Question

Laboratory Test: The patient was then subjected to a 2-hour water deprivation test followed by another blood chemistry profile and urinalysis. Of note, serum osmolality increased to 329mOsm/kg while serum ADH, urine specific gravity and urine osmolality remained unchanged.

Questions to Consider:

1. Why did the patient’s serum osmolality increase?

Follow-up and Diagnosis: The patient was then injected with a drug called DDAVP (desmopressin) which mimics the actions of ADH. One hour after the injection, serum osmolality decreased to 292mOsm/kg and urine osmolality increased to 480mOsm/kg. Based on the patient’s medical history, the results from lab tests and the, you make the differential diagnosis of idiopathic central diabetes insipidus.

Questions to Consider:

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2. Why do you suspect serum osmolality decreased? Why do you suspect urine osmolality increased? Based on these results, what is happening to water?

3. What do these results suggest is the underlying cause of diabetes insipidus?

4. How does the diuresis experienced in diabetes insipidus differ mechanistically from that experienced by a patient with diabetes mellitus?

Answer 1

1. The patient's serum osmolality increased because;
   • This patient is suffering from diabetes insipidus.
   • In this condition, there is a deficiency of the hormone antidiuretic hormone (ADH) or kidneys are not responsive to ADH.
   • ADH is a hormone that acts on the collecting ducts ( in the nephron ).
   • This hormone is released when the body is water-deprived.
   • Water-deprivation increases blood osmolarity. Increases in the blood osmolarity stimulate the release of ADH from the posterior pituitary.
   • ADH acts on the collecting duct and promotes water resorption.
   • When water is reabsorbed, it increases the blood volume.
   • This dilutes the blood, and therefore, reduces the blood osmolarity.
   • In diabetes insipidus, there is a deficiency of the hormone antidiuretic hormone (ADH) or kidneys are not responsive to ADH.
   • Therefore after water deprivation, there is
     • Release of ADH, or
     • ADH is released but the kidneys don't respond to it.
   • The net effect is no retention of water. This increases the blood osmolarity.
2. The patient's serum osmolarity decrease and urine osmolarity increase because:
   • After water deprivation, due to diabetes insipidus, blood osmolarity increases.
   • The increase in blood osmolarity may be due to:
     • Deficiency of ADH
     • ADH is released but the kidneys don't respond to it.
   • When desmopressin was administered, the patient's serum osmolality decreases and urine osmolarity increased
   • This means that
     • There is a deficiency of ADH in the body.
     • The kidney is responsive to the ADH hormone.
   • This means there is a central diabetes insipidus. [ If after desmopressin administration, there was no decrease in serum osmolarity and no increase in urine osmolarity - it would indicate nephrogenic diabetic insipidus ]
3. This patient is suffering from central diabetes insipidus:
   • There is deficiency of ADH secretion.
   • As a result, the body is unable to retain water in the body.
   • this water is lost in the urine.
4. The underlying cause of diabetes insipidus is ADH deficiency ( central diabetes insipidus)
   • Clues to the diagnosis:
     • Water deprivation didn't cause ADH release ( evident by no decrease in serum osmolarity )
     • After the administration of desmopressin, there was water retention. This caused a decrease in the serum osmolarity and an increase in urine osmolality.
5. Diabetes mellitus there is hyperglycemia. This hyperglycemia ( increase blood sugar levels ) induces osmotic diuresis. In the diabetes insipidus, diuresis is due to lack of anti-diuretic hormone.