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

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.