What is the most likely cause of a transient central scotoma that blurs vision but is not detected on routine ophthalmic examination?

Central Scotoma Not Visible on Routine Eye Examination

This patient is most likely experiencing a central scotoma from macular disease with eccentric fixation, which causes traditional eye exams to mislocalize or miss the scotoma entirely—specialized testing with microperimetry or careful binocular assessment is needed to accurately detect and map this defect. 1

Why Routine Eye Exams Miss These Scotomas

Central field assessment using traditional automated field tests is fundamentally inaccurate in patients with unstable or nonfoveal fixation secondary to macular disease. 1 When patients develop eccentric fixation, the scotoma location gets mapped in the wrong location during standard testing—a scotoma that appears paracentral may actually be a central scotoma that is displaced by the patient's eccentric fixation. 1

Additionally, scotomas can be either overestimated or underestimated by poor fixation stability during conventional testing. 1 The American Academy of Ophthalmology notes that Amsler grid testing detects only approximately 50% of central scotomas due to a phenomenon called "perceptual completion," where the brain fills in the missing visual information. 2

Most Likely Underlying Causes

The differential diagnosis for a central scotoma that blurs vision includes:

• Macular disease with preferred retinal locus (PRL) development: Patients with macular pathology naturally develop eccentric fixation using a PRL, which is a dynamic process where patients use multiple retinal locations that can change based on task, print size, or illumination. 1

• Central serous chorioretinopathy (CSC): Subretinal fluid can be so shallow that it cannot be detected on slit-lamp biomicroscopy, yet OCT imaging reveals the pathology. 1 This condition causes central visual blurring with a scotoma that may not be obvious on routine fundoscopy.

• Migraine-related scotoma: While typically transient, migraine can cause persistent monocular scotomas lasting beyond 10 days, though this is less common. 3, 4 The fortification spectrum with glimmering jagged lines is characteristic. 5

Recommended Diagnostic Approach

Fundus-related macular microperimetry (microperimetry) is the gold standard for accurately detecting both fixation patterns and scotomas in these patients. 1 During microperimetry, eye-tracking technology detects ocular movements during testing and ensures stimuli are presented to the correct retinal position, determining both fixation location and stability. 1

Alternative testing methods include:

• Optical coherence tomography (OCT): Critical for detecting shallow subretinal fluid or subtle macular pathology not visible on clinical examination. 1

• Binocular testing methods: The California Central Visual Field test with laser pointer on paper target, or observing the examiner's face/clock face binocularly, can provide useful information about fixation relative to scotoma during real-world viewing conditions. 1, 2

• Contrast sensitivity testing: Patients with poor contrast sensitivity may have normal visual acuity but significant functional impairment, increasing fall risk and difficulty with daily tasks. 1

Critical Pitfalls to Avoid

Do not dismiss this as a functional complaint or attribute it to simple refractive error when routine examination appears normal. 2 The American Academy of Ophthalmology emphasizes that scotomas appearing paracentral on testing may actually be central scotomas displaced by eccentric fixation. 1, 2

If the patient describes transient episodes, do not automatically assume migraine without excluding retinal vascular causes. Central retinal vein occlusion can present with initially normal fundoscopy and be misdiagnosed as migraine-related visual disturbance. 6 Retinal arterial ischemia (transient monocular visual loss) requires emergent cardiovascular workup as it carries the same risk for cardiovascular events and death as cerebral TIAs. 1

Management Algorithm

1. Obtain OCT imaging immediately to detect subtle macular pathology including subretinal fluid, even if fundoscopy appears normal. 1

2. Perform microperimetry if available to accurately map the scotoma and fixation pattern, or use binocular testing methods (California Central Visual Field test, face/clock observation) as alternatives. 1, 2

3. If CSC is identified: For acute CSC with symptoms less than 4 months, observation is appropriate unless rapid visual recovery is professionally necessary, in which case half-dose photodynamic therapy should be considered. 1

4. If no structural cause is found: Consider migraine-related persistent scotoma only after excluding retinal vascular disease, particularly if there is a history of migraine with visual aura. 3, 4 However, extensive vascular workup should be performed first.

5. Initiate vision rehabilitation: Patients benefit from training in eccentric viewing techniques to optimize their preferred retinal locus, contrast enhancement devices, and appropriate magnification tools. 1, 2