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Diabetes insipidus intracranial surg

I. What is DI?

A. Central DI – decreased secretion of antidiuretic hormone (ADH, aka AVP) – results in polyuria and polydipsia by diminishing the patient’s ability to concentrate urine – defect in one or more sites involving the hypothalamic osmoreceptors, supraoptic or paraventricular nuclei, or the supraopticohypophyseal trac

B. Nephrogenic DI – decrease in the ability to concentrate urine due to a resistance to ADH action in the kidney. – seen in chronic renal insufficiency, lithium toxicity, hypercalcemia, hypokalemia, and tubulointerstitial disease

-rare hereditary form of nephrogenic diabetes insipidus is transmitted as an X-linked genetic defect of the V2receptor gene

ADH: Actions are mediated through at least 2 receptors V1 mediates vasoconstriction, enhancement of corticotrophin release, and renal prostaglandin synthesis V2 mediates the antidiuretic response

C. ETIO -can result from trauma or CNS tumor (craniopharyngioma, pinealoma), CNS surgery (particularly to pituitary or hypothalamus) -10-20% of patients get DI following transsphenoidal removal of an adenoma, which increases to 60-80% with large tumors. Not all cases of diabetes insipidus are permanent. The most common causes of postoperative polyuria are excretion of excess fluid given during surgery and an osmotic diuresis as a result of treatment for cerebral edema

D. Manifestation: one of 3 patterns can be exhibited

1. Transient

2. Permanent

3. Triphasic: more often clinically observed

-1st: polyuric, lasts 4-5 days, ADH is inhibited -immediate increase in urine produced -2nd : antidiuretic phase, lasts 5-6 days, stored ADH is released -3rd: permanent DI, ADH stores are exhausted, cells that produce more ADH are absent or unable to produce it

E. DDx: psychogenic polydipsia, osmotic diuresis

F. L/S: usually clinical dx -urine specific gravity of 1.005 or less and a urine osmolality less than 200 mOsm/kg is the hallmark of diabetes insipidus. -Random plasma osmolality generally is greater than 287 mOsm/kg. -water deprivation test (ie, Miller-Moses test), a semiquantitative test to ensure adequate dehydration and maximal stimulation of ADH for diagnosis, is performed in ambiguous clinical circumstances, typically with more chronic forms of diabetes insipidus -MRI Brain

G. Rx: desmopressin, IVF if pt is not drinking enough

DI, usually appears 4-24 hrs after surgery, not intraoperatively. Dx made by hypo-osmolar urine w/ rising serum osmolarity. Replacement of fluid should be D5-1/2NS at normal maintenance rate plus 2/3 of prev hours UOP. Do not replace full uop, and monitor for hyperglycemia. If fluid requirements exceed 350-400ml/hr consider giving DDAVP.