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External anal sphincter

The external anal sphincter is a circular skeletal muscle that surrounds the anal canal and contributes to voluntary control of defecation. It is one of the primary muscles of continence, working together with the internal anal sphincter and pelvic floor muscles. It is under voluntary control but also maintains tonic contraction at rest, ensuring closure of the anal canal.

Synonyms

  • Sphincter ani externus

  • Voluntary anal sphincter

  • External sphincter of the anus

Origin, Course, and Insertion

  • Origin: The muscle has no fixed bony origin; it arises from surrounding perineal tissue and fibers blending with the anococcygeal ligament posteriorly.

  • Course: Encircles the anal canal as a circular skeletal muscle, extending from the lower edge of the levator ani to the perianal skin.

  • Insertion: Anterior fibers blend with the perineal body; posterior fibers attach to the coccyx via the anococcygeal ligament.

Nerve Supply

  • Inferior rectal nerve (branch of the pudendal nerve, S2–S4)

  • Additional contribution from perineal branches of S4 and the inferior rectal branch of the coccygeal plexus

Arterial Supply

  • Inferior rectal artery (branch of internal pudendal artery)

  • Perineal branches of internal pudendal artery

Venous Drainage

  • Inferior rectal vein → internal pudendal vein → internal iliac vein

Function

  • Maintains voluntary closure of the anal canal

  • Provides conscious control of defecation

  • Works synergistically with internal anal sphincter and pelvic floor muscles to maintain continence

MRI Appearance

T1-weighted images:

  • External anal sphincter appears with low-to-intermediate signal intensity in a ring-like configuration

  • Fibrous connective tissue bands may appear with slightly higher signal

T2-weighted images:

  • Low-to-intermediate signal intensity muscle encircling the anal canal

  • Tears or scarring appear as areas of altered signal

STIR (Short Tau Inversion Recovery):

  • Normal sphincter shows low-to-intermediate signal

  • Inflammation, edema, or sphincter injury show bright signal intensity

Proton Density Fat-Sat (PD FS):

  • Muscle shows low-to-intermediate signal intensity

  • Pathologic changes such as sphincter injury or fibrosis show bright or altered signals

T1 Fat-Sat Post-Contrast:

  • Mild, uniform enhancement in normal muscle

  • Pathologic muscle shows focal or heterogeneous enhancement

  • Abscesses show rim enhancement with central non-enhancing fluid collection

CT Appearance

Non-Contrast CT:

  • Appears as a soft tissue density ring encircling the anal canal

  • Hematomas appear as hyperdense areas within the sphincter or perianal space

Post-Contrast CT:

  • Mild homogeneous enhancement in normal muscle

  • Abscesses show rim enhancement with central hypodensity

  • Inflammatory changes show irregular enhancement

MRI image

External anal sphincter muscle MRI axial anatomy image-img-00000-00000

CT images

External anal sphincter muscle CT axial image