How the Cornea Keeps Your Eyes Healthy and Vision Clear
The clear, dome-shaped outer layer of the front of the eye is the cornea of the eye. The cornea covers the anterior chamber, the iris, and the pupil beneath it. Unlike most other tissues in the human body, corneal tissues don’t have any blood vessels (except at their margins). It gets its nourishment from the aqueous humor, posteriorly and by tears, anteriorly. Irregularity in the curvature of the cornea leads to astigmatism. Every time we blink our eyes, the tears wipe out the cornea throughout its surface, thereby keeping it moist and resists infection. It is very sensitive to touch and pain.
Composition of the Cornea
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The cornea is composed of five layers -
Epithelium, which is made of epithelial tissues, is regenerative in nature. Any irregularity in epithelium affects the total refractive power of the eye, thus unbalancing visual capability.
Bowman’s layer, which is composed of tightly-woven collagen fibrils, is a 14-micrometer thin layer that protects the corneal stroma.
Stroma, which is a thick collagen layer (comprises almost 90 percent of the cornea) plays a major role in the transparency of the cornea due to its arrangement of fibers.
Descemet’s membrane, the acellular membrane acts as the base layer for the corneal endothelium.
The endothelium, the innermost layer regulates the fluid transport thereby preventing the cornea from being over swollen. They don’t regenerate, rather the existing cells stretch to compensate for the void developed.
Function of the Cornea:
The cornea of the eye contributes mostly to the focusing power of the eye where it refracts the incoming light on the lens and focuses light on the retina; the epithelium provides a surface for optimum moisture and helps in clear and stable vision; the bowman’s layer prevents abrasions and scratches, and the endothelium maintains the fluidity within the cornea.
Corneal Disorders:
The disorders related to the cornea are –
1. Keratoconus
A. Condition – in such cases the collagen fibers of the cornea of the eye weakens and thins out due to which it can’t retain its shape and so, it bulges outwards giving a conical shape.
B. Causes – the exact reasons for keratoconus are unknown but environmental factors and genetics play a huge role in this.
C. Symptoms – a person suffering from keratoconus is likely to face symptoms like
i. blurred vision
ii. sensitivity to light
iii. poor dim light vision
iv. discoloration around the eye
D. Treatment – initially the victim is prescribed glasses when the symptoms are mild. For extensive cases, the prescribed methods are as follows
i. contact lenses which might be used like soft contact lenses, gas permeable contact lenses, hybrid contact lenses, and prosthetic lenses.
ii. Corneal cross-linking
iii. Corneal transplant
2. Corneal Arcus
A. Condition – in such cases the cornea is deposited with cholesterol and phospholipid at its periphery which forms a faint blue or grey ring.
B. Causes – due to decreased lipid metabolism, lipid deposits at cornea’s edge. Also, individuals with high cholesterol build up cholesterol crystals both in the central and peripheral cornea.
C. Symptoms – a person suffering from corneal arcus is likely to face symptoms involving
i. The arc sharpen at the outer border and starts fainting inwards
ii. The arc might grow to form a full ring, thereby covering the entire iris
D. Treatment – corneal arcus is generally not a matter of concern for victims aging above the 50s but if below that, chances are the victim has a high cholesterol level and needs an immediate check.
3. Corneal Dystrophy
A. Condition – it is a rare hereditary condition where abnormal material or foreign substances accumulate in the layers of the cornea progressively. There are primarily three types of dystrophies
i. Superficial or anterior corneal dystrophy
ii. Stromal corneal dystrophy
iii. Posterior corneal dystrophy
B. Causes – due to genetic mutations, transcription of aberrant protein occurs in the cornea.
C. Symptoms – the symptoms are as follows
i. Corneal erosion
ii. Blurry vision or loss of vision
iii. Sensitivity of light or glare
iv. Pain in the eye
D. Treatment – initially the doctor prescribes eye drops or ointments. In severe cases, corneal transplantation is required.
Solved Examples:
1. Is a Corneal Transplant Painful?
Ans: Generally, before the transplant surgery, eye drops and anesthesia are dosed to relax and numb the eye. During the surgery, there is a negligible sensation of pain but slight pain might be felt after the effect of anesthesia is over. Doctors prescribe eye drops for pain relief.
2. What is Corneal Blindness?
Ans: When the cornea is covered due to any reason, light is unable to pass through it to the light-sensitive retina and the victim is unable to see. This condition is called Corneal blindness.
