Ординатура / Офтальмология / Английские материалы / Notes on Veterinary Ophthalmology_Crispin_2005

GENERAL AND CANINE OPHTHALMOLOGY

PRIMARY GLAUCOMA

In this type of glaucoma there is no other recognised antecedent ocular disease process, and primary glaucoma is thought to be hereditary. It is invariably a bilateral condition, although the two eyes are not usually affected simultaneously. There are two forms: that associated with goniodysgenesis (abnormal drainage angle development) and open angle. For an up-to-date list of affected breeds see In Practice Supplement on Hereditary Eye Disease.

Goniodysgenesis

This is the commonest cause of primary glaucoma in dogs in the UK. It most commonly involves the drainage angle only, but more complex presentations are infrequently encountered in some types of anterior-segment dysgenesis. Affected breeds include the Basset Hound, Bouvier des Flandres, Great Dane, Samoyed, Siberian Husky, spaniels (English and American Cocker, English and Welsh Springer) retrievers (Flatcoated, Golden and Labrador) and terriers (Dandie Dinmont and Welsh).

‘Open-angle’ glaucoma

This is the type seen in the Norwegian Elkhound and possibly other breeds, including the Miniature and Toy Poodle in the UK. In the USA it has also been well documented in a laboratory strain of Beagle. It is rare, but may be under-diagnosed.

Management of primar y glaucoma

•Seek immediate referral to a specialist centre

•Both surgical and medical treatment may be needed (see Management of Glaucoma, this section, pp 150–151).

•Long term management may include prophylactic treatment of the second eye before any clinical signs of raised IOP are present

SECONDARY GLAUCOMA

Abnormal elevation of IOP associated with recognisable antecedent, or concurrent, eye disease. May be unilateral. Some types, notably primary lens luxation, are breedassociated, others are not.

Lens-associated

Lens-induced uveitis

Lens-induced uveitis with secondary glaucoma usually occurs because of a penetrating injury to the lens and consequent release of lens protein (phacoclastic uveitis).

Primary lens luxation (PLL)

Primary lens luxation is the commonest cause of secondary glaucoma in the dog. The lens may also luxate as a complication of other types of glaucoma, so advice should be sought if unsure.

•Terrier breeds are susceptible to primary lens luxation (Jack Russell Terrier, Wire and Smooth Fox Terrier, Sealyham Terrier, Tibetan Terrier, Miniature Bull Terrier

for example). PLL has also been recorded in the Border Collie, in which it is uncommon. The Shar Pei is rarely affected

•The condition has been shown to be an autosomal recessive in the Tibetan Terrier and is due to an inherent weakness of the lens zonules (suspensory ligament). It is probably inherited in similar fashion in other susceptible breeds and should always be considered as a possible diagnosis in young adult terriers (usually 3–6 years) with ocular redness or pain

•The condition is essentially bilateral, although only one eye may be affected at initial presentation

•When the lens has moved anteriorly, or is causing pupil block, secondary glaucoma results and the lens should be removed as a matter of urgency (i.e. as an emergency referral)

•Posterior lens luxations do not necessarily produce glaucoma and surgery in such cases can be delayed, because lens removal in this position is technically difficult and may not be necessary unless the lens moves

•The majority of luxated lenses move anteriorly with time

Clinical signs of lens luxation (Figure 2.25, p. 59)

•Glaucoma in most cases, therefore usual clinical signs with pain, photophobia, blepharospasm, lacrimation, redness and dilated unresponsive pupil*

•Greyish strands of prolapsed vitreous may be apparent in pupillary aperture

•The lens is not in its normal position. Look for signs such as instability of the lens (phacodenesis), abnormal location of the lens, aphakic crescent, refractile ring at lens equator*

•Localised corneal oedema if the lens is contacting the cornea, more extensive corneal

oedema if the intraocular pressure exceeds 40–50 mm Hg*

•Trembling of the iris (iridodenesis) is pathognomonic; it occurs because the iris has lost the support of the lens*

•May appear to be a unilateral problem, but early signs (e.g. vitreous prolapse or lens subluxation) or actual lens luxation may be present in the other eye, so always look carefully at both eyes

Inflammator y

•Following uveitis (a result of, for example, pre-iridal fibrovascular membrane formation; inflammatory debris, fibrin or haemorrhage in the filtration angle or trabecular meshwork; iris swelling)

•Adhesions may be in the form of posterior synechiae (iris to lens), if they ring the entire circumference iris bombé is present

•Adhesions may be peripheral anterior synechiae (iris base to corneoscleral tissue), usually because of pre-iridal fibrovascular membranes

