The Five Senses and Beyond: The Encyclopedia of Perception - Jennifer L. Hellier 2017

Astigmatism

Astigmatism is a visual condition that causes blurred vision. For most people, the cornea of the eye has a spherical shape so that when light hits the tissue in the back of the eye, or the retina, the vision is clear and even. Individuals with astigmatism have a retina with an irregular shape that causes steep or rounded areas in the cornea, such as the shape of a football. These irregularities in the cornea cause it to have a harder time properly focusing light to the retina, which results in blurred vision. Similar to nearsightedness and farsightedness, astigmatism is a type of refractive error that causes light to bend at a certain angle. Although they may not know it, many people have varying forms of astigmatism from mild to severe. Genetics plays a role because astigmatism is inherited and may decrease or worsen over time as the individual grows. Irregular astigmatism occurs when a degenerative disease called keratoconus causes the cornea to become thinner and more cone-shaped, resulting in protrusion of the cornea. Other causes of astigmatism may result from eye injuries.

Symptoms

For people with mild cases of astigmatism, symptoms are hard to distinguish and treatment may be limited to correcting the blurriness with prescription contact lenses or eyeglasses. Those with severe cases may report being unable to see fine details either nearby or far away and suffer from headaches, weariness, and variable vision. This may be due to added tension on the eyes from prolonged reading, staring at a computer or phone screen, or looking off into the distance.

Types

The three types of astigmatism include myopic astigmatism, hyperopic astigmatism, and mixed astigmatism. Myopic astigmatism occurs when the meridians of the eye, or the flattest parts of the eye, are nearsighted while hyperopic astigmatism occurs when the meridians are farsighted. Meridians are imaginary lines separated into one-degree units to determine where the most and the least curved sections of the cornea are present. The lines or curves bisect the sphere of the eyeball. The 90-degree angle is the vertical meridian of the eye while the 180-degree angle is the horizontal meridian. The meridians are classified as regular, at a 90-degree angle, or as irregular, not perpendicular. In myopia, light is focused in front of the retina due in part to the additional thickness of the eye. This results in nearby objects being seen as clear and objects far away being seen as blurry. Hyperopia results when the light rays are focused behind the surface of the retina, resulting in objects that are nearby seen as blurry and objects that are far away seen as clear. Mixed astigmatism involves one meridian being nearsighted while the other meridian is farsighted. Presbyopia occurs as the individual ages and the soft crystalline lens of the eyes loses its flexibility and starts to harden. This loss of flexibility results in the inability to focus light accurately, so nearby objects tend to appear more blurry.

Diagnosis and Treatment

Astigmatism is diagnosed through routine eye exams by an optometrist or ophthalmologist. Eye doctors perform a series of tests that involve having the patient read letters and numbers on a chart as well as using a keratometer to measure the amount of curvature to the surface of the cornea. Doctors may use a keratoscope device that includes a video camera to map the planes on the cornea’s surface to measure the variations on the surface. Usually eye doctors conduct a retinoscopy test that includes switching back and forth between various lenses while examining the eye with light shining at it.

From there, the doctor can determine the prescription to correct the blurred vision with noninvasive methods of prescribing either prescription glasses or contact lenses, or orthokeratology, a process that involves wearing rigid contact lenses to reshape the curve of the cornea. On an eyeglass prescription, cylinder or CYL indicates the lens power for astigmatism. Cylinder refers to the lens power so that one meridian has no added power to correct the lens curvature while the other meridian has the maximum power. If the individual decides to forgo the eyeglass methods, toric contact lenses are another but a more expensive means of correcting astigmatism. Invasive methods include eye surgery, laser in situ keratomileusis, or LASIK, which utilizes a laser to remove small amounts of tissue from the inner layer of the cornea while the outer layer is folded back during the procedure and then properly replaced after the procedure. Another invasive technique is photorefractive keratectomy (PRK), which corrects the cornea by removing tissue from both the inner and outer layers.

Simi Abraham

See also: Diplopia; Hyperopia; Myopia; Presbyopia; Retina; Visual Fields; Visual System

Further Reading

American Optometric Association. (n.d.). Astigmatism. Astigmatism. Retrieved from http://www.aoa.org/patients-and-public/eye-and-vision-problems/glossary-of-eye-and-vision-conditions/astigmatism?sso=y

Mayo Clinic. (2014). Astigmatism. Treatments and Drugs. Retrieved from http://www.mayoclinic.org/diseases-conditions/astigmatism/basics/treatment/con-20022003