Anal Abscess

Overview

Anorectal abscess is an infection of soft tissue around the anus. It causes severe perianal pain and swelling. 30% to 70% of anorectal abscesses are associated with an anorectal fistula. It is two times more likely to present in males.^{1 2}

Anatomy

Anal canal – channel connecting the rectum to the anus, located within the anal triangle of the perineum, between the two ischioanal fossae

Anal Sphincters

• Internal sphincter
  • Involuntary control – continuation of circular fibers of the colon so it receives autonomic supply
  • Surrounds upper 3/4 of anal canal
• External sphincter
  • Voluntary control
  • Surrounds entire length of anal canal
  • Consists of three parts – subcutaneous, superficial and deep

Intersphincteric plane

• Between external sphincter muscle laterally and the longitudinal muscle (inner sphincteric muscle) medially
• Contains anal glands
• Can be opened up surgically to provide access for operations on the sphincter muscles

Anal glands

• Apocrine glands
• Occasionally infected and act as a source of anal fistula

Dentate (pectinate) line

• Junction of the proximal and distal segments of the anal canal
• Divides the anus into proximal 2/3 and distal 1/3
• Situated at the middle of internal sphincter

	Above Dentate line	Below Dentate line
Embryology	Hindgut (endoderm)	Proctodeum (ectoderm)
Epithelium	Columnar	Stratified squamous
Arterial supply	Superior rectal a.	Middle rectal a.Inferior rectal a.
Venous drainage	Superior rectal v.	Middle rectal v.Inferior rectal v.
Innervation	Visceral – Inferior hypogastric plexus(insensitive to pain)	Somatic – Inferior rectal nerves: branches of Pudendal nerve(sensitive to pain)
Lymph drainage	Internal iliac	Superficial inguinal

Aetiology & Risk factors

• Cryptoglandular disease – blocked anal glands (most common)
• Crohn’s disease
• Hidradenitis suppurativa
• Trauma
• Anal fissures
• Malignancy
• Tuberculosis
• Immunocompromised states^{2 3} 

Pathophysiology

In the common cryptoglandular pathway, obstruction of an anal gland duct permits bacterial proliferation and pus collection in the intersphincteric space. Infection may extend into adjacent anorectal spaces, with abscess expansion and local tissue destruction.³

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Classification

Based on the relationship to anal sphincter from superficial to deep.

Divide into two groups: perianal and perirectal²

Perianal abscess:

• Most common
• Results from infection invading laterally and distally in the intersphincteric groove

Perirectal abscess – 4 types:

Type of Abscess	Anatomical location	Path of spread/Origin	Physical exam findings
Ischiorectal (ischioanal)	Ischiorectal space	Through external anal sphincter into ischiorectal fossa	Tender, fluctuant area in buttockLarge swelling
Intersphincteric	Intersphincteric groove	From infected anal gland	Fluctuant mass on DRE protruding in lumenMinimal skin changes
Supralevator	Above levator ani	Two origins:- Upward extension of cryptoglandular infection- Pelvic pathology	Fluctuant mass above anorectal ring on DREMinimal external findings
Horseshoe	Deep postanal space, extending around anal canal	Deep postanal space into bilateral ischiorectal fossae	Posterior tendernessPotential bilateral buttock swelling

Clinical Manifestations

• Constant severe pain in anal or rectal area
  • Worse when sitting
  • Usually not associated with bowel movements
• Skin irritation: swelling, erythema
• Systemic symptoms: fever, chills, malaise
• Purulent rectal drainage
• Urinary retention – rare, may be seen in supralevator abscess

Clinical Examination

• General inspection
  • Superficial abscess – area of fluctuance, erythema, indurated skin
  • Deep abscess – may not have any findings 
• Digital rectal exam
  • Severe pain
  • Induration
  • Fluctuance

Differential diagnosis

• Anal fissure
• Anal fistula
• Haemorrhoids
• Pilonidal disease
• Buttock skin abscess
• Bartholin abscess
• Hidradenitis suppurativa
• STI

Diagnosis

Clinical diagnosis is sufficient in most cases

Imaging – used when a deep abscess is suspected but cannot be palpated on physical exam:

• CT
• MRI
• Ultrasonography ^{2 4}

Treatment

Surgical incision and drainage – first line:

• Do not delay unless patient unstable
• Done under local or general anaesthetic

Antibiotics – suggested for all patients after incision and drainage

Complications & Prognosis

Complications 

• Anal fistula
• Fournier’s gangrene (necrotising soft tissue infection)
• Sepsis
• Recurrent abscess
• Faecal incontinence ^{1 4}

Prognosis

• 30 to 50% will develop anal fistula months to years following drainage
• Recurrence of abscess in about 11% of patients ¹

References

1. Rakinic J. Anorectal abscess. In: BMJ Best Practice [Internet]. 2022 [cited 2025 Nov 20]. Available from: BMJ Best Practice
2. Bleday R. Perianal and perirectal abscess. In: Weiser M, editor. UpToDate [Internet]. [updated 2024 Jul 22; cited 2025 Nov 20]. Available from: UpToDate
3. Kata A, Abelson JS. Anorectal Abscess. Clin Colon Rectal Surg. 2024;37(6):368-75. Available from: https://pmc.ncbi.nlm.nih.gov/articles/PMC11466523/ 
4. Conner JN, Eren S, Tuma F. Perianal Abscess. National Library of Medicine. 2023. Available from: https://www.ncbi.nlm.nih.gov/books/NBK459167/