CC: Diabetes insipidus

Question 1

Diagnosis

Answer 1

Excretion of large volume of dilute urine despite hyperosmolar serum

Question 2

Polyuria & polydipsia

Answer 2

1. Excessive urine production

2. Excessive intake of fluid

Question 3

Neurogenic (central) DI causes (3)

Answer 3

1. Often seen post-surgery of head trauma or pituitary surgery; damage to posterior pit
2. Autoimmune infiltration of lymphocytes; leads to swollen pituitary stalk
3. Langerhans’ cell histiocytosis: inflammation of immune cells

Question 4

Nephrogenic (renal) DI (2)

Answer 4

1. Majority of adult cases: acquired
   - high Ca, low K, drugs (lithium, demeclocyline)
2. Familial causes are rare
   - V2 receptor: x-linked recessive
   - AQP2: autosomal dominant

Question 5

Primary polydipsia (2)

Answer 5

1. Most common cause of polyuria in Western countries
2. Dipsogenic DI: reset thirst threshold due to mass lesions, granylomatous disease, idiopathic, aging)
3. psychogenic DI: drink for reasons other than true thirst (“good for you”), schizo= psychosis-intermittent-hyponatriemia-polydipsia (PIP) syndrome

Question 6

Screening

Answer 6

1. Hypotonic polyuria:
   24 hr urine volume >50ml/kg
   urine osmolality <300mOsm/kg
   urine dipstick negative for glucose (no solute diuresis)

Question 7

Water deprivation tests (2)

Answer 7

1. overnight: withhold fluids after dinner until next morning, measure serum Na and urine osmolality
2. Formal/inpatient: withhold until BW decreases by 3-5%, urine osmolality plateaus, or high serum Na >145
   -Administer AVP or dDAVP:
   increase in urine osmolality >50% -central DI
   <10% change = renal DI

Question 8

Differentiating between central DI and other types of polyuria

Answer 8

1. Measure plasma ADH levels
   - High ADH: nephrogenic DI
   - Low ADH: central DI
   - within normal levels: polydipsia
2. Posterior pituitary bright spot absent: central DI
3. Thick pituitary stalk: autoimmune; central DI

Question 9

Treatment

1. water
2. antidiuretic agents
3. antiduresis enhancing agents
   - chlorpropamide, carbamazepine, indomethacin

Answer 9

DDAVP: elimination of amino group at terminal and L–> D-Arg

1. Longer half-life than AVP
2. Selective to V2 receptor

*AVP has a short half-life b/c need to quickly save or excrete water; prevent intoxication

Question 10

Lesion in osmoreceptors

Answer 10

1. Can’t stimulate neurons –> no thirst –> no polydipsia

2. Still get polyuria –> no ADH secretion