Content Reviewers:Antonella Melani, MD, Lisa Miklush, PhD, RNC, CNS, Ashley Mauldin MSN, APRN, FNP-BC, Gabrielle Proper, RN, BScN, MN
Contributors:Talia Ingram, MSMI, Jennifer Montague, PhD, Kaylee Neff, Ahmed A. Abu Ajeene, Carla Clegg, BSN, RN
Amblyopia is a condition in which there’s reduced vision in one eye due to abnormal visual development. Amblyopia is commonly referred to as lazy eye, because the affected eye is weaker and often wanders inward or outward.
First, let’s quickly review the physiology of vision. Visual development begins during the first days of life, when the newborn starts receiving visual stimuli from the external world. This helps maturation of the visual pathways connecting the eyes and brain, until they reach maturity at about 4 years of age.
Okay, now, the visual pathways begin in the eye. If we zoom into the wall of the eye, there's an outer fibrous layer, which contains the cornea and sclera, and helps control the focus and entry of light. So the light that passes through the cornea is directed to the lens, which in turn collects light arrays and focuses them into the retina at the back of the eye. The retina houses photoreceptors that translate light into electrical impulses, which are then carried by the optic nerve into the visual cortex of the brain. Finally, the visual cortex processes the impulses coming from both eyes, and fuses them into one clear image.
Now, the causes of amblyopia include any condition that interferes with normal visual development, such as strabismus, or an abnormal alignment of the eyes; astigmatism, which is when light doesn’t focus properly on the retina, causing blurry vision; as well as refractive errors, like near- or farsightedness, and differences in visual acuity between the two eyes. Other causes include eye trauma; ptosis, which is drooping of the upper eyelid; congenital cataract, which is clouding of the eye lens; as well as corneal opacities and eyelid hemangioma. These conditions can also be considered risk factors for amblyopia, along with a family history of amblyopia, premature birth, and neurodevelopmental disorders .
Now, the pathology of amblyopia usually starts when one of the eyes does not correctly receive visual stimuli early in the client’s life. This results in abnormal visual development of the eye’s visual pathways, which prevents them from translating clear images to the visual cortex of the brain. As a result, the visual cortex is not able to fuse these images with the correct images from the unaffected eye, which can cause diplopia or double vision. To avoid diplopia, the visual cortex starts suppressing the impulses coming from the affected eye. So over time, the affected eye becomes weaker. Meanwhile, the other eye is doing all the work, which requires extra effort and, over time, causes eye strain. If untreated, amblyopia can progress to blindness in one or both eyes.
The typical clinical manifestation of amblyopia includes diminished vision in one eye, without pain. Also, clients often have a strong preference for the healthy eye, which is particularly evident when covering their healthy eye. On assessment, the clients may present with wandering eyes, squinting, or head tilting. In addition, the client may bump into things or seem unusually clumsy if they have a poor sense of depth. If untreated, amblyopia can progress to blindness in one or both eyes.
Now, all children should be screened for amblyopia and other visual problems starting at about 6 months of age. The diagnosis of amblyopia starts with the client's history and physical assessment, followed by tests to detect the underlying cause. These tests include the red reflex test, which evaluates conditions causing loss of transparency in the visual axis, such as cataract; as well as the stereopsis test, during which multiple images are shown to the client in order to detect abnormalities of visual acuity, which is typically seen in clients with strabismus.
Another test is the occlusion-objection test, which is when the child’s eyes are covered one at a time, and children with better vision in one eye typically become anxious or irritable when that eye is occluded. And then there's the Titmus test, during which a series of images are shown to the client in order to evaluate their ability to perceive visual depth. Finally, other tests include assessment of visual fixation, by asking the client to to aim their eyes to a specific object or spot, as well as visual acuity testing using the Snellen eye chart.
Treatment of amblyopia typically aims to strengthen the weaker eye, especially during the early years. This can be done by patching the stronger eye for several hours each day, encouraging the client to use the weaker eye in order for it to complete its visual development. Another technique involves the use of atropine drops in the stronger eye to blur its vision, thus encouraging the use of the weaker eye. Glasses can be used to correct refractive errors. Finally, the underlying cause of amblyopia should also be treated, which could include surgery for congenital cataract or severe ptosis.