Section

Preliminaries ONE

Background CHAPTER1 

1.1 Applied anatomy

1.1.1 The cornea

Corneal tissue is transparent and avascular, consisting of three layers and two membranes. It protects the interior ocular structures and contributes 70% of the refractive power of the eye. It has the following average dimensions:

Epithelium

Provides a layer of protective cells. It is enhanced by microvilli, which are prominent irregularities increasing the surface area and providing­ a roughened surface to assist the adherence of the precorneal tear film.

Very minor corneal insult is covered in about 3 hours by neighbouring cells. Larger areas of damage are covered by the migration of cells from all layers of the surrounding epithelium. Lesions close to the limbus show conjunctival cells taking part in the cell migration.

PRACTICAL ADVICE

Newly regenerated epithelium is very susceptible to damage. Lens wear should be suspended for a few days following any significant degree of corneal trauma such as overwear or severe abrasion.

©2010 Elsevier Ltd, Inc, BV

DOI: 10.1016/B978-0-7506-7590-1.00011-X

Section ONE Preliminaries

Bowman’s membrane

The relatively tough anterior limiting layer of the cornea. It consists of a very fine non-orientated fibrillar meshwork. If Bowman’s membrane is damaged, fibrous scar tissue is laid down, resulting in a permanent opacity.

Stroma

An avascular, regular structure which ensures both the mechanical strength and optical transparency of the cornea. Approximately 78% water, it represents about 90% of the corneal thickness. The basic component is made up of the lamellae which consist of collagen fibres that cross each other at various angles while maintaining an overall orientation parallel to the corneal surface. Keratocytes are the main cellular element in the form of flattened dendritic cells which are found in the interface between adjacent lamellae. They are indicators of stress to the tissue if they divide and also if their density diminishes; such changes have been reported as secondary to contact lens wear.1

Descemet’s membrane

The posterior limiting layer of the cornea. It is the basement layer of the endothelium and is elastic in nature.

Endothelium

A single layer of cells in direct contact with the aqueous humour. Its  pump mechanism maintains the cornea’s fluid balance, which is in turn responsible for transparency. No mitosis occurs, but enlargement and spreading of existing cells take place. Irregularity in the size of endothelial cells is termed polymegathism.

Corneal sensitivity

Innervation is by 70–80 sensory nerves entering the epithelium and usually losing their myelin sheaths within 0.5 mm of the limbus. The sensitivity of the cornea is greatest centrally and in the horizontal meridian. It reduces towards the vertical and is least at the periphery. Conjunctival sensitivity increases from a minimum at the limbus towards a maximum at the fornix and lid margins.

Corneal sensitivity reduces with age and with contact lens wear. The first indication of hypoxia is a drop in corneal sensitivity, although clinically this may not be evident until there is a significant and measurable decrease. Sensitivity varies in women during the menstrual cycle and there is also a diurnal variation, being greatest in the evening.

1.1.2 The conjunctiva

A mucous membrane, continuous with the corneal epithelium. It is divided into a bulbar portion, which covers the anterior sclera, and a palpebral portion,

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Background 1 Chapter

which lines the tarsal plate of the eyelids. The conjunctival glands or goblet cells secrete the mucoproteins found in the tears.

1.1.3 The eyelids

The orbicularis oculi muscle makes up almost one-third of the eyelid thickness. Behind lies the tarsal plate which consists of dense fibrous tissue. Closure on blinking is produced by relaxation of the levator muscle which is also responsible for the partial closure of the palpebral aperture on downward gaze.

The openings to the sebaceous meibomian glands lie in a single row along the lid margin. There are about 25 in the upper lid and 20 in the lower, and they are best observed by eversion. The meibomian glands if blocked reduce the  lipid content of the tears and give rise to meibomian gland dysfunction (MGD) (see Section 6.2.3).

PRACTICAL ADVICE

•Several contact lens problems relate to the eyelids so that lid eversion and thorough examination are essential prior to fitting.

•Lid eversion is an indispensable skill which the practitioner needs to develop.

•Meibomian gland dysfunction and blockage can contribute to dry-eye symptoms. These glands should always be investigated if patients report symptoms of dryness.

•Infection of meibomian glands causes styes or cysts.

•The average blink rate is about once every 5 seconds.

1.1.4 The tear film

Functions

•Maintains a smooth optical surface over the cornea.

•Keeps the surface of the cornea moist.

•Acts as a lubricant for eyes and lids on blinking.

•Provides bactericidal action to protect corneal epithelium.

