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Diabetes Insipidus is characterized by polyuria, polydypsia and hypernatremia. Patients excrete large amounts of dilute urine with osmolality typically < 300 mOsm/kg and urine specific gravity 1.001-1.005; urine output is typically > 250 cc/hr. If diagnosis is in doubt then do a water deprivation study where the patient with DI will continue to have a brisk diuresis. Endocrine dysfunction results from damage to the supraoptic nucleus, hypothalamo-neurohypophyseal tracts, and/or the posterior pituitary. Hypothalamic cell loss and retrograde axonal injury are key determinants. The number of vasopressin secreting magnocellular neurons correlates with the severity of DI; if less than 10-20% survives injury, permanent DI will result. Postoperatively, edema around the pituitary stalk, residual traction injury, and generalized hypotension/hypovolemia may all contribute to cell death.
Transient DI is the commonest pattern, especially following pituitary surgery, and accounts for 50-60% of cases. Onset is abrupt but short-lived (several days), with no residual loss of vasopressin secretion capacity. Permanent DI results from damage to the pituitary stalk, median eminence, or hypothalamus. Vasopressin may still be secreted, albeit in insufficient quantity, leading to chronic high urine output. Transfrontal resection of suprasellar masses involves high risk of permanent DI. Triphasic response DI is the least common yet most dangerous pattern. Initial diuresis occurs abruptly on post-operative day 0 or 1, as with the other patterns, but resolves spontaneously after 1-5 days. This is due to release vasopressin peptide from degenerating neurohypophyseal tissue rather than physiologic reconstitution. Commonly, DI progresses through the first two phases but resolves before the appearance of the third phase. Without neurohypophyseal regeneration the third phase of overt DI ensues, which may be transient or prolonged depending on the degree of damage. '
- start with 0.45% NaCl or 0.9% NaCl at a basal rate and replace urine output 1:1 with 0.25% NS
- aqueous vasopressin with a ½ life of 1-2 hours can be used when the patient is hypotensive
- DDAVP has a longer duration of activity (1/2 life 8-12 hours) but has none of the vasopressor effects of aqueous vasopressin (normal dose range 5-20 micrograms q12 hours) and may be used if unable to keep up with fluid losses as Na continues to climb