Periapical means "around the apex, or end, of the root."  This type of abscess occurs in a dead, or almost dead, tooth.  In the center of the tooth there is a hollow pulp chamber where the nerve tissue lives with it's blood supply.  When that nerve tissue dies (usually due to a deep cavity,  a deep filling, or a traumatic blow to the tooth in the past),  the dead tissue remaining becomes perfect food for  bacteria. The dead pulp  no longer has a blood supply and cannot defend itself and as long as the dead pulp tissue remains the infection will not resolve.

The infected central tissue in the tooth produces inflammation and debris (pus, or dead white blood cells) and swelling causes pressure on the tissue around the tooth. Bacteria can go on to invade the bone and surrounding tissues as well, causing facial swelling. The pressure of this inflammation on nerves is what causes the pain of an abscess.  Sometimes a person with a periapical abscess feels no pain, especially if the infection has grown gradually and slowly over time.  But sometimes,  the pain can be excruciating and the swelling can increase quickly.

Common symptoms of a periapical abscess include extreme pain that does not occur with cold but is sometimes relieved by cold.  Remember, a tooth's nerve tissue must be dead or very debilitated  before a periapical  abscess forms.  A dead tooth will not feel cold so an ice cube placed on such a tooth will not elicit a response. Heat can be (but is not always) painful because the dead tissue inside the tooth is producing gas byproducts that expand with heat and cause more pressure on surrounding tissue.  That is why applying cold on the tooth (contracting these gases) can often give some relief.

Typically, a tooth with a periapical abscess will hurt when pressed on , or with eating or chewing.  If it is tapped by a blunt dental instrument (usually the end of  mirror handle) that tooth will hurt much more than the others.   Patients will often say "That's the one"  when it is tapped.  Usually, an x-ray will show an area at the end of the root in the bone  where the pressure of the inflammation/infection is pushing normal tissue away and a pus sac has formed.

First, a dentist will need to diagnose the problem.  An x-ray of the tooth that includes the root area will be needed.  A history will taken from the patient and  some diagnostic tests done to determine if the tooth is vital, or alive.  For instance,  if the tooth feels an ice cube on it,  it  not dead or severely debilitated; ergo, probably does not have a periapical abscess.

Once the diagnosis of periapical abscess is made,  you will be told that the tooth cannot stay in your mouth in that condition. A periapical abscess cannot heal by itself.  An antibiotic can help in the short term. Usually that will be amoxicillin 500 mg 4 times a day for 7-10 days, or erythromycin, in case of penicillin allergy.  These first choice antibiotics may not work in a few cases, where a broader spectrum antibiotic, clindamycin, is needed.

If you plan on keeping the tooth in your mouth, you will need a root canal.  The root canal can usually be started immediately,  often relieving the pain quickly, or you can take an antibiotic and plan the root canal in the near future.  Many general dentists do root canals. Sometimes you will be referred to an endodontist, or root canal specialist.  Root canals are nothing to fear.  The vast majority of modern root canals are  virtually pain free.  (Check my Root Canal site.)

By the way,  baby teeth can also become abscessed and the treatment is essentially the same.  The tooth will be extracted or a "baby root canal,"  or pulpotomy (removal of  the top of the pulp)  or "pulpectomy" (removal of the pulp in its entirety will be done.