Along with clinical and ophthalmoscopic evaluation,
imaging,
especially magnetic resonance imaging,
plays an important role in the thorough evaluation of optic nerve (ON) and the entire visual pathway [1].
The knowledge of this detailed anatomy and its main pathologies is essential for the patient's highly accurate diagnosis.
The ON (the second cranial nerve) is an extension of the central nervous system (CNS) axons,
thus,
it’s myelinated by oligodendrocytes.
It originates from the retina,
where its first sensory bipolar cell body is located and it is circumscribed by glial cells and by the three meningeal layers,
measuring 4mm in diameter and approximately 5.0 cm in length.
The ON is divided into four segments (Fig 1):
a) intraocular (1mm): within the retina,
emerging through lamina cribrosa.
b) intraorbital (25mm): passes centrally within the orbit and is surrounded by dural layer and cerebrospinal fluid (CSF),
communicating with the subarachnoid space (Fig 2).
c) intracanalicular (9mm): where it exits through the optic canal inferior to the ophthalmic artery.
d) intracranial/ pre-chiasmatic (16mm): enters the middle cranial fossa and courses in the suprasellar cistern to join the contralateral nerve and form the optic chiasm,
where the nasal fibers from each ON decussate and temporal fibers do not (Fig 3).
The optic chiasm is located about 1,0 cm above the pituitary gland,
separated by the suprasellar cistern and it divides into two optic tracts that course posterolaterally along the cerebral peduncles to synapse at lateral geniculate bodies (LGB)[2].
The optic radiations arise from the LGB and divide into superior and inferior bundles.
The first ends at the cuneus and the last loops anteriorly to the temporal lobe (Meyer’s loop) and ends inferior to the calcarine sulcus.
The primary visual cortex is located around the calcarine sulcus while the secondary and tertiary cortices are located laterally.
The ON is supplied by small branches of the anterior cerebral artery (ACA) and the superior hypophyseal artery in its intracranial portion and the chiasm,
while small branches of the anterior choroidal and posterior communicating arteries supply the optic tracts.
Imaging protocols:
-
Computed tomography (CT): best for skull base and optic canal bony anatomy,
it can detect minimal calcified optic sheath meningiomas when MR may not.
-
Magnetic resonance (MR): is the imaging modality of choice for evaluating the ON pathway [2],
allowing excellent depiction of the anatomy,
its excellent soft tissue contrast without exposure to ionizing radiation,
and accurate evaluation of associated intracranial pathologies (1).
In our institution,
our protocol includes thin-sections of the following sequences: axial T2 TSE of the orbits,
axial and coronal T1 TSE fat-saturated pre and post-gadolinium injection of the orbits,
fat-saturated coronal short tau inversion recovery (STIR)/T2 of the orbits and fat-saturated volumetric fluid-attenuated inversion recovery (FLAIR) of the brain.