astigmatism

Corneal topography of someone with regular astigmatism.

The cooler tones indicate the flatter parts of the cornea while the warmer tones indicate the steeper parts.

As you can see, this cornea is more curved along the vertical plane. 

Uh-stig-muh-tizum. It sounds like a terrible disease, but it's not. It is a type of refractive error- it affects how light bends, or refracts, when it enters your eye. Nearsightedness (myopia) and farsightedness (hyperopia) are also refractive errors. Astigmatism is an irregular curvature of the front part of the eye (the cornea and/or the lens). Most people have at least a small amount of astigmatism, but not everyone's vision is affected by it. To explain astigmatism in more detail, let's take a quick step back and review refractive errors.

Image: NEI

A quick review of refractive errors: 
We refer to the curvature of the cornea and lens as either spherical or astigmatic. When you have a spherical cornea or lens, it has the same roundness all over, like a basketball. Because of this uniform curvature, light entering the eye focuses at one point.

• If that one point is in front of the back of the eye (the retina), you are nearsighted. You need minus power in your glasses/contacts to make the light focus ON the retina so you can see clearly. 
• If that one point is behind the retina, you are farsighted. You need plus power in your glasses/contacts to make the light focus ON your retina so you can see clearly. 

With an astigmatic cornea or lens, the curvature is NOT the same all the way around (more like a football than a basketball), and light entering the eye focuses at two different points. This distorts and blurs things up close and far away, and it's especially noticeable when looking at street signs or electronic displays. As someone who has a fair amount of astigmatism, reading signs at the airport is a nightmare without my astigmatism correction!

There are different types of astigmatism. If the irregular curvature is found in the clear part of the front of your eye (the cornea), it's called corneal astigmatism. If it is found in the part of the eye that sits behind the iris (the lens), it's called lenticular astigmatism. You can have either or both. Corneal astigmatism is more common.

Astigmatism can also be classified as either regular or irregular. Be warned: this part will bore you.

• Regular astigmatism occurs when the primary meridians/curvatures are 90 degrees apart, producing a bow-tie or figure-eight pattern on corneal topography (see below). This is much more common than irregular astigmatism. Regular astigmatism can be further classified based on which plane is steepest: 
• With-the-rule astigmatism is when the vertical plane is the steepest. This would be similar to a football laying on its side.
• Against-the-rule astigmatism is when the horizontal plane is the steepest. This would be similar to an upright football on a kick-off tee.
• Oblique astigmatism is when the principal planes are not at or close to (within 30 degrees) the vertical or horizontal. 

Types of regular astigmatism, as seen on corneal topography
Image: Optometric Management

• Irregular astigmatism is present when the curvature is not regular, and is generally a result of surgery, scarring, or disease. An example would be keratoconus. 

How do I know if I have astigmatism?
Astigmatism is diagnosed by your optometrist or ophthalmologist during a comprehensive eye exam. There are three instruments that may be used to detect astigmatism:

• Keratometer- this instrument measures the average curvature of the central part of the cornea. In most offices, an automated keratometer is combined with an autorefractor (AKA the thing that has a picture of a hot air balloon or barn in it). 
• Corneal topographer- this instrument maps the front surface of the eye, indicating where the cornea is steepest and flattest. The image at the top of this post is a corneal topogram.
• Phoropter- AKA the "better one or two" instrument. By using the dials below, your eye doctor can figure out how much astigmatic correction you need and where it needs to be.

How do you treat astigmatism?

• Glasses- Spectacle lenses can correct astigmatism. Unlike a spherical prescription, an astigmatic prescription has 3 numbers:

• The first number is the spherical component- the amount of nearsightedness (-) or farsightedness (+).
• The second number is the cylinder- the amount of the astigmatism. This can be written in either plus or minus form.
• The third number is the axis- a number between 1 and 180 that indicates the position of the astigmatism.
• Contact lenses- Contact lenses that correct astigmatism may be referred to as toric lenses. Soft contact lenses have come a long way in recent years, and are now available in a wide range of astigmatic prescriptions. However, they are not available in every axis and every cylinder, so they are not for everyone. With larger amounts of astigmatism, gas permeable contacts may give sharper, more consistent vision. Gas permeable contacts are an especially great option in patients with irregular astigmatism.
• Surgery- Refractive surgeries, such as LASIK, are also an option.

