What is the anatomy of the optic nerve, including its origin, course, and termination, and how can compression of the optic tract result in visual field defects in a patient with a mass at the optic nerve region?

From the Guidelines

The optic nerve originates from the retinal ganglion cells in the eye, and its compression can result in specific visual field defects depending on the location of compression, with prechiasmatic compression causing monocular vision loss, chiasmal compression causing bitemporal hemianopia, and post-chiasmatic compression causing homonymous hemianopia. The optic nerve, or cranial nerve II, sends axons that converge at the optic disc, forming the optic nerve which exits the orbit through the optic canal in the sphenoid bone 1. The nerve then travels posteriorly and medially to reach the optic chiasm, where partial decussation occurs. At the chiasm, fibers from the nasal (medial) retina cross to the opposite side, while temporal (lateral) retina fibers remain uncrossed. After the chiasm, the visual pathway continues as the optic tract, which carries information to the lateral geniculate nucleus of the thalamus. From there, visual information travels via the optic radiations to the primary visual cortex in the occipital lobe.

Visual Field Defects

Compression of the optic tract can result in specific visual field defects depending on the location of compression. If compression occurs before the chiasm (prechiasmatic), it affects all fibers from one eye, resulting in monocular vision loss or blindness. Compression at the optic chiasm typically affects crossing nasal retinal fibers, causing bitemporal hemianopia (loss of peripheral vision in both temporal visual fields) 1. Post-chiasmatic compression of the optic tract produces homonymous hemianopia, where the same visual field is lost in both eyes (right optic tract lesion causes left visual field defect and vice versa). The pattern of visual field defect is crucial for localizing the lesion along the visual pathway and guiding appropriate management of conditions like the mass identified in this patient.

Clinical Evaluation

A thorough neurologic and ophthalmologic examination can often accurately localize a defect along the visual pathway, and factors such as the age of the patient, the time course for onset, degree of visual loss at presentation, the presence of eye pain, and visible exophthalmos or enophthalmos determine if imaging is needed 1. Imaging analysis of orbital diseases is facilitated by a compartmental approach that establishes a differential diagnosis on the basis of lesion location along the visual pathway. CT and MRI are often complementary when assessing visual loss, and the inherent contrast provided by orbital fat allows for excellent anatomic definition with either technique.

Management

The management of conditions like the mass identified in this patient should prioritize minimizing the side effects of treatment and their impact on the patient’s vision, general health, and quality of life. This includes documenting the status of the optic nerve structure at baseline by clinical evaluation and imaging, and documenting visual function by visual field testing 1. The goal is to maintain quality of vision and preserve quality of life, and to adjust the target intraocular pressure to a lower level if deterioration occurs. Educating and involving the patient and appropriate family members/caregivers in the management of the disease is also crucial.

From the Research

Anatomy of the Optic Nerve

• The optic nerve originates from the optic disc, which is located at the back of the eye 2
• The optic nerve then courses through the optic canal, a narrow passageway in the skull, to reach the optic chiasm 2
• At the optic chiasm, the optic nerves from each eye cross over, allowing for the combination of visual information from both eyes 3, 4, 5
• The optic tracts, which are formed by the crossing of the optic nerves, then continue to the brain, where they transmit visual information for processing 3, 4, 5

Course and Termination of the Optic Nerve

• The optic nerve can be divided into three segments: the proximal intracranial, distal intracranial, and intracanalicular segments 2
• Each segment can be further divided into quadrants and subquadrants, which can be exposed to varying degrees through different surgical approaches 2
• The optic nerve terminates at the optic chiasm, where the optic tracts are formed 3, 4, 5

Compression of the Optic Tract and Visual Field Defects

• Compression of the optic chiasm can result in bitemporal hemianopia, a type of visual field defect where vision is lost in the outer half of each visual field 3, 4
• The precise mechanism of this visual field defect is unclear, but it is thought to be related to the selective damage of crossing nasal fibers in the optic chiasm 4, 5
• Studies have shown that the human optic chiasm is "H" shaped, with nasal fibers decussating paracentrally and running parallel to each other in the central arm of the "H" 4, 5
• This information can help explain the clinical observation of junctional scotoma and provide a basis for structural modeling of chiasmal compression 5