A 72-year-old man working on the roof of his house falls about 4 m to the ground. He is taken to the local emergency department. On physical examination vital signs show T 37 C, P 70/min, RR 17/min, and BP 140/90 mm Hg. There is a 4 cm contusion with swelling over the left parietal region scalp. There is no papilledema. A head CT scan shows no intracranial hemorrhage or midline shift. He remains in stable condition. Over the next week he develops increasing polydipsia and polyuria.

Questions:

3.1 What is suggested by these findings? What additional diagnostic findings should be sought?

He has developed diabetes insipidus. His serum osmolality is 320 mosm/mL and urine osmolality is 40 mosm/mL. Administration of ADH results in an increase in his urine osmolality.

3.2 What is the most likely diagnosis?

He has central diabetes insipidus, probably as a consequence of the head trauma, with loss of ADH secretion.

3.3 What is his free water clearance, if his urine flow is 16 mL/min?

C of H2O = urine flow rate - ((urine osmolality / plasma osmolality) X urine flow rate)

C = 16 mL/min - ((40 / 320) X 16 mL/min)

C = 14 mL/min

Normally, if the serum (plasma) osmolality increases (as with dehydration), the free water clearance decreases, the ADH increases, and the urine flow decreases. Drinking a lot of water will diminish serum osmolality, increasing free water clearance through an decrease in ADH with increase in urine flow.

If his urine osmolality would have been high (>300 mosm/mL) then excretion of a solute pulling the water with it would be likely, such as glucosuria or hypercalciuria.

If both his urine and serum osmolality are reduced, then psychogenic polydipsia is suggested.

3.4 What are normal body fluid compartments?

Total body water = 60% of body weight = 42 L.

Extracellular fluid = 20% of body weight = 14 L.

Intracellular fluid = 40% of body weight = 28 L.

Plasma = 4% of body weight = 3 L.

Interstitial fluid = 16% of body weight = 11 L.