Myopia Causes/Myopia or nearsightedness, as it is medically termed, is a vision condition in which close objects are seen clearly, but objects farther away appear blurred. Myopia occurs if the eyeball is too long or the cornea, the clear front cover of the eye, has too much curvature. As a result, the light entering the eye isn’t focused correctly and distant objects look blurred.
Myopia or nearsightedness, is the most common human eye disorder in the world, and is a significant global public health concern. Along with cataract, macular degeneration, infectious disease, and vitamin A deficiency, myopia is one of the most important causes of visual impairment worldwide. Severe or high-grade myopia is a leading cause of blindness because of its associated ocular co-morbidities of retinal detachment, macular choroidal degeneration, premature cataract, and glaucoma.
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The Epidemiology of Myopia[rx]–[rx]
|Age at onset||Congenital myopia – present at birth and persists through infancy|
|Youth onset myopia: prior to age 20 (school myopia appears during childhood, particularly the school-age years)|
|Adult onset myopia – aged 20 – 40|
|Late adult onset myopia – after age 40|
|Singapore – up to 80%|
|China – 77.3% in high school, more than 80% in college|
|India and Malaysia – up to 41-80%|
|Jordanian – 53.7% aged 17 to 40|
|United Kingdomm – 50% of British whites and 53.4% of British Asians|
|Greece – 36.8% at aged 15-18 in students|
|United States – 25.0% at aged 12-54|
|Australia – 2.5-17%|
|Brazil: 6.4% at aged 12 to 59|
|Ethnicity and race||For myopia|
|Asians – 18.5%|
|Hispanics – 13.2%|
|Caucasians – 4.4%|
|African American – 6.6%|
|Asians – 33.6%|
|Hispanics – 36.9%|
|Caucasians – 26.4%|
|African American – 20.0%|
|Education and IQ||The incidence of myopia increases with level of education. A correlation between myopia and a higher intelligence quotient (IQ)|
|Society and culture||Metaphorically to refer to cognitive thinking and decision making|
Types of Myopia
Various forms of myopia have been described by their clinical appearance:[rx][rx][rx]
- Simple myopia – Myopia in an otherwise normal eye, typically less than 4.00 to 6.00 diopters.[rx]
- Degenerative myopia – also known as malignant, pathological, or progressive myopia, is characterized by marked fundus changes, such as posterior staphyloma, and associated with a high refractive error and subnormal visual acuity after correction.[rx] This form of myopia gets progressively worse over time. Degenerative myopia has been reported as one of the main causes of visual impairment.[rx]
- Pseudomyopia – is the blurring of distance vision brought about by spasm of the accommodation system.[rx]
- Nocturnal myopia – Without adequate stimulus for accurate accommodation, the accommodation system partially engages, pushing distance objects out of focus.[rx]
- Nearwork-induced transient myopia (NITM) – short-term myopic far point shift immediately following a sustained near visual task.[rx] Some authors argue for a link between NITM and the development of permanent myopia.[rx]
- Instrument myopia – over-accommodation when looking into an instrument such as a microscope.[rx]
Induced myopia – also known as acquired myopia, results from exposure to various pharmaceutical drugs, increases in glucose levels, nuclear sclerosis, oxygen toxicity (e.g., from diving or from oxygen and hyperbaric therapy) or other anomalous conditions.
- Sulphonamide therapy – can cause ciliary body edema, resulting in anterior displacement of the lens, pushing the eye out of focus.[rx]
- Elevation of blood-glucose levels – can also cause edema (swelling) of the crystalline lens as a result of sorbitol accumulating in the lens. This edema often causes temporary myopia.
