Amblyopia

Introduction and Facts 

Amblyopia is a developmental disorder that affects vision. Characterized by neurodevelopmental changes, amblyopia causes physiological abnormalities in the development of the visual pathway during childhood, resulting in visual impairment that occurs unilaterally or, less commonly, in both eyes. This condition occurs when cortical visual development fails in one or both eyes early in life. Amblyopia is commonly known as “lazy eye” among the general public. If left untreated, amblyopia can cause permanent loss of vision in the affected eye, even if the underlying eye abnormality is resolved. Among children and young adults, amblyopia is the most common cause of reduced vision in one eye, affecting approximately 2% - 4% of children. The development of this condition is often associated with factors such as poor vision, inadequate quality of visual stimulation, or impaired binocular-eye interactions during critical periods of a child's development. Patients with amblyopia typically demonstrate decreased best-corrected visual acuity (BCVA), which cannot be attributed to structural changes in the eye.

Amblyopia presents a significant global health problem due to its prevalence and potential for permanent visual impairment if not promptly diagnosed and treated. Historically, various forms of amblyopia have been reported to affect up to 3% of the population, with a 1.2% lifetime risk of vision loss attributable to this condition. More recent data suggests that the global prevalence of amblyopia is in the range of 1.1% - 1.8%. Population-based studies have reported varying prevalence rates, with estimates ranging from 0.7% - 2.6% in children aged 30 - 70 months and 1.0% - 5.5% in older children. The reported prevalence of amblyopia can fluctuate based on factors such as age, ethnicity, race, the specific definition used for amblyopia, study methodology, and other contributing variables. A comprehensive meta-analysis of global prevalence data covering more than 1.8 million patients across 60 studies revealed a pooled prevalence rate of 1.44%, with a range of 1.17% - 1.78%. Prevalence rates vary across continents, with reporting rates in Europe, North America, Asia, and Africa of 2.90%, 2.41%, 1.09%, and 0.72%, respectively. The study estimates that in 2019, approximately 99.2 million people worldwide were affected by amblyopia, with an estimated increase to 175 million in 2030 and 220 million in 2040.

Classification and Etiology

The main types of amblyopia are refraction, strabismus, decreased vision, and occlusion.

• Refractive amblyopia. This type of amblyopia occurs when visual disturbances such as hypermetropia, myopia, or astigmatism are not corrected, causing vision problems.
• Strabismic amblyopia. Also known as strabismus or crossed eyes, this occurs when the eyes are not aligned correctly, so the brain ignores what comes in from one eye to avoid double vision. The two eyeballs may be directed in different directions. 
• Deprivation/decreased vision amblyopia. This occurs when something blocks normal visual input to the eye, such as cataracts, scar tissue on the cornea, living in complete darkness, or having the eyes closed for long periods of time.
• Occlusive amblyopia. Occurs when the healthier or stronger eye is closed or treated with eye drops for too long while being treated for amblyopia. Research shows that vision in a healthy eye can become worse when wearing an eye patch for more than 6 hours a day for 6 months. This condition will improve after stopping wearing eye patches or using eye drops.

Pathophysiology

Amblyopia is a neurodevelopmental condition that causes impairment of best-corrected vision in one or both eyes. The development of visual acuity in the postnatal period depends on continuous exposure to high-quality visual stimuli by both eyes and proper function of the visual pathways. Research has shown there is a sensitive or critical period in which amblyopia can develop due to incomplete development of the visual pathway, with a window of vulnerability until approximately 7 to 10 years of age. Any deviation from normal vision during the first decade of life caused by unilateral or bilateral blurry retinal images can lead to permanent vision loss if not promptly diagnosed and treated.

Disruption of the normal visual pathway can be caused by a variety of factors, including anisometropia, uncorrected refractive error, strabismus, and ocular media opacity. Early recognition and intervention is critical in addressing these underlying causes to prevent irreversible consequences of amblyopia.

Studies conducted using functional magnetic resonance imaging (fMRI) and optical coherence tomography (OCT) in individuals with amblyopia have revealed neuroanatomical structural and functional changes in the gray matter volumes of the corresponding hemispheres of the visual cortex, changes in cortical thickness in some white matter volumes, variations in ciliary muscle thickness, changes in choroidal thickness, differences in retinal nerve fiber thickness, variations in thickness of the central macula, and other structural modifications along the visual pathway. These findings highlight the widespread impact of amblyopia on structural and functional aspects of the visual system.

