Astigmatism is a refractive error where cornea is irregularly curved. Instead of being round, it may be shaped more oval. This irregular shape causes light to scatter and to bend, or refract, improperly as it passes through the cornea. Instead of focusing directly on the retina, some light rays focus in front of and some focus behind the retina. These multiple focal points distort vision. In some cases, an irregularly curved lens produces minor degrees of astigmatism.

What is physiological astigmatism?

Everyone has a small degree of astigmatism due to different refractive power of horizontal and vertical meridian of the cornea, but this astigmatism doesn’t cause any vision problems.

Can astigmatism be associated with some other refractive error?

Yes. Since in astigmatism there are two focal lines instead of one focus, it is possible that both lines focus in front of retina (myopic astigmatism), or behind retina (hypermetropic astigmatism).

Focus from one meridian can be on the retina, while other can be in front, or behind retina and it is also possible that light rays focus from one meridian in front of retina and from other meridian behind retina.

What is regular and irregular astigmatism?

When we talk about corneal refraction we mention two main meridians – one vertical and one horizontal. There are many meridians in different angles between these two. If refractive power of these meridians changes gradually from vertical to horizontal we talk about regular astigmatism. Irregular astigmatism doesn’t have gradual changes in refraction between meridians.

What is oblique astigmatism?

It is astigmatism where instead of vertical and horizontal meridian, the most affected meridians are some between vertical and horizontal.

What is the cause of astigmatism?

The main cause is unknown. It is generally considered that irregular shape of cornea is inherited. It can also result from pressure from the eyelids on the cornea, incorrect posture or an increased use of the eyes for close work. Some corneal diseases can leave scars on cornea and create astigmatism.

Astigmatism can be caused also by irregular shape of the lens.

What Are the Symptoms of Astigmatism?

People with undetected astigmatism often experience headaches, fatigue, eyestrain and blurred vision at all distances. While these symptoms may not necessarily be the result of astigmatism, you should schedule an eye exam if you are experiencing one or more symptoms.

How Are Astigmatisms Treated?

Almost all degrees of astigmatism can be corrected with properly prescribed eyeglasses or contact lenses. For a person with only a slight degree of astigmatism, corrective lenses may not be needed at all, as long as other conditions such as nearsightedness or farsightedness are not present. If the astigmatism is moderate to high, however, corrective lenses are probably needed.

Astigmatism can also be treated by refractive surgery. There is also an alternative form of treatment in a form of eye exercises that can reduce astigmatism. For more information about treatment visit this link HERE.

Hypermetropia (farsightedness) is refractive error where close objects are seen with difficulty, while distant objects are seen more or less clearly. This occurs because light rays coming from the distance focus behind retina.

The most common reason for hypermetropia is because eyeball is too short and normal refraction is too weak to focus light rays on the retina. This form of hypermetropia is also known as axial hypermetropia.

Hypermetropia is also possible when size of an eyeball is normal, but refractive power of cornea or lens or both is weak. This condition is very similar to presbyopia which occurs due to insufficiency of lens accommodation.

Since accommodation can increase the refractive power of the lens, some forms of hypermetropia can be compensated. Without accommodation both close and far objects are not seen clearly.

If accommodation can compensate hypermetropia why I see close objects with difficulty?

The power of accommodation is limited. To see clearly close object, person with hypermetropia require higher degree of accommodation than emmetrope.

What are the symptoms of hypermetropia?

Most common symptoms of hypermetropia are eye strain, difficulty with reading, and headaches. If you suffer from any of these symptoms, you should go to the eye exam to check your vision.

What is the cause of Hypermetropia?

The cause is unknown. It is considered that hypermetropia is inherited and eyes don’t grow enough and normal refractive power of lens and cornea is too weak.

How is hypermetropia treated?

Hypermetropia is usually treated with corrective eyeglasses and contact lenses. There are also some other forms of treatment like laser eye surgery and natural vision correction. Read more about treatment here.

Myopia (nearsightedness) is a refractive disorder where distant object cannot be seen clearly. This condition occurs because light rays coming to an eye from distant object focus in front of retina and the resulting image on the retina is blurry.

The most common reason for myopia is elongation of an eyeball and normal refractive power becomes too strong for an eye. The images formed on the retina are blurry and therefore cannot be seen clearly. This form of myopia is known as axial myopia.

