Criteria for Visual Field Defects Respecting the Vertical Midline
A visual field defect respects the vertical midline when the loss occurs in corresponding halves of both eyes (either both right or both left halves), creating a sharp boundary at the vertical meridian that divides the visual field into nasal and temporal halves. This pattern indicates pathology at or posterior to the optic chiasm 1, 2.
Key Anatomical Principles
Homonymous hemianopia represents loss in the same half of both visual fields (e.g., right half of both eyes or left half of both eyes), indicating chiasmal or post-chiasmal damage 1.
Bitemporal hemianopsia shows loss in the temporal fields of both eyes, suggesting a chiasmal lesion 1.
The vertical meridian boundary is anatomically determined by the decussation of nasal retinal fibers at the optic chiasm, where fibers from the nasal half of each retina cross to the opposite side 3, 2.
Distinguishing Features on Visual Field Testing
Sharp vertical boundary: The defect creates a precise line at the vertical meridian (0 degrees), with preserved vision on one side and loss on the other 2, 4.
Bilateral involvement: Both eyes show corresponding defects on the same side of the vertical meridian, though the defects may be congruent (identical) or incongruent (different shapes) depending on lesion location 4.
Congruency patterns: More posterior lesions (occipital cortex) produce highly congruent defects, while more anterior lesions (optic tract) produce less congruent defects 3.
Imaging Correlation
MRI of the brain without and with contrast is the preferred imaging modality for patients presenting with homonymous hemianopia or quadrantanopia, as these defects indicate post-chiasmal pathology 3.
For bitemporal defects or junctional scotomas, thin-slice MRI through the pituitary and suprasellar region is essential to evaluate the optic chiasm 3.
In acute presentations, consider vascular etiologies including anterior or posterior circulation stroke, which are common causes of sudden-onset hemianopic defects 3, 4.
Important Clinical Pitfalls
Glaucomatous defects cross the vertical meridian: Nasal steps and arcuate scotomas characteristic of glaucoma do NOT respect the vertical midline, which helps differentiate them from neurological causes 1.
Functional or testing artifacts: Rare cases of visual field defects respecting the vertical meridian may occur without distinct chiasmal or post-chiasmal lesions on MRI, potentially due to optic neuritis, testing algorithm artifacts, functional deficits, or optic disc abnormalities 5.
Bilateral occipital infarcts: These present with sudden, simultaneous onset of defects, normal funduscopic examination, and require CT or MRI confirmation 6.
Cortical vs. glaucomatous patterns: In cases of severe field constriction that might mimic glaucoma (pseudoglaucomatous pattern), examine the optic nerve head and perform OCT—normal structural findings indicate cortical rather than glaucomatous pathology 7.