)
Ectopic ADH secretion (eg, small cell lung cancer)
Pain &/or nausea
🔺Clinical features:
Mild/moderate hyponatremia: nausea, forgetfulness
Severe hyponatremia: seizures,coma .
Euvolemia (eg, moist mucous membranes, no edema, no JVD) .
🔺Laboratory findings:
Hyponatremia
Serum osmolality 100 mOsm/kg H2O
Urine sodium >40 mEq/L
🔺Management:
Fluid restriction ± salt tablets Hypertonic (3%) saline for severe hyponatremia
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