Perianal abscesses and fistulae (GI) Flashcards
Define perianal abscess.
Infection of the soft tissues around the anus - pus collection in perianal region
Define perianal fistula.
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
What is the difference between anal abscesses and anal fistulae?
Anal abscesses are the acute manifestation of a purulent infection in the perirectal area, while anal fistulas are the chronic manifestation of such infections
What can perianal abscesses lead to?
Can progress into fistulas (anorectal abscesses associated with anal fistulas in 37% of patients)
Describe the epidemiology of perianal abscesses & fistulae. (2)
- M>F
- 21-40 years old
What are some causes of perianal abscesses and fistulae? (5)
- 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
Describe the pathophysiology of perianal abscesses/fistulae.
- 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
What are the different types of anorectal abscesses? (4)
- 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
What rule should be applied to determine the internal opening of a perianal fistula?
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
What are the clinical features of a perianal abscess? (5)
- dull perianal discomfort/pain
- pruritus
- erythematous, subcutaneous, indurated mass near anus
- purulent discharge
- fever + malaise due to infection
What are the clinical features of a perianal fistula? (4)
- 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
What examination do we do for perianal abscess/fistula, and what would you find?
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
What are some risk factors for perianal abscesses/fistulae? (5)
- anal fistula - multiple recurrent anorectal abscesses are a common clinical manifestation or complication
- Crohn’s disease
- hard stools
- male
- 21-40y
How are perianal abscesses usually diagnosed?
Usually clinical diagnosis (Hx + anorectal examination), although occasionally examination under anaesthetic is required
What might WBC count show in perianal abscess/fistula?
May be elevated with increased proportion of granulocytes (left shift)
What might U&Es show in perianal abscess/fistula? (3)
- elevated urea and creatinine
- decreased HCO3-
- increased base excess
What might ABG show in perianal abscess/fistula?
Metabolic acidosis if life-threatening sepsis
What imaging can we do for perianal abscess/fistula? (4)
- 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
What is the investigation of choice for suspected perianal fistulae?
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)
What are some differential diagnoses for perianal abscess/fistula? (6)
- anal fissure - pain only with bowel movements
- thrombosed haemorrhoid - constant pain
- pilonidal abscess - inter-gluteal region
- infected epidermoid inclusion cysts
- perianal hidradenitis suppurativa
- STDs
What is the first-line treatment for perianal abscesses?
Surgical incision and drainage of abscess under local anaesthetic - to prevent spread of infection which may lead to sepsis
What post-operative care is there after surgical drainage of a perianal abscess? (6)
- Sitz baths
- analgesics
- stool softeners
- fibre
- Abx if immunocompromised
- absorbent dressings
What is the first-line management for perianal fistulae? (4)
- 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
When in perianal abscess/fistula do we give broad-spectrum Abx with anaerobic and gram -ve coverage? (6)
Ampicillin AND metronidazole/gentamicin if:
- elderly
- immunocompromised
- diabetes
- CVD
- cellulitis
- signs of systemic infection
What are some complications of perianal abscesses/fistulae? (6)
- necrotising soft-tissue infections (Fournier’s gangrene) + sepsis
- anal fistula
- recurrence
- damage to internal anal sphicnter
- incontinence
- persisting pain
Describe the prognosis of perianal abscesses/fistulae.
High recurrence rate without complete excision
