Keratoconus: Causes, Symptoms, Diagnosis, and Treatment – Eye Love Cares
Keratoconus is a vision disorder when the cornea, which is normally round in shape, becomes irregularly shaped and thinner. The tiny collagen fibers that make up the cornea and help maintain its shape become weak and the cornea begins to become deformed.
Out of the five senses, vision is often stated as the sense that people value the most, as it plays an important role in our ability to understand and interact with the world. Despite this commonly accepted understanding of the importance of vision, it is often under-appreciated, as most of us do not consider what life would be like if our vision was impaired or if we lost our ability to see all together.
In this article, we will be focusing on Keratoconus, a vision disorder that affects the cornea, resulting in impaired and blurry vision. We will be discussing the pathology of the disorder, its causes, how it progresses, and different treatment methods currently available to tackle this problem.
Before we can dive into our discussion of keratoconus and keratoconus treatment, we must first understand the anatomy of the eye when it is functioning properly and how the cornea’s dysfunction results in the symptomatology characterized by keratoconus.
Anatomy of the eye
Before we can go directly to understanding the anatomy of the cornea and how it leads to the development of keratoconus, we must have a general understanding of the eye as a whole. Outside of the eyeball itself, there are multiple organs and anatomical structures that play a large role in providing support for the eye. The anatomy surrounding the outside of the eye includes the orbit, eyelids, eyelashes, and extraocular muscles.
The orbit is the bony socket in the skull where the eyeball fits. It is not a single bone but is comprised of multiple bones, including the cheekbones, forehead, temple, and the side of the nose. Surrounding the orbit lies multiple muscles that innervate this space providing movement for not only the eyeball itself but also the blood vessels and nerves associated with the eye.
Most people misunderstand the eyelashes as a physiologically useless structure that only plays a cosmetic role. This is an incorrect perception of the purposes of the eyelids and eyelashes. They act as protective layers for the eye, preventing foreign matter or even extremely bright light from entering the eyes and causing subsequent damage. Eyelashes also help spread tears and lubrication over the surface of the eye as you blink.
As previously mentioned, there are extraocular muscles that surround the outside of the eye. Because there are multiple muscles attached to the eye, they allow the eye to move along different planes of motion and rotate along these different axes. This allows us to have a wider and broader field of vision that isn’t limited by the eyeballs’ ability to move within their sockets.
Without going too far into depth on the anatomy of these extraocular muscles, all you really need to know for the purposes of understanding basic eye anatomy is that there are a total of six extraocular muscles, divided into two classes: the rectus and oblique. The rectus muscles, of which there are four, allow the eye to move up and down and from side to side, towards both the periphery and towards the nose. The two oblique muscles act along a different plane of motion compared to the rectus muscles, acting along the sagittal plane, allowing the eye to rotate along this axis.
Now we will slowly dive deeper into the anatomy of the eye itself that is attached to these extraocular muscles. The first layer of the actual eye that we will discuss is the conjunctiva. The conjunctiva is a transparent membrane that not only covers the surface of the eye but also protects it. It is made up of two portions: the bulbar and palpebral conjunctiva.
The bulbar conjunctiva covers the anterior portion of the sclera. The bulbar conjunctiva stops where the sclera and cornea meet one another, and it is important to note that it does not cover the cornea. The palpebral conjunctiva covers the inner portions of both eyelids on each eye. These two palpebral conjunctivae are continuous. The continuous nature of these membranes is the reason why it is impossible for anything to get caught behind your eye, such as your contact lens. The purpose of these layers is to keep the front of the eye, and inner surfaces of the eyelids, well lubricated. Without this lubrication, we would experience a lot of irritation and possible damage from the eye moving within the socket.
There are two smaller anatomical structures that will be briefly mentioned here: the lacrimal gland and the Tenon’s capsule. The lacrimal gland produces tears while the Tenon’s capsule is another layer that lies between the conjunctiva and the surface of the eye.
Next is the sclera, the white layer of the eye, which surrounds the cornea. It is more commonly referred to as the “white part of the eye.” The sclera takes up a large majority of the total surface area of the eye, comprising more than 80 percent. It begins from the first point in which light enters, the cornea, and wraps behind all the way to the optic nerve at the back of the eye where the visual signals are sent to the brain to be processed. It is a dense layer of connective tissue with interwoven layers of collagen.
