Anal fistula overview

# Overview
The anal fistula is an abnormal connection between the anorectal canal and the perianal skin that is lined with granulation tissue. Anal fistula develops from infection of anal crypts gland. Anal fistulas are classified depending upon the location of sphincter into 4 types-intersphincteric, transsphincteric, suprasphincteric, extrasphincteric. Anal fistula are classified into two categories based on the risk factors associated- simple anal fistula and complex anal fistula. Anal fistulas are also classified according to primary tracks into two- high and low anal fistulas. Anal fistulas patient presents with rectal pain, discharge. On rectal examination, there is redness, tenderness and discharge are seen. The anal fistula is diagnosed clinically and endoanal ultrasonography and MRI helps in finding out the anatomy of fistula. The mainstay of treatment is surgery but medical therapy for symptomatic relief of pain and fever. Antibiotics are used if there are sepsis and treatment of underlying cause is very important to prevent recurrence of fistula. Various options for surgical management are  fistulotomy and Seton. Sphincter-saving methods are fibrin glue, endorectal advancement flap, LIFT procedure, BioLIFT, stem cells and defunctioning.

# Historical Perspective
In 1880, Herman and Desfosses described the anal glands within the internal sphincter, sub-mucosa and their opening into the anal crypts and demonstrated that the infection of these glands and the spread of the infection through the intersphincteric space can result in the formation of an anorectal abscess. In 1900, Goodsall found a rule of thumb that uses the location of fistula for the treatment of fistula.

# Classification
Anal fistula are classified into four types based on the relationship to sphincter- intersphincteric, transsphincteric, suprasphincteric, extrasphincteric. Anal fistula are classified into two categories based on the risk factors associated- simple anal fistula and complex anal fistula. Anal fistulas are also classified according to primary tracks into two- high and low anal fistulas.

# Pathophysiology
Anal fistula develops from infection of anal crypts gland. The initial infection occurs in the ducts of the anal glands and the spread of infection results in the formation of the abscess.If the abscess is ruptured, a fistula is formed. Anal fistulas are associated with following conditions are diverticulitis, foreign-body reactions actinomycosis, chlamydia, lymphogranuloma venereum (LGV), syphilis, tuberculosis, radiation exposure, HIV disease, Crohns disease, pilonidal disease, hidradenitis suppurativa, trauma, previous surgery (including ileoanal pouch surgery), presacral dermoid cysts, sacrococcygeal teratoma.

# Causes
Anal fistula is caused by crohn's disease, obstetric injury, retained foriegn body in rectum, radiation proctitis, gonorrhea, HIV, cryptitis, lymphogranuloma venereum, perirectal abscess sequel of rupture or surgery, syphilis, thrombosed hemorrhoids, tuberculosis, ulcerative colitis.

# Differentiating Anal fistula overview from Other Diseases
Anal fistula must be differentiated from other causes of anal pain including anal fissure, thrombosed hemorrhoids, levator spasm, sexually transmitted disease, proctitis, hidradenitis suppurativa, infected skin furuncles, herpes simplex virus, tuberculosis, syphilis, actinomycosis and cancer.

# Epidemiology and Demographics
The prevalence of anal fistula is approximately 1-2 per 10,000 individuals worldwide. In England, the incidence of anal fistula is approximately 18.4 per 10,0000 per year. Anal fistula commonly affects individuals in the third, fourth, and fifth decades, with a peak around 40 years of age. Men are twice more commonly affected by anal fistula than women.

# Risk Factors
Common risk factor in the development of anal fistula are diabetes, smoking, alcohol, obesity.

# Screening
According to The American Society of Colon and Rectal Surgeons, screening for the anal fistula is not recommended.

# Natural History, Complications, and Prognosis
Anal fistula usually develops in 20-30 years of age and peaks around 40 years. If left untreated, patients with anal fistula may progress to develop the perianal abscess and cancer. Common complications of anal fistula include: urinary retention, bleeding, perianal abscess, fecal incontinence, carcinoma. Prognosis is excellent after surgery and recurrence rate is 7-2.

# Diagnosis

## Diagnostic Criteria
There is no diagnostic criteria associated with anal fistula.

## History and Symptoms
The hallmark of anal fistula is rectal pain during defecation, sitting and cough. A positive history of Crohns disease, Rectal abscess, Obstetric injury and prior anorectal injury is suggestive of anal fistula. Common symptoms are intermittent rectal pain during defecation, sitting and any activity, pain is throbbing in quality and sometimes occur throughout the day and resolved by opening the track, recurrent perianal malodourous discharge, perianal bloody discharge, perianal pruritis. Less common symptoms of anal fistula are fever and pain referred to thighs, low back, or buttocks.

## Physical Examination
Patients with anal fistula usually appear in distress due to throbbing rectal pain. Patient presents with anal fistula is having normal vital signs but if anal fistula gets infected, it will lead to the formation of an abscess. Patient with an abscess presents with unstable vitals like High-grade fever, tachycardia, tachypnea, low blood pressure. On rectal examination, there is redness, tenderness and discharge is seen.

## Laboratory Findings
There are no diagnostic laboratory findings associated with anal fistula. The anal fistula is mostly diagnosed clinically but in case of complication like the anal abscess, tests done are - complete blood count with differentials, blood culture, ESR, wound culture.

## X-ray
Fistulography is used to diagnose anal fistula in the past but is replaced by MRI nowadays. This is used to find out the anatomy of fistulas.

## CT scan
There are no CT scan findings associated with anal fistula.

## MRI
MRI is the gold standard imaging study to know about fistula anatomy. Indications are recurrent fistulas and complex fistulas.

## ECG
There are no ECG associated with anal fistula.

## USG
Endoanal ultrasound used to know details of fistula anatomy, tracks, and the sphincters. This is used intraoperatively by surgeons to have better information of fistula. Accuracy of endoanal ultrasound is improved by injection of hydrogen peroxide into fistula tracks. Ultrasound has a limited use because probe can go 2 cm from the anus so it is poor at evaluating pathology beyond the sphincters.

## Other Imaging Findings
Anoscope is used to visualize the internal opening of fistula. Sigmoidoscopy is used to visualize the opening of fistula in the rectum.

## Other Diagnostic Studies
There are no other diagnostic studies associated with anal fistula.

# Treatment

## Medical Therapy
Pharmacotherapy used in anal fistula depends upon the location and symptoms of patient. Antibiotics are used in patient with comorbities like immunosuppression, diabetes, extensive cellulitis, prosthetic devices and high risk cardiac patients. Antipyretics and analgesic for symptomatic relief of pain and fever. Treatment of underlying causes is important to treat recurrent anal fistulas.

## Surgery
The mainstay of treatment of anal fistula is surgical treatment.The principles for the management of anal fistula are described by the acronym SNAP, which stands for sepsis, nutrition, anatomy, and procedure according to British Medical Journal. Various methods of surgery are Fistulotomy and Seton. Sphincter-saving methods are Fibrin glue, endorectal advancement flap, LIFT procedure, BioLIFT, Stem cells and defunctioning.

## Primary Prevention
Adequate treatment of crohn's disease, HIV infection, actinomycosis, cryptitis, gonorrhea, syphilis, tuberculosis, and ulcerative Colitis. Maintaining proper hygiene.

## Secondary Prevention
Proper treatment of the anal fistula will prevent the complications, such as perianal abscess and cancer.