Eyelids, conjunctivae and eyes

Abnormalities affecting the eyelids include swelling, position, movement, structure and other changes which may occur in the skin. Swelling of the eyelids is a feature in bowel oedema, photosensitization, allergy and trau­matic injury. Except in injury the swelling is invariably bilateral. In tetanus, particularly in the horse, the normal curvature of the palpebral border of the upper eyelid may form an obtuse angle. The eyelids may be kept closed in acute conjunctivitis and other painful eye conditions, and when there is gross swelling of the eyelids, as in local oedema due to photosensitization, allergy or purpura haemorrhagica. Excessive movement is seen in painful eye conditions in­cluding acute conjunctivitis, acute keratitis, and injury, or in cases of deranged nervous function including hypomagnesaemic tetany, lead poison­ing and sometimes encephalitis. Tumours on the eyelids are readily observable.

The palpebral conjunctiva—that part of the mucous membrane comprising the conjunctival sac which covers the inner surface of the eyelids —is examined by opening and gently everting each eyelid in turn with the forefinger and thumb of one hand while, at the same time, the eyeball is pushed back into the orbital cavity by pressure on the corresponding eyelid (Fig. 86). The membrana nictitans, which is covered with an extension of the conjunctiva, is exposed for examination, readily so in the horse and dog, by laying the forefinger and thumb along the upper eyelid and pressing it gently but firmly onto the eyeball, with, at the same time, the thumb pressing downwards on the lower eyelid (Fig. 87).

In the horse protrusion of the membrana nictitans (often noted only when the animal is tapped lightly under the jaw or walked on hard ground) is one of the early signs of tetanus (see Fig. 217, p. 312). Continuous protrusion occurs in painful eye diseases, in the later stages of tetanus and in encephalitis. In the cat, protrusion of the membrana nictitans is usually a sign of debility, but it may occur even in apparently normal animals.

The condition of the conjunctiva should be considered carefully, both eyes being included in the examination. On account of its accessibility and freedom from soiling with foreign matter such as foodstuffs or faeces, it is of particular value in assisting in the diagnosis of diseases which produce changes in the mucosae in general, such as jaundice (yellow colour), anae­mia (pallid and watery), shock (pallid and dry), arsenic poisoning (brick-red colour). Certain other diseases, including equine viral arteritis, malignant catarrhal fever, swine fever and canine distemper, affect the conjunctival mucosa with varying severity, causing it to become swollen and hyperaemic. Conjunctivitis occurs in association with inflammatory changes in other eye structures in infectious keratoconjunctivitis and contagious ophthalmia. When the swelling of the palpebral conjunctiva is so great that the mucous membrane protrudes beyond the free margin of the eyelid the condition is called chemosis.

Photophobia (Fig. 88) is a clinical sign which develops when light of normal daytime intensity causes pain in the eye, with the result that the eyelids are kept closed. It occurs in diseases of the eye itself, including all forms of acute con­junctivitis and keratitis. Blepharospasm is a feature of some cases of conjunctivitis and is usual when a foreign body irritates the cornea or conjunctiva. It is produced by reflex contrac­tion of the orbicularis oculi muscle which then causes spasmodic closure of the eyelids. Wrink­ling of the skin over the upper eyelid, which is observed in many pathological conditions of the eye, is the result of contraction of the corrugator supercilii muscle.

The detailed examination of the eye, more particularly of the deeper structures, is part of the subject of ophthalmology. Some of the general aspects relating to the more superficial structures will be given immediate consideration; those relating to examination of the deeper structures will be considered in Chapter 14. Structural abnormalities of the eyelids, including entropion (inversion of the palpebral margins), ectropion (eversion of the lower eyelid), trichiasis (abnormal deviation of eyelashes so that they impinge upon the cornea or conjunctiva), distichiasis (two rows of eyelashes), the incidence of which is probably highest in dogs and sheep, and various forms of maldevelopment including virtual absence of eyelids or eyelashes, are appreciated by direct observation.

Abnormalities of the eyeball are the result of local or systemic influences. In the former the changes may be unilateral; in the latter they are invariably bilateral, although both eyes may not be equally severely affected. Microphthalmos, which is usually associated with abnormality of various eye structures, is developmental in origin and may have a genetic or nutritional basis, as in the eye anomaly in rough collies and vitamin A deficiency in young pigs, respec­tively. Corneal conditions including keratitis, which varies in degree from slight haziness in mild cases and in the early stages of acute cases, to the dense white colour (leucoma) of the ad­vanced phase, with a variable degree of vascularization (pannus) as the result of blood vessels growing in from the conjunctiva, are frequently associated with conjunctivitis. Ulceration, scar­ring and focal pigmentation are other changes that may be observed following keratitis. In­creased convexity of the cornea occurs in a generalized form (keratoglobus) in hydrophthalmos and hypopyon, and in a localized form (keratoconus), causing a cone-shaped bulge, in a proportion of cases of keratitis. Protrusion of the eyeball (proptosis, exophthalmos) is a species or breed characteristic (it occurs to a mild degree in Jersey cattle and Pekingese dogs); it occurs as a clinical sign in cases of hyperthyroidism. In unilateral form it is due to pressure from behind the orbit; periorbital lymphoma in cattle, dislocation of the mandible and periorbi­tal haemorrhage are the common causes. Bi­lateral retraction of the eyeballs, with exposure of the sclera and conjunctiva, is associated with emaciation and severe dehydration, and is due to reduction in the quantity of fat and fluid in the retro-orbital tissues. Abnormal movements of the eyeball (nystagmus), which may take place in a horizontal, vertical or rotatory direc­tion, are periodic and involuntary, the initial movement being slow with a rapid return to the original position. Nystagmus occurs in hypoxia and when there is damage to the cerebellum or vestibular tracts. Limitation of movement of the eyeball occurs when there is paralysis of the motor nerves to the extrinsic ocular muscles with resulting malpositioning. In paralysis of the oculomotor, trigeminal and facial nerves, blink­ing may be restricted, infrequent or entirely absent, and the aperture between the eyelids is reduced in width (ptosis).