CliffsNotes: Astigmatism isn't a disease; it's a refractive error. It means the cornea and/or lens is not completely round, so light doesn't focus at one point. That distorts and blurs things, but it can be corrected with glasses, contact lenses, or refractive surgery. 

Additional recommended resources:

• AOA: Astigmatism
• All About Vision: Astigmatism
• NEI: Astigmatism
• AAO: Astigmatism

myopia control

What is myopia?
Myopia is the fancy name for near-sightedness. Myopia causes vision to be blurry in the distance. It is the result of light rays focusing in front of the light-sensitive tissue that lines the back of the eye (the retina) rather than on the retina. This can happen when the power of the front of the eye is too strong (refractive myopia) or when the eyeball is too long (axial myopia). Below is an illustration. A minus (concave) lens is used to focus the light rays on the retina so that images are clear. The most common type of myopia begins between the ages of 6 and 12.

A myopic eye focuses the image IN FRONT of the retina, producing a blurry image. Minus lenses are used to correct myopic eyes, putting the image ON the retina so you can see clearly. 

The prevalence of myopia has been increasing over the past decades. In 2000, 23% of the world's population was myopic, and researchers are predicting that 50% of the world's population will be myopic by 2050 (1). THAT'S HALF OF THE WORLD!! Higher rates of myopia, nearing 90%, occur in some Asian populations. The increase in myopia prevalence suggests that environmental factors play a role in its development, though there is certainly a genetic component as well. People with higher amounts of myopia are also at greater risk for eye health issues such as retinal detachment and myopic maculopathy (2), so the myopia "epidemic" is a public health concern.

How does myopia progress? 
The exact mechanism of myopia development is still unclear. The thought is that myopia progression is caused in large part by the elongation of the eye. Research suggests that peripheral hyperopic defocus causes elongation of the eye. The periphery can be blurred even if the center is clear, and this peripheral blur is not really something we notice. At near, the periphery is more out of focus than it is when looking in the distance, so near work may be implicated in the progression of myopia (3). Some more recent studies have shown a greater association between myopia development and the time spent outdoors than the time spent doing near work. Translation: increasing myopia is more closely related to how little time people spend outdoors than how much time people spend reading. So minimal time spent outdoors could be a risk factor for myopia development. Very recent research has identified a cell in the retina that may cause myopia when it dysfunctions. The dysfunction may be linked to the amount of time spent indoors/away from natural light (4).

image: Review of Optometry

Options that have been shown to slow the progression of myopia:

1. Orthokeratology (aka ortho-K, corneal reshaping technology, CRT). These are customized contact lens worn only at night, temporarily changing the shape and power of the front part of your eye (the cornea).  When you wake up, you take the contacts out and you can see! Ortho-K lenses involve reverse geometry, meaning the curves of the lens are structured so that the tear film beneath the lens essentially flattens the center of the cornea while steepening the midperipheral cornea. This creates peripheral myopic defocus that negates the peripheral hyperopic defocus that is linked to eyeball growth (5). Orthokeratology is FDA approved for myopia up to -6.00D and mild amounts of astigmatism (up to 1.75D). There is no minimum age; the child just has to be able to put the contacts in, take them out, and maintain them on their own. It all depends on the child's maturity level. As with any contact lens wear, there is a small risk for infection. Studies have found that ortho-K treatment produced an average of 30-50% reduction in the progression of myopia.(6). Ortho-K appears to be more effective for those with higher amounts of myopia and larger pupils. Both the LORIC and later the CRAYON study showed that ortho-K slowed the axial growth of the eye, thus reducing myopia progression (7). The SMART study is yet another recent study that supports the theory that ortho-K reduces myopia progression. At the conclusion of this 3-year study, the ortho-K group saw an average increase in myopia of 0.12D while the soft lens control group increased by an average of 1.01D (8).
2. Soft multifocal contact lenses. Multifocal contact lenses are those that correct your distance while also giving you plus power to help see up close when needed. The specific design of multifocal lenses that are used for myopia control are center distance design (see image below). Center distance means more plus power in the periphery of the lens, which decreases peripheral hyperopic defocus and induces peripheral myopic defocus, reducing axial elongation. Several study results have supported the use of distance-center soft multifocal contact lenses for myopia progression, averaging a 40% reduction in myopia progression (9, 10, 11). The CONTROL study found a whopping 72% reduction in progression of myopia over a one year period when compared to wearing single vision soft contact lenses (12), though this study involved myopic children with a specific focusing/postural issue (eso-fixation at near).





