Today’s topic is – Eye refractive errors! To recapitulate from the previous chapter, the human eye functions as a specialized camera that focuses light rays from objects as images on the light-sensitive film – the retina.
The focusing mechanism of the eye consists of a pliable crystalline lens, the cornea and the refracting media inside the eyeball. The various parts of the refracting system of the eye work together to act as a single lens. The eye can be compared to a camera in many ways.
In a camera, the focusing mechanism moves the lens forward or backward to focus the image clearly.
In the eye, the crystalline lens does this job by swelling up or thinning out. The thinned out lens focuses distant objects, while the swollen, thick lens focuses near objects. This process is called accommodation.
Similarly, the pupil works like the shutter of the camera by changing its size to optimize the entry of light.
An inability to focus images clearly on the retina, resulting in the formation of blurred images, is called a refractive error. Eye refractive errors are a group of disorders in which a precise image does not get focused on the retina.
An easily detectable and correctable problem like an Eye refractive errors still remains a significant cause of avoidable visual disability in our world.
Related Blog: How Does the Eye Work Properly?
Types of Eye Refractive Errors
There are four common types of refractive errors:
- Myopia or short-sightedness or nearsightedness
- Hypermetropia or long sightedness or farsightedness
Myopia and hypermetropia are caused by one or more of the following anomalies:
Abnormality in the size of the eyeball: The normal anteroposterior (front to back) length of the eyeball is around 24 mm.
The eyeball is too long in myopia and too small in hypermetropia.
Abnormality in the curvature of the cornea: The curvature of the cornea is too steep in myopia and too flat in hypermetropia. In addition, irregularity of the corneal surface may give rise to astigmatism.
Abnormality in the refractive index: As mentioned earlier, light passes through the major refracting surfaces of the cornea and lens before falling on the retina. The refractive index of these surfaces is their ability to bend light when it passes through them. A high refractive index may cause myopia, while a low refractive index may cause hypermetropia.
Of all these factors, an abnormality in the size of the eyeball is the most common cause of refractive errors especially in children and young adults.
Each of the types of refractive errors is now discussed in detail:
Myopia is a condition in which the person is unable to see distant objects clearly, but can see near objects well. This condition is caused when rays of light are focused in the front of the retina instead of on it.
As mentioned earlier, this happens when the eyeball is more elongated than normal or if the curvature of the cornea is very steep. Myopia is the commonest type of refractive error, and it usually develops at a young age.
The main complaint of a person with myopia is that he cannot see distant objects clearly. Other symptoms of the defect are a headache, watering from the eyes and constant itching of the eyes.
All these symptoms are a result of eye-straining induced by the refractive error. A child who is unable to articulate his complaints would tend to read his book by holding it very close to his face and sit closer to the blackboard or the TV.
The progression of myopia happens most often during the growing phase of a person’s life. Maximum growth occurs between the ages of 12 and 20. Frequent changes of spectacles may be required during these years but the myopic error usually stabilizes after the age of 20. Hence permanent surgical procedures like laser surgery should be performed only after documented stabilization of myopia.
There are two types of myopia. Simple myopia is a variant of the usual condition and does not pose a serious threat to vision. The power of the lens is expressed in diopters. In simple myopia, power is usually less than 6 diopters. Apart from the eye refractive errors, no anatomical or functional complications of the ocular structures are encountered in this type of myopia.
Pathological (or progressive) myopia, as the name suggests, is of a more serious nature. In this type of myopia, the retina becomes very thin and is stretched at the periphery to cover the elongated eyeball and is hence more likely to develop tears or separating from its base, a condition termed retinal detachment. Alternatively, progressive scarring of the retina and its underlying layers, called chorioretinal atrophy, can cause a chronic diminution of vision. If not diagnosed early and followed up regularly, pathological myopia can result in the permanent loss of vision.
Myopia can be corrected using lenses that bend light outward, that is, divergent or concave lenses. They are denoted by a minus ‘-‘ sign in the spectacle prescription.
Hypermetropia is a condition in which the person is unable to see both near and distant objects well.
In this condition, rhea light rays are focused behind the retina instead of on it. Headache and eye-straining following any prolonged near work may be encountered at desk jobs or while sewing can be symptoms.
These symptoms are collectively termed asthenopia. Although hypermetropia can be detected at any age, it generally becomes apparent with increasing age.
This condition can be corrected by using lenses which bend light inwards to fall on the retina, that is converging or convex lens, denoted by a plus ‘+’ sign in the spectacle prescription.
A precise, single point of focus on the retina is a prerequisite for clear vision. However, in astigmatism, two or more foci of light are focused on the retina due to abnormalities of the refracting surface, thus causing problems with vision. A person with astigmatism is likely to complain of headache or eye straining along with blurry vision.
