Anal Abscess

Notes

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}

Anal Abscess

Definition

Anorectal abscess: collection of pus in anorectal spaces resulting from infection of anal crypt glands or secondary causes.
Perianal abscess: superficial abscess located just below the skin near the anus.
Perirectal abscess: deeper abscess involving one of the internal anorectal compartments. It presents with more severe symptoms and less obvious external signs.
Fistula-in-Ano: abnormal connection of the anorectal epithelial surface to the perineal skin.

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

Hilton’s Line also called while line/anocutaneous line indicates lower end of the internal sphincter. Ischiorectal abscess when communicating with anal canal usually opens at or below Hilton’s line.

Aetiology and Risk factors

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

most common cause of anorectal abscess is cryptoglandular infection.

Pathophysiology

Anal gland duct obstruction

↓

Bacterial proliferation within the gland

↓

Pus collection in intersphincteric space

↓

Spread into other anatomical planes

↓

Expanding abscess causes local tissue destruction

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

perirectal abscesses present with more severe symptoms, systemic features and absence of external signs on exam.

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

Patients with concomitant anal fistula might undergo a primary fistulotomy during abscess drainage.

Complications and Prognosis

Complications

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

Anal fistula is the common complication of anorectal abscesses.

Prognosis

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

References

Rakinic J. Anorectal abscess. In: BMJ Best Practice [Internet]. 2022 [cited 2025 Nov 20]. Available from: BMJ Best Practice
Bleday R. Perianal and perirectal abscess. In: Weiser M, editor. UpToDate [Internet]. [updated 2024 Jul 22; cited 2025 Nov 20]. Available from: UpToDate
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/
Conner JN, Eren S, Tuma F. Perianal Abscess. National Library of Medicine. 2023. Available from: https://www.ncbi.nlm.nih.gov/books/NBK459167/