Astigmatism - all you need to know
Normal Vision and its Structure
Light focus on the eyes is the basis for Vision. Each eye contains a two-element optical system: The Cornea and The Lens. The Cornea at the front of the eye provides 80% of the focus (like a pair of glasses) and is fixed in place. The Lens changes its curved shape and provides the remaining variable 20% to adjust and focus for different visual differences. The objects viewed project onto the Retina behind which contains the network of neurons and visual receptors which react to the light, trigger electrical signals and by the optic nerve conducts signals to the brain.
The Normal Eye
Parts of the normal eye reveal the Cornea, Lens, Retina, Optic Nerve and other anatomy. Light rays are projected through the Cornea and Lens onto the Retina behind.
What is Astigmatism?
Astigmatism is a common vision problem caused by an error in the shape of the cornea. With astigmatism, the lens of the eye or the cornea, has an irregular curve. This can change the way light passes, or refracts, to your retina. This causes blurry, fuzzy, or distorted vision. Farsightedness and nearsightedness are two other types of problems with the way light passes to your retina. Farsightedness is called hyperopia. Nearsightedness is called myopia (LaFlamme, 2015)
The Astigmatic Eye
The Astigmatic eye shows multiple focal points (rather than a single focal point of the normal eye) and this gives the blurred vision and requires near-sighted or far-sighted correction.
The perception of Vision and how it is processed in the Brain
Visual receptors trigger electrical signals when the light is absorbed by light-sensitive visual pigment molecules and can also determine our ability to see dim light and light in different parts of the visual spectrum. (Goldstein, 2014, p26). The process of Transduction changes one form of energy to another and in this example visual pigments, each having two parts (Opsin and Retinal) and the resulting combined molecule changes shape by a process of isomerization, creating a chemical chain reaction. This amplified effect leads to activation of the receptor (Goldstein, 2014, p27). Most of our vision, however, comes from cones that work under most light conditions and are responsible for acute detail and color vision (Kibiuk, 2012). The human eye contains three types of cones (red, green and blue), each sensitive to a different range of colors and because their sensitivities overlap, cones work in combination to convey information about all visible colors.
The perception and organization of the brain is a process where the environment becomes perceptually grouped to create our perception of objects (Goldstein, 2014, p100). In the early 1900’s the Gestalt psychologists identified “configuration” combines a number of smaller sensations or images to create a more complex perception. Those scene elements as explained by Gestalt become organized by principles. The principle of good continuation, principle of pragnanz or good figure, the principle of simplicity, the principle of proximity, principle of common fate, principle of common region and principle of uniform connectedness all, according to Gestalt predict what we will perceive based on what usually happens in a scene. In effect what is usually observed is correct and what is assumed based on our visual system is organised correctly by our perceptual system.
How does Astigmatism affect your vision?
The two main types of astigmatism are corneal and lenticular. A corneal astigmatism happens when your cornea is misshapen. A lenticular astigmatism happens when your lens is misshapen (LaFlamme, 2015).Symptoms of astigmatism will vary per individual but indications include: blurry, distorted, or fuzzy vision at all distances (close-up and far away), difficulty seeing at night, eyestrain, squinting, eye irritation and headaches. At any time an individual has trouble seeing what is considered normal distance and/or near objects a visit to the Optometrist for an eye test may reveal astigmatism and provide for further resolution and treatment.
Treatment for Astigmatism
Mild cases of astigmatism may not require treatment. Your doctor may treat astigmatism that causes vision problems by using one of the following methods: Corrective Lenses, Orthokeratology (Ortho-K) which is a treatment that uses rigid contact lenses to temporarily correct the irregular curvature of your cornea and requires wearing rigid contact lenses for limited periods of time or refractive surgery (Lasik, PRK, RK) which permanently corrects astigmatism (LaFlamme, 2015). A new procedure from 2015 began termed a CILIARY SULCUS IMPLANTATION of a pinhole intraocular implant is showing promise as a safe and effective method for treating irregular corneal astigmatism (Ophthalmology Times, 2016). This implant provides improved visual field, corrects astigmatism and reduction is dysphotopic symptoms.
Correction of Astigmatism
In this example, astigmatism is corrected by a lens which then refracts the light back into focus (one focal point) at the back of the retina.
Goldstein, E. (2014). Sensation and perception (Ninth ed.). Belmont, CA: Wadsworth, Cengage.
Kibiuk, L. (2012, April 1). Retrieved January 14, 2016, from http://www.brainfacts.org/Sensing-Thinking-Behaving/Senses-and-Perception/Articles/2012/Vision-It-all-Starts-with-Light
LaFlamme, M. (2015, November 25). Astigmatism. Retrieved January 13, 2016, from http://www.healthline.com/health/astigmatism#Overview1
Novel implant may improve irregular astigmatism in pseudophakic eyes. (2015). Ophthalmology Times, 40(16), 26-28 2p.