Ординатура / Офтальмология / Английские материалы / Notes on Veterinary Ophthalmology_Crispin_2005

EQUINE OPHTHALMOLOGY

Diagnosis

•Affects non-pigmented or poorly pigmented third eyelids ± caruncle and the appearance is usually characteristic (e.g. plaque-like or follicular)

•The full extent of the tumour should be assessed by protruding the third eyelid

•Exfoliative cytology from impression smears can be helpful to aid diagnosis prior to surgery, but it is best to submit excised tissue for histopathology to confirm the diagnosis

Treatment

•Local resection of the third eyelid with a full-thickness wedge excision of the tumour and a wide margin of normal tissue, or complete removal of the third eyelid at its base if the tumour is more extensive

•Whichever technique is needed, the wound should be closed with a continuous buried suture of absorbable material (e.g. 5-0 to 6-0 polyglactin 910) because of the possibility of extraorbital fat herniation and because it produces a better postoperative and long-term appearance

•Additional forms of therapy are not usually required at this site, but would follow the lines outlined under upper and lower eyelids as set out below

UPPER AND LOWER EYELIDS

Entropion (Figure 7.10(a,b,c))

•May be perinatal or acquired

•Sick foals are at risk of the early onset type, which is bilateral

•Previous injury the commonest reason for acquired unilateral (cicatricial) entropion

Diagnosis

The appearance is characteristic and the history may be helpful.

Treatment

•Perinatal entropion in foals may correct spontaneously in the first few weeks of life and mild cases require no treatment other than management of any underlying disease (usually including rehydration), careful drying of the eyelids and manual correction of the entropion.

•For congenital entropion that is producing pain and corneal damage, temporary retention (‘tacking’) sutures (5-0 silk or polyglactin 910 vertical mattress or horizontal mattress) through the skin of the affected eyelid, close to the eyelid margin, are usually sufficient to evert the eyelid (Figure 7.10(c)).

•Non-responsive perinatal (and acquired) entropion may require standard skin –muscle resection techniques, or cosmetic surgery, according to cause and extent.

EQUINE OPHTHALMOLOGY

EQUINE OPHTHALMOLOGY

Figure 7.11 Periocular sarcoid. This was treated successfully using intralesional injections of BCG.

Figure 7.12 (a) Squamous cell carcinoma (SCC) of the lower eyelid. Note the lack of upper and lower eyelid pigment. (b) The other eye had normally pigmented upper and lower eyelids and no pathology. In both eyes, the third eyelid lacked pigment and it is important to assess this region carefully for any signs of solar-induced inflammatory change or SCC.

Treatment

Periocular sarcoids

•Periocular sarcoids are usually treated with intralesional injections of BCG in general practice

•Usually 1ml of intradermal BCG is injected into each lesion, and the treatment is repeated in 2–3 weeks

•Usually two treatments are sufficient, but occasionally three are required

•Intralesional cisplatin and radiotherapy are alternative forms of treatment

•Surgery should not be used, as the recurrence rate (80%) is unacceptably high

Squamous cell carcinoma

•Standard treatments for SCC of the upper and lower eyelids include surgical excision, chemotherapy (e.g. intralesional cisplatin), radiotherapy (e.g. iridium 192), immunotherapy (e.g. BCG) or cryotherapy.

•For cryotherapy, liquid nitrogen is the cryogen of choice and a double freeze–thaw cycle is used. The temperature should be lowered to -25°C throughout the affected tissue, and the ice ball should extend 2mm into healthy tissue.

Melanomas

Melanomas are usually surgically excised or treated using cryotherapy (as outlined for SCC). Cryotherapy can be used alone or after prior surgical debulking of the mass.

Other types of neoplasia

Most other tumours are surgically removed, as already described in the general and canine section. Advice should be sought if the management strategy is unclear, and a proportion of cases will require referral.

LACRIMAL SYSTEM

TEAR FILM PRODUCTION AND DISTRIBUTION PROBLEMS

The trigeminal nerve (Vth cranial nerve) provides the sensory supply to the eye and both sensory and autonomic nerve fibres to the lacrimal gland. The ophthalmic division of the Vth cranial nerve supplies the afferent arm of the trigeminal–lacrimal reflex pathway and the facial nerve (VIIth cranial nerve) supplies the efferent arm.

Damage to the motor fibres of the facial nerve may result in an inadequate blink and poor tear film distribution. Exposure keratopathy is the likely consequence.

