Interocular Fixation Instability Could Be Biomarker for Amblyopia

Fixational eye movement (FEM) refers to the tiny, involuntary, almost imperceptible adjustments that eyes make to focus on an object. A study at Cleveland Clinic Cole Eye Institute recently reported that FEM tends to stabilize from childhood to adulthood in people with normal vision.

The study also revealed that people with normal vision, regardless of age, have similar FEM patterns in both eyes. This symmetry is not observed in individuals with eye conditions such as amblyopia and strabismus.

“My lab studies FEM specifically in conditions that affect binocular vision, meaning when vision in one eye is worse than the other or when two eyes are not working in perfect synchronization,” says senior author Fatema Ghasia, MD, a pediatric ophthalmologist at the Cole Eye Institute. “It’s important for us to have a baseline, to know what’s normal in the eyes of people without disease, so we can better understand the differences in people with disease.”

Her recent study on FEM through the human lifespan builds on another study in which Dr. Ghasia linked interocular fixation instability with severity of amblyopia and strabismus.

“We find that in children with amblyopia, for example, there’s a difference in fixation instability between their two eyes,” she says. “Yes, children have more fixation instability overall, but in children with normal vision, the instability is seen symmetrically between the two eyes. In children (as well as adults) with a pathological condition, there’s greater instability in one eye compared to the other.”

Dr. Ghasia recently was awarded membership in the American Ophthalmological Society for her work on FEM in amblyopia and strabismus.

How eye movement affects vision

FEM plays an important role in visual function. Tiny adjustments within the eye enable clearer vision.

Without FEM, if the eyes were perfectly still, the brain’s neurons would struggle to process visual signals, causing visual stimuli to fade. Conversely, excessive eye movement during fixation can degrade visual acuity.

“It’s a paradox,” Dr. Ghasia says. “Vision deteriorates with excessive eye movement. But, if your eyes aren’t moving at all, you don’t sample your visual world accurately and your vision fades. There’s a fine balance.”

Normal FEM within a certain physiological range enables correct vision. Assessing what is “normal” has become more precise, Dr. Ghasia notes, thanks to advancements in eye-tracking devices, like the one used in her recent study published in Experimental Brain Research.

FEMs across lifespan

Dr. Ghasia’s team studied 20 children, 26 adolescents and 22 adults without eye disease. Each person sat in a dark room, with their head in a chin rest, and looked at a target on a monitor for 45 seconds. A high-resolution video-based eye tracker recorded how their eyes responded in different viewing conditions (i.e., when using both eyes, the right eye alone or the left eye alone).

“Some literature suggests that fixation instability is prevalent in children, while other literature does not,” Dr. Ghasia says. “We wanted to see if fixation instability differed by age in these healthy individuals and how fixation was affected by viewing condition.”

The researchers found that:

• Children had greater fixation instability than adults. The speed of their eye movement between fixation points (velocity) was faster, and the distance in eye movement (amplitude) was greater. Fixation appears to stabilize throughout adolescence and adulthood as visual systems mature.
• There was no significant difference in interocular fixation stability (steady gaze) or vergence stability (synchronized movement of both eyes) between the groups.
• Fixation and vergence stability was better in all age groups when using both eyes versus one eye.

“We know that people with normal vision perform 7%-8% better on visual acuity tests when they use both eyes rather than covering one,” Dr. Ghasia says. “That binocular summation came into play with FEM as well, indicating that FEM and visual acuity are tightly correlated.”

Clinical impact

While FEM is generally more unstable in children versus adults, people with amblyopia or strabismus show differences in FEM between their two eyes. This interocular asymmetry was not seen in the study participants (all of whom had normal vision) regardless of age.

“The eye with pathology will have greater fixational instability than the healthy eye,” Dr. Ghasia says. “So, we’ve established that it’s important to evaluate both eyes at the same time to see how one is different from the other.”

Interocular fixation instability could be used as a biomarker to detect eye diseases like amblyopia, she notes. This method could be more reliable than conventional vision tests.

“The current test requires a child to accurately read a vision acuity chart and a trained eye examiner to measure the child’s vision,” Dr. Ghasia says. “Now we have a nonverbal measure. We no longer need to rely on a child, who may not be sitting still or focused on the task, to communicate what they see. We can detect vision loss just by observing their eye movements.”

The next step in this research involves the use of artificial intelligence (AI). Another publication by Dr. Ghasia, exploring how AI algorithms can assess FEM to help diagnose eye disease, is pending.