In some rare cases it is possible that refractive power of an eye is too strong for a normal size of an eyeball and images from distant objects again focus in front of retina instead on it. The reason is either a strong refractive power of cornea (for example in condition known as keratoconus), or lens (for example condition known as lenticonus).

Can accommodation compensate strong refractive power?

Unfortunately no. Unlike some forms of hypermetropia accommodation cannot correct myopia. The reason is because the accommodation works by increasing refractive power of lens. It cannot weaken it over a certain point. When cilliary muscle is fully relaxed (like when we focus on distant objects), refractive power of the lens is maximally reduced. In myopia maximally reduced refractive power is still too strong.

When myopia mostly occur?

Myopia mostly occurs in school children, although it is not impossible that it occurs very early in childhood. It rarely occurs in the adulthood. Development of myopia in adulthood, especially after 40 years of age, may be the first sign of cataract.

What is the cause of myopia?

The main cause of myopia is unknown. Some studies show that it has genetic origin since it is more common in some families. For benign myopia it shows autosomal recessive heritage. Malign myopia is more likely to be inherited.

Some studies show the relationship between eye strain in intensive close work and development of myopia. Most people that spend a lot of time in front of computer screen, or in front of the book develop myopia at some point of their life.

Also more and more people become nearsighted which also could indicate that the stressful way of life and bad nutrition can also be responsible. Myopia was very rare condition in the past.

Why eyes in myopia grow?

During adolescence period when our entire body grows, eyes also grow and that is one reason that results in increasing of myopia.

Increased eye strain with too much close up work may lead to development of myopia. Some scientists believe that myopia occurs as some form of “long term” accommodation for close work. When we spend too much time looking at close objects, cilliary muscles are constantly tense. The need for cilliary muscle relaxation may lead somehow over time to elongation of an eyeball, so that the close up work could be done with reduced accommodation and reduced contraction of cilliary muscle.

People that suffer from myopia, especially without corrective glasses or contact lenses, strain to see distant objects. This strain may lead to the development of myopia.

However, eye strain is scientifically more considered to be a contributing factor in the development of myopia, rather than it’s cause. There are also some other factors that may be the reason of eyeball elongation, like dopamine deficiency, form deprivation and optical defocus.

You mentioned benign/malign myopia. What is the difference?

The main difference is in the pathologic changes on the eye fundus. Benign myopia usually starts in school, develops during adolescence period and finally stabilizes in the adulthood. It rarely grows beyond -7.0 D and it is never associated with any pathologic changes on the eye fundus.

Malign myopia usually starts in early childhood and is very progressive condition. It doesn’t show any tendency to stabilize over time and often grows beyond -10.0 D. Sometimes it can reach -15.0 D or even -20.0 D. Unlike benign myopia there are always pathologic changes on eye fundus. This myopia is very difficult to correct and it often requires surgery (scleroplasty) to stop further progression.

How do I know if I suffer from myopia?

The best way to know is to visit ophthalmologist or optometrist and take eye exam. If you are having difficulty seeing distant objects, reading small signs in the distance or recognizing other people, that could indicate that you suffer from myopia. Most people though don’t have any significant difficulties and they believe that they see perfectly.

In these cases the best indication that you may need glasses is if you notice that some people around you are able to see something in the distance (blackboard, signs on shops, car license plates, etc.) while you cannot. Standard eye exam will determine whether you suffer from myopia or not.

What is “temporal myopia”?

This term is used to describe a condition usually after a longer period of intense close work and accommodation. It is also known as pseudomyopia. The cilliary muscles become over-strained and unable to fully relax. The refractive power of an eye when focused on distance becomes temporary stronger and distant objects can’t be seen clearly. This condition is completely reversible after a period of rest when cilliary muscles relax.

The condition may last for a few days. When a person with temporal myopia undertakes eye exam, it will show the presence of myopia and it may be misdiagnosed as real myopia. That is why diagnose of myopia must be verified with pupil dilatation and cilliary paralysis (eye drops with some form of atropine derivate are administered). Paralysis of cilliary muscle is essential and will cure temporal myopia.

What is the tendency of myopia growth?

Unfortunately myopia has a tendency to grow. It can’t really be predicted how much it will grow, but for many people growth usually stops in adulthood. Wearing corrective eyeglasses and contact lenses little does to stop further progression.

How is myopia treated?

