Perianal abscesses and fistulae (GI)

Question 1

Define perianal abscess.

Answer 1

Infection of the soft tissues around the anus - pus collection in perianal region

Question 2

Define perianal fistula.

Answer 2

Abnormally chronically infected tract communicating between the rectum and the perineum

Abnormal tunnel with internal opening in anal canal and external opening in perianal skin - resulting from abscess that ruptures or is drained

Question 3

What is the difference between anal abscesses and anal fistulae?

Answer 3

Anal abscesses are the acute manifestation of a purulent infection in the perirectal area, while anal fistulas are the chronic manifestation of such infections

Question 4

What can perianal abscesses lead to?

Answer 4

Can progress into fistulas (anorectal abscesses associated with anal fistulas in 37% of patients)

Question 5

Describe the epidemiology of perianal abscesses & fistulae. (2)

Answer 5

• M>F
• 21-40 years old

Question 6

What are some causes of perianal abscesses and fistulae? (5)

Answer 6

• flow obstruction and bacterial infection of anal crypt glands (90%) - impaction of food, oedema from trauma (hard stool/foreign body/Crohn’s etc)
• IBD (Crohn’s –> oedema)
• diverticulitis
• appendicitis
• malignancy

Question 7

Describe the pathophysiology of perianal abscesses/fistulae.

Answer 7

• anal canal has 6-14 glands that lie in the plane between internal and external anal sphincters
• these glands have ducts that pass through the internal sphincters and drain into anal crypts at dentate line
• crypt occlusion –> infection of inter-sphincteric space –> inter-sphincteric, perianal or supra-levator abscess
• infection may also pass through the external anal sphincter –> perirectal abscess

Question 8

What are the different types of anorectal abscesses? (4)

Answer 8

• inter-sphincteric: in space between internal and external anal sphincter
• perianal: superficial soft tissues overlying inter-sphincteric space
• supra-levator: above the anorectal ring in supra-levator space
• perirectal: ischio-rectal or post-anal space

Question 9

What rule should be applied to determine the internal opening of a perianal fistula?

Answer 9

Goodsall’s rule - if external opening o/e is anterior to the transverse anal line, the tract will run straight, and the internal opening will lie radially anterior to the transverse anal line

Question 10

What are the clinical features of a perianal abscess? (5)

Answer 10

• dull perianal discomfort/pain
• pruritus
• erythematous, subcutaneous, indurated mass near anus
• purulent discharge
• fever + malaise due to infection

Question 11

What are the clinical features of a V fistula? (4)

Answer 11

• pus drainage (discharge) from anal canal or surrounding perianal skin
• pain on defecation
• constant, throbbing pain that may be worse on sitting/activity/defecation
• smelly discharge from near anus

Question 12

What examination do we do for perianal abscess/fistula, and what would you find?

Answer 12

DRE - fluctuant (application of pressure to one pole of a lesion transmits the pressure to every other point on the surface of the swelling), indurated mass

Perianal swelling and tenderness

Question 13

What are some risk factors for perianal abscesses/fistulae? (5)

Answer 13

• anal fistula - multiple recurrent anorectal abscesses are a common clinical manifestation or complication
• Crohn’s disease
• hard stools
• male
• 21-40y

Question 14

How are perianal abscesses usually diagnosed?

Answer 14

Usually clinical diagnosis (Hx + anorectal examination), although occasionally examination under anaesthetic is required

Question 15

What might WBC count show in perianal abscess/fistula?

Answer 15

May be elevated with increased proportion of granulocytes (left shift)

Question 16

What might U&Es show in perianal abscess/fistula? (3)

Answer 16

• elevated urea and creatinine
• decreased HCO3-
• increased base excess

Question 17

What might ABG show in perianal abscess/fistula?

Answer 17

Metabolic acidosis if life-threatening sepsis

Question 18

What imaging can we do for perianal abscess/fistula? (4)

Answer 18

• anal ultrasonography
• CT
• MRI (gold standard) - US/CT/MRI confirmatory for deeper abscesses/visualise fistulae
• fistula probe - inserted through external opening of fistula, visualise with anoscope/sigmoidoscope

Question 19

What is the investigation of choice for suspected perianal fistulae?

Answer 19

MRI - can be used to determine if there is an abscess, and if the fistula is simple (low fistula) or complex (high fistula that passes through/above muscle layers)

Question 20

What are some differential diagnoses for perianal abscess/fistula? (6)

Answer 20

• anal fissure - pain only with bowel movements
• thrombosed haemorrhoid - constant pain
• pilonidal abscess - inter-gluteal region
• infected epidermoid inclusion cysts
• perianal hidradenitis suppurativa
• STDs

Question 21

What is the first-line treatment for perianal abscesses?

Answer 21

Surgical incision and drainage of abscess under local anaesthetic - to prevent spread of infection which may lead to sepsis

Question 22

What post-operative care is there after surgical drainage of a perianal abscess? (6)

Answer 22

• Sitz baths
• analgesics
• stool softeners
• fibre
• Abx if immunocompromised
• absorbent dressings

Question 23

What is the first-line management for perianal fistulae? (4)

Answer 23

• simple fistula (superficial, involving <25% of sphincter): fistulotomy - cutting along whole length to open and drain it
• complex fistula: seton placement (piece of surgical thread through fistula which acts as drain, to allow continuous drainage while healing to ensure pus not contained)
• if Sx: oral metronidazole
• anti-TNF agents e.g. infliximab may also be effective in closing and maintaining fistula

Question 24

When in perianal abscess/fistula do we give broad-spectrum Abx with anaerobic and gram -ve coverage? (6)

Answer 24

Ampicillin AND metronidazole/gentamicin if:

• elderly
• immunocompromised
• diabetes
• CVD
• cellulitis
• signs of systemic infection

Question 25

What are some complications of perianal abscesses/fistulae? (6)

Answer 25

• necrotising soft-tissue infections (Fournier’s gangrene) + sepsis
• anal fistula
• recurrence
• damage to internal anal sphicnter
• incontinence
• persisting pain

Question 26

Describe the prognosis of perianal abscesses/fistulae.

Answer 26

High recurrence rate without complete excision