Ross & Wilson Anatomy and Physiology in Health and Illness (93 page)

Labyrinthitis

This complication of middle ear infection may be caused by development of a fistula from a cholesteatoma (see above). It is accompanied by vertigo, nausea and vomiting, and nystagmus. In some cases the spiral organ is destroyed, causing sudden profound sensorineural hearing loss in the affected ear.

Motion sickness

Repetitive motion causes excessive stimulation of the semicircular canals and vestibular apparatus and results in nausea and vomiting in some people.

Disorders of the eye

Learning outcome

After studying this section you should be able to:

describe the pathological changes and effects of diseases of the eye.

Inflammatory conditions

Stye

Also known as hordeolum, this is an acute and painful bacterial infection of sebaceous or tarsal glands of the eyelid margin. A ‘crop’ of styes may occur due to localised spread to adjacent glands. Infection of tarsal glands may block their ducts, leading to cyst formation (
chalazion
), which may damage the cornea. The most common infecting organism is
Staphylococcus aureus
.

Blepharitis

This is chronic inflammation of the eyelid margins, usually caused by bacterial infection or allergy, e.g. staphylococcal infection or seborrhoea (excessive sebaceous gland secretion). If ulceration occurs, healing by fibrosis may distort the eyelid margins, preventing complete closure of the eye. This may lead to drying of the eye, conjunctivitis and possibly corneal ulceration.

Conjunctivitis

Inflammation of the conjunctiva may be caused by irritants, such as smoke, dust, wind, cold or dry air, microbes or antigens and may be acute or chronic. Corneal ulceration (see below) is a rare complication.

Infection

This is highly contagious and in adults is usually caused by strains of staphylococci, streptococci or haemophilus.

Neonatal conjunctivitis

Sexually transmitted disease in the mother, including gonorrhoea, chlamydia and genital herpes, can infect the newborn infant’s eyes as the baby passes through the birth canal.

Allergic conjunctivitis

This may be a complication of hay fever, or be caused by a wide variety of airborne antigens, e.g. dust, pollen, fungus spores, animal dander, cosmetics, hair sprays, soaps. The condition sometimes becomes chronic.

Trachoma

This is a chronic inflammatory condition caused by
Chlamydia trachomatis
in which fibrous tissue forms in the conjunctiva and cornea, leading to eyelid deformity, and is a common cause of loss of sight in tropical countries. The microbes are spread by poor hygiene, e.g. communal use of contaminated washing water, cross-infection between mother and child, or contaminated towels and clothing.

Corneal ulcer

This is local necrosis of corneal tissue, usually associated with corneal infection (
keratitis
) following trauma (e.g. abrasion), or infection spread from the conjunctiva or eyelids. Causative organisms include staphylococci, streptococci and herpes viruses. Acute pain,
injection
(redness of the cornea), photophobia and lacrimation interfere with sight during the acute phase. In severe cases extensive ulceration or perforation and healing by fibrosis can cause opacity of the cornea requiring corneal transplantation.

Glaucoma

This is a group of conditions in which intraocular pressure rises due to impaired drainage of aqueous fluid through the scleral venous sinus (canal of Schlemm) in the angle between the iris and cornea in the anterior chamber (
Fig. 8.8
). Persistently raised intraocular pressure may damage the optic nerve by:

•
mechanical compression

•
compression of its blood supply causing ischaemia.

Damage to the optic nerve impairs vision, and the extent varies from some visual impairment to complete loss of sight.

Primary glaucomas

Primary open-angle glaucoma (POAG)

There is a gradual painless rise in intraocular pressure with progressive loss of vision. Peripheral vision is lost first but may not be noticed until only central (
tunnel
) vision remains. As the condition progresses, atrophy of the optic disc occurs leading to irreversible loss of vision. It is commonly bilateral and occurs mostly in people over 40 years of age. The cause is not known but there is a familial tendency.

Acute closed-angle glaucoma

This is most common in people over 40 years of age and usually affects one eye. During life the lens gradually increases in size, pushing the iris forward. In dim light when the pupil dilates, the lax iris bulges still further forward, and may come into contact with the cornea, blocking the scleral venous sinus (canal of Schlemm) suddenly raising the intraocular pressure. Sudden severe pain, photophobia, headache, nausea and blurred vision accompany an acute attack. It may resolve spontaneously if the iris responds to bright light, constricting the pupil and releasing the pressure on the scleral venous sinus. After repeated attacks spontaneous recovery may be incomplete and vision is progressively impaired.

