Pediatric Examination and Board Review (30 page)

(A) diabetes insipidus

(B) SIADH

(C) cerebral salt wasting

(D) congenital adrenal hyperplasia

(E) hypoglycemia

11.
The diagnostic studies that would best help narrow the differential diagnosis of hyponatremia would be which of the following?

(A) urine sodium, serum potassium, serum calcium, urine phosphate

(B) urine sodium, serum potassium, serum glucose, serum albumin

(C) serum potassium, serum calcium, serum albumin, bilirubin

(D) urine sodium, serum sodium, serum potassium, serum glucose

(E) none of the above

12.
The routine use of hypotonic IV fluids in hospitalized children is

(A) recommended because the infant cannot manage a sodium load

(B) recommended because the sodium requirement in ill children is the same as it is in healthy children

(C) not recommended because the ill infant cannot excrete a sodium load

(D) not recommended because the ill infant cannot routinely excrete excess free water

(E) none of the above

13.
The quantity of fluid that can best be called “insensible loss” in a patient with normal vital signs is

(A) 1200 mL/m
²
per day

(B) 500 mL/m
²
per day

(C) 2000 mL/m
²
per day

(D) 1000 mL/m
²
per day

(E) 100 mL/m
²
per day

14.
The quantity of IV fluids that can best be called “maintenance” in a child with routine ongoing loss and normal vital signs is

(A) 500 mL/m
²
per day

(B) 1200 mL/m
²
per day

(C) 2500 mL/m
²
per day

(D) 3000 mL/m
²
per day

(E) 4000 mL/m
²
per day

15.
Of the following IV fluids listed, which would be the best choice for the child in this case to be on if admitted for observation after seizures stopped in the emergency department?

(A) 5% dextrose, 2000 mL/m
²
per day

(B) 5% dextrose 0.2 normal saline 1200 mL/m
²
per day

(C) 5% dextrose 0.45 normal saline, 1200 mL/m
²
per day

(D) 5% dextrose 0.9 normal saline, 1200 mL/m
²
per day

(E) 5% dextrose in lactated Ringer’s solution, 3000 mL/m
²
per day

ANSWERS

1.
(B)
As a general principle, one must assure that the patient has a stable, patent airway and an appropriate hemodynamic status before proceeding to the next step in management.

2.
(A)
In a patient with seizures, it is important to obtain serum glucose and electrolytes to initiate the acute evaluation. Generalized seizure activity in an 8-month-old infant is relatively common, and there are a multitude of potential explanations. Metabolic derangements are among the most common causes of seizures in this age group. Although hypomagnesemia may cause seizures, hypoglycemia, hypocalcemia, and hyponatremia are the 3 metabolic disturbances that are most frequently associated with seizures. An examination of CSF is not unreasonable but can wait for consideration in an afebrile and previously healthy child. Consideration should be given to intracranial pathology related either to a congenital malformation or to a traumatic event resulting in intracranial or extracranial hemorrhage. A CT scan is appropriate to consider after the patient is stabilized and the seizure is controlled.

3.
(B)
Controlling the seizure should begin with the administration of lorazepam intravenously in a dose of 0.1 mg/kg. It is clear that at least part of the reason for this child’s ictal activity is hyponatremia. Thus it is prudent to correct the sodium. In general, if one administers 1 or 2 mEq/kg of sodium to a child with a seizure as a result of hyponatremia, the sodium will rise to a sufficient level that the seizures will be abolished. Hyponatremic seizures can be relatively resistant to anticonvulsant medications including phenytoin and phenobarbital, and IV sodium is often required. Although a cause for the seizure activity has been identified, the evaluation for intracranial pathology should be pursued as well. Hyponatremia might be a result of excess antidiuretic hormone resulting from intracranial malformation or injury.

4.
(A)
Patients with hyponatremia can be divided into those who present without accompanying dehydration and those who are hypovolemic. A history of fever or trauma should be sought from the caregiver. Additional important information to obtain from the caregivers concerns the manner in which the formula was prepared and the intake and urine output for the proceeding 24 hours. For this child, there is no evidence on examination or by history of any volume loss. Thus he falls into the category of euvolemia with hyponatremia. In this age range the commonest explanation for euvolemic hyponatremia is water intoxication from improper mixing of formula before feeding or exogenous administration of water. The alternative diagnosis in a hyponatremic child who is euvolemic is SIADH. This is generally associated with other pathology, most commonly in the CNS. For the hypovolemic child with a reliable history of volume loss and preexisting normal health, a diagnosis of hyponatremic dehydration is usually associated with acute gastroenteritis.

5.
(D)
Children with hyponatremic seizures, even from a relatively benign cause such as water intoxication, need to be admitted to hospital and have careful monitoring of the rise of serum sodium. Fortunately, this entity is associated almost uniformly with full recovery.

6.
(B)
When trying to distinguish water intoxication from SIADH, if the history is unavailable or unreliable, urine studies can be useful. SIADH is a diagnosis that is generally associated with other pathology, most commonly in the CNS. In SIADH, the urine volume is generally decreased because the underlying pathology is the excess absorption of free water at the collecting duct of the kidney. In water intoxication, the urine osmolality is quite low, as is the urine sodium concentration (<20 mEq/L). When hyponatremia is a result of antidiuretic hormone excess, urine osmolality is elevated and the urine sodium is 50-75 mEq/L.

