HYPONATREMIA
Definition
- <135
mmol/L (but usually only investigate at <130)
- state of excess total body water vs. total body sodium (not
necessarily secondary to sodium depletion)
Signs
and Symptoms
- hyponatremia is most often an incidental finding, especially
if mild
- the severity of symptoms is related to the degree of
hyponatremia and rate of fall of sodium levels
e.g. headache, nausea / vomiting, muscle cramps, restlessness, lethargy,
confusion, decreased reflexes, seizure, coma, death
DDx
ISO-OSMOLAR (pseudo-hyponatremia) (serum Osm 280-295):
hyperproteinemia, hyperlipidemia, post-TURP
HYPER-OSMOLAR (impermeable solutes) (serum Osm >295):
hyperglycemia (for every 10 increase in glucose,
Na decreases by 3), mannitol, glycerol, sorbitol, IVIg (secondary to maltose)
HYPO-OSMOLAR (serum Osm <280):
- EUVOLEMIC (primary water gain)
SIADH (see separate notes),
psychogenic polydispsia, hypothyroid, Addison's / glucocorticoid deficiency,
diuretics
- HYPERVOLEMIC (primary sodium gain)
CHF, CRF, nephrotic syndrome, cirrhosis, protein-losing
enteropathy
- HYPOVOLEMIC (primary sodium loss)
diuretics (thiazides), vomitting, diarrhea,
burns, hemmorhage, hypoaldosteronism, cerebral salt wasting, post-obstructive
diuresis, non-oliguric ATN, pancreatitis
Investigations
CBC, electrolytes, urea, creatinine, glucose, urine Osm and
lytes, serum Osm
Consider: TSH, random cortisol, LFTs, chest x-ray, CT head
Approach
1. Determine patient’s volume status. Is the patient
dehydrated (hypovolemic), euvolemic or fluid overloaded (hypervolemic)?
i.e. assess postural BP and HR, JVP,
mucous membranes, skin turgor, peripheral / sacral edema, basilar crackles
2. If patient is HYPOVOLEMIC:
- hydrate with IV normal saline
- discontinue diuretics
NOTE: Hypovolemia itself stimulates ADH release. ADH levels
will fall with volume resuscitation which may result in a rapid rise in serum
sodium. Modify the rate of infusion accordingly (see below).
3. If the patient is HYPERVOLEMIC or EUVOLEMIC:
For the asymptomatic patient or if the urine is
dilute (UOSM <200)
- fluid restriction to 1-1.5L / day
will usually be sufficient to correct the sodium level
- consider administering lasix
20-40 mg IV (lasix causes more water to be excreted than Na+)
For the patient with severe symptoms or concentrated
urine (UOSM >200)
- fluid restrict to 0.8-1.5L/d
- consider administering lasix
20-40 mg IV (lasix causes more water to be excreted than Na+)
- consider infusing hypertonic saline only until symptoms
improve or [Na+] = 125-130 mmol/L. Nephrology
or Internal Medicine should be consulted if hypertonic saline infusion
is required.
- other possible treatments:
lithium, urea
4. MONITOR serum lytes frequently to ensure that the
serum sodium level is being corrected and that the rate of rise is less than 8 mmol/L/day (see CAUTION below). E.g. q2-3h to
q6h initially then BID then OD
5. Treat seizures with anticonvulsants
6. Treat the underlying condition if possible
CAUTION
*** Rapid
sodium correction may result in central pontine myelinolysis!
***
Maximum rate of increase in serum Na+ is 0.5
mmol/L/h or 8 mmol/L/day
- In
patients with severe symptoms, the initial rate of correction can be increased
to 1 mmol/L/h for 2-3 hours, but then the rate should be decreased to the
aforementioned rate
- stop
correcting sodium at serum Na+ = 130-135 or if rate of rise is too
fast!
- even a small increase of 3-5 mmol/L of Na+ can be
enough to alleviate symptoms
Determining
the rate of saline infusion
1. Set
the target increase in [Na+] ie. 3 mmol/L in next
3 hours
2.
Calculate the change in serum sodium for administration of 1L of the chosen
infusate
Change in serum Na+ for 1L infusate = (infusate
Na+ - serum Na+) / (TBW + 1)
Infusate
Na+
-
0.9% saline (N/S) infusate Na+ = 154 mmol/L
- 3%
saline infusate Na+ = 513 mmol/L
Total
Body Water (TBW) = fraction x body weight (kg)
Where the
fraction is:
-
0.6 or 0.5 in non-elderly men or women, respectively
-
0.5 or 0.45 in elderly men or women, respectively
e.g. if serum Na+ = 120, therefore 1L N/S will increase the
serum Na+ by (154-120) / (0.6x70+1) = 0.8 mmol/L in a 70 kg
non-elderly man. If we had used 1L 3% saline, the increase would be 9.1 mmol/L.
3.
Calculate the rate of infusion. How much volume of chosen infusate is required
to meet desired target increase in serum sodium?
e.g. To obtain a 3 mmol/L rise in 3 hours
- using
3% saline, 330 ml would be required (3 / 9.1) at a rate of 110 ml/h (330ml / 3
hours)
- using
normal saline, 3.75 L (3 / 0.8) at a rate of 1250 ml/h (3750 ml / 3 hours)!
References
NEJM
342:1585 '00
Last
update: 2003-03-03
Reviewed
by: pending
Neurological
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