Astigmatism is a very common yet highly misunderstood vision problem – many people even pronounce it incorrectly, saying they have “a stigmatism” instead of “astigmatism”. Perhaps the reason most people don’t understand this condition very well is a lack of understanding about the anatomy of the human eye. To solve this problem, in this article we’ll look at the anatomy and optics of an eye with normal vision and contrast that with the anatomy of an eye with astigmatism. Later, we’ll discuss the different types of astigmatism, its epidemiology, symptoms, and how it is diagnosed and treated.
Normal eye anatomy and optics
A normal human eye is spherical, round like a baseball. Several of the components of the eye are also spherical, including the cornea and the lens. The cornea is a transparent dome that covers the iris and pupil, and this dome normally is equally curved in all directions, vertical, horizontal and every degree of diagonal. The cornea focuses light from the outside world into the pupil, where it will then travel through the aqueous humor until it reaches the lens.
Anatomy of a human eye with normal vision. Note the spherical shape of the cornea.
The lens is also spherical in a normal eye, and is responsible for focusing the light (i.e. image from the outside world) onto the retina, the small, light sensitive tissue on the back of the eye that transmits the visual signal to the brain.
Our eyes must process light coming in on multiple meridians or planes – vertical, horizontal, 30°, 157°, etc. The spherical shape of the cornea and the lens, having equal curvature in all directions, allows all of these different meridians to be focused equally. In the absence of near-sightedness (myopia) or far-sightedness (hyperopia), the result is that all meridians are focused precisely onto the retina, causing our brains to perceive a clear image of what we are seeing. In the case of myopia, all meridians are focused on a single point, but this point lies in front of the retina. Conversely, with hyperopia, all meridians are focused on a single point that lies behind the retina.
Astigmatism anatomy and optics
The difference between the anatomy of an eye with astigmatism and a normal eye lies in the curvature of either the cornea or lens, or possibly both. The curvature will be stronger or steeper in one plane than another, similar to the back of a spoon which curves more along from side to side than from top to bottom. With astigmatism, this difference in curvature can happen in any plane, but most commonly the horizontal or vertical planes (the principal meridians in a normal eye).
Astigmatism is usually caused by a cornea that is not perfectly round.
When the curve is steeper in one direction vs. another on either the cornea or lens, the light coming in from different meridians isn’t focused with the same strength. For example, stronger curvature on the cornea in the horizontal meridian (i.e. the line from 3 o’clock to 9 o’clock in your field of vision) would cause that meridian to be focused more strongly than the vertical meridian (12 o’clock to 6 o’clock). This would cause the light from these two meridians to focus on different points, with the horizontal meridian focusing in front of the vertical. Whether the two focal points land at different places in front of, behind, or on either side of the retina determines its type in one system of classification.
In all cases, having two different focal points causes an eye with astigmatism to have blurred vision at all distances, near and far, even if one of the focal points lands on the retina like it should. The brain is receiving different intensity signals for the horizontal plane and the vertical plane in your field of vision, causing a blurry image.
Types of astigmatism based on anatomy and optics
There are a few different ways in which astigmatism can be classified. The simplest is by whether it is caused by uneven curvature of the cornea (corneal astigmatism) or lens (lenticular astigmatism). Corneal astigmatism is more common than lenticular and is also easier to diagnose.
A second system names two types of astigmatism as “with-the-rule” or “against-the-rule”, depending on whether it is the vertical or horizontal meridian that has the steeper curve, respectively. If the steepest curve lies on the diagonal, it is known as “oblique astigmatism”. Finally, if the two principal meridians are not perpendicular to each other, this system terms it “irregular astigmatism”. The prevalence of these types varies with age, with children being more likely to have with-the-rule, and elderly people more likely to have against-the-rule.
The different curves of the “football shaped” cornea of an astigmatic eye cause two different focal points.