•Removes foreign bodies.

Composition

•An outermost oily, lipid layer secreted by the meibomian glands. Helps prevent evaporation.

•A central aqueous phase produced by the lacrimal gland and accessory glands of Krause and Wolfring.

•A mucoid layer, covering the epithelium, secreted by the conjunctival goblet cells.

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Section ONE Preliminaries

The tear film is approximately 0.7  m in thickness and about 90% of its volume is contained in the tear prism along the lid margin. The preocular tear film is adversely affected by the presence of a contact lens.

1.2 Applied physiology

A contact lens effectively occludes the cornea from its normal environment of oxygen, tears and ocular secretions. The effect depends upon lens thickness, size, method of fitting and material.

In this context, the following definitions are used:

•Anoxia occurs where no oxygen is present.

•Hypoxia occurs where there is reduced oxygen supply to the ocular tissues.

•Hypercapnia is the accumulation of carbon dioxide.

1.2.1 Corneal metabolism

Constant metabolic activity in the cornea maintains transparency, temperature, cell reproduction and the transport of tissue materials. The main nutrients needed for these functions are glucose, amino acids and oxygen. Glucose and amino acids are provided by the aqueous humour, whereas oxygen is mainly derived from the atmosphere via the tears.

Each layer of the cornea consumes oxygen at its own rate:

Epithelium 40%

Stroma 39%

Endothelium 21%

Oxygen enters the cornea from both surfaces so that there is minimum tension in the stroma. Oxygen tension is the driving force that moves oxygen into the cornea. At sea level, it is 155 mmHg for the open eye. Oxygen is supplied to the closed eye mainly by the palpebral conjunctiva, where the tension is about 55 mmHg.2

Corneal swelling as a result of anoxia can be explained by biochemical theory.2 In simple terms, there is not enough oxygen available to convert the glucose by means of glycolysis into sufficient energy and allow the waste product, lactic acid, to diffuse quickly out of the tissue. Less energy is therefore available for cellular activity, more lactic acid is produced and this builds up in the  stroma. Sufficient osmotic pressure is created to allow water to be drawn into the stroma faster than the endothelial pump can remove it, and so corneal  swelling occurs.

1.2.2 Oxygen consumption

The rate of oxygen consumption appears to vary. The range is 1 to 10 ml/h/ cm2, with different patients having varying oxygen needs.

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Background 1 Chapter

1.2.3 Corneal temperature

The normal corneal temperature of 33–36°C may alter during contact lens wear. The effect becomes more significant under closed eye conditions. The change in temperature may be only 3°C, but the rate of metabolic activity is so dependent on ambient temperature that the fine balance between available oxygen and corneal demands under the closed lid may be stressed by such a small alteration in temperature. Corneal temperature can be measured by thermography (see Section 2.6).

1.2.4 Stromal acidosis

A drop in stromal pH induces a state of acidosis in contact lens wearers as a result of corneal hypoxia and hypercapnia. It appears that hypercapnia accounts for about 30% of the total pH drop which can occur even without a change in corneal thickness.3 Chronic acidosis may explain some of the alterations seen in both corneal structure and function following contact lens wear.

1.2.5 Tear osmolarity

Corneal thickness is also affected by the osmolarity of the tears. In the normal, open eye, the salt content of the tear film is about 10% greater than that of freshly produced tears because of evaporation. When the eye is closed during sleep, there is a shift in tear tonicity from the open eye value of about 0.97 NaCl to 0.89 NaCl following 6 hours’ sleep. This overnight hypotonic shift may be explained by reduced tear evaporation. The cornea responds to the less concentrated solution by drawing water into the stroma faster than it can be pumped out by the endothelium. Hence, on waking, the cornea is found to have increased in thickness by about 4%.4 Deswelling occurs rapidly during the first 2 hours the eyes are open. See Section 6.1 for the measurement of tear osmolarity and its use in practice.

1.2.6 Tissue fragility

Reduced epithelial adhesion is found following contact lens wear. It appears to be directly related to the reduced numbers of hemidesmosomes5 due to loss of basal cell shape and chronic corneal hypoxia following contact lens wear. The hypoxia causes a decrease in the level of metabolic activity including the rate of cell mitosis. Cell life increases and those at the anterior surface of the epithelium may not retain normal functional resistance. As a result of these changes, the overall resistance of the epithelium is lowered and the risk of infection increased. The thickness of the epithelium is found to reduce as cell production rate and wastage reach a new equilibrium. Such thinning has been observed in longterm extended wear patients.

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