- Scleral buckles – used in the repair of retinal detachments may induce myopia by increasing the axial length of the eye.[rx]
- Index myopia – is attributed to variation in the index of refraction of one or more of the ocular media.[rx] Cataracts may lead to index myopia.[rx]
- Form deprivation myopia occurs – when the eyesight is deprived by limited illumination and vision range,[rx] or the eye is modified with artificial lenses[rx] or deprived of clear form vision.[rx][rx] In lower vertebrates, this kind of myopia seems to be reversible within short periods of time.[rx] Myopia is often induced this way in various animal models to study the pathogenesis and mechanism of myopia development.[rx]
The degree of myopia is described in terms of the power of the ideal correction, which is measured in diopters:[rx]
- Low myopia – usually describes myopia of −3.00 diopters or less (i.e. closer to 0.00).[rx]
- Moderate myopia – usually describes myopia between −3.00 and −6.00 diopters.[rx] Those with moderate amounts of myopia are more likely to have pigment dispersion syndrome or pigmentary glaucoma.[rx]
- High myopia – usually describes myopia of −6.00 or more.[rx][rx] People with high myopia are more likely to have retinal detachments[rx] and primary open angle glaucoma.[rx] They are also more likely to experience floaters, shadow-like shapes which appear in the field of vision.[rx]
Age at onset
Myopia is sometimes classified by the age at onset:[rx]
- Congenital myopia – also known as infantile myopia, is present at birth and persists through infancy.[rx]
- Youth onset myopia – occurs in early childhood or teenage, and the ocular power can keep varying until the age of 21, before which any form of corrective surgery is usually not recommended by ophthalmic specialists around the world.[rx]
- School myopia – appears during childhood, particularly the school-age years.[rx] This form of myopia is attributed to the use of the eyes for close work during the school years.[rx]
Adult onset myopia
Myopia is a refraction anomaly of the non-accommodated eye with a spherical equivalent of −0.5 dioptre (D) or lower.
Primary myopia – due to elongation of the visual axis OR a combination of primary and secondary myopia OR unknown causes
- Simple myopia – myopia up to −7.75 D
- High myopia – refraction ≥−8.00 D or more negative
Secondary myopia – due to too strong refractive ocular media, among others
- Corneal curvature too steep
- Lens swelling, higher refractive index or cataract
- Lens dislocation
- Medicine (eye drops) use
- Systemic diseases including genetic syndromes involving the eye
- Accommodation spasm
- Night myopia
More than 2 SD of the statistical average according to age, sex and ethnicity.
In daily practice, it is often not possible or practical to determine axial length. When no signs of secondary myopia are present, and essentially the cause is unknown, it is also called primary myopia.
‡−8.00 is an arbitrary and often used cut-off point.
Causes of Myopia
Nearsightedness happens when your eye is longer than normal, or, less often, when your cornea is too curved. Objects focus in front of the retina instead of on the retina, causing it to be blurred. The exact cause of myopia is not known.
Research about myopia supports two key risk factors:
- Family history – If one or both parents are nearsighted, the chance of their children developing it increases.
- Working up close – Long hours of near work such as reading or computer work may be at increased risk for developing myopia.
Temporary nearsightedness can also occur due to visual stress and disease:
- Night Myopia – Some people experience blurred distance vision only at night. When it’s dark, the eye may simply not be receiving enough light to focus, or too much light from the sides may be entering the naturally dilated eye, making it hard to focus.
- False Myopia – Some people cannot focus far after long periods of near vision tasks. Usually the focusing ability of the eye will return after some rest.
- Myopia symptoms – could also be a sign of variations in blood sugar levels in people with diabetes or an early indication of a cataract.
- The retina’s center (macula) thins with age – While this does not affect side (peripheral) vision, the ability to focus on a particular object (like reading a sign) diminishes and is eventually lost.
- Retinal thinning – can also lead to tears and holes, which can cause the retina to detach.
- The retina can split – a condition called myopic retinoschisis (MRS).
- Layers of the eye under the retina can degenerate or crack – allowing new, faulty blood vessels to grow under the retina (a process called choroidal neovascularization, or CNV). The vessels can bleed and scar, with small scars causing minor macular damage and larger ones more significant problems.
- Myopia occurs – when the eyeball is too long, relative to the focusing power of the cornea and lens of the eye. This causes light rays to focus at a point in front of the retina, rather than directly on its surface.
- Nearsightedness – can also be caused by the cornea and/or lens being too curved for the length of the eyeball. In some cases, myopia occurs due to a combination of these factors.
- Myopia typically begins in childhood – and you may have a higher risk if your parents are nearsighted. In most cases, nearsightedness stabilizes in early adulthood but sometimes it continues to progress with age.
- Increased eye pressure – may lead to glaucoma.
- Cataracts – are more likely to develop.
- The eyes become more susceptible to injuries and other traumas, like surgery
- Your eye care professional can evaluate your eyes to determine the reason and severity of your myopia.
Symptoms of Myopia
Some of the signs and symptoms of myopia include:
- Squinting to see properly, and
- Difficulty seeing objects far away, such as road signs or a blackboard at school.
- Recurrent headaches
- Incessant eye-rubbing
- Eye strain
- Eye fatigue when you try to see objects more than a few feet away
- Children with myopia often have trouble reading the blackboard at school.
