Emmetropia is the medical term for 20/20 vision without spectacles or contact lenses. When light enters an emmetropic eye, an image focuses perfectly on the retina giving perfect vision. Unlike in a myopic(nearsighted) eye, the light focuses on images in front of the retina, or in a hyperopic (farsighted) eye, the light is focused behind the retina. These are examples of Ametropia.
Myopia is also known as nearsightedness. Myopic patients usually have a good vision near, but blurry vision at distance. Due to the longer length of the myopic eye, it causes the light to focus in front of the retina, hence blurring the distance vision. Myopia is corrected with glasses, contact lenses, or laser correction. Because of ocular length elongation, Myopic patients are at greater risk of developing retinal detachment, cataracts, glaucoma, and macular degeneration.
Hyperopia is also known as farsightedness. Hyperopic patients see better at distance and worse at near. These patients have shorter eye length than normal, thus the light focuses behind the retina and gives them “out-of-focus” blurry vision at near. Because the eye is shorter, hyperopic patients are at greater risk for developing a specific type of glaucoma, which is called “ anatomically narrow-angle glaucoma”. Most babies are born hyperopic and by age 5 should grow out of it and become emmetropic. Just like Myopia, Hyperopia can be corrected with glasses, contact lenses, or refractive surgery.
Astigmatism is a medical term for variation in the curvature of the eye’s cornea or lens. Due to the irregular shape of the cornea, the light focuses at multiple points on the retina, thus patients with astigmatism experience blurry vision at all distances. In order for the cornea to focus light precisely, it has to have a spherical shape but the astigmatic cornea is shaped like a football. Mild astigmatism usually does not need to be corrected as it is not symptomatic. However, moderate to severe astigmatism needs to be corrected to achieve optimal vision and avoid amblyopia in children. Astigmatism is usually present at birth and can be presented with myopia, hyperopia, or alone. It is corrected with glasses, mild and moderate astigmatism can be corrected with some contact lenses or refractive surgery.
Presbyopia is the condition where the muscle that focuses vision at near becomes less flexible. It occurs when patients approach their 40s. It is an unavoidable natural aging process. Even emmetropic patients will start to experience Presbyopia. The symptoms include:
- Holding reading materials at arm’s length distance or further
- Fatigue with near work
- Strain with near work
- Frontal headaches with near work
It could be corrected with reading glasses or cataract surgery.
Amblyopia is also known as lazy eye. It is a problem with vision development. In order for a child to develop a normal and clear vision, the brain must receive clear and identical images from both eyes at all times. When amblyopia is present, the brain receives 2 different images. It could be due to a significant difference in refractive error between two eyes (Refractive Amblyopia) or an amblyopic eye is misaligned and receives a completely different image than a fellow eye (Strabismic Amblyopia). Over a sufficient length of time, the brain learns to ignore the “weaker” eye and favors the “stronger” eye. Hence, it is important to check a child’s vision as early as possible, especially if eye turn is noted. The treatment for Amblyopia may involve glasses to correct a significant difference in prescription, and/or patching or using eye drops in the stronger eye. In some cases, surgical involvement is indicated. Treating Amblyopia as early as possible is the most effective way to prevent vision loss and have the best possible outcome.