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Hyponatremic dehydration (dehydration with serum sodium < 140) may appear clinically similar to isotonic dehydration, but if the hyponatremia becomes pronounced, additional symptoms will become evident. These symptons reflect cellular overhydration which results from water movement from the relatively hypotonic serum into cells. Symptoms affect primarily the CNS and musculoskeletal systems. CNS effects range from headache, fatigue and anorexia to lethargy, confusion, disorientation, agitation, vomiting, seizures and coma. Musculoskeletal symptoms may include cramps and weakness. If these symptoms are present, the hyponatremia is fairly severe. However, people who have been chronically hyponatremic adapt to this condition, and their symptomatology underestimates the severity of their hyponatremia. Other physical findings related to the cause of the hyponatremia should be sought. For example, edema may be present in an patient with renal failure, or the patient may have sustained burns over a large surface area).
Although there are numerous etiologies for hyponatremia, they all boil down to three basic mechanisms related to water and sodium:
Decreased total body sodium accompanied by decreased of normal amounts
of body water.
Increased total body water with normal total body sodium.
An increase in both total body water and total body sodium, with the
increase in TBW exceeding that of sodium
Etiologies of hyponatremia can be divided into extrarenal and renal:
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There are four decision points in the diagnostic algorithm for hyponatremia:
1. Is the hyponatremia really a
sodium problem?
2. Is this a matter of water toxicity?
3. What is the volume status of the
patient?
4. Are the kidneys doing their job
of of retaining sodium?
1. Is this really a sodium problem?
There are several conditions in which a low serum sodium is misleading.
The reported lab value is usually in terms of sodium per volume of plasma,
rather than water. If large molecules such as lipids or protein are
present in large amounts, they decrease the amount of water in a given
volume of plasma; however, these large molecules contribute little to plasma
osmolality. What is truly important is the amount of osmoltically
active solute per volume of water. Newer ion-specific electrodes
report this value.
| For every unit increase in | this solute | measured sodium is decreased by |
| 1 g/dL | Triglycerides | 2 mEq/L |
| 100 mg/dL | Glucose | 1.6 mEq/L |
| Plasma Osmolality (mOsm/kg) = 2*[mEq/L Na+] + (mg/dL glucose)/18 + (mg/dL BUN)/2.8 |
| Solute | Add to plasma osmolality |
| Mannitol | (mg/dL)/18 |
| Ethanol | (mg/dL)/4.6 |
| Isopropanol | (mg/dL)/6 |
| Methanol | (mg/dL)/3.2 |
| Ethylene Glycol | (mg/dL)/6.2 |
3. What is the volume status of the patient? This is a major decision point in the hyponatremia algorithm, dividing patients into hypovolemic, euvolemia and hypervolemic states. It is basically a reflection of total body water (TBW). In hypovolemic hyponatremia, TBW is lost, but this loss is exceeded by the relative loss of sodium. In euvolemic states, the total body water is either usually increased. In hypervolemic states, total body sodium is increased but is exceeded by TBW.
4. Is the kidney doing its job in retaining sodium? As a matter of definition, there is too little sodium per volume of serum free water. The function of the kidney is the final branch point in the decision tree, helping to classify the different volume states according to etiology.
The following table is adapted from Pediatric Annals (1995) 24:23-30:
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1. Determine the Sodium Deficit
| Na deficit (mEq) = (135 - [Na+] ) * Bodyweight (kg) * 0.6 |
2. Emergency Treatment of Severe Hyponatremia
Treatment of hyponatremia involves gradual correction of the sodium deficit to avoid central pontine myelinolysis. However, if the patient is displaying signs of CNS involvement or is frankly seizing, a partial correction of the sodium deficit can be achieved rapidly.
If the patient is displaying altered mental status, the sodium can be
corrected rapidly to 125 mEq/L using 3% normal saline with or without loop
diuretics and/or water restriction depending on the clinical situation.
The goal is to prevent seizures. Substitute 125 instead of 135 in
the above equation to determine the number of mEq of Na needed, and administer
the equivalent amount of 3% saline over four hours. During this period,
monitor frequently for hypertension and signs of pulmonary edema.