The main options for myopia treatment are corrective eyeglasses, contact lenses, refractive surgery (LASIK, PRK, lens implantation, etc.) and orthokeratology (Ortho-K). Eyeglasses and contact lenses are temporary solutions, while refractive surgery and orthokeratology permanently reduce myopia.

There is also an alternative method of curing myopia consisting of eye exercises and eye relaxation techniques. It was first introduced by ophthalmologist dr William Bates, but due to the inconsistency of his work and some potentially dangerous methods (staring in direct sunlight) it was rejected by the mainstream of medicine. His original techniques were adopted and modified so that they pose no danger anymore and now exist as alternative method of myopia treatment.

I am not aware of any scientific study that proves that these methods work or not, but that doesn’t change the fact that many people claim that they have been helped by these exercises. In my professional opinion, it doesn’t hurt to try. Read more about eye exercises HERE.

Accommodation is the process by which the eye increases optical power to maintain a clear image (focus) on the retina. The principal focusing ability of the eye is due to the difference in refractive index between and the curved cornea, but the variable curvature of the lens allows for an additional adjustment. This varies from a maximum of over 15 diopters in an infant to only about 1.5 diopters in a person 70 years old, as the lens becomes less flexible with age.

What is the mechanism of accommodation?

When viewing a far object, the circularly arranged ciliary muscle relaxes causing the lens zonules and suspensory ligaments to pull on the lens, flattening it. Flatter lens has reduced refractive power. When viewing a near object, the ciliary muscles contract causing the lens zonules to slacken which allows the lens to spring back due to elasticity into a thicker, more convex, form. This convex form has increased refractive power.

This theory was introduced by Helmholtz and can be easily verified through the use of atropine, or some other atropine derivate. There was also some other theories that involved the use of extraocular muscles, but these are rejected as there is no scientific proof.

Can prolonged accommodation cause some refractive error?

There is a theory that prolonged accommodation can be responsible for axial eye elongation and inducing nearsightedness (myopia). There are some scientific studies that do show that accommodation can induce a very slight axial elongation. Prolonged accommodation is currently only considered as a factor that can worsen myopia. Many people do intense near work for a very prolonged period of time and never develop myopia.

What is a connection between accommodation and aging vision – presbyopia?

Presbyopia is caused by inadequate accommodation. Over time lens loses its elasticity and becomes rigid, thus unable to change its curvature. People become unable to see clearly near objects and often require reading glasses.

What is aphakia?

Aphakia is complete absence of the eye lens due to surgical removal, perforating wound or ulcer, or congenital anomaly; causes total loss of accommodation, extreme farsightedness (hypermetropia) and a deep anterior chamber.

People with aphakia are unable to accommodate and suffer from high degree of hypermetropia.

I suffer from cataract and my lens will have to be replaced with artificial lens. Will I be able to accommodate?

Unfortunately no. Artificial lens cannot be adequately incorporated into the ciliary apparatus and therefore it is unable to change refractive power. People with artificial lens permanently lose accommodation ability.

Refraction is bending of light rays that occur when passing from one medium to the other. With law of physics light has a constant speed when traveling through some transparent medium, but light travels with different speeds through mediums like vacuum, air, glass, water, etc. The fastest speed of light is through vacuum and somewhat slower through other mediums.

Because of this difference of speed, light will either “slow down”, or “speed up” when passing from one medium to the other. If the light enters different mediums from angle of 90 degrees, it won’t change its direction – only speed. But if light enters different mediums from some other angle, the effect of “speeding up” or “slowing down” will make it change its original direction.

If light enters from less dense material to more dense material, it will refract toward normal. If light enters from more dense material to less dense material, it will refract away from normal. The intensity of refraction will depend on the optical density of medium and angle of light.

What is lens?

Lens is an object designed to either converge or diverge light. It is created from optically denser material than air (usually glass) and has at least one surface curved. This curvature is essential and will determine whether the lens will diverge or converge light. Convex lenses converge light, while concave lenses diverge light.

How is this convergence and divergence explained through refraction?

Imagine many parallel light rays coming from the air to the curvature of the lens (denser material). Each of these light rays will enter lens from different angle and will refract differently. Only the central ray will not refract since its angle is 90 degrees. All other rays will enter lens at different angle and will refract toward normal of the lens curvature at that point. As I mentioned, if the lens is convex, angles will cause the light to converge and if it is concave, angles will cause it to diverge.

