Peritonsillar abscess (PTA) is a suppurative infection of the tissues between the capsule of the palatine tonsil and pharyngeal muscles and is the most common abscess of the head and neck region. It is usually unilateral but can be bilateral in about 6% of instances.  Peritonsillar abscess (also known as quinsy) is the commonest of all deep neck infections.  Other deep neck infections include retropharyngeal abscess and parapharyngeal (lateral pharyngeal) space abscess. 
Peritonsillar abscess generally occurs in the superior pole of the tonsil. It can also be present at the midpoint or inferior pole of the tonsil or have multiple loculations within the peritonsillar space. The development of the abscess is often gradual, with an early stage of peritonsillar cellulitis. If not properly treated, an abscess emerges.
Two mechanisms have been proposed to explain the development of a collection of pus in the loose connective tissue of the supratonsillar fossa. The more common explanation is that a peritonsillar abscess develops from an inadequately treated bacterial tonsillitis. An alternative explanation is that a peritonsillar abscess is an abscess formed in a group of salivary glands in the supratonsillar fossa, known as Weber glands.
Lymphatic drainage from an infected peritonsillar abscess is to the ipsilateral jugulodigastric nodes. Bacterial cultures that are also adequate for the recovery of anaerobic bacteria usually yield polymicrobial aerobic and anaerobic bacteria. Group A beta-hemolytic streptococci is recovered in 25-40% of the abscesses. Anaerobic bacteria is isolated in over 90% of aspirated pus,  and elevated antibody levels to these organisms is detected in most patients with peritonsillar abscess.  Methicillin-resistant Staphylococcus aureus (MRSA) has been isolated with greater frequency in peritonsillar abscesses in recent years. 
The estimated incidence in the United States is 30 cases per 100,000 person-years in patients aged 5-59 years.  The incidence in children younger than 18 years is 14 cases per 100,000 population. Approximately 25-30% of patients with peritonsillar abscess are in the pediatric age group. 
A study by Qureshi et al analyzed temporal trends in the incidence and surgical management of children with peritonsillar abscesses. The study found that there was no change in the incidence from 2000 to 2009 but there was a change in surgical management, with a significant decrease in the rate of tonsillectomy and significant increase in the rate of incision and drainage procedures. 
The incidence of peritonsillar abscess in Ontario, Canada was 12 cases in 100,000 population. 
The mean annual incidence of peritonsillar abscess in Europe was 41 cases in 100,000 population. 
The mortality rate is unknown. Mortality is often due to aspiration of a ruptured abscess or sequelae of sepsis. Morbidity stems principally from pain and dehydration. See Complications in the Follow-up section.
No race predilection is known.
No sex predilection is reported.
Peritonsillar abscess most commonly occurs in the third and fourth decades of life. Pediatric cases are more common in children older than 10 years, although cases have been described in children younger than 1 year.
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