Outer sheath of optic nerve

Outer sheath of the optic nerve refers to the dural sheath, which is the outermost meningeal layer surrounding the optic nerve. The optic nerve, as an extension of the central nervous system, is enveloped by three meningeal layers continuous with those covering the brain: the pia mater (innermost), the arachnoid mater (middle), and the dura mater (outermost). The dura mater constitutes the outer sheath and provides a tough, protective covering for the optic nerve as it courses from the globe to the optic chiasm.

Key anatomical features:

• The outer (dural) sheath fuses anteriorly with the sclera of the eye at the posterior aspect of the globe and posteriorly becomes continuous with the periosteum of the optic canal and the intracranial dura mater.

• Between the arachnoid and pia mater lies the subarachnoid space, which contains cerebrospinal fluid (CSF) and is in direct communication with the intracranial subarachnoid space. This is clinically significant because elevated intracranial pressure can be transmitted along this space, leading to optic nerve sheath distension and papilledema.

• The optic nerve sheath complex (including the outer dural sheath) can be measured via ultrasound or MRI. An optic nerve sheath diameter (ONSD) greater than approximately 5.0–5.5 mm on ocular ultrasound is considered suggestive of elevated intracranial pressure.

Clinical relevance:

• Optic nerve sheath meningiomas arise from the meningothelial cells of the dural sheath and represent the most common primary tumor of the optic nerve sheath.

• Optic nerve sheath fenestration is a surgical procedure in which slits are made in the dural and arachnoid layers to allow CSF drainage, used in the management of vision-threatening papilledema (e.g., in idiopathic intracranial hypertension).

• The outer sheath serves as an important landmark in orbital imaging and surgery, delineating the boundary of the optic nerve complex within the orbital fat.