The third system classifies astigmatism based on where the focal points fall in relation to the retina. In simple myopia and simply hyperopia, one focal point lands on the retina while the other lands either in front of or behind the retina, respectively. In compound myopia or compound hyperopia, both focal points land in front of or behind the retina, respectively. Mixed astigmatism occurs when one focal point lands in front of the retina and the other lands behind it. Myopic astigmatism appears to be more common than hyperopic, at least in some populations.
Incidence of astigmatism in different populations
What causes the changes in curvature of the cornea or lens is not precisely known, but appears to have an inherited component, and rates of astigmatism are different in different races and ethnicities. In a recent study on children in the United States with astigmatism (ages 5-17), children of Hispanic origin had the highest rates of astigmatism (36.9%), followed by Asian (33.6%), Caucasian (26.4%) and African-American (20.0%). Overall, 28% of children in the study were found to have clinically significant astigmatism (1 diopter or more of difference between focal strength of the two meridians).
People tend to develop more astigmatism with age.
Symptoms of astigmatism
It is possible to have a very slight degree of astigmatism that causes no noticeable symptoms whatsoever. However, people with more serious degrees of astigmatism (generally 0.75 diopter or greater difference between focal strengths) often experience these symptoms:
- Blurry or distorted vision at near and far distances
- Squinting
- Eye strain
- Headaches, including migraines
- Difficulty with night vision, such as driving at night
A recent study found a correlation between migraine headaches and astigmatism, finding that the case group (migraine sufferers) were significantly more likely to have some form of astigmatism than the control group (people without migraines).
Diagnosis of astigmatism
Astigmatism is diagnosed by an ophthalmologist during a routine eye exam. The most common method of diagnosing corneal astigmatism is using an instrument called an ophthalmometer or keratometer, where the patient braces their forehead and chin against a headrest (this is to control the exact distance between the eye and the instrument), and the ophthalmologist is able to measure the curvature of the patient’s cornea by looking through the opposite end of the instrument. Some practitioners have automated corneal topography machines which perform essentially the same function, and may be used alone or in conjunction with the manual instrument during an eye exam.
Testing a variety of corrective lenses is another way to measure and diagnose a patient’s astigmatism, particularly in the case of lenticular astigmatism where the cornea appears to be normal. This is also used to confirm or refine diagnosis of corneal astigmatism before writing a prescription for corrective lenses. The patient usually sits in front of a standard eye chart, and the ophthalmologist will test different lenses on each eye, one at a time, asking the patient which of the lenses makes the letters on the eye chart look the least blurry.
Astigmatism treatment options
Most types of astigmatism can be corrected with glasses or contact lenses. Unlike simple lenses which are used to treat near-sightedness or far-sightedness with no astigmatism, the different curvatures of an astigmatic eye requires a toric lens, or a lens with two different curvatures. The goal is to balance out the different in curvature of the eye to achieve normal vision. For example, in “with-the-rule” astigmatism where the vertical meridian is steeper than the horizontal, the corrective lens would have a steeper curvature for the horizontal meridian (or a flatter curvature for the vertical) to balance the eye’s focal strength along these meridians.
Measuring the angle of the two principal meridians is critical for correcting astigmatic vision.
To have good results from corrective lenses, it’s important to know where the patient’s principal meridians lie; while they lie on the horizontal and vertical planes for normal eyes, this may not be the case for all patients. The angle at which these meridians lie can be measured by the ophthalmometer, essentially the way we might use a protractor to measure any other angle. This measurement must be included in the prescription for eyeglasses or contact lenses to properly balance the uneven curvature of the eye.
It used to be that many patients with astigmatism weren’t good candidates for contact lenses, and the toric lenses necessary to correct astigmatic vision were only available as rigid lenses. Today, however, soft contact lenses are available for patients with astigmatism as well.
Laser refractive eye surgery (like LASIK) can be used to treat astigmatism. This has the advantage of providing a permanent solution to the vision problem, as opposed to glasses or contact lenses which only manage symptoms and must be worn throughout a person’s lifetime. LASIK can be used to reshape the cornea so that it becomes spherical like a normal eye, thereby eliminating the problem of astigmatism. Magnitudes of up to 3.0 diopters can usually be corrected in a single session.
