Anatomy Atlas

Ischioanal fossa

Fossa ischioanalis
Also: Ischiorectal fossa
Space / regiondraftGI

Paired fat-filled wedge-shaped spaces on either side of the anal canal. The pudendal canal lies in their lateral wall, and anorectal abscesses spread through them, sometimes in a horseshoe pattern.

Core anatomy

Location

In the anal triangle of the perineum, between the anal canal and the ischium on each side.

Boundaries
lateral
Ischium and obturator internus with its fascia (containing the pudendal canal)
medial
External anal sphincter and the sloping levator ani
apex (superior)
Where levator ani arises from the obturator fascia
floor (inferior)
Perianal skin and superficial fascia
anterior
Posterior edge of the perineal membrane (the anterior recesses extend forward above it)
posterior
Sacrotuberous ligament and gluteus maximus
Contents
  • Fat (the fat body of the ischioanal fossa), which lets the anal canal distend during defecation
  • Pudendal canal in the lateral wall, containing the pudendal nerve and internal pudendal vessels
  • Inferior rectal nerves and vessels, crossing from the pudendal canal medially to the anal canal
  • Perineal branches of the posterior femoral cutaneous nerve and the perforating cutaneous nerve (S2–S3)
Communications

The right and left fossae communicate behind the anal canal (deep postanal space, above the anococcygeal body), so infection can spread from one side to the other (horseshoe abscess). Anterior recesses extend forward above the perineal membrane.

Found in

Mentioned in

Generated automatically from links in other entries.

Perineum: urogenital and anal triangles (inferior view)Diamond-shaped perineum bounded by the pubic symphysis, ischiopubic rami, ischial tuberosities, sacrotuberous ligaments and coccyx. A line between the ischial tuberosities separates the anterior urogenital triangle (urethra and vagina) from the posterior anal triangle (anus and ischioanal fossae). The perineal body lies at the center. The pudendal canal runs in the lateral wall of each ischioanal fossa.Pubic symphysisIschiopubic ramusIschialtuberosityIschialtuberosityUrogenital triangle(perineal membrane,superficial & deep pouches)UrethraVagina (♀)Perineal bodyIschioanalfossaIschioanalfossaAnus + externalanal sphincterPudendal canal(pudendal n. +int. pudendal vessels)Inferior rectal n.Sacrotuberous lig.Anal triangleCoccyxAnterior ↑ (inferior view, lithotomy)Dashed line: between the ischial tuberosities

Swipe sideways to see the whole diagram →

Perineum: urogenital and anal triangles. Inferior view with the patient in lithotomy (anterior at the top). A line between the ischial tuberosities divides the urogenital triangle from the anal triangle; the pudendal canal runs in the lateral wall of each ischioanal fossa.
Original schematic, created for this atlas.
Coronal section through the bladder, prostate and urethra showing levator ani, the fascial layers of the urogenital region, bulbospongiosus, ischiocavernosus and the ischioanal fossae
Coronal section through the pubic arch: the ischioanal (ischiorectal) fossae on each side, lateral to levator ani.
Henry Vandyke Carter, Gray's Anatomy of the Human Body, 20th ed. (1918), Fig. 407. Source · Public domain (Public domain in the US: published before January 1, 1931 (per Wikimedia Commons))
Male perineum from below with the layers of superficial perineal fascia reflected, showing the inferior rectal vessels and nerve crossing the ischioanal fossa
Male perineum with superficial fascia reflected: the inferior rectal vessels and nerve cross the ischioanal fossa.
Henry Vandyke Carter, Gray's Anatomy of the Human Body, 20th ed. (1918), Fig. 405. Source · Public domain (Public domain in the US: published before January 1, 1931 (per Wikimedia Commons))

Clinical correlations

Ischioanal (ischiorectal) abscess PANCE

Mechanism / lesion

Spread of a cryptoglandular infection from an anal gland through the external sphincter into the fossa (or from a perianal abscess).

Classic presentation

Deep throbbing buttock/perianal pain, fever, and a tender indurated or fluctuant area lateral to the anus (external signs may be subtle because the fossa is deep). Treat with prompt incision and drainage; antibiotics are added for cellulitis, immunosuppression or diabetes.

Fistula-in-ano PANCE

Mechanism / lesion

A persistent tract between an anal gland at the pectinate line and the perianal skin after an abscess; associated with Crohn disease when complex or multiple.

Classic presentation

Chronic intermittent discharge from a perianal opening. Goodsall rule: an external opening anterior to a transverse line through the anus usually has a straight radial tract; one posterior to it usually curves to the posterior midline. Treated surgically (fistulotomy, seton), taking care to preserve continence.

High-yield PANCE pearls

  • Perianal pain + fever + fluctuance → incision and drainage (antibiotics alone are not enough).
  • Goodsall rule: anterior openings → straight tracts; posterior openings → curve to the posterior midline.
  • Horseshoe abscess = spread between the two ischioanal fossae behind the anal canal.
  • Complex or recurrent fistulas → think Crohn disease.

Imaging anatomy

CT

CT pelvis locates deep abscesses (ischioanal, supralevator).

MRI

MRI (fistula protocol) is the gold standard for mapping complex fistula tracts and horseshoe extensions.

Ultrasound

Endoanal ultrasound (with hydrogen peroxide enhancement) shows fistula tracts relative to the sphincters.

Key views & landmarks
  • MRI pelvis fistula protocol (axial and coronal STIR/T2)

Physical exam

Surface landmarks

The fossa lies between the anus and the ischial tuberosity on each side.

Palpation

Inspection and palpation of the buttock and perianal skin for induration and fluctuance; DRE for tenderness or a mass lateral to the anal canal and for an internal opening.

Self-quiz

Q1.

A 40-year-old man with diabetes has 4 days of throbbing left buttock pain and fever. There is a tender, indurated, fluctuant area 3 cm lateral to the anus. What is the most appropriate management?

Show answer

B

This is an ischioanal abscess. Definitive treatment is incision and drainage; antibiotics are an adjunct in patients with diabetes, immunosuppression or cellulitis.

Q2.

Which structure runs in the lateral wall of the ischioanal fossa?

Show answer

B

The pudendal (Alcock) canal is a split in the obturator internus fascia on the lateral wall of the ischioanal fossa; it carries the pudendal nerve and internal pudendal vessels.

Q3.

How can an abscess in one ischioanal fossa spread to the other side?

Show answer

A

The two ischioanal fossae communicate behind the anal canal through the deep postanal space, above the anococcygeal body, so infection can spread from side to side — a horseshoe abscess.

Flashcards

See also