A multifocal lens with a center-distance design
image: Review of Optometry





3. Atropine. This method is different from the above two because it is not about changing the stimulus that contributes to eye elongation, but rather interfering with a biochemical pathway.  How does it work? We don't really know for sure. But it is thought to act on the white part of the eye (the sclera) or the tissue that lines the back of the eye (the retina) to prevent the sclera from thinning or stretching (13). If you have had your eyes dilated during an eye exam, the doctor likely used tropicamide.  This drop made your pupils big and your vision up close was blurry for 3 or 4 hours. Atropine is a similar drop from the same family of drugs, but the effects last for much longer. Atropine 1% has been shown to produce a 90% average reduction in myopic progression, from 0.5 D/yr to 0.05 D/yr (14). The downside: blurry near vision, light sensitivity, and large pupils. Other studies have shown comparable results using lower concentrations of atropine, which produce less side effects. Atropine 0.01% has been shown to slow myopia progression by 50%(15, 16).  Pirenzepine has also been tested, showing slightly less efficacy (44%), but with fewer side effects. Unfortunately, it's not commercially available as an eyedrop or gel in the US. More research is being done on the long-term effects of atropine therapy, and whether or not its effects are permanent.

Options that are NOT GREAT for slowing the progression of myopia:

1. Undercorrection of myopia. I've had some parents specifically ask me to give their child less powerful glasses in hopes that that would make the child need glasses less. There is no validity to this claim; actually, the undercorrection of myopia has been shown to INCREASE its progression (17, 18, 19).
2. Progressive addition lenses (PALs). Commonly called "no-line bifocals," PALs are lenses that have your distance prescription at the top and gradually become more plus-powered as you go vertically down the lens. Many people over the age of 40-ish wear these to help them see clearly at all distances. For the purposes of myopia control, they're not a top option. The COMET study found a small, statistically significant decrease in myopia progression in children wearing PALs vs children wearing regular, single-vision lenses, but only in the first year. So the conclusion was yes, it produces a little decrease in progression, but not enough to warrant a change in how we prescribe for myopic children (20). Also, other aspects of the child's binocular vision should be taken into consideration. 
3. Spherical soft contact lenses or rigid gas-permeable (RGP) contact lenses (aka regular distance vision contacts). These two choices are excellent forms of vision correction, but they have shown little value in terms of controlling myopia progression. The CLAMP study showed some reduction in myopia with RGPs in year 1 in comparison to soft contact lenses; however, it was not clinically significant because it didn't change axial length and was likely due to the flattening of the cornea, which is not permanent (21).

CliffsNotes: Glasses and regular contact lenses don't help slow myopia progression. Undercorrection of myopia doesn't help either; it actually makes it worse. Atropine therapy, orthokeratology, and soft multifocal contact lenses have been shown to be effective in controlling myopia progression. Ask your optometrist for more information!


Additional recommended resources:

• All About Vision: Myopia Control
• Myopia Prevention
• Ortho-K Academy
• CRT Vision

back-to-school eye exams

For many parents, the next few weeks will be filled with orientations, school supply lists, sports physicals, and immunizations.  If you are in that boat, be sure to include a visit to your eye doctor to have your child's eyes examined before starting the school year.  Experts estimate that as much as 80% of learning occurs through the visual system (1), so give your child every possible opportunity to succeed in school.  Learning, particularly in the form of reading, requires a combination of many visual skills.  There is far more to vision than just seeing 20/20.  

Vision screenings at the pediatrician's office and at school, though helpful in flagging some potential problems, usually only assess one or two aspects of vision.  Screenings are limited and non-diagnostic, so they are not a substitute for a comprehensive eye exam.  I regularly see kids who pass school screenings but have vision problems that need to be addressed.  Some states actually require a comprehensive eye exam before entering kindergarten, which is a fantastic idea.  The visual system is not fully developed in young children, and equal input from both eyes is necessary for proper development.  The earlier problems are detected, the better the chance of treatment being successful.  The American Optometric Association (AOA) recommends an eye exam at 6 months old, 3 years old, and again before entering first grade.  While in school, a child/teen should have an eye exam at least every 2 years if no issues exist, or yearly if he/she wears glasses or contact lenses.  Depending on the child and the condition, your eye doctor may recommend more frequent examinations.  

