Overview
Definition:
Preverbal vision screening involves assessing visual function in infants and young children before they can reliably communicate their visual experiences or participate in standard acuity tests
fixation preference tests are a key component of this, evaluating which eye a child prefers to use for visual tasks.
Epidemiology:
Visual impairment can affect up to 3 in 1,000 children
Early detection is crucial, as treatable causes of vision loss like refractive errors, strabismus, and amblyopia can lead to permanent visual deficits if missed
Fixation preference tests help identify potential unilateral visual deficits in preverbal children.
Clinical Significance:
Early identification of visual problems in preverbal children is paramount to prevent irreversible amblyopia (lazy eye) and developmental delays
Fixation preference tests are non-invasive, readily applicable in a clinical setting, and provide an objective measure of ocular preference, guiding further ophthalmologic evaluation.
Age Considerations
Infants 0-6 Months:
Focus on tracking of objects, blink response to approach, and pupil light reflexes
Fixation preference tests are often inferred by observing visual orienting behavior.
Infants 6-12 Months:
Can observe preference for looking at toys or faces with one eye covered
Examiner can use toys to elicit fixation.
Toddlers 1-3 Years:
More cooperative for some visual tasks
Fixation preference can be assessed more directly
May start to use some basic language.
Preschoolers 3-5 Years:
Can often perform modified acuity tests, but fixation preference remains a valuable tool for identifying unilateral deficits.
Diagnostic Approach
History Taking:
Ask about family history of strabismus, amblyopia, or significant refractive errors
Inquire about any observed eye crossing, wandering, or if the child seems to favor one eye
Note developmental milestones.
Physical Examination General:
Assess general appearance for signs of genetic syndromes associated with ocular anomalies
Observe spontaneous eye movements and pupil symmetry
Perform red reflex testing using a direct ophthalmoscope to screen for media opacities.
Fixation Preference Tests:
Cover-uncover test: Observe which eye maintains fixation when the other is covered
Alternating cover test: Rapidly alternate covering each eye to assess for phorias or tropias that induce a fixation switch
Optokinetic nystagmus (OKN) drum: Assess for smooth pursuit with each eye separately.
Further Investigations:
If fixation preference indicates a problem, referral to an ophthalmologist is necessary for a comprehensive eye exam, including cycloplegic refraction, fundus examination, and strabismus evaluation
Consider B-scan ultrasonography if media are opaque.
Fixation Preference Tests Detailed
Basic Fixation Observation:
Present a visually engaging object (toy, light) at a comfortable distance and observe if the child consistently uses one eye to look at it, especially if their head is turned
This is more for very young infants.
Cover Test Methodology:
Hold a colorful, interesting object for the child to fixate on
Cover one eye firmly with an occluder
Observe if the uncovered eye maintains fixation or if the covered eye drifts
If the covered eye drifts, it implies the uncovered eye is suppressing or has reduced vision
Repeat by covering the other eye.
Interpreting Cover Test Results:
If the child maintains fixation with the same eye when the other is covered, and then switches to the previously covered eye when its turn comes to be covered, this suggests orthophoria or a small phoria
If one eye consistently maintains fixation while the other drifts, it suggests a unilateral visual deficit in the eye that drifts when uncovered.
Alternating Cover Test And Tropias:
This is performed with the child fixating on a distant or near target
The examiner quickly covers one eye, observes any movement of the fellow eye, and then quickly moves the occluder to cover the fellow eye, observing movement of the previously covered eye
Any manifest deviation (tropia) will be revealed
This is crucial for diagnosing strabismus.
Optokinetic Nystagmus Assessment:
Using an OKN drum with alternating black and white stripes, move the drum horizontally
Observe the nystagmus response
A normal response involves a slow pursuit phase and a quick saccadic reset phase
If the pursuit is jerky or absent with one eye, it suggests a visual deficit in that eye
This is more challenging in preverbal children but can be adapted.
Management Of Detected Visual Impairment
Referral To Ophthalmology:
Any concern raised by fixation preference testing warrants referral to a pediatric ophthalmologist or optometrist for comprehensive evaluation.
Refractive Errors:
Correction of significant myopia, hyperopia, or astigmatism with spectacles is the first-line management for amblyopia due to uncorrected refractive error.
Amblyopia Treatment:
Treatment involves patching the sound eye or using atropine drops in the sound eye to force the use of the amblyopic eye
The duration and intensity of treatment depend on the severity of amblyopia and the child's age.
Strabismus Management:
Management may include spectacle correction (for accommodative esotropia), patching, vision therapy, or surgical correction of the muscle imbalance
Early intervention is critical for visual development and binocularity.
Prognosis
Factors Affecting Prognosis:
The prognosis for visual recovery is generally excellent if visual impairment is detected and treated early, ideally before 6-8 years of age
The severity of the visual deficit, the underlying cause, and compliance with treatment are also crucial factors.
Outcomes With Early Intervention:
With prompt diagnosis and consistent management, most children can achieve good visual acuity and develop normal or near-normal binocular vision, preventing permanent amblyopia and associated developmental issues.
Long Term Follow Up:
Regular follow-up with an ophthalmologist is essential to monitor visual status, ensure compliance with treatment, and detect any recurrence or new visual problems.
Key Points
Exam Focus:
Understanding the principles of fixation preference tests and their role in screening for unilateral visual deficits in preverbal children is high-yield for DNB and NEET SS exams.
Clinical Pearls:
Always use an interesting, brightly colored object to engage the child during fixation preference testing
Be systematic and patient
some children require time to show a clear preference
Remember that red reflex testing is an essential part of any infant eye examination.
Common Mistakes:
Failure to perform vision screening in preverbal children
Misinterpreting the results of fixation preference tests
Delaying referral for further evaluation when abnormalities are suspected
Not considering the age and developmental stage of the child when assessing vision.