Chapter 9
The Uveal Tract
Introduction
The uveal tract comprises the middle vascular pigmented layer of the eye. It is composed of three areas:
•the choroid, which forms the posterior five-sixths
•the ciliary body
•the iris.
These two latter structures (Fig. 9.1) together form the anterior one-sixth.
The choroid
The choroid lies between the sclera and retina and extends from the optic nerve forwards to the ora serrata where it joins the ciliary body. It is composed of four layers:
•the suprachoroid, containing pigment cells, elastic tissue and collagen
•the vascular layer, comprising large and small blood vessels with pigment cells contained in the stroma surrounding the vessels; the large vessels are mainly veins
•the choriocapillaries, comprising fenestrated capillary vessels
•Bruch’s membrane, which is a barrier with fenestrations which allow nutrients through to the underlying retina; it is also a supportive membrane.
Function of the choroid
The function of the choroid is to provide nourishment to the outer layer of the underlying retina.
Blood supply
The blood supply and drainage is via the short posterior ciliary artery; and the choroidal and vortex veins.
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Epithelium and |
Cornea |
Descemet and |
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endothelium |
Pigment |
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Bowman's |
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membrane |
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Iris |
layer |
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Conjunctiva
Canal of Schlemm
Trabecular
meshwork
Episcleral
vessels
Zonule
Capsule Lens
Ciliary body
Ciliary epithelium
Fig. 9.1 Section through the iris and ciliary region.
Nerve supply
The posterior ciliary nerve from the oculomotor nerve provides the nerve supply to the choroid.
The ciliary body
The ciliary body is a triangular structure lying between the choroid and the iris, being 6 mm wide. It has three areas:
•The pars plana is the posterior aspect lying next to the ora serrata and is 4 mm wide.
•The pars plicata is the area which lies between the pars plana and the iris and is 2 mm wide. It contains 70–80 radiating strips, the ciliary processes. These processes are composed of vascular tissue, mainly veins and capillaries. They are 2 mm long and 0.5 mm wide. Their function is to produce and secrete aqueous which fills the posterior chamber and then flows through the pupil into the anterior chamber. The zonular fibres or suspensory ligaments, which hold the lens in place, originate in the valleys formed by the processes.
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•The ciliary muscles lie in the anterior section of the ciliary body, underneath the sclera. The ciliary muscles are known as the muscles of accommodation. They contract and relax to change the shape of the lens so that light rays can be brought to a focus on the retina when looking at objects at varying distances. When the ciliary muscles contract, the zonules relax and decrease the tension on the lens capsule. The lens thus becomes more spherical and light rays can be focused on the retina for near vision. When the ciliary muscles relax, the zonules tighten and there is increased tension on the lens capsule. The lens thus becomes less spherical and light rays are focused on the retina for distance vision.
Blood supply
The blood supply to and drainage from the ciliary body is via:
•anterior ciliary arteries and veins
•long posterior ciliary arteries and veins
•vortex vein.
Nerve supply
The nerve supply is through the short ciliary nerve from the oculomotor nerve.
The iris
The iris is the coloured circular diaphragm situated behind the cornea and in front of the lens. It is attached at its periphery to the ciliary body. The pupil is the aperture in the middle of the iris. The iris forms the posterior wall of the anterior chamber and the anterior wall of the posterior chamber. There are two zones:
•the ciliary zone on the periphery
•the pupillary zone on the central aspect.
Three layers of the iris
•the endothelium
•the stroma containing connective tissue, pigment cells, blood vessels, nerves and muscles
•pigment epithelium which is an extension of the pigment epithelium of the retina (note that the epithelium of the iris is situated at the back of the structure).
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Muscle of the iris
There are two muscles in the iris, whose actions are either to constrict or dilate the pupil:
•The sphincter muscle is a circular muscle lying around the pupillary zone. This muscle constricts the pupil. It is served by the short ciliary nerve of the oculomotor nerve.
•The dilator muscle is a radial muscle lying under the pigmented layer of the iris. As its name indicates it is the muscle that dilates the pupil and is supplied by the long ciliary nerve from the nasociliary nerve, the third branch of the ophthalmic division of the trigeminal nerve.
The sphincter muscle is more powerful than the dilator muscle, so if both muscles are equally affected by intra-ocular inflammation the pupil will tend to constrict. The sphincter muscle and the ciliary muscle both have their nerve supply from the oculomotor nerve, therefore drugs stimulating or paralysing this nerve will affect both dilation and accommodation.
