With the segregation of nasal and temporal retinal fibers at the chiasm, visual field loss due to chiasmal and retrochiasmal lesions is characterized by temporal defects that align along the vertical meridian. The most common visual field defect of chiasmal compression is a bitemporal hemianopia. These defects may appear relative or complete. They may involve the paracentral temporal field alone, and visual acuity is often unaffected until late (Fig 4-25).
In chiasmal syndromes, the ONHs may appear normal initially, even with significant visual field loss. Early on, peripapillary retinal RNFL dropout and mild ONH pallor develop. With progressive damage, the ONHs show typical atrophy, often in the temporal portion of the ONH. Cupping of the ONH may increase. A tumor compressing the chiasm almost never produces ONH edema. Parasellar lesions that involve the chiasm—whether compressing or infiltrating this area—result in gradually progressive, bilateral, often asymmetric vision loss. The peripheral (temporal) visual fields are usually involved first. In any case of bilateral visual field loss, the clinician must carefully evaluate perimetry testing results for respect of the vertical midline. Any of the variations on bitemporal visual field loss (described in the following sections) may occur. An affected optic nerve may cause more central vision loss, with impaired visual acuity, dyschromatopsia, and an RAPD on the affected side. Markedly asymmetric visual field loss without direct optic nerve damage may also produce an RAPD.
Visual Field Loss Patterns
Lesions that injure an optic nerve at its junction with the optic chiasm cause junctional scotomas. Diminished visual acuity and central visual field loss occur in the ipsilateral eye, and a temporal hemianopia develops in the opposite eye (Fig 4-26). A unilateral temporal hemianopia that respects the vertical midline, with no involvement of the visual field in the opposite eye (also known as a junctional scotoma of Traquair), can also indicate a chiasmal abnormality. Presumably, the mass compresses only the crossing nasal fibers from 1 eye.
Figure 4-25 Schematic illustration of the visual pathways with numbered locations of the most common visual pathway lesions. The visual field patterns shown on the right represent potential findings corresponding to lesions of the same number shown along the visual pathways.
(Illustration by Christine Gralapp.)
Excerpted from BCSC 2020-2021 series: Section 5 - Neuro-Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.