Eye Movements During Perimetry and the Effect that Fixational Instability Has on Perimetric Outcomes

Area of Science:

• Ophthalmology
• Neuroscience
• Visual Psychophysics

Background:

• Stable fixation is a key assumption in perimetry, crucial for accurate visual field assessment.
• Limited research exists on fixation stability during prolonged perimetric testing.

Purpose of the Study:

• To quantify horizontal eye movements during automated perimetry in normal subjects.
• To evaluate the impact of fixational instability on perimetric outcomes, particularly in the presence of scotomas.

Main Methods:

• Infrared eye monitoring of 12 normal subjects during automated threshold perimetry.
• Simulated intentional eye movements (5 degrees) in four trained observers with and without a scotoma.

Main Results:

• No observer maintained fixation within +/-3 degrees of the target throughout testing.
• All subjects showed fixational instability of +/-5 degrees, with some reaching +/-10-15 degrees.
• Intentional eye movements significantly increased short-term fluctuation and false negative rates, especially when a scotoma was present.

Conclusions:

• Fixational instability is inherent in automated perimetry, exceeding typical assumptions.
• Eye movements can 'blur' small scotomas, reducing their detectability.
• Clinicians should consider elevated fluctuation and false negative rates as potential indicators of subtle, mobile scotomas.