Lazy Eye Causes/Amblyopia is a relatively common developmental disorder (affecting 2-4% of the population) that results in a dramatic loss of spatial acuity in the affected eye and subsequent binocular dysfunction. The condition is caused by disruption of normal visual input during the critical period(s) of visual development – post-natal windows of experience-dependent neural plasticity [rx]. The neural locus of the amblyopic deficit is widely thought to be primary visual cortex [rx–rx] although extrastriate areas may also have a supplementary role [rx,rx]. Amblyopia is usually associated with amblyogenic factors such as anisometropia (unequal refractive errors between the eyes), strabismus (misalignment of the visual axes) or a combination of the two.
Amblyopia also called lazy eye is a disorder of sight due to the eye and brain not working well together.[rx] It results in decreased vision in an eye that otherwise typically appears normal.[rx] It is the most common cause of decreased vision in a single eye among children and younger adults.[rx]
Amblyopia is reduced visual acuity not immediately correctable by glasses, in the absence of ocular pathology.
- It is associated with complete or partial lack of clear visual input to one eye (stimulus deprivation amblyopia or unilateral/anisometropic refractive amblyopia), or,
- Less often, to both eyes (bilateral refractive amblyopia), or
- To conflicting visual inputs to the two eyes (strabismic amblyopia).
Types of Amblyopia/Lazy Eye
- Strabismic – by strabismus (misaligned eyes)
- Refractive – by anisometropia (difference of a certain degree of nearsightedness, farsightedness, or astigmatism), or by significant amount of equal refractive error in both eyes
- Deprivational – by deprivation of vision early in life by vision-obstructing disorders such as congenital cataract.
The severity of amblyopia is often classified according to the visual acuity in the affected eye
- Mild amblyopia – is often classified as being visual acuity of 6/9 to 6/12,
- Moderate amblyopia – as being worse than 6/12 to 6/36, and
- Severe amblyopia – as being worse than 6/36. Different studies use different definitions of severity, but most assume normal vision (6/6 or better) in the fellow eye.
Causes of Amblyopia/Lazy Eye
- Anisometropia
- Strabismus
- Strabismic anisometropia
- Visual deprivation
- Organic causes- problems with the retina or the optic nerve
- Near-sightedness (short-sightedness or myopia) – The eye can only focus clearly on objects that are close by.
- Far-sightedness (hyperopia) – The eye can only focus clearly on objects that are far away.
- Astigmatism – Everything appears blurry because the lens or the cornea is deformed.
- Muscle imbalance (strabismus) – The most common cause of lazy eye is an imbalance in the muscles that position the eyes. This imbalance can cause the eyes to cross in or turn out, and prevents them from tracking together in a coordinated way.
- Difference in sharpness of vision between the eyes (refractive anisometropia) – A significant difference between the prescriptions in each eye — often due to farsightedness but sometimes to nearsightedness or an imperfection on the surface of the eye called astigmatism — can result in lazy eye. Glasses or contact lenses are typically used to correct these refractive problems. In some children lazy eye is caused by a combination of strabismus and refractive problems.
- Deprivation – Any problem with one eye – such as a cloudy area in the lens (cataract) – can deprive a child of clear vision in that eye. Deprivation amblyopia in infancy requires urgent treatment to prevent permanent vision loss. Deprivation amblyopia often results in the most severe amblyopia.
- Refractive errors – are more likely to lead to a lazy eye if each eye is affected in different ways: for example if one eye has a refractive error and the other one doesn’t, or if one eye is near-sighted and the other is far-sighted.
- Lazy eye – is only rarely caused by an eye disorder. Eye disorders that can cause lazy eye include cataracts, a droopy eyelid (called ptosis) or an eye that does not have a lens (aphakia).
Symptoms of Amblyopia/Lazy Eye
Signs and symptoms of lazy eye include
- An eye that wanders inward or outward
- Eyes that appear to not work together
- Poor depth perception
- Squinting or shutting an eye
- Head tilting
- Abnormal results of vision screening tests
- Amblyopia is characterized by several functional abnormalities in spatial vision, including reductions in visual acuity, contrast sensitivity function, and vernier acuity, as well as spatial distortion, abnormal spatial interactions, and impaired contour detection.