After this initial correction, calculate the remaining deficit, subtracting
the fluid and sodium from this bolus. If the patient is seizing,
calculate the total sodium deficit and administer 1/3 of this amount of
3% normal saline over one hour.
| Example: Severe symptomatic hyponatremia, no seizures
On a hot summer day, a 5 year old patient with cystic fibrosis presents in the ER. He is lethargic and disoriented. His parents state that he was playing outside earlier in the day, and that after lunch he complained of a headache. He is tachycardic, afebrile, and has no pulmonary distress. A stat serum chemistry shows Na of 117. He weighs 18 kg, and is 115 cm tall. Calculating the Na deficit for emergency treatment:
3% NS has 513 mEq Na per liter, so only about 168 mL would be needed to supply 86 mEq. This can be infused over 4 hours. Since there is no concern regarding water restriction in this patient who must have a hypovolemic hyponatremia, NS could also be used to give this amount of sodium. Since normal saline is 154 mEq Na, 86 mEq would be 558 mL, which could be given instead over a four hour period. |
| Example severe hyponatremia with seizures
A Cushingoid 14 year old heme-onc patient with a history of doxorubicin-induced cardiomyopathy, brain metastases, and liver failure presents is transported to the ER while seizing. A stat ABG reveals a sodium of 117. His weight is 70 kg. In this complicated patient, the source of the hyponatremia is not clear. Regardless, a rapid correction of the serum sodium is necessary to stop the seizures. Calculate the total deficit and give 1/3 of it by rapid push of 3% saline. Deficit = (135-117) * 70 * 0.6 = 756 mEq sodium One third of this is 252 mEq, which would be given in about 500 mL of 3% saline. This amount could be bolused over one hour. In additional, 10 mg of furosemide could be given for his history of congestive heart failure. |
| Example: Severe hyponatremia, corrected over two days:
An 8 year old is found by a rescue crew after several days being
trapped in a mountain lodge with nothing to drink but water from melted
snow. He appears about 5% dehydrated, but his sodium is 112.
He is disoriented, but otherwise healthy. His weight is 20.9 kg.
As he is not seizing, he receives a 380 mL bolus of 3% saline over a four
hour period designed to correct his serum sodium to 125. The
remainder of his fluid and sodium deficit can be given over 48 hours:
After the initial bolus, sodium repletion can be accomplished using relatively hypotonic fluids. In this case, 1/3NS would be a good choice. The formula used for correction of hyponatremia uses the lower end of the normal sodium range (135-145). If 140 had been used, 43 mEq/L would have been required which is more than could be supplied by 0.2% NS. |
3. General Treatment of Hyponatremia
The above emergency considerations apply only to patients who are symptomatically
hyponatremia. The majority of hyponatremic patients will demonstrate
signs of dehydration alone. In these patients, the method of treatment
is similar to isotonic dehydration: half of the deficit fluids and electrolytes
are repleted in the first 8 hours, with the remainder over the next 16
hours:
| Example: Dehydration with asymptomatic hyponatremia case #1
A 3 year old with diarrhea is brought to the hospital. Clinically, she appears about 10% dehydrated. Her sodium is 128. Calculate the deficit due to hyponatremia: (135-128) * 12.5 kg * 0.6 = 52.5 mEq Na Calculate the proportional losses as in isotonic dehydration:
Using the Holiday-Segar formula, the maintenance fluids would be calculated
to be 1187.5 mL per day assuming a starting weight of 13.75 kg.
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| Example: Dehydration with asymptomatic hyponatremia, case #2
A 16 year old basketball player presents to the ER after a game.
She's had diarrhea all week. By clinical examination, she appears
moderately dehydrated. Her weight last week was 72 kg; her present
weight is 67 kg. Her serum sodium is 128, her bicarbonate is 17.
She received two boluses of 500 mL of lactated Ringer's solution in the
ER, and is now admitted to the floor for i.v. rehydration.
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