Question 17

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)