How refraction of an eye work?

Refraction of an eye is no different from every other form of refraction. Eye has some optically transparent mediums: cornea, humor aqueous, lens and vitreous body. Cornea is optically thicker than air, while humor aqueous is less dense. Lens of an eye is denser than humor aqueous and vitreous body.

From the anatomy of eye and laws of refraction we can see that refraction only happens when light enters and exits cornea and enters and exits the lens. Most important factor in refraction is curvature where light enters the medium, so we can simplify and say that the two parts of an eye responsible for refraction are frontal curvature of cornea and frontal curvature of lens. Since they are both convex, we can easily deduct that both cornea and lens converge the light.

What is focus of a lens?

Only convex lenses have real focus. Focus is a point where all light rays that pass through the lens meet. Concave lenses don’t have real focus, since they diverge light. Their focus is imaginary and can be projected in front of the lens.

What is the power of lens?

The power of lens is the capability of refraction. Lenses that are more curved and have thicker optical density possess high capability of refraction and are more powerful.

How is this power measured??

The power of lens is measured in diopters. Convex lens that focus parallel light rays one meter behind lens has optical power of 1 diopter. Lens with 3 diopters focus light 1/3 of meter behind lens. Since convex lenses have real focus, diopters for convex lenses usually have plus prefix. For example: +1.00, +2.00, +3.00, etc.

Concave lens diverge light and they have no real focus, but imaginary focus that can be projected in front of the lens. Concave lens with the power of 1 diopter has imaginary focus 1 meter in front of it and lens with power of 3 diopters has imaginary focus 1/3 meter in front of it. Because of imaginary focus diopters for concave lenses have minus prefix. For example: -1.00, -2.00, -3.00, etc.

What is the power of human eye?

Human eye when focused for distance has refractive power of around +60.00 diopters. With accommodation this power can increase by around +12.00 diopters.

Many people are affected with at least some form of refractive eye disorder at some point of life that prevents them to see clearly. My goal in creating this website is to help those people and everyone else interested to learn more and understand better these conditions that seriously affect the quality of life.

What is Refraction?

Refraction is bending of light rays. Our eyes have precise optical elements that bend light rays and focus them on the retina of an eye. Light rays must be focused perfectly on the retina in order to see clearly. Eye with optical ability to focus parallel light rays from infinity directly on retina is called emmetropic. [Read more about Refraction]

What is Accommodation?

Accommodation is the ability of an eye to change it’s refractive power. Light rays coming to an eye from distant objects are normally focused on the retina, but light rays coming from short distances would be focused behind retina if it weren’t for accommodation. [Read more about Accommodation]

What are Refractive Eye Disorders?

Refractive Eye Disorders are conditions where optical elements of an eye are unable to focus light rays on the retina which results in blurry vision. Eyes with some form of refractive disorder are called ametropic. Most common refractive eye disorders are:

Nearsightedness – Myopia
Farsightedness – Hypermetropia
Astigmatism
Aging vision – Presbyopia

What is Nearsightedness – Myopia?

Nearsightedness is refractive eye disorder where distant objects cannot be seen clearly, while closer objects are seen more, or less clearly. In nearsightedness optical elements of an eye focus light rays comming from distance in front of retina. The result is blurry image formed on retina. [Read more about Nearsightedness]

What is Farsightedness – Hypermetropia?

Farsightedness is refractive disorder where distant objects are seen more or less clearly, while close objects are seen with difficulty. In farsightedness optical elements of an eye focus light rays behind retina. While in nearsightedness eye is unable to correct defect, in farsightedness cilliary muscle of an eye is able to correct more or less this defect and focus light rays on the retina. [Read more about Farsightedness]

What is Astigmatism?

Astigmatism is a refractive disorder with blurry vision caused by irregular shape of cornea or lens. The refractive power in one meridian of cornea or lens is greater than that of the perpendicular meridian which results in losing focus. Instead of one focus point, there are two focal lines. Person with astigmatism have difficulty in seeing fine detail. [Read more about Astigmatism]

What is Aging vision – Presbyopia?

Aging vision, or presbyopia is a refractive disorder caused by the eye’s reduced ability to focus objects at various distances. It is more a disorder of accommodation and affects nearly everyone at certain age. It is usually noticed between 40 and 50 years of age and slowly progresses. [Read more about Presbyopia]