Colour of the iris
The pigment melanin gives the colour to the iris. The colour depends on the amount of pigment laid down in the stroma after birth. This is genetically determined. The pigment in the pigment epithelium is present at birth and is consistent throughout life. This gives newborn babies light-coloured eyes. After a few days of life pigment begins to be laid down in the stroma and the baby’s eyes become darker. The more melanin laid down, the darker the eyes become. All babies are therefore born with light-coloured eyes, despite what some doting parents may say. The amount of pigment produced in the stroma can vary during life, so that the colour of eyes can alter. Dark irises with dense pigment cause the pupil to take longer to dilate following instillation of mydriatics.
Blood supply
The arterial blood supply is via the long posterior ciliary arteries. The capillaries from these arteries anastomose with the anterior ciliary arteries to form the arterial circle of the iris. The venous drainage is through the anterior ciliary veins and vortex veins.
Conditions of the uveal tract
Anterior uveitis or iritis
Anterior uveitis or iritis is inflammation of the iris or iris and ciliary body (see Fig. 9.2). It is usually a recurring condition in which the cause is unknown in 70% of cases.
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Fig. 9.2 Acute iritis.
Causes
Causes of anterior uveitis include:
•ankylosing spondylitis
•Still’s disease or childhood arthritis
•seronegative rheumatoid disease
•ophthalmic surgery
•trauma – perforating injury, corneal foreign body
•corneal ulcer
•sarcoid
•tuberculosis
•syphilis
•ulcerative colitis and Crohn’s disease
•rarely, neovascularisation from diabetes mellitus
•also rarely, heterochromic uveitis: patients with different coloured irises may develop this chronic, progressive condition in which the pigment of the affected eye is dislodged, with the iris becoming gradually paler.
Signs
•The visual acuity may be reduced.
•Ciliary limbal injection.
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•Cornea is usually clear. Keratic precipitates may be present on the posterior surface of the cornea if the inflammation is severe. These are plaques of precipitates from the inflamed iris. In tuberculosis or sarcoid these are particularly marked and are called ‘mutton fat’ because of their appearance.
•The anterior chamber is of normal depth but ‘flare and cells’ may be seen in the beam of the slit lamp. Flares are as a result of exudative protein from the inflamed iris and cells are seen as leucocytes. Inflammatory cells may be sufficient in number to settle and form a hypopyon.
•The iris vessels may be dilated. A nodule may be present if the cause is tuberculosis.
•The intra-ocular pressure may be elevated.
•The pupil is small because the iris muscles are in spasm and the sphincter muscle is the stronger of the two iris muscles. The pupil will be irregular if posterior synaechiae has occurred when the posterior surface of the swollen iris adheres to the anterior surface of the lens.
•There may be cells in the vitreous
•Macular oedema may be present in severe anterior or intermediate uveitis
Patient’s needs
•Relief of symptoms:
pain due to the spasm of the nerves of the iris
photophobia due to irritation of the nerves of the iris
watering eye again due to irritation of the nerves of the iris
reduced visual acuity due to the presence of flare and cells in the anterior chamber.
•Investigation of the cause in recurrent cases, and treatment, if applicable.
•Institution of treatment.
Nursing action
•Dilation of the pupil to prevent posterior synaechiae from forming or to break down any that have formed.
Instil prescribed mydriatic drops, often a ‘cocktail’ will be used, e.g. G. Phenylephrine Hydrochloride 10% and G. Cyclopentolate 1%. These may need to be repeated if synaechiae are present.
The application of heat by means of eyepads soaked in hot water and wrung out then placed over but not on closed lids will enhance the action of the mydriatics and cause the pupil to dilate quicker. The heat will also afford some pain relief. Do not apply the hot pads directly onto the closed eyelids.
A subconjunctival injection of mydriatics, e.g. Mydricaine may need to be given (see p. 43). This may be given in conjunction with steroids such as Betnesol.
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•Ensure the patient understands the treatment to be instilled at home. This will be a:
mydriatic, e.g. G. Cyclopentolate 1% twice a day;
steroid, e.g. Prednisolone Forte 1% two-hourly or four times a day depending on severity.
•If investigations are to be carried out, ensure the patient understands where and when to attend for these. These tests will include: X-rays – skull, chest and joints to exclude sarcoid, tuberculosis, arthritis and ankylosing spondylitis; blood tests – haemoglobin, full blood count, erythrocyte sedimentation rate, serology and autoimmune profile.
•Ensure the patient knows when to return for follow-up treatment which will probably be in one or two days.
Often the management of patients with uveitis can be complex and time consuming and according to Jones (1998), the management of these patients must be optimised by early setting of targets for achievement, gaining patient’s understanding and co-operation and by setting clear plans for management. Ophthalmic nurse practitioners have a large role to play in achieving these goals.