Conclusion:
The cornea of the eye can repair itself from minor abrasions but in severe cases, it might lead to the damaging of the corneal tissues which affects its transparency and the visual capability of the person.
If neglected, corneal disorders can lead to eye pain and loss of vision in the victim. Hence, consulting a doctor at the earliest is advisable.
FAQs on Cornea: Anatomy, Function, and Disorders
1. What is the cornea and where is it located in the eye?
The cornea is the transparent, dome-shaped outer layer at the very front of the eye. It is positioned directly over the iris (the coloured part of the eye), the pupil (the black dot in the centre), and the anterior chamber. It serves as the eye's primary protective barrier against dirt, germs, and other foreign particles.
2. What are the main functions of the cornea?
The cornea has two primary functions that are critical for vision:
- Refraction: The cornea is responsible for about 65-75% of the eye's total focusing power. Its curved surface bends, or refracts, the light that enters the eye, directing it towards the lens, which then fine-tunes the focus onto the retina.
- Protection: It acts as a shield, protecting the sensitive inner structures of the eye from physical harm, UV radiation, and pathogens.
3. What are the different layers that make up the cornea?
The cornea is composed of five distinct layers, each with a specific role:
- Epithelium: The outermost layer that provides a smooth surface for the tear film and blocks foreign material. It has a remarkable ability to regenerate quickly if injured.
- Bowman's Layer: A tough, transparent sheet of collagen that lies just beneath the epithelium, providing structural integrity.
- Stroma: The thickest layer, making up about 90% of the cornea's thickness. It consists of uniformly arranged collagen fibres that are crucial for the cornea's strength and transparency.
- Descemet's Membrane: A thin but strong inner layer that serves as a protective barrier against infection and injury.
- Endothelium: The innermost layer, which functions as a pump to maintain the correct fluid balance within the stroma, keeping the cornea clear and transparent.
4. Why is the cornea transparent and avascular (lacking blood vessels)?
The cornea's transparency is essential for clear vision and is maintained by two key factors. Firstly, the collagen fibres in the stroma are arranged in a precise, lattice-like pattern that allows light to pass through without scattering. Secondly, the cornea is avascular, meaning it has no blood vessels. Blood vessels would obstruct and scatter light, making vision cloudy. Instead, the cornea receives its nourishment from the tears on its outer surface and the aqueous humour in the anterior chamber behind it.
5. How does the cornea's dome shape contribute to vision?
The cornea's fixed, curved dome shape acts like a powerful primary lens for the eye. This curvature is precisely what allows it to bend incoming light rays significantly—a process called refraction. This initial, strong refraction is a critical first step in focusing an image. The lens, located behind the cornea, then makes finer adjustments to bring the image into sharp focus on the retina. Without the cornea's specific curvature, light would not be focused properly, leading to blurry vision.
6. How does the cornea's function differ from that of the lens and iris?
While all three are crucial parts of the eye, they have distinct roles:
- Cornea: Provides the majority of the eye's fixed focusing power. Its shape does not change. It also acts as the primary protective barrier.
- Lens: Provides adjustable focusing power. It changes its shape (a process called accommodation) to fine-tune focus for objects at varying distances, from near to far.
- Iris: It does not focus light at all. Instead, it functions like the aperture of a camera, controlling the size of the pupil to regulate the amount of light that enters the eye.
7. What are some common corneal disorders and their symptoms?
Several conditions can affect the cornea. Common disorders include:
- Keratitis: Inflammation of the cornea, often caused by infection. Symptoms include severe pain, redness, and blurred vision.
- Keratoconus: A progressive disease where the cornea thins and bulges outwards into a cone shape, causing distorted vision.
- Corneal Ulcer: An open sore on the cornea, typically from an infection, which can cause intense pain, discharge, and a white spot on the cornea.
- Corneal Dystrophies: A group of genetic disorders where abnormal material accumulates in the cornea, leading to a loss of clarity over time.
Common symptoms across many corneal issues include pain, blurred or cloudy vision, light sensitivity (photophobia), and the sensation of having something in the eye.
8. Why is a corneal transplant often successful with a low risk of rejection?
A corneal transplant, or keratoplasty, has a high success rate primarily due to the cornea's immune privilege. Unlike most organs in the body, a healthy cornea is avascular (it has no blood vessels). This is significant because immune cells, which are responsible for identifying and attacking foreign tissue (causing organ rejection), travel through the bloodstream. Since these cells cannot easily reach the transplanted corneal tissue, the risk of an immune rejection response is significantly lower than in transplants of other organs like the kidney or liver.
