Myopia, commonly known as nearsightedness, is a widespread vision condition where distant objects appear blurry while close-up objects remain clear. It occurs when the eye’s shape prevents light from focusing correctly. By 2050, nearly half the world’s population could have nearsightedness, highlighting its growing prevalence.
The Healthy Eye’s Structure and Function
The human eye is designed to capture light and convert it into images the brain can interpret. Light first enters through the cornea, a clear, dome-shaped outer layer at the front of the eye, which provides about 70% of the eye’s focusing power. Behind the cornea, light passes through the pupil, an opening in the iris (the colored part of the eye) that regulates incoming light.
Next, light travels through the lens, a clear structure behind the pupil. The lens works with the cornea to fine-tune the focus, bending light rays to converge precisely onto the retina, a light-sensitive tissue lining the back of the eye. The retina contains specialized cells called photoreceptors, which convert light into electrical signals. These signals are then sent via the optic nerve to the brain, where they are interpreted as the images we see.
Primary Anatomical Alterations in Myopia
Myopia primarily results from structural changes in the eye that disrupt its ability to focus light accurately. The most common anatomical alteration is an elongated eyeball, also known as increased axial length. In a myopic eye, the distance from the front to the back of the eye is longer than average, causing light to converge before reaching the retina. For instance, an average non-myopic eye is about 23 mm long.
Another contributing factor to myopia can be an excessively curved cornea, the transparent front surface of the eye. A steeper corneal curvature bends light more sharply, causing it to focus too soon within the eye, contributing to the overall refractive error.
The lens of the eye can also play a role if it has excessive focusing power. In myopic eyes, the lens may not adequately counteract the effects of an elongating eyeball, further contributing to the myopic condition.
How Anatomical Changes Affect Vision
The anatomical changes in a myopic eye directly impact how light is focused, leading to blurry distant vision. When the eyeball is too long, or the cornea and lens bend light too strongly, light rays from distant objects converge at a point in front of the retina. By the time these light rays reach the retina, they have already crossed their focal point and begun to spread out.
The image projected onto the retina is therefore not a sharp point but a diffused, blurry circle of light. This prevents the retina’s photoreceptors from receiving a clear, focused image. Close-up objects, however, may still appear clear because their light diverges more, allowing it to focus correctly or closer to the retina.
Long-Term Anatomical Complications
Significant or progressive myopia can lead to several long-term anatomical complications due to the stretching and thinning of eye tissues. As the eyeball elongates, the retina, the light-sensitive tissue lining the back of the eye, becomes stretched and thinner, making it more susceptible to tears or holes. If a tear or hole occurs, it can lead to a retinal detachment, where the retina separates from its underlying support, requiring immediate medical attention to prevent permanent vision loss.
Myopic maculopathy is another potential complication, affecting the macula, the central part of the retina responsible for detailed vision. The stretching of the eyeball in high myopia can cause degenerative changes in the macula, including thinning of the retina and the growth of abnormal blood vessels. These changes can result in a gradual loss of central vision, impacting the ability to see fine details.
Individuals with high myopia also face an increased risk of developing certain types of glaucoma. The elongated eyeball and changes around the optic nerve head may contribute to optic nerve damage. Regular eye examinations are advised for those with high myopia to monitor optic nerve health and detect early signs of glaucoma.