Chronic closed-angle glaucoma

The intraocular pressure rises gradually without symptoms. Later, peripheral vision deteriorates followed by atrophy of the optic disc and loss of sight.

Secondary glaucoma

The most common cause is anterior uveitis (inflammation of the iris, ciliary body and/or choroid) with the formation of adhesions. Other predisposing primary conditions include intraocular tumours, enlarged cataracts, central retinal vein occlusion, intraocular haemorrhage, and trauma to the eye.

Congenital glaucoma

This rare abnormal development of the anterior chamber is often familial or due to maternal infection with rubella in early pregnancy.

Strabismus (squint, cross-eye)

In normal binocular vision, the eyes are aligned so that each eye sees the same image, meaning that both eyes send the same image to the brain. In stabismus only one eye is directed at the observed object and the other diverges (is directed elsewhere). The result is that two different images are sent to the brain, one from each eye, instead of one. It is caused by one-sided extrinsic muscle weakness or impairment of the cranial nerve (III, IV or VI) supply to the extrinsic muscles. In most cases the image from the squinting eye is suppressed by the brain, otherwise there is double vision (
diplopia
).

Presbyopia

Age-related changes in the lens lead to loss of accommodation as the lens loses its elasticity and becomes firmer. This interferes with the light path through the eye and prevents focussing of light on the retina, giving blurred vision. Correction is achieved using reading glasses with convex lenses (see
Fig. 8.26
).

Figure 8.26 
Common refractive errors of the eye and corrective lenses. A.
Normal eye.
B
and
C.
Farsightedness.
D
and
E.
Nearsightedness.

Cataract

This is opacity of the lens which may be age-related or congenital, bilateral or unilateral.

In
age-related cataract
there is gradual development of lens opacity that usually develops during older age as the result of exposure to a variety of predisposing factors including: UV light, X-rays, cigarette smoke, diabetes mellitus, ocular trauma, uveitis, drug therapy, e.g. corticosteroids, chlorpromazine.

Congenital cataract
may be idiopathic, or due to genetic abnormality, or maternal infection in early pregnancy, e.g. rubella. Early treatment is required to prevent permanent loss of sight. The extent of visual impairment depends on the location and extent of the opacity.

Retinopathies

Vascular retinopathies

Occlusion of the central retinal artery or vein causes sudden painless unilateral loss of vision.
Arterial occlusion
is usually due to embolism from, e.g., atheromatous plaques, endocarditis.
Venous occlusion
is usually associated with increased intraocular pressure in, for example, glaucoma, diabetes mellitus, hypertension, increased blood viscosity. The retinal veins become distended and retinal haemorrhages occur.

Diabetic retinopathy

This occurs in type I and type II diabetes mellitus (
p. 227
) and is the commonest cause of blindness in adults aged between 30 and 65 years in developed countries. Changes in retinal blood vessels increase with the severity and duration of hyperglycaemia. Capillary microaneurysms develop and later there may be proliferation of blood vessels. Haemorrhages, fibrosis and secondary retinal detachment may occur, leading to retinal degeneration and loss of vision.

Retinopathy of prematurity (ROP)

This condition affects premature babies. Known risk factors include: birth before 32 weeks’ gestation, birth weight less than 1500 g, requirement for oxygen therapy and serious illness. There is abnormal development of retinal blood vessels and formation of fibrovascular tissue in the vitreous body causing varying degrees of interference with light transmission. The prognosis depends on the severity and many cases resolve spontaneously. In severe cases there may also be haemorrhage in the vitreous body, retinal detachment and loss of vision.

Retinal detachment

This painless condition occurs when a tear or hole in the retina allows fluid to accumulate between the layers of retinal cells or between the retina and choroid. It is usually localised at first but as fluid collects the detachment spreads. There are visual disturbances, often spots before the eyes or flashes of light due to abnormal stimulation of sensory receptors, and progressive loss of vision, sometimes described as a ‘shadow’ or ‘curtain’. In many cases the cause is unknown but it may be associated with trauma to the eye or head, tumours, haemorrhage, cataract surgery when intraocular pressure is reduced or diabetic retinopathy.