The Water Switch: Fourteen Days After a Transsphenoidal Hypophysectomy
The posterior pituitary is a switch: SIADH holds water and drowns the brain in dilute sodium, diabetes insipidus dumps water and dries it out — and one patient can do both in a fortnight. Scrub through Mr Ferreira's post-operative course, read the monitor, and call each phase before you look at the answer. Urine specific gravity beside a serum sodium settles it in two minutes.
1 · Scrub the post-operative course
h 2h 14POD 1d 3d 7d 9d 13
0 of 7 called
The switch, in one table
SIADH
Diabetes insipidus
ADH
Too much
Too little, or kidney cannot respond
Urine output
Low, concentrated
Massive, dilute
Specific gravity
Above 1.030
Below 1.005
Serum sodium
Below 135 mEq/L
Above 145 mEq/L
Serum osmolality
Below 275 mOsm/kg
Above 295 mOsm/kg
Weight
Gains, without oedema
Loses
Main risk
Cerebral oedema and seizure
Hypovolaemic shock and hypernatraemia
Treatment
Fluid restriction; hypertonic saline if seizing; vaptans
Replace water; desmopressin for central; thiazide for nephrogenic
Whatever the sodium is doing, the nose rules do not change. Head of bed 30 degrees. No nose blowing, no sneezing with the mouth closed, no straws, no coughing without support, no straining — give stool softeners and antiemetics preemptively, because a Valsalva can blow open the sellar repair. Change and document the nasal drip pad. Watch for a cerebrospinal fluid leak: clear drainage positive for glucose or showing a halo sign on gauze, persistent postnasal drip, a salty or sweet taste, or a severe positional headache — report immediately, because meningitis is the consequence. Oral care by rinses only, no toothbrushing near the incision line for about ten days, and warn him that upper-lip and gum numbness may last weeks.
2 · The bedside monitor
Urine output
mL/hr
Urine specific gravity
Serum sodium
mEq/L
Serum osmolality
mOsm/kg
Weight, same scale, same time
3 · Your call
Read the monitor, then call the phase on the left. The answer comes back with the physiology and with what the nurse does about it.