The deeper structures of the eye, consisting of the iris, lens and fundus, are only satisfactorily examined by means of an ophthalmoscope (Figs 85в, с, 89). Certain pathological changes in these structures can result from systemic diseases. The presence in the anterior chamber of the eye of blood (hyphaema) or pus (hypopyon) is readily recognized by direct vision when changes are moderate to severe. (In hypopyon there is a yellow to white opacity with, in the early stages, a horizontal upper border obscuring the iris.) The former condition is occasionally observed in warfarin poisoning, and the latter in most forms of severe keratitis.

Determining the size, shape and position of the pupil is important in recognizing certain diseases. Mobility of the pupil, determined by exposure to variations in light intensity, is decreased in functional disorders including severe hypoxia and botulism, in diffuse encephalopathies and when there are lesions affecting the oculomotor nucleus. In these circumstances the pupils are bilaterally overdilated (mydriasis). Unilateral dilatation suggests the existence of a lesion in the orbital region or in the eye itself; both pupils are dilated in peripheral blindness due to bilateral lesions of the orbits. Drugs with a parasympatholytic action, e.g. atropine, have a mydriatic effect of variable duration following systemic or local absorption. Conversely, over-dosage with parasympathomimetic drugs inclu­ding organophosphorus compounds produces excessive constriction of the pupil (miosis). The shape and position of the pupil are abnormal when the iris is adherent to the posterior surface of the cornea (anterior synechia), or to the lens (posterior synechia). Iritis, which produces these conditions, frequently occurs during the course of equine viral arteritis, canine distemper, some other virus infections and purpura haemorrhagica. Partial absence of the iris (coloboma) may vary in extent and in some cases does not affect the shape or size of the pupil. Opacity of the lens (cataract), most commonly observed in aged dogs, is in most cases readily visible, particularly in advanced cases when the whole of the pupil­lary area is greyish-white in colour. Retinal changes are seen in tuberculosis, progressive retinal atrophy (Irish setters, miniature poodles) and hydrophthalmos, when detachment occurs. The conjunctival, corneal and pupillary reflexes, and the various nervous disorders affecting the eye, are discussed in Chapter 14.

Diseases of the eye may cause changes in the surrounding skin, e.g. periorbital dermatitis, caused by the discharge in conjunctivitis, or the lacrimal secretion when the tear duct is obstruc­ted or the punctum lacrimalium abnormally situated (entropion).

A certain quantity of lacrimal fluid is always present in the conjunctival sac and is regularly distributed over the cornea by the normal blink­ing movements of the eyelids. An overflow of tears down the face (epiphora) may be caused by excessive secretion, obstruction of the lacrimal duct or deviation of the punctum lacrimalium. A discharge from the conjunctival sac may also consist of inflammatory exudate or blood. Cer­tain nematode parasites (Thelazia spp.) occa­sionally occur in the conjunctival sac; the larval stage of the worms is deposited in the conjuncti­val sac by flies (Musca spp.) which are the inter­mediate hosts. The worms are associated with the appearance of conjunctivitis, keratitis, oph­thalmia and abscessation of the eyelids during the summer months. Inanimate foreign bodies, such as grass seeds, cereal husks, chaff, etc., are frequently found in the conjunctival sac in cattle.

Laboratory aids may be of assistance in diag­nosing infective conditions of the eye. When normal the conjunctiva is almost free of bacteria because of the activity of lysozymes and other factors. Microscopical examination of stained smears prepared from conjunctival scrapings may be found useful in recognizing intracytoplasmic inclusion bodies in canine distemper, or rickettsia in contagious ophthalmia. Cultural techniques are more useful for confirming the presence of Moraxella bovis in cattle and Moraxella spp. in infectious keratoconjunctivitis and contagious ophthalmia in cattle and sheep res­pectively. As these organisms can be isolated from the eyes of normal animals, their signifi­cance in typical cases of eye disease is deter­mined by assessment of the clinical features and the numbers of organisms present. Other orga­nisms that may be recovered from conjunctival swabs include streptococci, staphylococci, Nocardia spp. and fungi. In many cases these may be of secondary origin, so that a detailed clinical examination is essential in order to discover the type of primary lesion present.

The behaviour and actions of the patient may indicate that it is suffering from a defect of vision for which, as yet, there is no describable cause. Tests for visual acuity can be performed at this stage, including those for eye preservation involving the corneal and conjunctival reflexes, and an obstacle test. In peripheral and central blindness, reflex closure of the eyelids does not occur when a blow is aimed at the eye; similarly in facial nerve paralysis when, however, there may be withdrawal of the head. An obstacle test is best arranged in unfamiliar surroundings so that the animal's behaviour in relation to the obstacles can be more readily assessed. The test for night-blindness (nyctalopia), which is one of the earliest clinical signs of avitaminosis A, should be performed at dusk or in moonlight conditions. Reduced visual acuity in the absence of a demonstrable eye lesion is termed amblyopia; complete blindness in similar circumstances is called amaurosis.