Keratoconjunctivitis sicca (KCS)

Aetiology

•Trauma associated with fractures of the stylohyoid bone or mandible is the commonest cause of damage to the parasympathetic fibres that run in the superficial petrosal nerve to supply the lacrimal gland

•Guttural-pouch pathology, middle ear disease and vestibular disease may also be associated with keratoconjunctivitis sicca

•Damage to the lacrimal gland itself (e.g. toxic, eosinophilic and chronic dacryoadenitis) can affect lacrimal-gland function so that keratoconjunctivitis sicca results

Clinical signs of KCS (Figure 7.13)

•Ocular discomfort or more obvious pain

•Blepharospasm, but without any signs of excessive lacrimation

•Lacklustre cornea – the corneal light reflex is disrupted. Subtle epithelial defects or frank ulceration may be present, as well as fine neovascularisation

•There is often rather sparse ocular discharge and any accompanying conjunctivitis is mild

EQUINE OPHTHALMOLOGY

EQUINE OPHTHALMOLOGY

Diagnosis

•History and clinical signs

•Mean normal STT I in the horse is approximately 13mm ± 9mm/minute. Consistent values of 10mm/minute or less should be regarded as indicative of KCS, especially in conjunction with relevant clinical signs

Treatment

•KCS is usually treated with topical ocular lubricants (e.g. polyvinyl alcohol or carbomers such as polyacrylic acid)

•Neurogenic dry eye is the commonest type of KCS encountered in the horse, but there has been no clinical evaluation of the efficacy of parasympathomimetics such as pilocarpine (topical or oral)

•Immune-mediated KCS has not been identified as a specific entity in the horse, so it is unlikely that immunosuppressive drugs, such an important mode of treatment in the dog, would achieve a great deal

Figure 7.13 Unilateral keratoconjunctivitis sicca following head trauma. The Schirmer tear test reading was 6 mm wetting/minute in the affected eye and 14 mm wetting/minute in the fellow eye.

TEAR-FILM DRAINAGE PROBLEMS

These problems are not uncommon, and they can be congenital or acquired. Investigative procedures are as for small animals, with the important difference that in horses the nasal ostium is visible and accessible and so can be used instead of the lacrimal puncta for cannulation and retrograde irrigation. Some drainage problems are quite complex and affected animals will be best referred.

Congenital dysgenesis

Dysgenesis (abnormal development), including atresia (absence) of the nasal ostium and varying portions of the rostral (nasal or distal) nasolacrimal duct, is the commonest abnormality of the lacrimal system encountered in foals.

Clinical diagnosis

•Veterinary advice is usually sought when the animal is quite young; the problem is usually unilateral

•Clinical examination reveals no sign of a nasal ostium and considerable epiphora, or a mucopurulent discharge, may be present at the medial canthus and on the side of the face (Figure 7.14(a))

•If the nasal ostium is absent it is likely that surgery will be required, therefore the additional investigations are usually performed under general anaesthesia so that surgery can follow immediately

Additional investigations

•Congenital dysgenesis cases are often referred, following the initial clinical examination, for additional investigations and treatment

•A sterile catheter (e.g. feline urinary catheter) can be passed via the upper lacrimal punctum and down the nasolacrimal duct to establish the degree of abnormality

•Dacryocystorhinography may also be required to establish the extent of any dysgenesis (Figure 7.14(b)). Note that it is not uncommon to encounter diverticula, both patent and impatent, at the rostral (nasal) end of the nasolacrimal duct in some horses

(a)

(c)

EQUINE OPHTHALMOLOGY

•External reasons for obstruction of the nasolacrimal duct include trauma, neoplasia, rhinitis, sinusitis and upper-arcade dental disease

Clinical signs of acquired drainage problems

•Epiphora or some form of ocular discharge

•Identifiable cause of obstruction by direct inspection (lacrimal puncta or nasal ostium) or by indirect inference (e.g. dental disease, sinusitis)

Clinical signs of habronemiasis

•Usually a rapid onset of swelling, which is both painful and pruritic. The lesion is usually raised and ulcerated, or of caseous plaque-like appearance (Figure 7.16)

•In addition to causing lacrimal drainage obstruction and secondary dacryocystitis, habronemiasis may also be associated with granulomas of the eyelids and conjunctiva, especially in the region of the medial canthus

•The diagnosis is confirmed by biopsy

Treatment of parasitic causes of dacryocystitis

A single dose of oral ivermectin (0.2mg/kg) is usually effective.