Astigmatism can be corrected by using a cylindrical lens that rectifies the error in one particular meridian only.
In this condition, the eye is unable to focus near objects. The problem is caused by the aging of the lens and the accommodating system, which fail to focus near objects on the retina. Hence, it is a disorder of aging.
This is a physiological phenomenon which happens to almost everybody and starts during the fourth decade of a person’s life.
People with presbyopia find it increasingly difficult to read the newspaper at their usual working distance in dim light and tend to keep it at a distance to make out the letters. Simple tasks like threading a needle, signing a document or making out the fine print on a visiting card become increasingly difficult and sometimes embarrassing.
In an adult, the uncorrected disability hampers a person’s financial productivity and compromises his occupational skills. Headaches and eye-straining may occur after long hours of near work. Left uncorrected, the person loses interest in performing near work like reading and writing, leading to a compromise in the quality of life.
Presbyopia is corrected using convex lenses, which can be used for reading and near work.
Correction of Refractive Errors
Refractive errors can be corrected through several means. Some methods offer a permanent cure while others need to be adhered to life-long. Treatment can be in the form of spectacles, contact lens or surgery.
Contact lens and surgery will be described in detail:
An attractive alternative to spectacles for correcting refractive errors, the contact lens is also used for various therapeutic, pharmacological and cosmetic procedures.
A contact lens is an optical device that is placed directly over the corneal and scleral surfaces of the eye. Modern contact lenses are polymers that are relatively safe, non-toxic and on-allergic. Since the cornea derives its oxygen requirement mainly from the atmosphere, placing a lens on the corneal surface can impede the oxygen supply to the cornea. Newer types of lenses, however, allow oxygen to pass through them to the cornea.
Types of Contact Lenses
There are three types of contact lenses: soft, semi-soft and hard. Below here:
Soft contact lenses: These are very flexible lenses made of a polymer called hydroxyl ethyl meta acrylate (HEMA). They are very comfortable to wear and permit oxygen to reach the cornea.
Semi-soft lenses or rigid gas permeable lenses: These lenses are made from special materials like cellulose or silicon acrylate. They are less flexible than soft contact lens but allow some oxygen to pass through to the cornea. Thoughtless comfortable than soft lenses, they can be used to correct steep corneas and high astigmatism.
Hard contact lens: Made of a polymer called polymethyl metha acrylate (PMMA), hard lens are the least flexible of all lenses and permit almost no oxygen to pass through to the cornea. However, the advantage of a hard contact lens is its durability.
Advantages of Contact Lenses
- Contact lenses provide a larger field of vision than spectacles and a better range of mobility. Hence, they are ideally suited for sportspersons and people engaged in outdoor physical activities.
- In people with uniocular refractive error (power difference of more than 3 dioptres between the eyes), spectacles will produce double vision due to the discrepancy in the size of the image. Contact lens can be used in such cases.
- Cosmetically, they offer an advantage over spectacles.
- Apart from correcting refractive errors, a contact lens is also used to deliver drugs to the eyes and treat corneal diseases
- Nowadays, contact lenses are available in various colors and can even be used by people with a normal vision for cosmetic purposes.
With all these advantages, there are also some precautions to be taken before opting for contact lenses:
- An ophthalmologist should examine the eyes to rule out infections and other disorders of the eye
- Only a qualified contact lens specialist should fit the lens. The tests include measuring the corneal curvature and diameter and issuing a suitable pair of contact lens. After wearing them, the specialist examines the eyes to check for a good, comfortable fit.
- Hand-washing with soap before inserting and removing the contact lens from the eye is mandatory.
- Proper maintenance of the contact lens by cleansing it with the appropriate lens fluid is necessary.
- If the eyes are red or there is a white discharge from the eyes, the contact lens should not be used and the eye care professional should be contacted for further instructions.
- One should not sleep overnight with the contact lenses on. The oxygen supply to the cornea is totally cut off by the contact lenses during sleep. The oxygen-deprived cornea becomes vulnerable to damage and ulceration.
- The eyes should be periodically examined by an eye care professional to check for any infection or corneal damage due to contact lens wear.
The latest innovation in the field is extended wear contact lenses which can be used over a longer period of time. These lenses allow the free passage of oxygen to the cornea, but even they must be used with care.
Contact lenses offer a lot of advantages but they can also cause serious complications if not handled properly. A little care and attention will go a long way in enabling a person to enjoy the benefits and avoid the problems.
Types of Eye Refractive Errors topic is now ended! If you like this article, please share this via your social media platform and if you have any question, please comment below. Thank you all!