Clinical Scenario:
A 7-year-old boy is admitted with a 5-day history of profuse watery diarrhea. After initial fluid resuscitation, he develops progressive difficulty in walking and is unable to rise from a sitting position. On examination, he is alert, blood pressure is normal, muscle power is reduced symmetrically in all four limbs, and deep tendon reflexes are diminished. Cranial nerve examination is normal. An ECG demonstrates ST-segment depression with prominent U waves.
What is the most likely electrolyte abnormality?
A. Hypercalcemia
B. Hyperkalemia
C. Hypocalcemia
D. Hypokalemia
E. Hyponatremia
Correct answer & Explanation:
Correct answer: D. Hypokalemia
The child most likely has hypokalemia secondary to excessive gastrointestinal potassium losses from prolonged watery diarrhea.
The diagnosis is suggested by the combination of:
- Recent history of prolonged diarrhea
- Progressive, symmetrical muscle weakness
- Difficulty standing and walking (proximal muscle weakness)
- Depressed deep tendon reflexes
- Normal cranial nerve examination, making a primary neurological disorder such as Guillain-Barré syndrome less likely
- ECG findings of ST-segment depression and prominent U waves, which are characteristic of hypokalemia
Potassium plays a crucial role in maintaining the resting membrane potential of skeletal and cardiac muscle. Potassium depletion leads to reduced neuromuscular excitability, resulting in muscle weakness, diminished reflexes, and, in severe cases, respiratory muscle paralysis and cardiac arrhythmias.
Why the other options are incorrect
A. Hypercalcemia
Usually presents with constipation, polyuria, dehydration, abdominal pain, and muscle weakness. ECG typically shows a shortened QT interval, not prominent U waves.
B. Hyperkalemia
Produces characteristic ECG changes such as tall peaked T waves, PR prolongation, widening of the QRS complex, and eventually a sine-wave pattern. It does not produce prominent U waves.
C. Hypocalcemia
Typically causes increased neuromuscular excitability with tetany, carpopedal spasm, laryngospasm, and seizures. ECG shows QT interval prolongation, not U waves.
D. Hypokalemia
Correct. The combination of diarrhea, flaccid muscle weakness, depressed reflexes, and ECG findings of ST-segment depression with prominent U waves is classic for hypokalemia.
E. Hyponatremia
Primarily causes neurological manifestations such as headache, confusion, seizures, and altered consciousness due to cerebral edema. It is not associated with the characteristic ECG changes seen in this patient.
Learning Point
The hallmark ECG changes of hypokalemia include:
- Flattened or inverted T waves
- ST-segment depression
- Prominent U waves
- Apparent QT prolongation (actually prolongation of the QU interval)
In a child with acute flaccid weakness following diarrhea, always consider hypokalemia before pursuing neurological causes, especially when the ECG demonstrates prominent U waves. Severe hypokalemia is a medical emergency because of the risk of life-threatening cardiac arrhythmias and respiratory muscle weakness.
