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Ischioanal fossa

The ischioanal fossa is a wedge-shaped fat-filled space located on each side of the anal canal within the anal triangle of the perineum. It provides a distensible compartment that allows for expansion of the anal canal during defecation. The fossa also serves as a conduit for neurovascular structures supplying the perineum. Clinically, it is important because infections here may spread widely, forming ischioanal abscesses.

Synonyms

  • Ischiorectal fossa

  • Lateral anal fossa

  • Perianal fat space

Boundaries

  • Base (inferior): Skin of the perineum

  • Apex (superior): Convergence of obturator internus and levator ani muscles

  • Medial wall: External anal sphincter and levator ani (puborectalis)

  • Lateral wall: Ischial tuberosity and obturator internus muscle (covered by obturator fascia)

  • Posteriorly: Sacrotuberous ligament and gluteus maximus muscle

  • Anteriorly: Continues forward into the anterior recesses between pelvic diaphragm and urogenital diaphragm

Contents

  • Ischioanal fat pad (loose adipose tissue)

  • Inferior rectal (anal) vessels and nerves (branches of internal pudendal vessels and pudendal nerve)

  • Perforating cutaneous branches of the posterior femoral cutaneous nerve

  • Pudendal canal (Alcock’s canal) running on the lateral wall carrying pudendal nerve and internal pudendal vessels

  • Lymphatics from anal canal and perineum

Relations

  • Medially: Anal canal and external anal sphincter

  • Laterally: Obturator internus muscle and ischial tuberosity

  • Superiorly: Levator ani muscle

  • Inferiorly: Perineal skin

Function

  • Provides space for expansion of the anal canal during defecation

  • Acts as a fat-filled cushion protecting anal and perineal structures

  • Serves as a conduit for pudendal neurovascular structures to perineum and anal canal

  • Provides a potential space for spread of infection or abscess formation

Clinical Significance

  • Site of ischioanal abscesses and fistulas-in-ano, which may extend across the midline (“horseshoe abscess”)

  • May be involved in perianal sepsis, trauma, or surgical complications

  • Important landmark in perineal surgery and drainage procedures

  • Imaging helps differentiate between benign inflammatory vs malignant lesions

MRI Appearance

T1-weighted images:

  • Normal ischioanal fossa fat appears bright with low-signal vessels and nerves

  • Abscesses appear as low signal areas within the fat

T2-weighted images:

  • Fat shows bright  signal

  • Fluid, abscess, or edema shows bright signal intensity

  • Fibrosis  appears low signal

STIR (Short Tau Inversion Recovery):

  • Fat suppressed to dark signal

  • Abscesses, inflammation, or edema appear bright against the dark background

T1 Fat-Sat Post-Contrast:

  • Fatty tissue shows no enhancement

  • Abscesses show rim enhancement with central non-enhancing cavity

  • Malignant infiltration shows heterogeneous enhancement

CT Appearance

Non-Contrast CT:

  • Appears as a fat-filled space of low attenuation with streaks of soft tissue representing vessels and nerves

  • Abscesses appear as soft tissue density masses with or without air pockets

Post-Contrast CT:

  • Fat remains low attenuation

  • Abscesses show peripheral rim enhancement

  • Neoplastic infiltration appears as irregular enhancing soft tissue masses

MRI image

Ischioanal fossa MRI axial anatomy image-img-00000-00000

CT image

Ischioanal fossa CT axial anatomy image-img-00000-00000