Refractive Errors of eye

Introduction

This occurs when the eye loses the ability to correctly focus the light rays coming from the object onto the plane of the retina^[1],[2] In short, the eye doesn’t correctly bend the light.^[3] As a result, the error causes blurred vision and needs a refractive correction.^[1] Refractive error can affect people of all age, sex, socio-economic status, According to the data estimated by WHO in Global data on visual impairment 2010, 285 million are visually impaired, and 39 million among them were blind. In a research paper according to 2010 data, 80% of patients are at a curable stage. 90% of visually impaired people live in developing countries.^[4] Generally, refractive errors cannot be prevented. However, it can easily be cured by corrective eyeglasses, contact lenses, or in some cases, corrective surgery. But eyeglasses are the safest and common way to treat the refractive error. Contact lenses provide a wider range of vision however have a high risk of infection. Refractive surgery is the expensive and permanent method of treatment as it permanently changes the shape of the cornea.

Generally, there are four common types of refractive errors, these are as follows: –

1. Nearsightedness (Myopia), makes far-away objects look blurry
2. Far-sightedness (Hyperopia), makes nearby objects look blurry
3. Astigmatism causes far away and nearby objects to look blurry or distorted.
4. Presbyopia causes middle-aged and older adults’ difficulty in seeing things close by.

Refractive errors

 

Hyperopia

 

This is a refractive error of the eye, also known as farsightedness or long-sightedness or Hypermetropia. In Hyperopia, the parallel light rays, coming from infinity are focused behind the light-sensitive layer of the retina.

In Hyperopia the cornea becomes flattened or the axial length of the eye becomes shorter than the normal eye. This is the reason why the image doesn’t focus on the retina. In other words, the Axial Length (AL) shows a stronger correlation than corneal power and lens power, with the refractive error of the eye. The magnitude of hyperopia is described as the additional dioptric power of the converging lenses required to advance the focusing of light rays onto the retinal plane, while accommodation is relaxed.^[5]

The hyperopia is categorized by the structure or function of eyes, into three types: –

1. Simple Hyperopia: It occurs due to the decreased ability of the cornea or the lens to diverge the light or due to reduction in axial length. The flattened cornea or decreased curvature results in decrease in converging power of cornea or lens.
2. Pathological Hyperopia: It is caused due to unusual development, trauma, or any disease related to the eyes. It may be of:

1. 1. Senile or acquired hyperopia: It occurs in old age due to
      1. Curvature Hyperopia: In old age, the curvature of the outer fiber of    the lens decreases.
      2. Index Hyperopia: At a young age, there is a difference between refractive indices of the cortex and the central part (nucleus) of the lens. With age, the difference of refractive index diminishes thus cause reduced converging power.
   2. Positional hypermetropia: It is the result of the posterior subluxation of the eye lens.
   3. Aphakic Hypermetropia: It occurs due to the dislocation of lens from the pupillary position in the eye. It may be an inborn or acquired condition (surgery, etc.)
   4. Consecutive Hypermetropia: It is due to surgically overcorrected myopia with under correction.
2. Functional Hyperopia: It happens due to the paralysis of accommodation. It generally occurs by birth.^[6]

The effect of Hyperopia can vary greatly, and it depends upon the magnitude of Hyperopia, convergence ability of the lens, and the strain level in the visual system. However, affected people may experience:

• Blurred vision
• Headaches
• Binoculars dysfunction
• Asthenopia

Myopia

It is also called short-sightedness; this is the most common type of refractive error in the world. WHO has declared Myopia as an uncorrected refractive error with cataract, macular degeneration, infectious disease, and vitamin A deficiency among the leading causes of blindness and vision impairment in the world ^[7].

The people with myopia are classified into two types.

1. Simple myopia: Low to a modest degree of myopia also referred to as simple or ‘school’ myopia (0 to 6 diopters). It can be corrected with the use of spectacles and contact lenses.
2.  High myopia: The second is high or ‘pathological’ Myopia (above 6 diopters). Moreover, pathological myopia is often associated with potentially blinding conditions, for example, retinal detachment, macular degeneration, and glaucoma. ^{[7] [8]}

Astigmatism

In this refractive error, the eye is not able to see clearly the far away and also nearby objects. This is caused due to the distortion in the cornea of the eye.

Astigmatism was first discovered in the 1800s by Thomas Young when he claimed his own Astigmatism. However, in 1825 George Airy used the first cylindrical lens for correction of his astigmatism.

