Cornea and Tear Film Structure

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Questions and Answers

What are the two most important refractive surfaces of the eyeball?

The cornea and the lens.

How much total refractive power does a normal emmetropic eye have?

60-63 diopters.

Why is the cornea considered more important than the lens in focusing light on the retina?

The cornea has a steeper anterior surface and light passes through air before reaching it, which has a lower refractive index than aqueous humor for the lens.

What is emmetropia?

Emmetropia is the refractive state where parallel rays of light from a distant object focus directly on the retina. Signup and view all the answers

Define ametropia.

Ametropia refers to the absence of emmetropia, where light does not focus properly on the retina. Signup and view all the answers

What is the primary difference between axial and refractive ametropia?

Axial ametropia involves an abnormal length of the eyeball, while refractive ametropia has a normal length but abnormal refractive power. Signup and view all the answers

What characterizes hypermetropia?

Hypermetropia, or far-sightedness, occurs when parallel rays of light focus behind the retina. Signup and view all the answers

What type of lens is required to assist an ametropic eye in focusing on distant objects?

An ametropic eye may require either a diverging or a converging lens. Signup and view all the answers

What is the axial length growth of the eye from birth to age 6 years?

The axial length of the eye grows by approximately 5 mm from birth to age 6 years. Signup and view all the answers

What characterizes pathologic myopia?

Pathologic myopia is characterized by excessive growth in axial length while other eye components grow normally. Signup and view all the answers

At what age does the prevalence of myopia begin to increase?

The prevalence of myopia begins to increase at about the age of 6 years. Signup and view all the answers

What happens to the ciliary muscles during accommodation?

During accommodation, the ciliary muscles contract, which increases the curvature of the lens. Signup and view all the answers

What is presbyopia and when does it typically occur?

Presbyopia is the difficulty with near vision that typically occurs around age 40. Signup and view all the answers

How does the amplitude of accommodation change with age?

The amplitude of accommodation decreases with advancing age. Signup and view all the answers

What is the relationship between hypermetropes and the onset of presbyopia?

Hypermetropes typically experience presbyopia earlier than myopes. Signup and view all the answers

What is the refractive power increase necessary for close vision?

The refractive power must increase by more than 17 D to visualize objects closer than 6 meters. Signup and view all the answers

What happens to the amplitude of accommodation as a person ages?

It decreases with age, from 14D early in life to 1D by age 60. Signup and view all the answers

At what age does the amplitude of accommodation drop to 6D?

At age 36. Signup and view all the answers

What is presbyopia and how does it relate to aging?

Presbyopia is the recession of the near point due to decreased amplitude of accommodation with age. Signup and view all the answers

What are the types of contact lenses mentioned for correcting astigmatism?

Hard contact lenses for high astigmatism and soft contact lenses for low-grade astigmatism. Signup and view all the answers

What is keratoconus and at what age does it typically onset?

Keratoconus is a progressive disorder that typically begins at puberty. Signup and view all the answers

What are the contraindications for LASIK surgery?

Keratoconus, active inflammation, thin cornea, and large diameter pupils with high ametropia. Signup and view all the answers

What surgical method is used for mild to moderate myopia under 7D?

Photorefractive keratectomy (PRK). Signup and view all the answers

How much can LASIK correct up to in terms of myopia?

LASIK can correct up to 15D of myopia. Signup and view all the answers

What are the three layers that compose the precorneal tear film?

The three layers are the superficial oily layer, the middle aqueous layer, and the deep mucin layer. Signup and view all the answers

What is the significance of the cornea's transparency and avascularity?

The cornea's transparency allows for light entry, while its avascularity prevents obstructions to vision and minimizes immune rejection. Signup and view all the answers

How much of the total diopteric power of the human eye does the cornea contribute?

The cornea contributes about 74% or 43.25 diopters of the total diopteric power. Signup and view all the answers

What role does the tear film play in corneal health?

The tear film lubricates the cornea, provides a smooth optical surface, and facilitates the diffusion of oxygen and nutrients. Signup and view all the answers

What is the thickness range of Bowman’s layer in the cornea?

Bowman’s layer is 8-14 micrometers thick. Signup and view all the answers

Discuss the nutritional sources for the cornea.

The cornea receives its nutrition primarily from glucose diffusing from the aqueous humor and oxygen from the tear film and peripheral blood supply. Signup and view all the answers

What sensory features distinguish the cornea from the conjunctiva?

The sensitivity of the cornea is 100 times greater than that of the conjunctiva due to its dense supply of sensory nerve fibers. Signup and view all the answers

Describe the composition and function of the corneal epithelium.

The corneal epithelium consists of stratified squamous epithelial cells and serves as a protective barrier for the cornea. Signup and view all the answers

What are the visual impairments commonly associated with keratoconus?

Myopia and irregular astigmatism are the common visual impairments. Signup and view all the answers

What is Munson sign and how is it related to keratoconus?

Munson sign is the bulging of the lower lid during down gaze, indicating corneal protrusion in keratoconus. Signup and view all the answers

Which diagnostic tool is used to detect irregular reflex during retinoscopy for keratoconus?

