When interpreting any visual field, it is helpful to ask the following questions:

• Is the test reliable?
• Is the test normal?
• Does the visual field defect involve one eye or both eyes?
• If both eyes, does the defect respect the vertical meridian?
• If the defects respects the vertical meridian, is it bitemporal or junctional (optic chiasm) or homonymous (retrochiasmal)?

In this case, there are several red flags in the history and exam that suggest a diagnosis other than a previous episode of NAION:

• No documentation of a swollen optic disc
• Gradual rather than acute onset
• Lack of a small, crowded optic disc (cup-to-disc < 0.2)
• Visual field defect that respects the vertical meridian
• Progression of vision loss in the absence of disc swelling

Reasons to have a high index of suspicion for a compressive optic neuropathy in such patients:

• Early detection of a mass can result in better prognosis for vision and survival (if neoplastic).
• In many cases, the mass lesion can pose a risk to the unaffected eye.
• Visual loss can recover once the compressive lesion is treated.