What is the most appropriate diagnostic study for a 55-year-old man with increased thirst and urine output (polyuria) of over 2000 milliliters (mL) in 12 hours after pituitary adenoma resection?

From the Guidelines

The most appropriate diagnostic study for a 55-year-old man with increased thirst and polyuria after pituitary adenoma resection is serum and urine osmolality testing, along with serum sodium levels. This patient is likely experiencing diabetes insipidus, a common complication following pituitary surgery due to damage to the posterior pituitary or pituitary stalk, which disrupts antidiuretic hormone (ADH) production or release, as suggested by the potential changes in water metabolism and regulation of arginine vasopressin (AVP) post-surgery 1. The diagnosis can be confirmed by finding high serum osmolality (>295 mOsm/kg) with inappropriately dilute urine (urine osmolality <300 mOsm/kg), and often elevated serum sodium (>145 mEq/L).

Key points to consider in this diagnosis include:

• The patient's increased urine output (polyuria) and thirst, which are hallmark symptoms of diabetes insipidus.
• The importance of monitoring fluid and electrolyte balance peri-operatively and post-operatively in patients with pituitary adenoma who undergo surgery, as strongly recommended 1.
• The potential for transient or permanent AVP deficiency, which can lead to diabetes insipidus, as a complication of pituitary surgery 1.
• The need for close observation and management in a setting where concerns can be flagged and addressed by an expert endocrinologist at an early stage 1.

A water deprivation test may be performed if the diagnosis remains unclear, but is often unnecessary in the post-neurosurgical setting where the clinical picture is typically evident. Once diabetes insipidus is confirmed, treatment with desmopressin (DDAVP) can be initiated, typically starting at 0.1-0.2 mg orally twice daily or 10 mcg intranasally twice daily, with dose adjustments based on symptom response and serum sodium monitoring. Prompt diagnosis is essential as untreated diabetes insipidus can rapidly lead to severe dehydration and hypernatremia.

From the FDA Drug Label

Central Cranial Diabetes Insipidus: Desmopressin nasal spray solution, USP 0. 01% is indicated as antidiuretic replacement therapy in the management of central cranial diabetes insipidus and for management of the temporary polyuria and polydipsia following head trauma or surgery in the pituitary region. Patients are selected for therapy by establishing the diagnosis by means of the water deprivation test, the hypertonic saline infusion test, and/or the response to antidiuretic hormone.

The most appropriate diagnostic study for a 55-year-old man with increased thirst and urine output (polyuria) of over 2000 milliliters (mL) in 12 hours after pituitary adenoma resection is the water deprivation test or the hypertonic saline infusion test to establish the diagnosis of central cranial diabetes insipidus, as these tests are directly mentioned in the drug label as means of selecting patients for desmopressin therapy 2.

• Key considerations:
  • The patient's symptoms of polyuria and polydipsia following pituitary surgery suggest central cranial diabetes insipidus.
  • The water deprivation test and hypertonic saline infusion test are used to diagnose this condition.
  • These diagnostic studies will help determine if the patient has central cranial diabetes insipidus, which would be treated with desmopressin.

From the Research

Diagnostic Approach

The patient's symptoms of increased thirst and polyuria after pituitary adenoma resection suggest a possible diagnosis of diabetes insipidus (DI) 3, 4. To confirm the diagnosis, the following diagnostic studies can be considered:

• Measurement of urine and serum osmolality to assess the concentration of the urine and the level of antidiuretic hormone (ADH) in the blood 3, 5
• Water deprivation test to evaluate the patient's ability to concentrate urine in response to dehydration 3, 6
• Desmopressin test to assess the patient's response to exogenous ADH 3, 4
• Pituitary magnetic resonance imaging (MRI) to evaluate the pituitary gland and stalk for any abnormalities or lesions 3, 4

Rationale for Diagnostic Studies

The patient's increased urine output and thirst suggest a possible defect in ADH secretion or action, which can be evaluated through measurement of urine and serum osmolality 3, 5. The water deprivation test can help differentiate between central DI, nephrogenic DI, and primary polydipsia 3, 6. The desmopressin test can confirm the diagnosis of central DI by assessing the patient's response to exogenous ADH 3, 4. Pituitary MRI can help identify any underlying lesions or abnormalities in the pituitary gland or stalk that may be contributing to the patient's symptoms 3, 4.

Most Appropriate Next Step

Based on the patient's symptoms and the potential diagnoses, the most appropriate next step would be to measure the patient's urine and serum osmolality to assess the concentration of the urine and the level of ADH in the blood 3, 5. This can help guide further diagnostic testing and management. Additionally, a pituitary MRI may be considered to evaluate the pituitary gland and stalk for any abnormalities or lesions 3, 4.