Introduction of a New Test for Standardizing and Quantifying Monocular Fixation: The Aalen-Pacific Fixation Accuracy & Stability Test (AP-FAST)

Background: This is a two-part paper, in which fixational eye movements are reviewed in part one, and a new clinical test of fixational eye movement accuracy and stability is introduced in part two. Part two also contains a partial validation study and normative results for this new test. The study was designed to explore the following questions: 1. Is there a significant difference in clinically measurable fixation stability and accuracy between symptomatic individuals who have sustained mTBI versus visually normal individuals who have not sustained TBI? 2a. Could the Macular Integrity Tester (MIT) be useful as a diagnostic instrument for differentiating fixation stability and accuracy with mTBI patients versus those without TBI? 2b. If so, could introduction of a standardized test protocol (AP-FAST) allow for more reliable and descriptive quantification of monocular fixation behavior? Methods: To answer the first question posed in our study, data were compared between non-mTBIs and mTBI patients using two different fixation testing methods. One of the methods employed the eyetracker EyeLink II comparing optometry students to mTBI patients. Fixation accuracy and stability were measured with a custom fixation pattern program. Fixation results were compared for both groups. To answer question 2a, we compared fixation behavior for both groups using the MIT diagnostically by having each subject rate accuracy and stability of his/her own fixation posture. To address question 2b, a 15-point rating system protocol was created to help describe and quantify monocular fixation behavior via Haidinger’s brush perception. Two groups of test subjects: mTBI patients versus visually normal subjects were compared using the AP-FAST grid and fixation behavior survey. Results: Regarding question one, there was a significant and clinically relevant difference in monocular fixation stability and accuracy between the visual normal subject group and mTBI patient group, with normals demonstrating better accuracy and stability. Regarding questions 2a and 2b, the mTBI patient group had poorer mean ability (statistically significant) to perceive, localize, and center the Haidinger’s brush than those in the non-TBI group. Based on our results, we propose that the newly developed AP-FAST is useful for helping describe, standardize and quantify how the Haidinger’s brush entoptic phenomenon is perceived monocularly by both mTBI and nonmTBI subjects.