Hyponatremia

Today we discussed a common internal medicine problem, hyponatremia.

Acute vs. chronic cutoff is 48h.

Hyponatremia relative excess of free H2O for a given total body Na or ECFV (whether it's hypo/eu/hyper).

Traditional first breakpoint is by serum osmolality:
1) Hyperosmolal: hyperglycemia (10:3 adjustment), mannitol (IV)
2) Euosmolal: hyperproteinemia, hyperlipidemia (both pseudo; not usually an issue with modern lab techniques), bladder irrigation (dilutional)
3) Hypo-osmolal: Most common

Within Hypo-osmolar, divide by volume status:
Hypo: vomiting, diarrhea, DKA, diuretics, hypercalcemia
Eu: post-op, pain, nausea, pregnancy, SIADH (paraneopl from lung/CNS/pancreas/HL/leukemia or non-neoplastic), beer potomania, psychogenic polydipsia, hypothyroidism, adrenal insufficiency
Hyper: CHF, cirrhosis, nephrotic syndrome, renal failure

A word on SIADH:

Drugs that cause it: The "C's":
cyclosporin, cyclophosphamide, carbamazepine, cisplatin, 1st gen sulfonylureas (like chlorpropramide), antipsychotics (like clozapine), SSRI (OK...not a "C")

SIADH criteria
serum osm less than 275
urine osm more than 100
urine na more than 40
euvolemia
no diuretics
normal thyroid and adrenal function

NB- 'SIADH' with low UNa is not inappropriate b/c hypovolemia should turn on ADH.

Rate of correction:

Be very careful with the hypovolemic, hyponatremic patient (which is very common!). Danger is to volume replete and turn off ADH, causing dilute urine excretion, and rapid increase in Na. Consider ddAVP here; e.g. 2-4mcg IV or SC, even possibly before volume resuscitation. Always monitor urine output and serum lytes closely in this setting.

Aim for an increase of less than 8mEq per 24h (truly dangerous range is 12mEq/24h)
Highest CPM risk: malnourished, elderly, EtOH use (i.e. most people who get hyponatremic!)

Acute or severely symptomatic (e.g. sz, coma, cerebral edema):
3% saline at 1-2ml/kg/h
aim for 2mM/h increase
check Na q2h
d/c when symptoms improve

Othewise, fluid restrict and watch carefully.

Links:

Click here for a NEJM review of SIADH

Click here for the evidence for ddAVP in preventing overcorrection