List the potential causes of polyuria during the immediate post-operative period in a patient who has undergone surgery for a pituitary tumour.
For each cause listed, outline the following:
i.     Mechanism of action,

ii.     Approach to diagnosis, and

iii.     Management

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College answer

Central Diabetes Insipidus-

Mechanism of action- 
Anatomic injury to hypothalamus, pituitary stalk or posterior pituitary gland. Usually partial or total disruption of pituitary stalk-severs connections between cell bodies of ADH secreting neurons in hypothalamus and their nerve endings in posterior pituitary preventing ADH secretion.  
ADH acts at the V2 receptor via cAMP and is responsible for the insertion and removal of water (aquaporin) channels into luminal membrane of renal tubules thereby altering permeability to water.  
 

Approach to diagnosis- 
Hypotonic polyuria- Urine output >300ml/hr x 3hrs OR >2.5ml/kg/hr OR >3L/day. Urine osmolality<200mOsm/kg (urine specific gravity <1.010), Hypernatraemia. Lab parameters may be affected if patient has access to free water. 
 

Management-  
DDAVP (Desmopressin) 1-4mcg SC or IM (OR short acting AVP analogue), monitor urine output, watch for hyponatraemia developing. Allow to drink to thirst if possible and monitor. Exclude other causes of polyuria (as below). Monitor urine and serum osmolality and electrolytes. 
 
Hyperglycaemia- 
Mechanism of action-  
Osmotic diuresis from high blood glucose levels.  
Approach to diagnosis-  
Presence of high blood glucose levels in the context of glucocorticoid administration following pituitary tumour resection or from Cushing’s disease or GH-secreting tumour.

Management- 
Correct BGLs with insulin. Correct volume deficit and monitor fluid balance, electrolytes. 
 
High volume IV fluid administration- 
Mechanism of action- 
High volume IV fluid administration intra-operatively 

Approach to diagnosis- 
Evidence of high volume IV fluids, electrolytes measured may vary with fluid administered

Management- 
Should be self-limiting. Monitor urine output/electrolytes. 
 
Acute fall in Growth Hormone-

Mechanism of action- 
Drop of blood levels of GH and IGF-1 that cause fluid retention when inappropriately high due to a GH-secreting tumour. 
 

Approach to diagnosis- 
Context of GH secreting tumour and acromegaly. Serum and urine electrolytes and osmolalities should be in reference range.

Management- 
Should be self-limiting. Monitor urine output/electrolytes. 
 
Mannitol administration-

Mechanism of action-  
Osmotic diuresis 

Approach to diagnosis- 
Administration of mannitol due to large tumour/cerebral oedema (unusual). Osmolar gap may be seen. 

Management- 
Should be self-limiting. Monitor urine output/electrolytes 
 
Examiner Comments: 
 
Maximum score if Diabetes Insipidus not mentioned was 4 marks. Must have mentioned at least two causes to achieve passing mark. 

 

Discussion

As a tabulated answer, this has the distinct disadvantage of requiring a level of detail which does not lend itself to elegant distribution into narrow columns. There is simply too much to write about. The table which follows represents distilled information from the excellent articles by Prete et al (2017) and Hensen et al (1999)

Mechanisms of Polyuria following Pituitary Surgery
Mechanism Diagnosis Management

Central diabetes insipidus

   
  • Secretion of vasopressin from the posterior pituitary is interrupted;
  • Aquaporin expression on the luminal surface of the collecting duct is inhibited
  • Water resorption 
  • Urinalysis: maximally dilute urine
  • Urinary sodium: minimal
  • Serum sodium: increasing
  • Serum osmolality: increasing
  • Serum ADH levels: minimal
  • DDAVP
  • Vasopressin infusion
  • Free water replacement (NG or intravenously as 5% dextrose)

Cerebral salt wasting

   
  • Brain natriuretic peptide-mediated hypovolemic hyponatremia 
  • Polyuria is the consequence of natriuresis
  • Urinary sodium: 20-40 mmol/L
  • Serum sodium: decreasing
  • Hypertonic saline
  • Volume replacement with sodium-rich fluid, eg. normal saline

Perioperative IV fluids

   
  • Iatrogenic fluid overload
  • Perioperative fluid accumulation is mobilised after the patient has been extubated and raised intrathoracic pressure is removed, allowing venous return. 
  • Stable serum sodium
  • Stable haemodynamic performance
  • Urinary sodium is low
  • Nil

Osmotic diuresis

   
  • Perioperative mannitol leads to diuresis
  • Urinary osmolality: high
  • Serum osmolality: high
  • Serum osmolar gap: high
  • Serum sodium: increasing
  • Fluid replacement with isotonic fluid
  • Hyperglycaemia leads to diuresis
  • Due to steroid use perioperatively, or ACTH-secreting adenoma, or insulin resistance due to GH-secreting adenoma
  • BSL: elevated
  • Urinary osmolality: high
  • Serum osmolality: high
  • Serum osmolar gap: normal
  • Serum sodium: increasing
  • Fluid replacement with isotonic fluid
  • Insulin infusion
Acute fall in growth hormone (GH) levels
  • Removal of GH-secreting adenoma 
  • GH hypersecretion tends to cause sodium and water retention (Kamenicky et al, 2013)- polyuria represents the mobilisation of retained fluid.
  • Clinical features of acromegaly
  • Urinary osmolality: normal
  • Serum osmolality: normal
  • Serum osmolar gap: normal
  • Serum sodium: normal
  • Nil

References

References

Guerrero, R., et al. "Early hyponatraemia after pituitary surgery: cerebral salt-wasting syndrome." European journal of endocrinology 156.6 (2007): 611-616.

Hensen, Johannes, et al. "Prevalence, predictors and patterns of postoperative polyuria and hyponatraemia in the immediate course after transsphenoidal surgery for pituitary adenomas.Clinical endocrinology 50.4 (1999): 431-439.

Hans, Pol, Achille Stevenaert, and Adelin Albert. "Study of hypotonic polyuria after trans-sphenoidal pituitary adenomectomy." Intensive care medicine 12.2 (1986): 95-99.

Kamenický, Peter, et al. "Growth hormone, insulin-like growth factor-1, and the kidney: pathophysiological and clinical implications." Endocrine reviews 35.2 (2013): 234-281.