So, what does the sclera do? It helps maintain the round shape of the eyeball and intraocular pressure (IOP). The sclera protects the eye from serious damage through the multiple layers of collagen. The extraocular muscles also connect to the sclera to control the finer movements of the eye.
Now, we have finally reached the cornea. The cornea is the first point through which light enters the eye. It lies in front of the iris and pupil. If the eye is considered an anatomical version of a camera, it is the lens. It may appear clear in nature, but it plays multiple roles. Although it lacks blood vessels, it is able to receive nourishment from tears. The cornea is organized into five different layers: the corneal epithelium, Bowman’s layer, corneal stroma, Descemet’s membrane, and corneal endothelium.
The corneal epithelium is the outermost layer of the cornea and comprises around 10 percent of the cornea’s total thickness. It has two main functions. It protects the eye from foreign materials and also acts as a surface through which oxygen and nutrients from the tears can be absorbed. Oxygen and nutrients are distributed to the other layers of the cornea through this layer.
The Bowman’s layer is an extremely thin layer made up of collagen that serves as a connective layer between the corneal epithelium and stroma.
The stroma is the thickest layer of the cornea and makes up around 90 percent of the cornea’s total thickness. This layer is responsible for the cornea’s strength, elasticity, and shape. It is made up of water and collagen. The cornea appears to be transparent in nature because the collagen fibers are arranged in a uniform manner.
Similar to the Bowman’s layer, the Descemet’s membrane acts as another protective layer between the stroma and corneal endothelium. It is also made up of collagen fibers, but the distribution and composition of the collagen fibers differ from the Bowman’s layer.
The corneal endothelium is the innermost layer of the cornea. The posterior end of this layer is bathed in a clear fluid called the aqueous humor. The aqueous humor lies in the space between the cornea and iris and pupil. Endothelial cells help keep the cornea clear by pumping the excess fluid out of the stroma. They are responsible for this role as this fluid leaks slowly. Without this, the stroma would swell with fluid and lose its clear nature.
It was previously discussed that the cornea acts as the human’s outermost “lens.” In fact, the cornea is responsible for around 75 percent of the eye’s ability and power to focus. Light is bent, or refracted, from the cornea onto the lens, which then further focuses light onto the retina.
Now we will discuss the remaining intraocular anatomy simply to understand how the eye is able to transform light.
There are two fluid-filled spaces called the anterior and posterior chambers that lie behind the cornea. The anterior chamber is immediately behind the cornea until the front of the iris. This is filled with fluid that provides nourishment to both the cornea and lens. The posterior chamber is behind the lens and is filled with vitreous humor.
The uvea is the middle-pigmented layer of the eyeball, made up of three different components: the iris, ciliary body, and choroid. The iris is a thin, circular structure that surrounds the pupil. The iris is the layer responsible for your eye color, which is based on the amount of pigment. The iris also controls the size of the pupil. Two muscles are responsible for helping the iris constrict in bright light and dilate in the dark.
The ciliary body holds the lens in place. It surrounds the iris but cannot be seen as it is behind the sclera. It also secretes the aqueous humor, the fluid which lies in the anterior chamber. The choroid is the innermost layer of the uvea, located between the sclera and the retina. Its role is to provide nourishment to the retina. It contains numerous tiny blood vessels.
Ironically, the pupil, which is well known as an important part of the eye, is an open space. The pupil serves as an opening in the center of the iris through which light can enter the eye. This is so that the light can be further focused on the retina and allow us to visualize the world. Unlike the iris, the pupil appears black in color. This is because the light that passes through the pupil is not reflected back by the retina and is rather absorbed. It works hand in hand with the iris, and together, they control how much light enters the eye. Using the previous camera analogy, the pupil acts as the aperture and the iris acts as the diaphragm which controls the size of the aperture. The size of the pupil is also controlled by the muscles of the iris.
Directly behind the pupil is the lens. As previously mentioned, it is held in place by the ciliary bodies. The lens acts to further bend light, allowing it to better focus light onto the retina.
