A root canal treatment is necessary when decay or trauma affects the pulp or nerve of a tooth. The blood flow in the tooth rushes to the site of inflammation to aid the tooth. A pressure is created on the nerve endings, generating pain, which usually occurs at night or when lying down, and can be triggered by hot or cold. What occurs is a slow progression of decay traveling through the canal and settling at the end of the root. This results in an abscess (or swelling) at this site.