In astigmatism, the curve of the lens or cornea is mismatched. For a normal eye, there is only one curve like a ball, but in astigmatism the shape of the eye becomes oval. It is often present by birth or it may occur with farsightedness, short-sightedness, eye injury, or surgery.

Mainly Astigmatism is of two types.

Corneal Astigmatism: When the curve of the cornea is mismatched.

Lenticular Astigmatism: when the curve of lens is mismatched. ^{[9] [10]}

 Sign of Astigmatism may include:

• Squinting
• Difficulty with night vision
• Eyestrain or discomfort
• Blurred or distorted vision
• Headaches.

Presbyopia

Presbyopia is a type of refractive error in which the ability to focus on near objects is lost. As a result, the patient feels difficulty in seeing the objects which are kept closer like reading, stitching, etc. This happens due to age factor. People start feeling its symptoms after the age of around 40.

People often get confused in presbyopia and Hyperopia. However, both of them are entirely different from each other. In Hyperopia, because of short axial length or distortion in the shape of the cornea or lens, light rays are focused behind the retina instead of onto the retina. On the other hand, in presbyopia, the natural lens of the eye gets less flexible due to aging. ^{[11] [12]} Patients of presbyopia mainly feel

• Headaches while reading.
• Fatigue while doing close work.
• Need to keep reading material at arm’s distance away from the eyes.
• Blurred vision on normal reading distance. ^[11

Causes

 What are the causes of myopia?

Normally light rays pass through the cornea and focus exactly on the retina. However, in the case of myopia, the light rays focus in ‘front of the retina’ instead of on the retina. This irregular bending of light results in too much focusing power than required. This can be either because of the eyeball being too long or the cornea being excessively curved than normal.

Indeed, this can be due to mutation of the SLITRK6 gene ^[13]. It regulates the growth of the eye after birth. Improper functioning of this protein is not only responsible for uncontrollable eyeball growth but can also result in hearing problems. Thus, myopia can be either due to overstressing, short eyeball or genetic issues.

What are the causes of Hyperopia? 

In this situation, light rays entering the eye focused behind the retina instead of on the retina. There can be generally following influencing factors.

First is the abnormal shape of the cornea. The size of the cornea increases than normal which alters the refractive index of the eye.

Second is, the shortening in the size of the eyeball, that causes the light rays to be focused beyond the retina.

Third is the low convergence of the eye lens. It involves a misalignment of the eye when focusing on nearby objects. This is the consequence of weak action of the ciliary muscle. It can be the consequence of:

• Axial Hypermetropia: It is the most common type of hypermetropia. There is an axial shortening of the eyeball in it.^[14]
• Index Hypermetropia: Occur due to alteration in the refractive index of the lens due to age.
• Positional Hypermetropia: It is due to mispositioning of the eye lens.
• Absence of crystalline lens: This absence could be either innate or amassed over a period of time.

However, most infants are born with a mild degree of hyperopia. This generally cures on its own in this case but if it persists and becomes more severe, can even result in amblyopia ^[16] and strabismus ^[17].

What are the causes of Astigmatism? 

This occurs when light rays entering the eye don’t focus at a single point on the retina. It obstructs both near and far vision.

The most common astigmatism is corneal astigmatism but there is also another type called lenticular astigmatism. Lenticular astigmatism is a result of the distortion of the lens inside the eye. The most common corneal astigmatism is due to the irregular shaped cornea. The symmetrical circular shape of the cornea tends to be oval.

There is not any exact cause known for these changes so far, but this disorder is common in certain categories of people. These categories may include the person with a family history of astigmatism, people suffering from either far or nearsightedness and also it can be the consequence of any eye injury.

What are the causes of presbyopia? 

At a young age, the eye lens is soft and the muscles around it are flexible. These muscles govern the amount of light entering the eye and easily change the shape of the lens by either contraction or relaxation. This phenomenon is known as accommodation.

In old age the lens becomes stiff and the muscles around it become less contractile and change their configuration and don’t allow voluntary movement. Consequently, instead of focusing light rays on the retina, lens focuses light behind the retina. As a result, restrict the eye to see close objects clearly.

Diagnoses

The cornea reaches adult size by the age of 2 years. However, the eye continues to grow in anteroposterior length until age 7-8 years old. Children between 2 to 6 year of age are 80% hyperopic although only 10% need refractive correction before 8 years. Myopia develops mostly between 6 – 9 years old and frequently increases throughout adolescence whereas astigmatism is common in babies. Since these errors keep on fluctuating, there is the need of frequent diagnoses.

