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Oculomotor cistern

The oculomotor cistern is a small, tubular extension of the subarachnoid space that envelops the oculomotor nerve (cranial nerve III) as it passes from the interpeduncular cistern toward the cavernous sinus. It is a key anatomical landmark located at the medial aspect of the cerebral peduncle, just lateral to the posterior clinoid process, and inferior to the posterior communicating artery. The cistern provides a protective sheath for the oculomotor nerve and allows it to travel through the subarachnoid space before piercing the dura mater to enter the cavernous sinus.

Synonyms

  • Oculomotor nerve cistern

  • Third nerve cistern

  • CN III cistern

Function

  • Envelops and protects the oculomotor nerve as it passes from the brainstem to the cavernous sinus

  • Provides a pathway for the oculomotor nerve through the subarachnoid space

  • Contains cerebrospinal fluid (CSF), offering cushioning and facilitating nerve mobility

  • Serves as an important radiological and surgical landmark for identifying the course of the oculomotor nerve

MRI Appearance

  • T1-Weighted Images

    • Appears as a CSF-intensity (hypointense/dark) tubular structure adjacent to the cerebral peduncle

    • The oculomotor nerve itself may appear as a thin, linear, isointense (relative to brain) structure within the cistern

  • T2-Weighted Images

    • The cistern appears hyperintense (bright), consistent with CSF signal

    • The nerve remains as a linear, hypointense (dark) structure within the bright cisternal space

  • FLAIR (Fluid-Attenuated Inversion Recovery)

    • CSF in the cistern is suppressed and appears hypointense (dark)

    • The oculomotor nerve is not clearly visualized; the suppression of CSF can help to differentiate cisternal lesions or pathology (e.g., nerve enhancement, cisternal masses)

CT Appearance

  • Not distinctly visible on non-contrast CT due to small size and similar attenuation of surrounding tissues

  • May be inferred as a low-density (hypodense) linear space adjacent to the cerebral peduncle if large or if there is pathological enlargement (e.g., by mass, hemorrhage, or CSF leak)

  • The oculomotor nerve itself is generally not visualized on CT

MRI images

oculomotor cistern mri 3t coronal image

CT image

Oculomotor cistern CT coronal anatomy image -img-00001-00001