Retinoscopy shows an irregular reflex. Signup and view all the answers

What role do rigid contact lenses play in the management of keratoconus?

Rigid contact lenses reshape the cornea to correct higher degrees of irregular astigmatism. Signup and view all the answers

Describe the purpose of corneal collagen cross-linking in keratoconus treatment.

Corneal collagen cross-linking aims to stabilize or reverse ectasia by strengthening corneal tissue. Signup and view all the answers

What visual symptoms may indicate acute hydrops in keratoconus patients?

Acute hydrops may cause sudden decrease in visual acuity, discomfort, and watering of the eye. Signup and view all the answers

What indicates the progressive nature of keratoconus in terms of corneal structure?

Signs of progressive thinning of the cornea and irregular astigmatism signify the disease's advancement. Signup and view all the answers

In which circumstances is keratoplasty considered as a treatment for keratoconus?

Keratoplasty is considered in advanced progressive disease, especially with significant corneal scarring. Signup and view all the answers

Study Notes

Precorneal Tear Film

Composed of three layers: an oily layer, an aqueous layer, and a mucin layer
Oily layer produced by meibomian glands
Aqueous layer produced by main and accessory lacrimal glands
Mucin layer derived from conjunctival goblet cells
Essential for normal corneal function
Lubricates the cornea and conjunctiva, creates a smooth optical surface, allows for diffusion of oxygen and other nutrients

Cornea

Transparent, avascular tissue
Measures approximately 11-12 mm horizontally and 10-11 mm vertically
Located at the center of the anterior pole of the globe
Contributes about 74% (43.25 diopters) of the total diopteric power of the eye
Major source of astigmatism
More sensitive than the conjunctiva (100 times)
Receives nutrition from glucose diffusing from the aqueous humor, oxygen from the tear film, lid vasculature, and aqueous humor
Peripheral cornea also receives supply from limbal circulation.
Composed of six layers: epithelium, Bowman's layer, Stroma, Dua's layer, Descemet's membrane, and endothelium.

Bowman's Layer

Acellular layer located beneath the basal lamina
Composed of collagen fibrils
Secreted by anterior stromal keratocytes and epithelium during embryogenesis
Not replaced and can become opacified by scar tissue following injury

Dua's Layer

15 micrometers thick
Found between corneal stroma and Descemet's membrane

Refractive Power of the Eye

Cornea and lens are the two main refractive surfaces of the eye.
The power of both depends on the curvature of their surfaces.
Total refractive power (converging power) of a normal eye (emmetropic eye) is approximately 60-63 diopters.
The cornea contributes the highest portion (43D), with the lens contributing the remaining 15-20D (in non-accommodative state).
Cornea is more important for focusing light on the retina because:
- It has a steeper anterior surface compared to the lens
- Light passes through air, with a lower refractive index, to reach the cornea
- Light passes through aqueous humor, with a higher refractive index, to reach the lens.

Refractive States of the Eye

Emmetropia: Parallel rays of light from a distant object are focused on the retina when the eye is at rest.
Ametropia: Deviation from emmetropia. This can be axial or refractive.
- Axial ametropia: Eyeball length is abnormal (longer in myopia, shorter in hyperopia).
- Refractive ametropia: Eye length is normal, but refractive power of the cornea and/or lens is abnormal.
Myopia (nearsightedness): Eyeball is too long, so light focuses in front of the retina.
Hypermetropia (farsightedness): Eyeball is too short, so light focuses behind the retina.

Accommodation

Mechanism by which the eye changes its refractive power by altering the shape of the lens.
Contraction of ciliary muscles leads to increased lens curvature and a greater refractive power.
Amplitude of accommodation: Difference between the eye's power with maximum accommodation and the unaccommodated state (rest).
This decreases with age due to atrophy of muscles and loss of lens elasticity.
Presbyopia: Recession of near point with age due to decreased amplitude of accommodation.

Keratoconus

Progressive disorder where the cornea assumes a conical shape.
Onset usually occurs around puberty, with slow progression and eventual stabilization.
Causes progressive and irregular changes in the cornea, making it more convex and thinner.
Presentation:
- Unilateral impairment of vision due to progressive myopia and irregular astigmatism.
- Frequent changes in spectacle prescription or decreased contact lens tolerance.
- Usually affects one eye first, but most cases are bilateral.
Signs:
- Stromal thinning
- Apical protrusion
- Vogt's lines (fine, vertical, deep stromal striae)
- Reduced corneal thickness
- Munson sign (bulging of lower lid in down gaze)
- Acute hydrops (acute influx of aqueous into cornea due to a rupture in Descemet's membrane and Dua’s layer)
Management:
- Spectacles: Early cases with mild irregular astigmatism
- Rigid contact lenses: Higher degree of irregular astigmatism
- Keratoplasty: Advanced cases with significant corneal scarring
- Intracorneal ring segment (Intacs) implantation: Correct abnormal corneal curvature.
- Corneal collagen cross-linking: Stabilize or reverse ectasia in some patients.

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Description

Explore the intricate anatomy of the precorneal tear film and cornea. This quiz covers the composition, function, and importance of these eye structures. Test your knowledge on how these components contribute to visual health and clarity.