The vitreous cavity makes up a large portion of the back of the eye, filled with a fluid called the vitreous humor. This fluid nourishes the inner structures of the eye. This cavity is the portion of the inner eye behind the lens that leads to the retina.
The retina is the anatomical structure that allows us to essentially see. It is a layer of nerve cells lining the back portion of the eye where raw light stimuli are converted into signals that the brain interprets, resulting in our sense of vision. It is composed of multiple layers of millions of specialized photoreceptor cells packed closely together. Humans have two different types of photoreceptor cells that play different roles in our vision: rods and cones.
Rod photoreceptors detect motion and provide black and white vision. Because they allow for black and white vision, they are able to function fairly well in low light. They are located throughout the retina, and we have a much larger number of photoreceptor cells compared to cone cells.
Cone photoreceptors are responsible for color and work best in medium and bright light. They are also responsible for central vision. Because of the varying frequencies in wavelength within the electromagnetic spectrum that we can see which represent different colors, humans have three types of cone photoreceptors that vary in specific sensitivity to these different wavelengths. Unlike rods, cones are concentrated into a small localized area in the middle of the retina called the macula. There is another central small portion of the macula called the fovea where only cone photoreceptors lie. Because of this density of cone cells in the fovea, maximum visual acuity and color vision occurs here. It is estimated that each retina has around 120 million rods and 6 million cone photoreceptors. Each of these photoreceptor cells contains an axon.
The retina is made up of ten different layers, and we will discuss them from the innermost layer, closest to the vitreous, and moving outwards toward the sclera.
- The inner limiting membrane is the layer between the vitreous body and retina.
- The nerve fiber layer, which contains the optic nerve fibers, the ganglion cell axon fibers.
- The ganglion cell layer contains the nuclei of the ganglion cells and the retinal ganglion cells.
- The inner plexiform layer contains the synapses between the dendrites of the ganglion and amacrine cells and the axons of the bipolar cells.
- The inner nuclear layer houses the nuclei of the horizontal, bipolar, and amacrine cells.
- The outer plexiform layer has the axons of the rod and cone cells and the dendrites of the horizontal and bipolar cells.
- The outer nuclear layer houses the rod and cone cell bodies.
- The outer limiting membrane separates the cell nuclei of the photoreceptors from their inner portions.
- The inner and outer segments of photoreceptors cells, both rods, and cones are located in the rod and cone layer.
- Lastly, the pigment epithelium is the outermost layer of the retina. The Retinal Pigment Epithelium (RPE), which is a layer of cells located deep in the retina, helps to maintain the function of photoreceptor cells by processing Vitamin A, absorbing light, and transporting nutrients between the photoreceptor cells.
For the purposes of this article, it is important to know that there are numerous types of other photoreceptor cells and supporting neural cells that play important roles in our ability to not only see but ensure that signals are transmitted properly. The visual signals then travel to the brain through the optic nerves, allowing us to understand different visual information. For more details, please refer to our previous article on eye anatomy.
Keratoconus – What Is It?
Now that we have an understanding of the general anatomy, we can discuss keratoconus and how it affects the cornea. Keratoconus is a vision disorder when the cornea, which is normally round in shape, becomes irregularly shaped and thinner. The tiny collagen fibers that make up the cornea and help maintain its shape become weak and the cornea begins to become deformed.
Due to the deformation of its shape, the cornea becomes dysfunctional, and light is not able to properly enter into the eye or be focused correctly. This results in a distortion of vision. It usually affects both of your eyes, although the severity of symptoms may differ between each eye, which is called asymmetric development. This means that one eye might be noticeably worse than the other although both are affected. For most people, the disorder begins in younger people, typically in your late teens and early 20s. The symptoms may continually get worse in severity over the course of 10 to 20 years after which the condition’s progression slows down.
In the earlier stages of developing keratoconus, you might experience slight blurring or distortion of your vision. You might also notice an increased sensitivity to light and glare. As previously mentioned, the cornea has two main functions: acting as a protective layer from foreign material and also acting as the eye’s outermost lens.
