Dehydration is loss of body water, and with it a fall in plasma volume. In a child with acute diarrhea, that loss turns an ordinary illness into a dangerous one.
What must the bedside settle first?
The body keeps sodium and water in balance with ADH, which makes the kidney concentrate urine, and with thirst.
That gives the bedside two jobs.
- Grade how much fluid has been lost, so that the rehydration plan matches the deficit.
- Identify the sodium pattern: is the serum sodium likely to be high, normal or low?
Why does the second job matter? Because a serum sodium that has moved away from normal shows which side of the sodium-water balance has failed. The sodium pattern decides how quickly the deficit may be corrected without causing harm. Correcting the sodium too aggressively can harm the brain.
How much fluid is missing?
The grade answers the first question, but the usual measure of it needs a starting weight. The best single measure of dehydration is the percentage of body weight lost. That figure requires a reliable pre-illness weight, and in practice the pre-illness weight is often unknown.
Despite this, classifying the child as having no or minimal, mild-to-moderate, or severe dehydration is the essential basis for choosing treatment. The guideline grades this statement as a strong recommendation.
Why can no test give the grade?
Without a reliable weight, and with no laboratory test that estimates the percentage of body weight lost accurately, the grade has to come from the child.
Clinical tests of dehydration are imprecise. They show only fair-to-moderate agreement between examiners. The conventional clinical features describe a continuum. The grade is a considered judgement built from several signs, not a reading off one sign.
Which signs carry the grade?
The signs are not equal. The grade is built from several signs, and some carry more information than others. Among individual examination signs, the three that assess dehydration best are prolonged capillary refill time, abnormal skin turgor and abnormal respiratory pattern.
What does each sign look like at each grade?
Each sign is read at three grades. Two of them need a plain description. Capillary refill is the time for colour to return to a patch of skin after it has been pressed pale. Skin turgor is how quickly a pinched fold of skin flattens again.
Some signs are the body’s own response to water shortage. A thirsty child is showing that response. When water is short, ADH makes the kidney concentrate urine, so a fall in urine output is the kidney conserving water.
| Sign | No or minimal | Mild to moderate | Severe |
|---|---|---|---|
| General appearance | Normal | Thirsty, restless or tired, irritable | Drowsy, limp, apathetic or unconscious |
| Tears | Present | Reduced | Absent |
| Oral mucosa | Moist | Dry | Very dry |
| Capillary refill | Normal | Prolonged | Prolonged, markedly delayed |
| Eyes | Normal | Sunken | Very sunken |
| Skin turgor | Returns immediately | Returns slowly | Returns very slowly |
| Peripheral perfusion | Warm | Cool | Cool, mottled, cyanotic |
| Urine output | Normal | Reduced | Minimal |
How is a score built from the signs?
A score adds the signs together rather than trusting one. Where a numerical grade is wanted, the Clinical Dehydration Scale is the tool the guideline points to, and it is the validated scoring tool that should be relied on for grading. It has four items, each scored 0 to 2, giving a total from 0 to 8.
- General appearance: 0 normal; 1 thirsty, restless, or lethargic but irritable when touched; 2 drowsy, limp, cold or sweaty, with or without coma.
- Eyes: 0 normal; 1 slightly sunken; 2 extremely sunken.
- Mucous membranes (tongue): 0 moist; 1 sticky; 2 dry.
- Tears: 0 present; 1 decreased; 2 absent.
The total gives the band. A score of 0 means no dehydration, 1 to 4 means some dehydration, and 5 to 8 means moderate to severe dehydration.
The scale predicts the need for intravenous rehydration, weight gain, blood testing, admission and length of stay. It has moderate-to-good agreement between observers. Even so, it is a weak recommendation on low-quality evidence. It should be used alongside the rest of the assessment, not instead of it. No single scoring system has been shown to be superior in every setting, and different tools suit outpatient and inpatient use.
Why is no single sign enough?
A score helps, but a single sign alone is still unreliable. How far can one sign be trusted?
A useful operational threshold comes from a prospective study of 186 children aged 1 month to 5 years. The study defined dehydration as a deficit of at least 5% of body weight. Four findings predicted that deficit as well as the full set of ten signs did:
- capillary refill longer than 2 seconds
- absent tears
- dry mucous membranes
- ill general appearance
Two or more of these four findings indicated a deficit of at least 5%.
Individual findings in the study had generally low sensitivity and high specificity. In plain terms, many dehydrated children lacked any given sign, while a sign that was present seldom occurred in a child who was not dehydrated. Low sensitivity with high specificity is the underlying reason any single sign is unreliable on its own.
Why does the sodium come next?
Once the grade is set, the next question is the serum sodium, because a sodium that is too high or too low changes the safe rate of correction before it changes anything else.
Hypernatremic dehydration is a serum sodium above 145 mmol/L. It is uncommon in acute gastroenteritis, ranging from under 1% to 4% of cases depending on the setting.
What happens when the sodium is high?
Hypernatremic dehydration is dangerous partly because it is under-recognised. The usual clinical signs are less prominent, so the degree of dehydration is easily underestimated.
Infants, mainly those under 6 months, may instead show doughy skin, tachypnea and neurological signs. These neurological signs are increased muscle tone, hyperreflexia, convulsions, drowsiness or coma.
So a child who looks relatively preserved while severely dehydrated should raise the possibility of hypernatremia rather than lower it.
What happens when the sodium is low?
The same logic runs the other way, and the choice of fluid matters. Hyponatremia is a low serum sodium. Hypotonic intravenous solutions lower plasma sodium and so increase the risk of acute hyponatremia. In one retrospective series, 19% of children with a normal sodium on admission developed mild hyponatremia during treatment.
Isotonic fluid does not dilute the sodium the way a hypotonic fluid does. It is recommended for initial rehydration in most cases, and it is used first in severe dehydration or shock before free water is given.
What can the bedside tell about sodium?
The bedside can raise the suspicion of an abnormal sodium. Only a blood measurement gives the value of the serum sodium. So which tests should be sent, and when does a result change what is done for the child?