Figure 7.16 Habronemiasis as a cause of acquired obstruction of nasolacrimal drainage.

CONJUNCTIVA

Conjunctivitis

Aetiology

Causes of conjunctivitis include foreign debris (e.g. dust), allergies (e.g. fly spray and drugs), parasites (e.g. Habronema spp, Thelazia spp and Onchocerca spp), viruses (e.g. equine viral arteritis, equine herpes virus and adenovirus), bacteria (almost exclusively secondary pathogens) and mycoses in tropical countries (e.g. blastomycosis) (Figure 7.17).

EQUINE OPHTHALMOLOGY

EQUINE OPHTHALMOLOGY

Figure 7.17 Acute allergic conjunctivitis in a Hanoverian. Acute blepharoconjunctivitis is often a painful condition in horses, as is apparent here.

Treatment

•Treatment of conjunctivitis depends upon establishing and eliminating the cause

•Environmental factors are the commonest causes (physical, chemical and allergic)

•As horses live in a relatively contaminated environment, growth of organisms from conjunctival swabs is common, but they may not be pathogenic, so expert assistance may be required if the clinical significance of any isolates is unclear

Neoplasia involving the bulbar conjunctiva

•Squamous cell carcinoma is the commonest and may be confirmed by histopathology (Figure 7.18)

•Other tumours are uncommon and include papilloma, haemangioma, haemangiosarcoma, melanoma and lymphoma

•The extent of the tumour, and any indications of local or distant spread, should be established before contemplating removal

Figure 7.18 Squamous cell carcinoma involving the conjunctiva, limbus and cornea.

Differential diagnosis

Tumours that involve the conjunctiva (bulbar, palpebral or nictitating) should be differentiated from amyloidosis, which is very rare.

Treatment

• For SCC involving the conjunctiva of the lids, see under Eyelids (this section

pp 276–281)

•For SCC involving the bulbar conjunctiva, limbus and cornea, keratectomy and conjunctivectomy can be used to debulk or excise the tumour

•Surgery can be combined with radiotherapy (e.g. beta irradiation with a strontium 90 applicator) or radiotherapy can be used on its own

•The presence of unrecognised micrometastases can complicate the management of these cases, as local ischaemia and limbal keratomalacia may result

•Referral should be considered for any case with corneal involvement (e.g. SCC and carcinoma in situ)

•Other tumours can usually be removed using wide-based excision, and, as with SCC, the diagnosis should be confirmed by histopathology

CORNEA

Non-specific corneal opacities

Band opacities

Band opacities (linear keratopathy), in the form of linear streaks crossing the cornea, are an occasional finding. They represent thin regions of Descemet’s membrane and are most commonly associated with glaucoma ± globe enlargement, so the rest of the eye should be checked very carefully.

Corneal oedema

Corneal oedema is not uncommon in horses. It may be a primary ‘dystrophic’ condition, but is more likely to be a sequel to a variety of ocular insults (e.g. blunt trauma, keratitis, uveitis and glaucoma).

Keratitis

Keratitis accounts for a high proportion of all eye cases examined in equine practice

– and donkeys and mules, as well as horses, are involved. There are a number of possible causes that include trauma (physical and chemical), infection, hypersensitivity, immune-mediated disease and neoplasia, and it is important to try to establish the aetiology if treatment is to be effective. On occasions it is impossible to establish the cause, so early specialist advice may be required to help with such cases.

When pain and blepharospasm are features of the clinical presentation, topical local anaesthesia, systemic sedation ± analgesia and an auriculopalpebral nerve block may be required for examination. If indicated, Schirmer I tear tests should be performed before local anaesthetic is applied and swabs and scrapes should be taken before agents such as fluorescein and rose bengal are applied. The insertion of an indwelling medication device after initial assessment may make subsequent management easier.

Non-ulcerative keratitis

Eosinophilic keratoconjunctivitis

Eosinophilic keratoconjunctivitis is an unusual form of limbal-based keratoconjunctivitis of possible immune-mediated origin. It bears comparison with human vernal disease, canine chronic superficial keratoconjunctivitis and feline proliferative keratoconjunctivitis. Horses present with conjunctival hyperaemia and chemosis, as well as characteristic superficial, white, corneolimbal plaques. There is usually some ocular discomfort with blepharospasm and lacrimation and, sometimes, a mucoid discharge.

Treatment consists of topical 0.1% lodoxamide four times daily. Alternatively, topical corticosteroids may be effective.

EQUINE OPHTHALMOLOGY