Topics

Topic

design image
Sacral hiatus

The sacral hiatus is an opening at the caudal end of the sacral canal, formed due to the failure of the laminae of the fifth (and sometimes fourth) sacral vertebra to fuse. It is located on the posterior surface of the sacrum, just superior to the coccyx. The hiatus provides access to the sacral canal and is of major clinical significance for procedures such as caudal epidural anesthesia.

It serves as a landmark for anesthesiologists, surgeons, and radiologists, as well as a site for the passage of terminal sacral and coccygeal nerves.

Synonyms

  • Caudal opening of the sacral canal

  • Caudal hiatus

  • Sacral canal termination

Location and Boundaries

  • Location: On the dorsal (posterior) surface of the sacrum, just above the coccyx, at the lower end of the sacral canal

  • Boundaries:

    • Superiorly: Sacral cornua (horns of S5)

    • Laterally: Sacral cornua and margins of sacral laminae

    • Inferiorly: Coccyx

    • Anteriorly: Posterior sacral surface and sacral canal contents

Relations

  • Posteriorly: Covered by skin, subcutaneous tissue, and sacrococcygeal ligament

  • Anteriorly: Sacral canal with dural sac (ending usually at S2), epidural venous plexus, and terminal sacral nerves

  • Laterally: Sacral cornua and sacral laminae

  • Inferiorly: Coccygeal body and intercoccygeal disc

Function

  • Provides entry into the sacral canal for clinical procedures (e.g., caudal epidural block)

  • Serves as a landmark in spinal and pelvic anatomy

  • Allows passage of terminal filum terminale, coccygeal nerve, and surrounding connective tissue

Clinical Significance

  • Caudal epidural anesthesia: Common site for injection of anesthetic agents in pediatric and some adult procedures

  • Access point: For administration of drugs into the epidural space

  • Variability: Size, shape, and bony morphology vary greatly between individuals, affecting procedural success

  • Pathology: May be altered by trauma, congenital anomalies, or degenerative changes

  • Imaging relevance: Must be differentiated from fractures or lytic lesions of sacrum on imaging

MRI Appearance

T1-weighted images:

  • Sacral hiatus appears as a low-signal bony defect at caudal sacrum

  • Surrounding fat within sacral canal appears bright, highlighting the opening

T2-weighted images:

  • Hiatus appears as a low-signal gap in the posterior sacrum

  • CSF within the dural sac (ending above hiatus) appears bright

  • Epidural fat is hyperintense, aiding identification

STIR (Short Tau Inversion Recovery):

  • Fat suppressed, so epidural fat appears dark

  • Hiatus still seen as a bony gap, with surrounding soft tissue signal

T1 Fat-Sat Post-Contrast:

  • No enhancement of the bony hiatus itself

  • Pathological processes (e.g., cysts, tumors, infection) in or around hiatus may enhance

3D T2 SPACE / CISS:

  • Shows hiatus as a clear bony defect with surrounding bright CSF and epidural fat

  • Excellent for identifying small nerve roots exiting or abnormalities at the caudal canal

CT Appearance

Non-Contrast CT:

  • Hiatus seen as a triangular or oval bony defect in the dorsal sacral canal at S4–S5

  • Epidural fat appears as low attenuation inside the canal

  • Helpful for detecting congenital variations, fractures, or lytic lesions

Post-Contrast CT:

  • Hiatus itself does not enhance

  • Surrounding pathological tissue (infection, neoplasm, vascular lesions) may enhance

  • Useful for surgical and interventional planning

MRI image

Sacral hiatus mri coronal anatomy image

MRI image

Sacral hiatus mri coronal anatomy image

CT image

Sacral hiatus ct axial anatomy image