Case Study 3: Diabetes Insipidus

Inexpensive tests for frequency urination patient?
Checking hormones levels – problems with pituitary or adrenal gland
Blood test to check for hormone levels
Measure urination quantity
Physical exam for hydration status, checking blood pressure
Perform a prostate examination
Giardia does not cause increased urination, may be due to excessive consumption of water
Have not ruled out diabetes, could perform sugar urine test as well as blood sugar

Higher levels of glucose results in…
Decreased reabsorption of water due to osmosis flow, results in more urination

What does the dipstick urinalysis indicate
Specific gravity – shows whether the urine is diluted or not; ratio of the density of a substance to the density (mass of the same unit volume) of a reference substance

Used to rule out structural defects of kidneys, UTI

Urine normally has a specific gravity between 1.003 and 1.035.

Low specific gravity = dilute urine

Blood normally has a specific gravity of ~1.060.

Urine Solute Test
Measurement of urinary osmolarity

Low osmolarity = more water = dilute urine

High osmolarity = less water = concentrated urine

24 Hour Urine Volume Levels
Should be between 800-2000 mL

Elevation above this value is known as polyuria

What conditions decrease urine osmolarity
Overhydration – excess fluid intake

Kidney failure – inability to excrete excess fluid

Malfunctions with ADH hormone – inability to reabsorb water (remains in the collecting tubule)

Infection of the bladder or kidney – Post Streptococcal glomularnephritis

Renal tubular necrosis

Diabetic Causes of Decreased Urine Osmolarity
Diabetes insipidus – excess urine output, typically dilute urine; usually blood glucose levels aren’t changed

Nephrogenic DI – insensitivity to ADH despite release from CNS

Neurogenic DI – affecting the hypothalamus, lack of ADH

Addison’s Disease
no secretion of aldosterone, therefore no Na+ or H20 reabasorption, leads to decreased urine osmolarity

Cushing’s Disease
increased ACTH levels, which increases cortisol secretion. Increased glucose levels in blood draws H20 out and decreases reabsorption of water, leading to decreased urine osmolarity

Over-hydration due to psychogenic polydispsia
Constant sensation of dry mouth, urge to consume water, could be due to medications

What would you expect a dipstick urinalysis to show for Diabetes Insipidus?
Constant urine output regardless of water consumption, due to hormonal regulations instead of water consumption urges

What would you expect a dipstick urinalysis to show for Psychogenic Polydipsia?
Following doctor’s instructions to refrain water intake for 24 hours, urine collection levels should eventually decrease near the end of the 24 hour cycle.
Osmolality should increase and stabilize

What are the three main forms of diabetes insipidus in adults?
Neurogenic / Central DI

Nephrogenic DI

Dipsogenic DI

Neurogenic / Central DI
Decreased production of ADH
Results from damage to the pituitary gland
Could be caused by head trauma / infectious process
Medication used for depression

Nephrogenic DI
Insensitivity to ADH release

Dipsogenic DI
Defect to thirst mechanism in hypothalamus

Inability to trigger ADH secretion

What are the causes of central diabetes insipidus?
Results from damage to the pituitary gland – metastatic pituitary hypothalamic tumor

Could be caused by head trauma / infectious process – destruction of hypothalamus or some part of the supraoptic hypothalamic tract leading to the posterior pituitary gland

Medication used for depression that the patient did not inform the physician of

Vascular autoimmune infection (sarcoidosis)

Genetic disorders may play small role

What are the causes of nephrogenic diabetes insipidus?
Insensitivity to ADH

Could be lithium medication complication

Genetic disorder – V2 class of vasopression receptors, lack of aquaporin receptors to reabsorb water

Chronic disorders – polycystic kidney disease, sickle cell disease, amyloidosis, inherited genetic disorders

Kidney failure


How might Central and Nephrogenic Diabetes insipidus be differentiated?
Desmopression – synthetic hormone, used to treat neurogenic DI (mimics ADH)

Medication does not work in nephrogenic DI since body is insensitive to ADH

Measure levels of ADH:
Would be decreased in neurogenic DI compared to nephrogenic DI, complete by using hormone blood test

Head MRI – examination of potential malfunctions or damage of the pituitary gland / hypothalamus

What is desmopressin?
Mimics ADH function

Structural analog to vasopression, works through second messenger system by initially binding to a V2 receptor on the basolateral surface

Results in phosphorylation of proteins, frees aquaporin receptors from enclosed vesicle within the luminal cell

Aquaporin receptors transported to apical membrane of the collecting duct, allows for reabsorption of water

Decreases urine output

Typically in the nephron, hormones function near the distal end of the tubular system (collecting ducts)

Why high sodium levels critical in patient with central diabetes insipidus?
Lack of ADH results in decreased water reabsorption

Hypothalamus recognizing increased sodium levels, results in activation of thirst mechanism, kidneys attempting to reabsorb remaining water

By consumption of chips and his underlying condition without medication, the patient will become extremely dehydrated

What precaution should be taken in treating diabetes insipidus?
Avoid excess salts within IV fluid
Recommended to give hypotonic saline solution

Negative side effect – destruction of nerve cells by lysis
IV drip possibility, continual assessment of the patient, several blood samples. Attempts to correct low blood volume / blood pressure

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