• trauma

• non-traumatic red eye.

Orbit

The orbit is a large bony socket that contains the eyeball or globe with its associated muscles, nerves, blood vessels, fat and most of the lacrimal apparatus. Each of the two orbits is roughly pyramidal in shape with the apex lying posteriorly. The orbit is made up of seven individual bones: the maxilla, palatine, zygoma, sphenoid, frontal, ethmoid and lacrimal bones.

Eyelids

The lids are layered structures covered on their outer surfaces by skin and on their inner surfaces by conjunctiva (Fig. 31.2). In between is subcutaneous tissue, the orbital septum of which thickens within the lids to form fibrous tarsal plates that give structure to the lid. The upper lid contains the levator muscle and the lower contains the inferior tarsal muscle, which retracts it. The lids are maintained in position by the medial and lateral canthal tendons that attach to the periosteum. The lids are closed by the orbicularis muscle.

Within the lid structure are a number of glands. Tarsal or Meibomian glands are arranged perpendicular to the lid margin on the conjunctival surface of the tarsal plate; when blocked and infected, these are known as chalazia (singular, chalazion). The eyelashes are more numerous on the upper lid than on the lower. Sebaceous and modified sweat glands open into each lash follicle – infection produces a hordeolum or stye.

Behind the lashes is the join between the conjunctiva and the skin of the lids. This is known as the grey line because of its relative avascularity. The lids protect the eye by preventing contact with foreign bodies and by preventing drying of the cornea and conjunctiva. Lid closure and blinking help to spread the tear film over the front of the eye and move it into the lacrimal drainage apparatus.

Lacrimal system

The tear film is composed mainly of watery fluid from the lacrimal gland (99 %), which is situated in the lacrimal fossa of the frontal bone in the orbit. The other important components of the tear film are mucin from the conjunctival goblet cells and oil from the Meibomian (tarsal) glands and the glands of Möll and Zeiss. The tear film is distributed over the surface of the eye by gravity, capillary action of the puncta and canaliculi and the eyelids. The tears leave the eye by evaporation and by way of the puncta, the upper of which takes around 30 % of the unevaporated tears and the lower around 70 %. From the puncta, the tears flow into the canaliculi, into the common canaliculus and then into the lacrimal sac and through the nasolacrimal duct (Fig. 31.3).

Conjunctiva

The conjunctiva is a thin, transparent mucous membrane lining the inner surface of the eyelid (palpebral conjunctiva), reflecting back on itself at the upper and lower fornices and covering the sclera as far as the corneoscleral junction (bulbar conjunctiva). The conjunctiva is adherent to the lid and rather less so to the Tenon’s capsule overlying the sclera. It is most adherent at the corneoscleral junction (limbus). The conjunctiva is quite mobile in the fornices and over the globe and can absorb a large volume of fluid and become oedematous. The epithelium of the conjunctiva is continuous with the corneal epithelium. It contains goblet cells which secrete mucus. The main body of the conjunctiva is connective tissue housing blood vessels, nerves and other glands.

Cornea

The transparent cornea forms the anterior one-sixth of the globe. Its curvature is higher than that of the rest of the globe and it is the main structure responsible for the refraction of light entering the eye. It is an avascular structure which is nourished by the aqueous humour, the capillaries at its edge and from the tear film. Microscopically, it consists of five layers:

Sclera

Sclera forms the posterior five-sixths of the eye. It is 1 mm thick posteriorly and thinnest (0.3 mm) immediately posterior to the insertion of the recti muscles. The sclera forms the ‘white’ of the eye; its outer surface is smooth, except for where the six orbital muscles are attached. It is perforated posteriorly by the optic nerve at an area known as the lamina cribrosa. In this area, the sclera forms a meshwork rather than a solid structure to allow nerve fibres and the central retinal artery and vein to pass through it. The sclera is weakened at this point. Raised intraocular pressure can make the lamina cribrosa bulge outwards, producing a cupped disc.

The sclera is composed of two main layers: the episclera, a loose connective tissue that provides most of the nutritional support of the sclera via a vascular plexus; and the main body of the sclera, which is a dense fibrous tissue that is relatively avascular. The function of the sclera is to protect the intraocular contents and preserve the shape of the globe, maintaining the placement of the optical system. It provides the insertion for the muscles.

