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Optic nerve sheath

The optic nerve sheath is a specialized, tubular extension of the cranial meninges that surrounds the optic nerve as it travels from the eyeball to the optic canal. It provides mechanical protection, facilitates the transmission of visual information, and acts as a conduit for cerebrospinal fluid (CSF) around the optic nerve. The sheath comprises three layers—dura mater, arachnoid mater, and pia mater—and plays a crucial role in several clinical conditions, including raised intracranial pressure, optic neuritis, and orbital trauma.

Synonyms

  • Optic nerve meningeal sheath

  • Sheath of the optic nerve

  • Perineural sheath (of optic nerve)

  • Optic nerve dural sheath

Function

  • Protects the optic nerve from mechanical injury

  • Provides a CSF-filled subarachnoid space, allowing pressure transmission between the intracranial compartment and orbit

  • Supports vascular and lymphatic supply to the optic nerve

  • Facilitates immune surveillance in the optic nerve region

MRI Appearance

  • T1-Weighted Images

    • Optic nerve sheath appears as a hypointense (dark) rim around the slightly hyperintense (relative to CSF) optic nerve itself.

    • CSF within the sheath is low signal (dark).

  • T2-Weighted Images

    • CSF within the optic nerve sheath is hyperintense (bright), resulting in a "tram-track" or "double donut" appearance, especially in the coronal plane.

    • The optic nerve itself remains isointense or slightly hypointense compared to orbital fat.

  • STIR (Short Tau Inversion Recovery)

    • Optic nerve sheath and CSF remain  isointense or slightly hypointense.

    • Edematous or inflamed sheaths (e.g., in optic neuritis or raised ICP) show increased signal intensity.

CT Appearance

  • Optic nerve sheath is not directly visible on non-contrast CT.

  • Can be inferred as a thin, low-attenuation rim (representing CSF) surrounding the optic nerve.

MRI images

Optic nerve sheath mri image