Ординатура / Офтальмология / Английские материалы / Notes on Veterinary Ophthalmology_Crispin_2005

EQUINE OPHTHALMOLOGY

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ANATOMY

•The supraorbital fossa and direction of the well-developed eyelashes on the upper eyelid should always be inspected as part of ocular examination. The supraorbital fossa should also be palpated.

•Obvious indentations, the dorsal and ventral orbital sulci, are apparent close to and parallel to the eyelid margins. A rounded prominence, the lacrimal caruncle, located in the medial canthus proximal to the third eyelid is an obvious feature of many horses; it may be pigmented or non-pigmented and usually bears fine hairs.

•The nasal palpebral aperture and cornea are wider medially than laterally and the pupil is horizontally oval.

•The grey lines that are such a striking feature of the medial and lateral cornea in some horses mark the insertion of the pectinate ligaments into the termination of the cornea at Descemet’s membrane. The pectinate ligaments and entrance to the drainage angle can be directly observed in these regions.

•The commonest iris colour is dark brown, although heterochromia of the whole iris, or a sector of the iris, are common (this section, pp 294–295) and persistent pupillary membrane remnants are a very common finding (this section, p. 294). Colour dilution is associated with blue, cream or tan iris colour and there is usually focal or diffuse lack of stromal pigment as well as a relatively under-developed iris.

•The granula iridica are better developed on the dorsal pupillary border.

•Although the pupillary light response to a penlight is often somewhat slow and incomplete, the pupil responds briskly and completely when exposed to bright sunlight.

•The equine lens is large and may demonstrate a number of non-progressive minor aberrations. The lens suture lines are usually apparent: the anterior suture lines are usually in the shape of an upright Y, whereas the posterior suture lines may be in the form of an inverted Y or of stellate or sawhorse configuration. The tips of the suture lines sometimes have a somewhat feathered appearance. It is very common to be able to visualise the nuclear–cortical interface.

•As the eye is relatively large, some details of vitreal structure, such as the fibrillar condensations, are often quite clearly defined. For example, the anterior hyaloid face is readily visualised with a slit lamp and appears as an irregular, grey, veil-like structure just posterior to the posterior lens capsule. Hyaloid remnants persist to some four months of age in most foals.

•The fundus has a number of features of note – the retinal blood supply is paurangiotic, the optic nerve head is located within the non-tapetal fundus and two regions of higher visual acuity (approximating to the human macula) have been identified.

EXAMINATION

ASSESSMENT

The initial assessment is important and should include details of the horse’s age, breed, coat colour and sex, plus family history and any previous illnesses, together with details of their treatment. The animal’s lifestyle, management, performance and behaviour should also be recorded.

The nature of the present problem should be established, especially in relation to the clinical signs observed, including the rapidity of onset and whether there have been similar problems in the past. Any treatment that the animal has received should be documented, as should the response to treatment.

Ophthalmic examination is usually carried out in a loose box from which light has been excluded. Any stray light entering may complicate interpretation as the light reflects on the cornea and lens.

Proper handling is essential and a twitch is occasionally required. Sedation and analgesia will be needed on occasions, particularly if the eye is painful, but in most cases routine ophthalmic examination is performed without chemical restraint. Local nerve blocks, if needed, should be administered after systemic sedation has been given, although motor nerve blocks may be unnecessary after effective sedation.

Note that if the significance of any ophthalmic finding at a pre-purchase examination is not apparent, then referral should be considered.

SEDATION AND ANALGESIA

The drugs (Table 7.1) are given by slow intravenous injection, and additional, smaller doses can be given if deeper sedation is required. The horse should be left undisturbed to allow these drugs to take effect before proceeding to the next stage. Young foals should not be given injectable sedatives and analgesics.

EQUINE OPHTHALMOLOGY

EQUINE OPHTHALMOLOGY

•Some motor paralysis of the facial nerve may also be produced, as there are palpebral branches at this site.

•The supraorbital block is useful for reducing blepharospasm and prior to minor eyelid surgery, including eyelid biopsies.