- In addition, individuals with amblyopia suffer from binocular abnormalities such as impaired stereoacuity (stereoscopic acuity) and abnormal binocular summation.[rx] Also, a crowding phenomenon is present.[rx]
- These deficits are usually specific to the amblyopic eye. However, subclinical deficits of the “better” eye have also been demonstrated.[rx]
Diagnosis of Amblyopia/Lazy Eye
There are several tests that eye doctors can do to find out whether a child has amblyopia:
- They can use eye charts to see how good the child’s vision is. The exact kind of test will depend on several factors, including the child’s age. There are special tests for babies and toddlers.
- A slight squint isn’t always visible to the naked eye. To tell whether a child has a squint, the position of their eyes can be measured – for instance, with a test in which the eyes are covered one after the other. The doctor then checks whether the other eye moves.
- A physical check-up can help to see whether the poor eyesight is caused by something else, like a cataract.
- This involves shining light into the eye with a special instrument called a retinoscope, and seeing how the light reflects off the retina at the back of the eye. By holding different corrective lenses in front of the light, it is possible to determine exactly how well the eye can focus.
- Amblyopia is diagnosed by identifying low visual acuity in one or both eyes, out of proportion to the structural abnormality of the eye and excluding other visual disorders as causes for the lowered visual acuity.
- It can be defined as an interocular difference of two lines or more in acuity (e.g. on Snellen chart) when the eye optics is maximally corrected.[rx]
- In young children, visual acuity is difficult to measure and can be estimated by observing the reactions of the patient reacts when one eye is covered, including observing the patient’s ability to follow objects with one eye.
Treatment of Amblyopia/Lazy Eye
- Glasses- Glasses are prescribed when amblyopia is caused by severe refractive errors and/or anisometropia (when one eye sees more clearly than the other). Glasses help send clear, focused images to the brain, which teach it to “switch on” the weaker eye. This allows the brain to use the eyes together and develop normal vision.
- Eye patches – In many cases, kids with amblyopia must wear an eye patch over the stronger or unaffected eye. The patch is worn for 2–6 hours a day while the child is awake for several months or years, depending on the condition. There are two types of eye patches: one works like a band-aid and is placed directly over the eye; the other, designed for kids who wear glasses, is a cloth patch that fits securely over one lens.
- Atropine eye drops – These may be used to blur vision in the unaffected eye. Atropine dilates the pupil, resulting in blurring when looking at things close up. This makes the lazy eye work more. Atropine is usually less conspicuous and awkward for the child, compared with a patch, and can be just as effective. Children who cannot tolerate wearing a patch may be prescribed eye drops instead.
- Vision exercises – This involves different exercises and games aimed at improving vision development in the child’s affected eye. Experts say this is helpful for older children. Vision exercises may be done in combination with other treatments.
- Pharmacological therapy for amblyopia – The drugs in this review include levodopa-carbidopa combination and antidepressants such as fluoxetine, GABA antagonists, and cytidine 5’-diphosphocholine (choline or citicoline).
- The effect of levodopa was studied on many aspects – It can increase endogenous expression of nerve growth factor, increase expression of N-methyl-D-aspartate receptor-1-subunit in visual cortical neurons which is reduced in amblyopia, improve visual evoked potential response, increase visual acuity, and decrease fixation point scotomas.
- Chronic administration of fluoxetine – promotes the recovery of visual functions in adult amblyopic animals by reducing the intracortical inhibition and increasing the expression of brain-derived neurotrophic factor in the visual cortex.
- GABA antagonist – was found able to restore binocularity. However, it also has serious adverse effect. Significant visual improvement was found in citicoline administratio
- Fluoxetine – Fluoxetine is a selective serotonin reuptake inhibitor, used as antidepressant. It acts by altering the cortical expression of various heat shock proteins and neurofilaments which are important for synaptic functions. Guest et al. demonstrated an increase in the percentage of synapses with split postsynaptic densities, a phen
- Cytidine 5’-diphosphocholine, CDP-choline, or citicoline – Citicoline is an intermediate by-product involved in the biosynthesis of cell membrane phospholipids. Following systemic administration, it gets degraded into its constituents, cytidine and choline. Citicoline, once absorbed, crosses the blood–brain barrier and gets incorporated into the cell membrane phospholipids. It has been shown to increase the levels of norepinephrine and dopamine levels in CNS, offering neuroprotection in hypoxic and ischemic conditions. In addition, citicoline has been shown to restore the activity of mitochondrial ATPase and membrane Na+/K+ ATPase, thereby accelerating resorption of cerebral edema in various experimental models.
- Surgery – Sometimes, eye surgery is performed to improve the appearance of an eye turn, resulting in better alignment of the eyes. This may or may not improve vision.
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