During root canal irrigation with sodium hypochlorite (NaOCl), the patient suddenly experiences severe pain and significant bleeding from the canal. What is the most likely cause?

A sodium hypochlorite (NaOCl) accident occurs when the irrigant is inadvertently extruded beyond the apex into the periapical tissues. Classic signs include immediate, severe burning pain, profuse bleeding through the canal, and rapid swelling. NaOCl is cytotoxic and causes tissue necrosis, vascular damage, and histamine release when it contacts periradicular tissues. This is a dental emergency. Management includes allowing the canal to bleed (do not seal it), cold compresses, analgesics, and monitoring. Vital pulp remnants may cause pain, but not the bleeding described. Tooth fracture and needle trauma are less likely to produce this acute presentation.