Disorders of Water Balance

Question 1

what are the two key effectors in the maintenance of plasma osmolality?

Answer 1

1. circulating ADH/AVP - rise linearly as plasma osmolality increases
2. water intake

Question 2

in patients with really low BP or volume, baroreceptors stimulation induces non-osmotic release of _____

Answer 2

ADH and AVP - regulate plasma osmolarity, but in situations of low BP/volume will be high even if plasma osmolarity is low

Question 3

osmotic demyelination syndrome

Answer 3

demyelination occurring from correcting plasma [Na+] too quickly, symptom onset 2-6 days after correction

—> lethargy, confusion, disorientation
—> dysarthria, dysphagia
—> paraparesis or quadriparesis
—> obtundation, coma

Question 4

Defects in water excretion are almost always associated with an inability to suppress ____

Answer 4

ADH secretion

persistent ADH release can be caused by volume loss (GI or renal), SIADH, depletion of effective circulating volume (CHF, cirrhosis), or cortisol deficiency (cortisol suppresses ADH release)

Question 5

what are the common etiologies of SIADH?

Answer 5

SIADH: persistent vasopressin secretion in absence of osmotic or hemodynamic stimulus, most common cause of euvolemic hyponatremia

must exclude other causes (like low cortisol or hypothyroidism)

etiologies: CNS disturbance (head trauma), malignancy, drugs

Question 6

what is the cause of nephrogenic syndrome of inappropriate anti-diuresis (NSIAD)?

Answer 6

NSIAD: X-linked GOF mutation in vasopressin V2 receptor —> recurrent episodes of hyponatremia

vasopressin is not detectable, but urine osmolarity is high - receptor constitutively active

manage with fluid restriction

Question 7

hypernatremia + high plasma osmolarity + low urine osmolarity is indicative of…..

Answer 7

Central diabetes insipidus: ADH synthesis is impaired, associated with production of large volumes of dilute urine

Commonly due to head trauma or CNS infection

Question 8

50% of patients who take lithium develop this renal disorder. What is?

Answer 8

Nephrogenic diabetes insipidus: tubule segments do not respond to ADH, there is normal or increased ADH

Question 9

What is the cause of X-linked vs autosomal congenital nephrogenic diabetes insipidus?

Answer 9

X-linked: LOF mutation in vasopressin V2 receptor

autosomal: LOF mutation in aquaporin 2

infants present with polyuria + polydipsia and recurrent episodes of hypernatremia —> irreversible neurological deficits can occur