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What is Anorectal abscess and explain in detail?
Anorectal abscess (also known as an anal/rectal abscess, or perianal/perirectal abscess) is an abscess adjacent to the anus.  Most cases of perianal abscesses are sporadic, though there are certain situations which elevate the risk for developing the disease, such as diabetes mellitus, Crohns disease, chronic corticosteroid treatment and others. It arises as a complication of paraproctitis. Ischiorectal, inter- and intrasphincteric abscesses have been described.

Signs and symptoms
Pain in the perianal area is the most common symptom of an anorectal abscess. The pain may be dull, aching, or throbbing. It is worst when the person sits down and right before a bowel movement. After the individual has a bowel movement, the pain usually lessens. Other signs and symptoms of anorectal abscess include constipation, drainage from the rectum, fever and chills, or a palpable mass near the anus.

The condition can become extremely painful, and usually worsens over the course of just a few days. The pain may be limited and sporadic at first, but may worsen to a constant pain which can become very severe when body position is changed (e.g., when standing up, rolling over, and so forth). Depending upon the exact location of the abscess, there can also be excruciating pain during bowel movements, though this is not always the case. This condition may occur in isolation, but is frequently indicative of another underlying disorder, such as Crohns disease.

Complications
If left untreated, an anal fistula will almost certainly form, connecting the rectum to the skin. This requires more intensive surgery. Furthermore, any untreated abscess may (and most likely will) continue to expand, eventually becoming a serious systemic infection.

Cause
Abscesses are caused by a high-density infection of (usually) common bacteria which collect in one place or another for any variety of reasons.  Anal abscesses, without treatment, are likely to spread and affect other parts of the body, particularly the groin and rectal lumen.  All abscesses can progress to serious generalized infections requiring lengthy hospitalizations if not treated.
Historically, many rectal abscesses are caused by bacteria common in the digestive system, such as E. coli. While this still continues often to be the case, there has recently been an increase in the causative organism being staphylococcus, as well as the difficult to treat community-acquired methicillin-resistant S. aureus. Because of the increasing appearance of more exotic bacteria in anal abscesses, microbiological examination will always be performed on the surgical exudate to determine the proper course of any antibiotic treatment.

Diagnosis
Diagnosis of anorectal abscess begins with a medical history and physical exam. Imaging studies which can help determine the diagnosis in cases of a deep non-palpable perirectal abscess include pelvic CT scan, MRI or trans-rectal ultrasound. These studies are not necessary, though, in cases which the diagnosis can be made upon physical exam.

Classification
Anorectal abscesses are classified according to their anatomic location and the following are the most common types: perianal abscess, ischiorectal abscess, intersphincteric abscess and supralevator abscess.
Perianal abscess, which represents the most common type of anorectal abscesses accounting for about 60% of reported cases, are superficial collections of purulent material just beneath the skin of the anal canal.Ischiorectal abscess is formed when suppuration transverses the external anal sphincter into the ischiorectal space.Intersphincteric abscess results from suppuration contained between the internal and external anal sphincters.Supralevator abscess forms from cephalad extension of the intersphincteric abscess above the levator ani or from caudal extension of a suppurative abdominal process like appendicitis, diverticular or gynaecologic sepsis.

Differential diagnosis
This condition is often initially misdiagnosed as hemorrhoids, since this is almost always the cause of any sudden anal discomfort. The presence of the abscess, however, is suspected when the pain quickly worsens over one or two days and usual hemorrhoid treatments are ineffective in bringing relief.  Furthermore, any serious abscess will eventually begin to cause signs and symptoms of general infection, including fever and nighttime chills.
A physician can rule out a hemorrhoid with a simple visual inspection, and usually appreciate an abscess by touch.

Treatment
Anal abscesses are rarely treated with a simple course of antibiotics