Dehydration is one of the most worrisome complications of diarrhea in children. Mild diarrhea usually doesn't cause significant fluid loss, but moderate or severe diarrhea can. Severe dehydration is dangerous; it can cause seizures, brain damage, even death. Know the signs of dehydration.
Cough and cold medications and decongestants may cause the eyes to dilate. In general, any medication that is a stimulant may cause the eyes to dilate. While it's not particularly common, this could include ADHD medications such as Adderall or Ritalin. Of course, certain illegal drugs do this as well, such as cocaine.
Epilepsy and stroke can also cause your pupils to dilate when they would not normally do this. This symptom will usually dissipate after a period of time, but it does signal something serious. Eye doctors can dilate eyes during an exam with eye drops. Doctors will use mydiatric drops during the exam.