Amblyopia is influenced by the anatomical relationship between photoreceptors and ganglion cell receptive fields, the relationship between ganglion cell receptive fields and deep layers of the lateral geniculate, and the relationship between the lateral geniculate and layers of the visual cortex. These complex nerve connections play an important role in developing and manifesting amblyopia. 

The depth and severity of amblyopia are closely related to the age of onset of amblyopia and the duration of treatment. In general, the earlier the onset of amblyopia and the longer the amblyopia persists without intervention, the more difficult it is to correct and reduce its effects.

Risk Factors   

A child is more likely to experience lazy eye if:

• Premature birth
• Born smaller than average
• Have a family history of amblyopia
• There are developmental disorders

Signs and symptoms

Amblyopia begins in childhood, usually between the ages of 6 and 9. Identifying and treating it before age 7 provides the best chance of correcting the condition completely.

Common symptoms include:

• Difficulty knowing how close or far an object is (distance estimation)
• Squinting or closing one eye
• Head tilt

Diagnosis

Despite various deficits in visual function, the diagnosis of amblyopia is still made by measuring visual acuity on an eye chart using optotype-based recognition.

In preverbal children who cannot complete this task, diagnosis can be made using behavioral methods, such as fixation preference by observing how strongly the child resists closing one eye compared to the other eye. Scoring schemes can be used to quantitatively measure fixation preferences, and grating acuity can be determined using Teller acuity cards. Visual acuity testing for optotype-based recognition (letters, numbers, or symbols) should be performed as soon as the child can perform this task well.

Because amblyopia is a common and preventable visual deficit, there is great concern regarding early diagnosis and determination of more effective treatment for this condition. The American Academy of Pediatrics recommends amblyopia screening as part of a routine well-child examination performed by a pediatrician or family care practitioner, including the use of instrument-based vision screening techniques for preverbal children.

Treatment and Management

Initial management of amblyopia depends on the cause. Some of the main considerations in the management of amblyopia are as follows:

• Deprivation/decreased vision amblyopia: In this case, the first step that must be taken is to address and correct the pathological obstruction that causes amblyopia. This may include surgical interventions such as cataract surgery, retinal detachment repair, corneal surgery, or treatment for various other eye pathologies. 
• Refractive amblyopia: An important intervention involves treating the patient's complete refractive error with corrective lenses. In many cases, the use of corrective lenses is sufficient to reverse amblyopia to normal.
• Strabismus amblyopia: Correction of strabismus with intervention to align the eyes may be considered. However, strabismus repair alone can rarely reverse amblyopia completely.

The reason for treating amblyopia is to limit the risk of permanent visual loss at an early stage of development. In general, treatment is more effective and in some cases resolved if diagnosis and therapeutic measures are started as early as possible in childhood. Timely treatment can provide many benefits during early development and adulthood, including improvements in best-corrected visual acuity (BCVA), binocular vision, oculomotor deficits, stereopsis, fixation, contrast sensitivity, and quality of life.

In treating various forms of amblyopia, administering a visual “penalty” to the eye without amblyopia is often necessary to stimulate vision development in the amblyopic eye. The most common visual “penalty” method is to use an eye patch on the non-amblyopic eye. The duration of eye patch wear can vary depending on the severity of the amblyopia, and healthcare practitioners recommend wearing the eye patch for 2 hours, 6 hours, or even a full day in some cases. Encouraging visual activities such as reading, watching television, or playing video games during “patching” may be beneficial, as it forces the patient to engage the amblyopic eye and may increase the patient's cooperation with the “patching” program, especially if it is associated with enjoyable activities.

References:

Referensi:

1. Blair K, Cibis, G, Zeppieri M, Gulani AC. Amblyopia. National Library of Medicine [Internet]. 2024. Available from: https://www.ncbi.nlm.nih.gov/books/NBK430890/
2. Whitbourne K, Sheikh Z. Lazy eye (amblyopia). WebMD [Internet]. 2024. Available from: https://www.webmd.com/eye-health/amblyopia-child-eyes
3. Zagui RM. Amblyopia: Types, diagnosis, treatment, and new perspectives. American Academy of Opthalmology [Internet]. 2019. Available from: https://www.aao.org/education/disease-review/amblyopia-types-diagnosis-treatment-new-perspectiv