There is a series of examinations of painless evaluation and analysis by the experts. Observation and information made through these examinations are needed to decide the direction of further diagnosis. Doctors often use various instruments like an autorefractometer to direct the future procedure. There are various tests used to detect refractive errors of an eye. Here are few of these:

 

Visual Acuity Test 

Visual Acuity Test is needed to check the ability to distinguish or identify the details of figures or symbols placed at a certain distance.

 

Anyone experiencing vision problems can go for this test. Moreover, no specific preparation needed regarding this test.^[17]

 

The two most common acuity tests are Snellen and random E tests.

 

Snellen 

Source: Google images

In this test, a chart of letters and symbols is used. Letters of different shapes and sizes are arranged in rows and columns. This chart is kept about 6 meters or 20 feet away from the eye. One eye is covered, and the other eye reads the letters and the vise versa. You are asked to read smaller and smaller symbols until visible.

This test is to check the distant vision. If a person is not able to spot symbols, then he is asked to wear glasses and then spot the symbol. The test is done on one eye at a single time.

If needed the doctor may ask to spot symbols from a card. Card Is to be placed about 14 inches (36 centimeters) from the eyes. This test is done to check the near vision.^[18]

Random E Test  

In this test, a random direction of ‘E’ in which it is facing is indicated by the viewer. This direction can be either up, down, right, or left. The Doctor can ask you to look through various charts required. If the chart is not visible, the Doctor switches various lenses, until the chart is visible properly.

 

Corneal Topography 

The cornea is responsible for about 70% of eye focusing power. If on regular examination the doctor notices unusual irregularity in the shape of cornea then they suggest Corneal Topography. In this test, we can get the three-dimensional graphics of the curvature of the cornea.^[19] 

 

Corneal topography is a painless test. There is a computer-based instrument, designed with the proper concavity to keep your face in front of a lens. You are asked to rest your chin in this cavity such that there is a lensed hole just in front of the eye. There is an illuminated pattern focused on the eye through this hole. The part of this illuminated pattern is reflected back through the cornea. The computer collects this data and forms a digital image of the cornea by using this information.

Refraction

 This is an eye test that measures the child’s deficiency of long-sightedness or short-sightedness. In this process first of all child’s eye needs to be dilated by using the eye drops and further optometrist uses a different type of lenses which is brought close to the child’s eyes and will focus a little light in the child’s eye. Most of the time this is done by a trial frame by putting lenses in it to get accurate results. This lets us know the exact strength of glasses a child needs to be prescribed.

Auto-refractometer

 An auto-refractometer or automated refractor is a machine controlled by computer and is used during an eye examination for the measurement of person’s refractive error and for prescribing correct glasses or contact lenses.

The instrument works on three principles all together:^{[19] [20]}

 

• Placido Disc Reflection ^[21]
• Scanning Slit elevation evolution ^[21]
• Scheimpflug principle-based assessment ^[22]

 The corneal topography test can identify the type of refractive error. Individual eye diagnosis with correct lens power needed can also be checked through corneal topography.

Treatment

Refractive errors can be easily cured if diagnosed on time. However, serious refractive errors can even result in permanent vision loss if neglected. So far there are three general treatment ways:

1. Glasses (Spectacles).
2. Contact lenses.
3. Eye surgery.

Glasses

 There are various different probabilities and dilemmas regarding the invention of spectacles. But many people suggest that they were first used in the 13^th century in Italy. However, today the most common treatment is the use of vision correction. This method is least expensive and easily accessible in most of the countries.

Glasses can control the light reflecting on the retina by letting the light rays pass through them. The Doctor suggests the exact focal length and type of lens according to the error diagnosed. These lenses are framed in the desired design of the frame.

People suffering from presbyopia have trouble in seeing the closeup objects that need Reading Glasses.^[23]

Previously, the higher the refractive error was, the thicker the lens used to be. But with time many advanced lenses are introduced. These lenses can correct more than two errors like:

Bifocal lens– glasses with two distinct optical powers.

ARC Lens – Anti-reflective coating lens cuts glare and allows more light through to your eyes, resulting in less need to strain and focus hard to see.

Blue Cut Lenses– These lenses reflect back the harmful blue light which is basically emitted from computers and mobile screens.

Trifocal lens- It covers three regions, distance, intermediate, and near vision.

Progressive lenses– These are multifocal lenses.

High Index Lenses – These lenses ensure comfort. They are very light and thin as made of high index plastic material.