- Squinting in an attempt to help vision
- Unexplained drop in school performance
- Blurry vision when looking at distant objects
- The need to squint or partially close the eyelids to see clearly
- Headaches caused by eyestrain
- Difficulty seeing while driving a vehicle, especially at night (night myopia)
Nearsightedness is often first detected during childhood and is commonly diagnosed between the early school years through the teens. A child with nearsightedness may:
- Persistently squint
- Need to sit closer to the television, movie screen or the front of the classroom
- Seem to be unaware of distant objects
- Blink excessively
- Rub his or her eyes frequently
Diagnosis of Myopia
- Short-sightedness is simple to diagnose – An optometrist or ophthalmologist usually asks you to read a special chart (Snellen chart) that has large letters at the top and smaller letters at the bottom. If you can see all the letters clearly you have 6/6 vision. If you have 6/12 vision, it means that you can read letters at 6 m that a person with normal vision can read from a distance of 12 m.
- In everyday language – people still talk about 20/20 vision, which is a throwback to when Australia, like the USA, used imperial measurements and so 20/20 is the same as 6/6 vision.
- Doctors diagnose – most cases of high myopia by age 13. Patients then get regular eye exams to check the retina for further problems. We may use a special test called fluorescein angiography to look for new blood vessels, with doctors injecting dye and then taking photographs.
Treatment of Myopia
Available Surgical Treatments
- Refractive surgeries are becoming popular since they can reshape the cornea of a myopic patient and eliminate the need for Rx eyewear. PRK and LASIK are two of the available refractive surgeries, both of which use an excimer laser. While PRK just removes a layer of corneal tissues and flattens the cornea, a LASIK procedure involves a flap cut through the top of the cornea. Other laser eye surgeries for vision correction include by LASIK, LASEK, and Epi-LASIK etc. Each of these surgeries costs several thousand dollars per eye[rx]–[rx].
Ortho-k Contact Lenses and Phakic Intraocular Lenses
- For night wear, special contact lenses applying orthokeratology (ortho-k) technology can reshape the cornea overnight and provide clear vision during daytime without the help of Rx contact lenses or glasses. As implantable lenses, phakic intraocular lenses (IOLs) can deal with special situations that are beyond LASIK and PRK. These IOLs are permanently placed in the eye during a surgery, eliminating any maintenance[rx]–[rx].
Single Vision Lenses
- An active emmetropization mechanism regulated by optical defocus is supported by results of numerous studies (reviewed in [rx]). Strong evidence is provided by compensatory ocular growth seen in response to lens-induced defocus in animal models. [rx] Based on these results, it has been suggested that spectacle intervention in myopic children with the commonly prescribed single vision lenses (SVLs) might lead to increased progression and axial elongation.
Bifocals and Progressive Addition Lenses
- The use of bifocals or progressive addition lenses (PALs), sometimes called no-line bifocals, for slowing the progression of myopia has produced relatively small treatment effects overall, on the order of 0.15 to 0.50 D over 1.5 to 3 years, [rx–rx]
- The use of bifocals or progressive addition lenses (PALs), sometimes called no-line bifocals, for slowing the progression of myopia has produced relatively small treatment effects overall, on the order of 0.15 to 0.50 D over 1.5 to 3 years, although treatment effects are reported to be larger in certain subgroups of myopic children, as described below[rx].
- The largest of the treatment trials with this type of lens was the Correction of Myopia Evaluation Trial (COMET), a multi-center, randomized, double-masked clinical trial to evaluate whether PALs slow the rate of progression of myopia compared to conventional SVLs[rx],[rx].
- Many early investigations of rigid gas permeable contact lenses (RGP) for myopia control suffered from lack of randomization and a high drop out rate from the contact lens group[rx]–[rx].
- In an attempt to eliminate the high loss to follow-up found in previous studies, a recent randomized clinical trial, the Contact Lens and Myopia Progression (CLAMP) study, implemented a run-in period to ensure good compliance with rigid contact lens wear.
Prism bifocal spectacle lenses
- Prism bifocal spectacle lenses are bifocal spectacles with 3-Δ base-in prism in near addition of + 1.50D. With this device, the effort for convergence and accommodation during near work can be attenuated. It appears to work best for myopic children with low lags of accommodation. In these patients, 0.99D/year of myopia retardation as compared with control was observed.[rx]
- Recent well-designed studies using topical atropine, a non-selective muscarinic antagonist, have demonstrated statistically and clinically significant reductions in the progression of myopia.[rx,rx] Shih et al [rx] reported that myopia progression was significantly slowed (p<0.0001)over 18 months in 6-13 year old children randomized to 0.5% atropine with multi-focal glasses (0.41 D) compared to multi-focal glasses alone (1.19 D) or SVLs alone (1.40 D). Chua et al[rx] reported similar results in a two-year study of 400 6-12 year-old myopic children in Singapore, although this study used a different experimental paradigm.