PRE-PURCHASE AND INSURANCE EXAMINATION

At examinations for purchase and insurance purposes, findings that may be difficult for non-specialists to interpret include opacities of the ocular media, especially those involving the cornea, lens or vitreous, iris variations including those affecting the granula iridica and normal and pathological alterations in the appearance of the ocular fundus. Many of the findings are normal variants – indeed this type of examination is usually conducted on the assumption that the eye will be normal. There is no doubt that, as in other species, examination of as many normal animals as possible is the best way of gaining confidence in interpreting the findings.

The intended use of the animal is obviously of relevance when purchase and insurance examination is carried out: for example, what is expected of an animal used for quiet hacking places quite different demands on the eye than those that would be required of a three-day eventer. It is always sensible to categorise changes identified at these examinations as anomalies or abnormalities of no clinical significance, abnormalities of minor clinical significance and abnormalities of major clinical significance.

Abnormalities of major clinical significance include:

•Any abnormality that may become, or already is, painful

•Any abnormality that affects, or may affect, vision

•Any abnormality that is progressive, chronic or recurrent

•Any abnormality that cannot be cured by a course of medical treatment or by surgical correction

LAVAGE DEVICES

Highly-strung animals, particularly those in pain, can be difficult or impossible to treat with topical medication applied directly to the eye and it is sensible to consider inserting some form of indwelling medication device at the initiation of treatment. These devices can be inserted in the sedated conscious horse following infiltration with local anaesthetic, or at the end of any surgical procedure under general anaesthesia.

Subpalpebral lavage devices

Two types are available: single entry and double entry. The single-entry type consists of delivery tubing at the end of which is a small central hole surrounded by a footplate. Inserting this type is associated with fewer complications for the occasional user than the double entry type and its insertion is described below (Figure 7.5(a–d)).

EQUINE OPHTHALMOLOGY

•In the conscious animal, topical anaesthetic is applied to the cornea and local anaesthetic is infiltrated at the site where the trochar (or needle) will exit.

•The trochar is placed as deeply as possible in the dorsal or ventral conjunctival sac at almost 90 degrees to the eyelid margin and then pushed decisively to emerge through the skin (Figure 7.5(a,b)). This is slightly more difficult to achieve when using a ventral approach, but it is probably the position of choice, because the location of the third eyelid makes inadvertent corneal damage from the footplate less likely (Figure 7.5(c)).

•The tubing is then threaded through the trochar, after which the trochar is removed and the tubing is adjusted so that the footplate rests snugly against the conjunctiva.

•The tubing is retained in place with skin sutures or cyanoacrylate adhesive, applied to ‘butterflies’ made from zinc oxide tape wrapped around the tubing. The first suture is placed close to the eyelid, the second one closer to the midline between the eyes and a third, if required, close to the ear. It is usually convenient to lead the tubing between the ears, or to some other point where it may be attached to the head collar (Figure 7.5(d)).

•Topical solutions are injected into the system and flushed through with air. If multiple solutions are used in therapy, at least five minutes should be allowed to elapse between different drugs.

•These devices should be checked regularly as a number of complications can occur, including local haemorrhage when they are inserted and simple blockage during use.

•Eyelid swelling and corneal damage can result if they are improperly located, or become displaced from their original position.

Indwelling nasolacrimal cannula (Figure 2.20, p. 53)

A commercial cannula is available, which is inserted into the nasolacrimal duct via the nasal ostium after applying local anaesthetic drops to the lower lacrimal punctum and local anaesthetic spray or gel to the nasal ostium. Indwelling nasolacrimal cannulae and catheters may be used as a means of delivering drugs or other preparations to the eye, or to the nasolacrimal drainage system, or as a means of retaining patency within the nasolacrimal drainage system. The volume of lavage solution used in this system is greater than that required for subpalpebral devices, although if air is used to drive the bolus of drug the difference is insignificant.