7.
(C)
Ideally, the serum sodium should be corrected no more than 0.5-1 mEq/L/hour to a normal value with isotonic fluid. The exception is the need to rapidly respond to epileptic activity, at which time a rapid minimal correction of the serum sodium is indicated as noted above. In this child, should the seizures continue after anticonvulsant therapy, it would be appropriate to raise the serum sodium to about 125 mEq/L at which time the seizure activity should stop.

8.
(C)
Severe neurologic injury resulting from central pontine myelinolysis has been associated with rapid correction of serum sodium in some patients.

9.
(D)
Central pontine myelinolysis has been most associated with too rapid correction of serum sodium in young women with who receive excess free water in the perioperative period.

10.
(A)
In addition to SIADH, 2 other pathologic entities deserve mention in a pediatric discussion of hyponatremia: cerebral salt wasting and adrenal insufficiency. In diabetes insipidus, the urine is hypotonic and hypernatremia may develop.

11.
(D)
The determination of the underlying cause of hyponatremia can be made by evaluating the hydration status of the child in conjunction with electrolyte and urine studies. In cases of hyponatremia with robust urine volume and euvolemic state, water intoxication is almost always the diagnosis. Urine osmolality is quite low, as is the urine sodium (<20 mEq/L). The child is euvolemic with low urine output, a relatively high urine sodium, and high urine osmolality. When hyponatremia is a result of antidiuretic hormone excess, urine osmolality is elevated and the urine sodium is 50-75 mEq/L, which is clearly inappropriate in a child whose serum sodium is quite low. For the hypovolemic child with a reliable history of volume loss and a normal preexisting health status, a diagnosis of hyponatremic dehydration is usually associated with gastroenteritis.

In the case of cerebral salt wasting, the patient is likely hypovolemic, has a robust urine output, and a very high urine concentration of sodium. This occurs almost uniformly in the postoperative care of the patient who underwent a neurosurgical procedure or after severe head trauma.

In the case of adrenal insufficiency, the patient also presents with volume loss. In addition to hyponatremia, the patient is usually hyperkalemic and hypoglycemic and may present in shock. When the etiology of the adrenal insufficiency is congenital adrenal hyperplasia, there are often physical findings consistent with virilization.

12.
(D)
It is common among hospitalized patients or following surgical procedures to have a self-limited period of excess antidiuretic hormone (ADH) secretion, signaling the kidney to retain free water. With the addition of hypotonic IV fluids in this state, hyponatremia may develop.

Isotonic fluid is often the most appropriate choice for children hospitalized for numerous conditions because the risks of giving isotonic fluid are often less than the risks of giving hypotonic fluid. Hyponatremia is found among 3-5% of hospitalized children. The most common cause is the administration of hypotonic IV fluids. If these children were restricted to water and salt intake that exactly meets their physiologic maintenance, they would be normonatremic. It is sometimes difficult to exactly determine a child’s maintenance needs for water and salt. If a clinician overestimates the volume of fluid needed and a child has increased ADH secretion, that child will retain much of the water given in IV fluids. If the fluids chosen are hypotonic, the serum sodium will be driven down over time.

13.
(B)
Insensible loss of about 500 mL/m
²
per day occurs in the absence of excess insensible loss, as found in fever or with radiant warmer use.

14.
(B)
The hospitalized child requires approximately 700-800 mL/m
²
per day of water to excrete a normal solute load. In addition, children require replacement of insensible losses of about 500 mL/m
²
per day. The sum of these requirements leads to a total IV fluid rate of approximately 1200 mL/m
²
per day. Additional IV fluids in excess of 1200-1300 mL/m
²
per day in a child without elevated ADH will generally result in more dilute urine. Excess hydration may be required to replace excess water losses, as occur with dehydration or ongoing losses from diarrhea, fever, or other causes.

Often it is taught that the appropriate IV fluid for children who are younger than 1 year of age is 0.2 N saline and for those older than a year of age, 0.45 N saline at a rate that approximates 2000 mL/m
²
per day. Weight-based formulas found in textbooks of 100 mL/kg/day (4 mL/kg per hour) for the first 10 kg, followed by 50 mL/kg (2 mL/kgper hour) for the next 10 kg, and by 20 mL/kg (1 mL/kg per hour) for each kilogram greater than 20 kg approximate this volume of 2000 mL/m
²
per day for all weights. These formulas are derived from data published in the 1950s that suggested IV fluids be administered in accordance with metabolic needs of healthy children. However, many hospitalized patients today are not “healthy children” and have increased ADH secretion.

15.
(D)
Once seizure free, the only intervention necessary in the water-intoxicated child would be to restrict free water. However, if IV fluids are required, an isotonic fluid should be chosen and given at a “maintenance” rate.

The alternative fluid regimen now proposed for the hospitalized child is the administration of isotonic fluids at a maintenance rate that more closely matches the volume required to excrete a solute load and the amount needed to replace insensible losses as discussed in answer 14. However, this provides more than the sodium requirement for any age. The consequences of the administration of excess salt are usually inconsequential for most children without underlying cardiovascular or renal disease but can include pulmonary edema and peripheral edema. Children who require sodium restriction, including those with renal failure, congestive heart failure, and chronic obstructive pulmonary disease such as chronic lung disease following prematurity and cystic fibrosis, are certainly exceptions, and hypotonic fluids are often appropriate.