- Pirenzepine, like atropine, is a muscarinic antagonist but it is less likely to produce mydriasis and cycloplegia. Two clinical trials of pirenzepine have been conducted, one in Singapore, Hong Kong, and Thailand[rx] and the other in the United States.[rx–rx] In the Singapore study, myopia in children increased over a one-year period by 0.47 D for those using pirenzepine ophthalmic gel twice a day, 0.70 D for those using it once a day, and 0.84 D for the control group.[rx]
- There is also pathologic myopia, which is extremely severe and can not be corrected by any of the above treatments. This type of myopia occurs in children with extremely elongated eyeball by age 12. The situation will worsen as children age and unfortunately develop abnormal growth of new blood vessels. There was no effective treatment during a long period until the approval of drug Visudyne along with non-thermal laser application in 2001. This treatment named photodynamic therapy has been proved to be effective[rx]–[rx].
- This is a microscope-like device purported to improve the vision of emmetropes, myopes, and even presbyopes, to prevent the worsening of myopia and to treat amblyopia. The retail price is around US$580. Users are to peer into the eye-pieces of the device for 5 minutes per eye, where they will see a kaleidoscope of brightly colored lights that focus and defocus. However, there is no is no evidence that it can retard the progression of myopia.
Vision Therapy Eyewear/Pinhole Glasses
- These are black opaque lenses which have multiple small holes in them. A 10% to 20% improvement in vision—even elimination of myopia—is advertised. However, there is no evidence that this can retard myopia progression. What it actually uses is the pinhole effect where only coherent rays of light pass through. As the pinhole blocks most of the light rays, there is a smaller circle of blur on the retina.[rx]
Vision therapy for people with stress-related myopia
- Vision therapy is an option for people whose blurred distance vision is caused by a spasm of the muscles that control eye focusing. Various eye exercises can improve poor eye focusing ability and regain clear distance vision.
Laser Eye Surgery
Laser eye surgery involves using a laser to burn away small sections of your cornea to correct the curvature so light is better focused onto your retina.
- Photorefractive keratectomy (PRK) – where a small amount of the cornea’s surface is removed, and a laser is used to remove tissue and change the shape of the cornea. Also called PRK , a laser is used to remove a layer of corneal tissue, which flattens the cornea and allows light rays to focus closer to or even on the retina.
- Laser epithelial keratomileusis (LASEK) – similar to PRK, but involves using alcohol to loosen the surface of the cornea so a flap of tissue can be lifted out of the way, while a laser is used to alter the shape of the cornea; the flap is then put back in place afterwards
- Laser in situ keratectomy (LASIK) – similar to LASEK, but a smaller flap of cornea is created
- Laser-assisted in situ keratomileusis – Commonly called LASIK , a laser is used to cut a flap through the top of the cornea, a laser removes some corneal tissue, then the flap is dropped back into place. LASIK is the most common surgery used to correct nearsightedness.
- Corneal rings – Plastic corneal rings, called Intacs, are implanted into the eye to alter the shape of the cornea. One advantage of the rings is that they may be left in place permanently, may be removed in case of a problem, or adjusted should a prescription change be necessary.
Nearsightedness is associated with a variety of complications from mild to severe, such as:
- Reduced quality of life. Uncorrected nearsightedness can affect your quality of life. You might not be able to perform a task as well as you wish. And your limited vision may detract from your enjoyment of day-to-day activities.
- Eyestrain. Uncorrected nearsightedness may cause you to squint or strain your eyes to maintain focus. This can lead to eyestrain and headaches.
- Impaired safety. Your own safety and that of others may be jeopardized if you have an uncorrected vision problem. This could be especially serious if you are driving a car or operating heavy equipment.
- Financial burden. The cost of corrective lenses, eye exams and medical treatments can add up, especially with a chronic condition like nearsightedness. Vision reduction and vision loss also can affect income potential in some cases.
- Other eye problems. Severe nearsightedness puts you at a slightly increased risk of retinal detachment, glaucoma, cataracts and myopic maculopathy — damage in the central retinal area. The tissues in long eyeballs are stretched and thinned, causing tears, inflammation, new blood vessels that are weak and bleed easily, and scarring.
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