Complications
•Secondary glaucoma from three causes:
Posterior synaechiae, if not broken down, can cause a ring synaechiae when all of the pupillary zone of the iris is bound down to the anterior lens surface. The aqueous cannot flow through the pupil, so as the pressure builds up in the posterior chamber, the iris is pushed forward. This condition is known as iris bombe. Ring synaechiae can be divided surgically if mydriatics do not cause the pupil to dilate and thus break the synaechiae.
The peripheral anterior surface of the iris bombe adheres to the peripheral posterior surface of the cornea causing peripheral anterior synaechiae. These block the drainage angle.
Debris from the inflamed iris blocks the drainage angle.
•Cataract formation from impairment of the metabolism of the lens.
•Hypopyon of sterile pus.
•Cystoid macular oedema.
Choroiditis
Choroiditis is a condition manifesting itself as patches of inflammation on the choroid. On examination with an ophthalmoscope fluffy white patches can be seen through a hazy vitreous. When these patches heal, they leave pigmented areas of scar tissue.
Symptoms
The patient complains of reduced vision due to infiltrates in the vitreous and of an increased number of vitreous floaters. In 60% of cases, the cause is
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unknown. It can be caused by toxocara, toxoplasmosis (see Colour Plate 8) or syphilis. If the cause is known, this should be treated. A short course of high-dose steroids is also given.
Complications
•Cataract due to defective nourishment of the lens.
•Optic neuritis and secondary optic atrophy.
•Retinal changes and progressive degeneration, resulting in retinal atrophy – a decrease in the size of the visual field will be noted.
•Cystoid macular oedema (see p. 181).
Ophthalmic manifestations of HIV infection
Patients with HIV maybe presented with ocular manifestations involving the anterior or posterior segment of the eye. Due to recent advances in therapeutic agents for treating such infections, early diagnosis is important. All HIV patients should undergo an ophthalmological review so that prompt treatment can be instigated where appropriate. Anterior segment diseases of HIV includes Kaposi’s sarcoma of the eyelids, conjunctiva and rarely the orbit. Herpes zoster ophthalmicus, herpes simplex virus and fungal infections can be associated with early clinical manifestations of HIV infection. Uveitis, Reiter’s syndrome and syphilis are frequently seen in HIV patients. Posterior segment disease afflicting HIV sufferers involved the retina, choroid and optic nerve and are categorized into two: those associated with noninfectious causes and those infected with a variety of infectious disorders. Cytomegalovirus retinitis (CMV) is found in 25–40% of patients with HIV and is the most common retinal infection. Treatment for CMV included Ganciclovir or Foscarnet or a combination of both these agents.
Tumours
Benign naevi
These can be present in the uveal tract and must be observed carefully and regularly for malignant changes. They can be removed by laser. Melanomas are the variety of malignant tumour affecting the uveal tract. They are more common in the choroid but can occur in the iris and, more rarely, in the ciliary body where they carry a higher mortality.
Melanoma of the choroid
This can occur at any age but is more common over the age of 55 years. It usually occurs in the posterior pole and as it grows it pushes the retina forwards. A retinal detachment thus caused is often the first sign of a melanoma, so careful differential diagnoses must be made between a malignant and a simple retinal detachment. The edge of a malignant detachment is usually smoother than that of a simple detachment. Investigations include
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transillumination, colour fundus photography and ocular ultrasound. Treatment is with ruthenium or iodine plaques, proton beam radiotherapy, transscleral or trans-vitreal local resection or laser photocoagulation. The aim is to conserve the eye. Enucleation is reserved for patients who have visual loss, pain and poor cosmetic appearance, and those who are unable to cope with the thought of tumour spread or with prolonged treatment and follow-up. A length of optic nerve must also be removed with the whole eyeball. If the optic nerve is found to be involved on histological examination, radiotherapy should be given to the socket. The five-year mortality rate varies from 16% for small tumours less than 10 mm in diameter, to 53% for tumours larger than 15 mm (Damato, 1995). For nursing care following enucleation, see p. 220.
Melanoma of the iris
This is illustrated in Colour Plate 9. If a naevus in the iris is noted to be enlarging, local excision should be performed. The prognosis, providing treatment is prompt, is usually good.
Melanoma of the ciliary body
Ciliary body melanoma comprises of only 12% of uveal melanoma (Kanski, 2003). Due to the location of the ciliary body, small ciliary body melanomas are often quite difficult to locate and patients are often asymptomatic. Patients presenting with a large ciliary body melanoma may complain of a painless blurred vision due to secondary lens subluxation or astigmatism. Other secondary complications include hyphaema, cataract, retinal detachment and haemorrhage.