•The indwelling cannula is passed into the nasal ostium via a stab incision over the cranial aspect of the false nostril and fed up the nasolacrimal duct until the moulded collar abuts against the ostium

•Non-absorbable sutures are used to retain the cannula in place at this point

•The remainder of the tubing is led up between the ears and kept in place as already described for the subpalpebral lavage device

OCULAR AND ADNEXAL CONDITIONS

Box 7.1 Common and important equine ocular and adnexal conditions

•Ocular and adnexal trauma

•Entropion (foals)

•Non-ulcerative and ulcerative keratitis, including melting ulcers

•Uveitis from a variety of causes; the immune-mediated aspects are of particular significance

•Neoplasia, particularly periocular sarcoids and ocular and adnexal squamous cell carcinoma. Less common, but clinically significant, tumours include uveal melanoma and lymphoma

•Differentiation of normal variants and pathological changes affecting the cornea, iris, lens, vitreous and fundus

•Space-occupying orbital and paraorbital neoplastic and non-neoplastic lesions are uncommon, but important in terms of differential diagnosis and prognosis

GLOBE AND ORBIT

PHTHISIS BULBI

The commonest sequel to severe ocular damage in horses is phthisis bulbi – a small, shrunken, blind, painless globe (Figure 7.6). It should be distinguished from a congenitally small eye (e.g. microphthalmos).

No treatment is required, unless there is an unacceptable amount of ocular discharge, which may, in turn, attract flies and lead to chronic infection. In such cases the eye should be removed.

EQUINE OPHTHALMOLOGY

EQUINE OPHTHALMOLOGY

swelling in this region. It is comparison of each side that is most revealing. This is an early and important sign in many cases

•Exophthalmos is a frequent finding ± an associated strabismus

•The third eyelid may become prominent

•Note, also, the direction of the eyelashes – they may be more horizontal than usual when exophthalmos develops

Aetiology and diagnosis

•Neoplastic, inflammatory, cystic (hydatid disease) and other space-occupying lesions within the orbit (retrobulbar) and outside the orbit (paranasal, retro-orbital and paraorbital). For example, both ethmoidal haematoma and carcinoma involving the ethmoidal labyrinth can produce exophthalmos as the ethmoidal labyrinth is anatomically close to the orbit (Figure 7.7).

•Extraorbital lesions are a commoner cause of exophthalmos in horses than intraorbital lesions

•Radiology, ultrasonography, sinus trephination and endoscopy (including of the guttural pouch) can help in differentiation, and such cases are often best referred for comprehensive investigation

Figure 7.7 Exophthalmos and third-eyelid prominence because of an ethmoidal carcinoma. Note that the pupil is widely dilated as a result of chronic compressive optic neuropathy – optic atrophy was apparent on ophthalmic examination.

EYELIDS

THIRD EYELID (NICTITATING MEMBRANE)

Prominence – summar y

•Orbital, retro-orbital and paraorbital space-occupying lesions

•Prolapse of the extraorbital fat pad (corpus adiposum extraorbitale)

•Inflammation of the third eyelid (including solar-induced inflammation in nonpigmented lids)

•Neoplasia of the third eyelid, most commonly SCC or lymphoma

•Tetanus: in all species there is bilateral prominence of the third eyelid in this disease, but it is more likely to be seen in herbivores than carnivores.

•Horner’s syndrome is rare in horses. Ptosis is usually the most obvious feature, and the third eyelid is often not prominent. Increased skin temperature and sweating

may also be observed and give a useful insight into the location of the damage to the neurological pathway.

Neoplasia (Figures 7.8(a,b) and 7.9(a,b,c))

See also under Conjunctiva and Upper and Lower Eyelids (this section pp 285–287 and pp 278–281). Commonest tumour is squamous cell carcinoma (SCC) of the third eyelid and this condition (and pre-cancerous solar-induced inflammation) should always be considered in animals with an unexplained ocular discharge. Animals in which the eyelids and caruncle are poorly pigmented are most at risk.

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Figure 7.8 (a) Squamous cell carcinoma of the third eyelid in a Thoroughbred. The third eyelid has been protruded prior to excisional surgery of the tumour and part of the third eyelid. (b) The immediate postoperative appearance of the eye in (a). The third eyelid conjunctiva was closed with a continuous buried suture of 5-0 polyglactin 910.

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Figure 7.9(a–c) Squamous cell carcinoma of the third eyelid in a Shire X Thoroughbred. Appearance at rest (a) and after manual protrusion of the third eyelid (b). The third eyelid was resected at its base and there were no problems relating to either squamous cell carcinoma or loss of the third eyelid during a six-year follow up (c).

(b) OPHTHALMOLOGY

EQUINE

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