The uveal tract

The uveal tract is composed of the iris, the ciliary body and the choroid. The iris is a thin-pigmented diaphragm with a central aperture or pupil. It is located between the cornea and the lens. The pupil varies in size from 1–8 mm and differs in size on the two sides in 25 % of ‘normal’ people. The iris divides the anterior segment into anterior and posterior chambers. Its periphery is attached to the ciliary body. The colour of the iris is produced by pigment in melanocytes within its structure. The main body of the iris consists of highly vascular connective tissue; it also contains nerve fibres, the muscle of the sphincter pupillae and the dilator pupillae. The sphincter forms a ring of smooth muscle around the pupil. When it contracts in bright light and during accommodation, the pupil constricts. The dilator is a thin layer of muscle extending from the iris root to the sphincter pupillae. When the dilator pupillae contracts in low-intensity light and during sympathetic activity such as fear, the pupil enlarges.

The ciliary body is continuous with the choroid and the margin of the iris. It contains the ciliary muscle used to change the shape of the lens during accommodation. Its outer, pigmented layer is continuous with the retinal pigment epithelium. Its inner, non-pigmented layer produces aqueous humour. The lens attaches to the ciliary body by a suspensory ligament whose fibres are known as zonules.

The choroid is a thin, soft, brown coat covering the inner surface of the sclera. It extends from the optic nerve to the ciliary body at the ora serrata. The inner surface of the choroid is firmly attached to the pigment layer of the retina. The main body of the choroid is a vascular layer, the choriocapillaris, which supplies nutrition to the external half of the retina and the macula. Its outer layer consists of larger vessels and collecting veins.

The angle and aqueous

The anterior segment of the globe is divided into two chambers. The anterior chamber lies between the cornea and the root of the iris. At the periphery of the anterior chamber is a junction between the cornea, sclera, ciliary body and iris, known as the angle. Within this angle is the trabecular meshwork. The posterior chamber is a slit-like cavity between the back of the iris and the ciliary processes and lens.

Aqueous humour is a clear fluid which fills both of these chambers. It is formed by the ciliary processes of the ciliary body. From the ciliary processes, the aqueous flows through the pupil into the anterior chamber and from there through the trabecular mesh-work into a sinus, the canal of Schlemm. From this structure, it drains into the aqueous veins and into the general circulation. There is a continuous dynamic production and drainage of aqueous which supplies the metabolic needs of the lens and cornea. Pathologically high pressure, such as glaucoma, is usually due to reduced outflow of aqueous and causes damage to the retina.

The lens

The lens is a transparent, biconvex structure situated behind the iris and in front of the vitreous. It is flexible and kept in position by suspensory ligaments attached to the ciliary body. The convexity of its anterior surface is less than that of its posterior surface and it contributes to the refractive power of the eye. The lens consists of a capsule, an epithelial layer on its anterior surface and the lens fibres. The capsule is elastic and encloses the whole lens. The lens fibres constitute the main part of the lens. Epithelial cells change to become lens fibres throughout life. No cells are lost and therefore the centre of the lens becomes denser and less pliable over time. With age, the nucleus becomes dense and yellow; if it becomes opaque, it is known as a cataract.

When in its normal state, the lens is designed to focus light onto the retina. In order to focus on a near object, the lens must become more powerful. Contraction of the ciliary muscle moves the ciliary body forwards. This relieves pressure on the fibres of the zonule and allows the lens to relax and become more spherical. At the same time, the sphincter pupillae contracts, allowing light to enter through the thickest part of the lens. Light from a near source is therefore enabled to focus on the retina. This is known as accommodation and the amount of accommodative power possible reduces as the lens becomes less flexible with age, resulting in the need for ‘reading glasses’ in middle age.

Retina

The retina is the nervous coat of the eye and the internal layer of the globe. It is a thin, transparent membrane, continuous with the optic nerve and extending to the ora serrata behind the choroid. The retina consists of a pigmented layer next to the choroid which absorbs light and releases vitamin A, which is necessary for the functioning of the photoreceptors. The neural retina consists of photoreceptors and then a number of layers of nerve cells which serve to amplify and transmit the impulses from the photoreceptors to the optic nerve and from there to the brain.