 

Contact lenses

 Contact lenses are made up of different kinds of plastic polymers. These are small, bowl-shaped, and can be directly placed on the eye. There are a variety of contact lenses available. For example,

Soft contact lenses – These are made up of soft and flexible plastic. ^[24]Soft lenses are easy to adjust and comfortable. They include a soft lens material named silicone-hydrogels. These provide comparatively more oxygen to the eye, decreasing the risk of eye infection.

Rigid Gas Permeable (RGP) ^[25] contact lenses- These are more durable, clearer, and give sharp vision. They are affordable and easy to handle. But initially they are uncomfortable as they are hard. Their composition is of plastic combined with few other materials. However, they can also allow a certain amount of oxygen to pass to the cornea.

Extended wear contact lenses(disposable) – They are made for the continuous and long-wearing duration. These are usually made of soft and flexible plastic material. As continuous use of lenses can result in various consequences, these lenses are designed to address this problem. However, still, go with professional recommendations and let your eye rest without lenses at least for a night on each scheduled removal.

Toric Lens – It is used to correct astigmatism.  This lens could facilitate different optical fiber and focal length due to its shape. It works as a combination of spherical and cylindrical lens.

Eye surgery

Eye surgery is increasing day by day as they can replace spectacles and contact lenses. The corrective surgeries involve a change in the shape of the cornea. Generally, the surgeons suggest performing these sorts of surgeries on the people at least 21 years old. As to this age, there is a possibility of alteration in the shape and size of the cornea.

In most cases, the surgeries like LASIK surgery and surface ablation surgery have given good results. However, it often takes time to determine the visual improvements.

LASIK 

 LASIK stands for Laser-Assisted in Situ Keratomileuses ^[26]. It is a widely used eye surgery. Though doctors differ in what they consider the maximum amount of myopia, or nearsightedness that should be treated. Some doctors consider -8 D to be the maximum, but in the United States the FDA has approved LASIK to treat up to -12 D. The vision can even improve within just two or three weeks. Moreover, the process is not at all painful. It is hardly a half or an hour issue in case both eyes are to be treated.

The surgeon numbs the eye and keeps the eyelid open with a small instrument. Then the exposer point is made using a laser or small blade. The outer flap of the cornea is separated from the rest of the eye. This may feel painful though this is not painful. To reshape the cornea precisely, the laser is used at the exposed point. This procedure is precise and based on a three-dimensional image of the cornea. After the reshaping surgeon replaces the outer flap and may place a clear contact lens that works like a bandage, it is removed a few days later.

Also, the range of hyperopia LASIK can treat is between +1.00D and +6.00D

 Surface Ablation 

It is also called photorefractive keratectomy (PRK). PRK is a better alternative in case of thin or scarred cornea or even dry eye syndrome. It is much similar to LASIK however in it there is no need of removing the thin flap of the cornea. The time taken in this procedure is also much similar, but recovery time is comparatively long as new corneal cells take time in growing to cover the surface of the eye.

ICL (Implantable Contact Lens) surgery- It is also an effective, safe and quick procedure to treat myopia, hyperopia and astigmatism. The surgeon makes a small incision in your eye. They insert the ICL through this incision. The lens is very thin, so it might be folded then unfolded in the eye. The procedure takes hardly 20-25 minutes.

The ICL can cure vision correction for myopia from -0.50 to -20.00 D and Hyperopia from +0.50D to +10.00D.

Clear Lens extraction surgery–

This procedure is usually reserved for patients over 40 year of age and who are not good candidates for LASIK. This procedure is often used to correct vision loss from mild cataracts and presbyopia. There is no such change in shape of cornea, but the original lens is removed and replaced by a synthetic lens.

Conclusion

The data of the world health organization states that 285 million people are suffering from refractive error. So, taking it lightly is not a good option as it may lead to blindness if not cured. However, it will never be a serious disorder and cause any serious problem if treatment is done at an appropriate time. Myopia is the most common disorder among all four. The symptoms of all four errors are almost the same and easy to catch. The diagnosis of the error is also available almost everywhere. The screening of refractive error for school level children should be done at the community level. Because finding error at such an early stage or at a young age has a higher recovery rate than any adult or senior citizen. The school must include the refractive index as a chapter to spread awareness among the children. Campaigns must be done to remove their mental and religious barriers about the Disorder and the correction of disorder. For example, wearing spectacles is neither a means of a joke, nor it means the person with spectacles is blind. This will be beneficial for the upcoming generation to prevent the refractive errors.

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