Some common signs and symptoms for parents and teachers to keep an eye out for:

• squinting
• eye(s) turning in or out
• sitting close to the TV 
• holding books close to the face
• complaining of seeing double
• complaining of headaches
• tilting head 
• avoiding reading and/or reading slowly
• losing place when reading and/or skipping words

While some children will have symptoms and signs that parents and/or teachers can pick up on, it is not uncommon to find children with issues that do not report any symptoms at all.  That is why the objective tests done during a comprehensive eye exam are so important for young children.    

According to the American Public Health Association, about one in four school-aged children has a vision problem that interferes with learning.  Here is a brief overview of some eye conditions that are relevant to this age group:

• Hyperopia, myopia, and astigmatism:  These are all types of refractive error.  A refractive error exists when the eye doesn't focus light exactly on the back of the eye (the retina).  An eye doctor measures the refractive error of the eyes and prescribes glasses/contact lenses if needed.  Correcting a child's refractive error when appropriate is the first step in addressing problems with the visual system because it may be the root cause of other vision problems, like those described later in this post.  
• Hyperopia, or farsightedness, is when the eye focuses light behind the retina.  Small amounts of hyperopia are normal in young children.  Kids can often compensate for this by using their eye muscles to bring images into focus.  But when the amount of hyperopia is substantial, it creates an unnecessary burden on the system and can result in blur, fatigue, eye strain, and headaches, especially when reading.  High amounts of uncorrected hyperopia can cause an inward eye turn to develop as well (accomodative esotropia).  
• Myopia, or nearsightedness, is when the eye focuses light in front of the retina.  Those with myopia have difficulty seeing far away.  Myopia is often first detected in school-aged children, and typically progresses through the adolescent years.
• Astigmatism is when the curvature of the cornea and/or the lens of the eye is different in one direction than it is in another, and light gets focused at two different points.  This can cause blur and distortion up close and far away.    
• Strabismus:  Commonly called crossed eyes, strabismus is a misalignment of one or both eyes, present in 2-5% of the general population.  The eye may turn in (esotropia), out (exotropia), up (hypertropia), or down (hypotropia).  It may happen all the time, or it may happen only some of the time (constant vs. intermittent).  The turned eye may be the same eye all the time, or it may alternate between the two (unilateral vs. alternating).  When the eyes are pointing in two different directions, they are sending the brain two different images.  Depending on the frequency and severity of the eye turn, it may cause double vision.  Alternatively, the brain may learn to ignore the image from the turned eye in an attempt to relieve the confusion.  This adaptation is called suppression.  If left uncorrected, strabismus can result in decreased vision in the turned eye (ambylopia) and loss of depth perception.     
• Amblyopia:  Sometimes referred to as lazy eye, amblyopia affects 2-3% of the general population.  Amblyopia is defined as reduced vision in one eye, or less commonly both eyes, in the absence of disease or structural abnormalities.  It can be caused by an eye turn (strabismic amblyopia), or a high prescription in one or both eyes that hasn't been corrected (refractive amblyopia), or something physically obstructing the line of sight (deprivation amblyopia).  Amblyopia occurs in the brain during the developmental stage, and it is often a preventable and treatable cause of vision loss.  The sooner it is diagnosed and treated, the better the chance of achieving normal visual function.  This is a big reason why eye exams are recommended early in life. 
• Ocular motor dysfunction (OMD):  Patients with OMD have problems keeping their eyes on a target (fixation), moving their eyes quickly and accurately from one target to another (saccades), and following a moving target with their eyes (pursuits).  These eye movements play an important role in reading and sports.  OMD usually does not present by itself; a child with OMD may also have issues with focusing and/or eye teaming.  The primary treatment here (assuming the appropriate glasses prescription has been given) is vision therapy.    

Vision therapy is a program of activities designed to help correct deficiencies and improve efficiency of the visual system.  Vision is a developed skill, and thus can be enhanced with guided training.  Vision therapy can be prescribed to help treat some conditions I mentioned above, as well as others relating to focusing and/or eye coordination.  Not all optometrists offer vision therapy, but all optometrists can direct you to one in your area that does provide such services.

CliffsNotes: Be sure to add a comprehensive eye exam to your child's yearly back-to-school checklist.  Clear, comfortable, single vision is important to a child's academic success!

     Additional recommended resources:

• Vision and Learning
• College of Optometrists in Vision Development

(1) Gazzaniga MS, Ivry RB, Jangun CR. Cognitive Neuroscience, the Biology of the Mind. New York, NY; WW Norton & Co, 1998.