Two types of photoreceptor are present within the retina: ‘rods’, which allow vision in dim light and in black and white; and ‘cones’, which are adapted to bright light and can resolve fine detail and colour. Rods are absent at the fovea and rise rapidly in numbers towards the periphery of the retina. Cones are most dense at the fovea and reduce in number towards the periphery. Light impinges on the photo-receptors, producing a chemical reaction which results in an electrical impulse. This is amplified by the various nerve cells and synapses in the neural retina and transmitted through the nerve fibre layer to the optic nerve.

Vitreous

The vitreous body fills the posterior segment of the eye. It is a clear, jelly-like substance consisting of a collagen framework with hyaluronic acid. Collagen fibrils attach the vitreous to the retina at the ora serrata and the optic disc. Its function is to transmit light and to contribute slightly to the resolving power of the eye. It supports the posterior surface of the lens and assists in holding the neural part of the retina in place against its pigment layer.

History

As in any presentation, establishing the exact history of a patient’s condition is fundamental to making an accurate diagnosis. The history of the presenting problem should include:

Discussion of systemic problems and medication is important as it can point to possible ophthalmic problems. For example, there is a link between ankylosing spondylitis and uveitis, and a link between rheumatoid arthritis and dry eyes, and there are many ophthalmic side-effects of systemic drugs. The assessing nurse needs to investigate any pre-existing ophthalmic or other medical conditions. Of particular importance are conditions such as glaucoma, iritis (uveitis) and blepharitis; systemic conditions such as diabetes and rheumatoid arthritis; and any drug therapy, as all of these may affect the health of the eye.

Visual acuity

Assessment of visual acuity should be undertaken at initial assessment for any ophthalmic patient, before any other investigations or treatment, except irrigation or instillation of topical anaesthetic. The patient’s affected or poorer-seeing eye should be tested first, and the other occluded with a card or the patient’s hand. Any distance glasses should be worn. He should be asked to read down from the top of the Snellen chart, making an attempt at all possible letters. Visual acuity should be recorded as:

The number for this line is indicated on the Snellen chart, just above or just below the letters. If part of a line only is read, this may be recorded as the line above plus the extra letters, or the line below minus the missed letters. For example, if the patient reads the ‘12’ line except for one letter, at 6 m, it should be recorded as 6/12 – 1.

If the patient’s vision appears poor (less than 6/9), a pinhole (a small hole in a card or a commercial pin-hole) can be held in front of the eye to negate the effects of any refractive error. The visual acuity should be recorded with and without pinholes and a note should be taken of whether distance glasses or contact lenses are worn. If the patient is unable to read the top letter, the distance should be reduced until the patient can see the top letter on the chart, i.e., 5/60, 4/60, etc. to 1/60. If the patient cannot see the top letter at 1 m, it should be ascertained whether he can count fingers (CF), see hand movements (HM) or just perceive light (PL) at 1 m. Lack of light perception is recorded as NPL. Normal visual acuity is 6/6, but normal visual acuity for the patient may be less for a variety of reasons.

Problems in accurate visual acuity assessment may occur if the patient does not speak English or is not able to read. Strategies to overcome this may include:

Patients who are in pain should have a drop of topical anaesthetic instilled so that any corneal pain is alleviated and the patient can cooperate more fully with the procedure, thus achieving an accurate visual acuity. Patients sometimes feel that this is a test that they have to pass and ‘cheat’ by looking through their fingers etc. It should be explained that the nurse is attempting to obtain an accurate assessment of their vision and that it is important that they are not tempted to make it seem better than it really is.

Examining the eye

Eye examination must be systematic. It is very easy to assume a diagnosis from the history and, in that way, miss less obvious problems. The eye should be examined from the ‘outside’ – the eye position and surrounding structures – working ‘in’ to consider the globe itself. Considerations for a thorough eye examination are given in Box 31.2.

Remember to compare the findings with the normal eye. What appears to be an abnormality may be bilateral and normal for the patient.

Equipment to aid assessment