Diarrhea is the passage of frequent watery stools. In children, bacteria and viruses are the most common causes of diarrhea. According to the Duke University Health System, fainting, also known as syncope, refers to when a child suddenly losses consciousness and the ability to stay upright and then recovers spontaneously. Diarrhea and syncope can occur in the setting of dehydration, as a result of cardiac arrhythmias and electrolyte imbalances, and as a result of vasovagal syncope.
Dehydration
Dehydration is common in diarrhea. Among microbes, rotavirus is the most common cause of diarrhea and a leading cause of childhood hospitalizations. In children, the symptoms of dehydration include decreased urine output, rapid heart rate or tachycardia, dry lips and mouth, sunken eyes with lack of tears, and lethargy. Dehydration can lead to fainting when the lack of fluid affects the distribution of blood in the body, especially to the brain. According to the Merck Manuals, severe dehydration can lead to orthostatic hypotension, in which blood pressure falls, causing lightheadedness and fainting.
Electrolyte Imbalances
Dehydration can affect the level of electrolytes in the blood. Sodium and potassium are two of the electrolytes affected by dehydration. Loss of sodium in diarrhea leads to hyponatremia or low levels of sodium in the blood. Too much fluid loss in diarrhea can lead to hypernatremia, or high concentration of sodium. Any of these conditions can lead to problems with brain swelling or shrinking, respectively, which can then lead to fainting and seizures.
Cardiac arrhythmias
Abnormal heart rhythms are a concerning cause of fainting in children with diarrhea. This could indicate a problem with potassium, another electrolyte that can be affected in severe dehydration. High levels of potassium in the blood can cause a dysrrhythmia, or abnormal heart rhythm, that could manifest itself as fainting, since the heart is unable to pump blood adequately to the brain.