The development of an abnormal cone-shape of your cornea changes your eyes’ ability to focus light appropriately onto the retina. You might also notice that you have difficulty seeing at night with poor night vision. Although it is rare, some people may notice that they are experiencing double vision, known as diplopia, or multiple images or partial images, that look similar to ghost images. You may experience a sudden loss of visual acuity as well.
As the condition develops, worsening vision may result in other conditions, including nearsightedness or myopia. In extremely rare cases, some individuals may develop a corneal blister when the cornea thins too much. This causes subsequent swelling of the cornea due to a fluid accumulation in the various layers of the cornea and can result in pain, redness, and even scarring of the cornea.
Causes of Keratoconus
Unfortunately, the underlying mechanisms responsible for causing keratoconus are not well known. Researchers have suggested that the majority of cases studied up until now have been for unknown reasons and appear to have occurred sporadically. There are some suggestions that there might be an underlying familial link to the condition, but it is currently thought that the condition occurs from a multifactorial model, including genetics, environment, and other factors.
There is currently research being done to identify any specific genes that might be linked with the development of keratoconus. Researchers believe that even if you might have these unidentified genes, it does not mean you will automatically develop the condition. These genes must be activated through certain circumstances, including environmental factors, such as contact lens use, consistent eye-rubbing, allergic predisposition, or other factors. But it is currently unknown if there is a specific cause underlying the development of keratoconus.
Diagnosis of Keratoconus
Keratoconus can be diagnosed based on a complete medical history of the patient, including familial history, and a thorough physical examination of the eye. This examination may include multiple components, including evaluation of the external appearance of the eyes, a visual acuity test, assessment of eye movements, and other procedures.
Sometimes physicians utilize corneal topography, which is an imaging test used specifically for the front of the eye, to aid their diagnosis. Corneal topography uses an instrument to shoot a series of light rings directly onto the surface of the cornea. These light rings are reflected back, and the instrument is able to record this data to identify if there are any changes in the shape and structural stability of the cornea.
Fortunately, keratoconus can be treated. The treatment suggested by your physician will vary based on the severity of your condition and how quickly it is progressing. For those with mild symptoms, treatment may simply require the use of glasses or contact lens. However, because the condition continually worsens, individuals suffering from keratoconus may need to change their glasses frequently in order to accommodate their changing vision.
Once it has developed to a certain point, you may have to get prescriptions for more rigid contact lenses or even large scleral contacts. In cases where the condition has progressed to advanced, a cornea transplant may be necessary. Although it might seem silly if you notice that you are consistently having to rub your eyes repeatedly and begin noticing that your vision is starting to become blurry, seek the immediate consultation of your eye doctor. The earlier you can diagnose the condition, the greater the likelihood of you being able to minimize its progression and deal with the symptoms more easily.
There are some preventative procedures that are being used that have not been approved by the FDA to treat keratoconus. One of these procedures is called corneal collagen cross-linking, CXL. Although CXL has not been approved, there is significant clinical evidence to suggest the efficacy of the procedure and it is being used in the United States as well as in other countries. The procedure involves the use of ultraviolet A (UVA) light and riboflavin, one type of B vitamin. The use of these stimuli can help slow or even halt the progression of keratoconus. Riboflavin is first placed on the cornea through the use of eye drops, and this substance allows the cornea to absorb the UVA light. This procedure creates new bonds within the collagen of the cornea, strengthening the weakened collagen and helping prevent the progression of the condition. Although this procedure has potential, it is not able to return the cornea to a completely normal state. There are other similar procedures that can help you if you are suffering from keratoconus.
This article focused on helping inform you about keratoconus, understanding how it affects the cornea and how this leads to the development of symptoms. Obviously, keratoconus is not a life-threatening condition; however, its symptoms can be uncomfortable and affect your life.
As more research is being done to better understand the condition, it is important that you stay proactive on maintaining the health of your eyes to ensure that you keep your sense of vision working properly. You want to be able to enjoy the sense of vision without any complications, as it allows us to better understand the world.
As with all of our articles, please do not use this article to self-diagnose or treat yourself or anyone else. If you are ever concerned or feel that you might be at risk for developing the condition, please speak with your eye care provider to obtain a proper consultation and diagnosis.