Master the board step 3

Master the Boards: USMLE Step 3
Hypernatremia
Elevated serum sodium always implies a free water deficit. Dehydration is
treated with normal saline replacement at first. Step 3 does not require knowledge
of specific dosing. However, fluids should be first ordered as a bolus, then
given continuously.
Besides simple dehydration, which can occur from poor oral intake, fever,
pneumonia, or other types of increased insensible losses, the other main cause
is diabetes insipidus (DI). Diabetes insipidus can be caused by either:
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failure to produce antidiuretic hormone (ADH) in the brain (central); or
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insensitivity of the kidney (nephrogenic). Nephrogenic DI can result from
hypokalemia, hypercalcemia, or lithium toxicity.
Hypernatremia leads to neurological abnormalities, such as confusion, disorientation,
or seizures. The worst manifestation is a coma. Sodium disorders
do not cause cardiac rhythm disturbance.
Both central and nephrogenic DI give the following results:
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Low urine osmolality
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Low urine sodium
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Increased urine volume
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No change in urine osmolality with water deprivation
The following table summarizes specific diagnostic tests and treatment for
central DI and nephrogenic DI.
Urine volume
Urine osmolality
Central DI
Prompt decrease in urine
volume with administration of
vasopressin (DDAVP)
Prompt increase in urine osmolality
with DDAVP
Nephrogenic DI
No change in urine volume with DDAVP
No change in urine osmolality with
DDAVP
Treatment Treat with DDAVP or vasopressin Correct underlying cause, such as
hypokalemia or hypercalcemia. Thiazide
diuretics are used in other cases.
Hyponatremia
Hyponatremia presents with neurological abnormalities, such as confusion,
disorientation, seizures, or coma. There will be neither edema nor signs of
dehydration.
The first step in the management of hyponatremia is to assess volume status
to determine the cause and, therefore, the treatment.
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