•Anterior synechiae (iris to cornea) are not usually extensive enough to produce secondary glaucoma

•Secondary glaucoma associated with uveitis and multiple iridociliary cysts is an unusual problem in the Golden Retriever and Great Dane

GENERAL AND CANINE OPHTHALMOLOGY

GENERAL AND CANINE OPHTHALMOLOGY

Neoplastic

•Variety of primary intraocular tumours (most commonly melanoma). Metastases from primary tumour elsewhere (most commonly mammary adenocarcinoma)

•Glaucoma is a consequence of fibrovascular membrane formation, blood–aqueous barrier breakdown, inflammatory material and solid tumour or tumour cells obstructing the drainage angle

Traumatic

•Blunt, non-penetrating trauma may result in extensive hyphaema, complicated by glaucoma

•Penetrating trauma may result in glaucoma as a complication of inflammation associated with the injury and the disruption of the normal intraocular relationships

Haemorrhagic

Extensive haemorrhage may compromise the drainage angle with subsequent glaucoma. Note, however, that recurrent haemorrhage may itself be a feature of chronic glaucoma.

Pigment-associated

The Cairn Terrier and occasionally other breeds of dog (e.g. Labrador Retriever, Boxer) are subject to a form of abnormal pigment deposition, termed diffuse ocular melanosis, in middle-aged and older dogs. Glaucoma may develop when accumulated pigment-filled epithelioid cells, presumed to be melanocytes, block the drainage angle.

Diagnosis of secondar y glaucoma

Breed, history, aetiology and clinical signs

Management of secondar y glaucoma

•Seek early expert advice as to initial treatment and subsequent management. All cases will require referral for initial assessment if the practice lacks facilities for accurate measurement of intraocular pressure. The majority of cases will require early referral if there is a possibility of restoring and retaining vision

•The underlying cause must be identified and, whenever possible, treated

•Surgical and medical options are available, often a combination of both is needed

MANAGEMENT OF GLAUCOMA

Summar y of surgical techniques available for glaucoma management

•Aqueous outflow can be increased by means of bypass procedures that provide an alternative aqueous outflow pathway. Bypass can be achieved with or without the aid of drainage implants; medium to long-term results are better if implants are used.

•Patients with primary lens luxation invariably require surgery to remove the luxated lens by intracapsular extraction. When the lens is subluxated, phacoemulsification may be the treatment selected for removal of the lens contents

•Patients with lens luxation secondary to primary glaucoma rarely benefit from lens

removal, as the eye is often blind and enlarged by the time the lens luxates. Enucleation is the preferred option for permanently blind and painful eyes with this pathogenesis

•Aqueous production can be reduced using laser cyclophotocoagulation or cyclocryotherapy. The former technique tends to be used in sighted eyes and the latter in blind eyes as an alternative to enucleation

•For end-stage glaucoma where eyes are painful and permanently blind, irrespective of whether they are enlarged or not, enucleation is the most simple and effective choice

Medical management of acute glaucoma

Osmotic agents

•Mannitol 20% is usually used as an emergency therapy to reduce IOP rapidly in acute glaucoma and also during surgical intraocular procedures

• Mannitol is administered intravenously at a dose rate of 1–2 g/kg over 20–30 minutes, and will produce ocular hypotension within minutes

•When used prior to intraocular surgery, mannitol should be used some 30–60 minutes prior to surgery for maximum effect. Repeat in six hours if necessary

•State of hydration should be monitored, particularly in small dogs, and considerable caution should be exercised if there is pre-existing renal disease

Carbonic anhydrase inhibitors

•Usually used as part of long-term management

•Topical carbonic anhydrase inhibitors (dorzolamide and brinzolamide) work well for moderate increases of IOP

•There appear to be no systemic side effects, which represents a considerable advance on previous treatments

•For dorzolamide, one drop three to four times daily is required for 24-hour control and, for brinzolamide, one drop two to four times daily in the same time period

Prostaglandin analogues

•These drugs increase uveoscleral outflow, but should be used with caution when there is a possibility of uveitis

•The best known is latanoprost, which is expensive, but it is used only once or twice daily. Travaprost is an alternative choice used at the same application rate

•They may be used in conjunction with topical carbonic anhydrase inhibitors and are usually reserved for the more severe cases of glaucoma

•Both agents are very effective at lowering the IOP, albeit with intense pupil constriction as a side effect of treatment in dogs

LENS

EMBRYOLOGY

GENERAL AND CANINE OPHTHALMOLOGY

GENERAL AND CANINE OPHTHALMOLOGY

3Formation of the primary lens fibres

4Formation of the secondary lens fibres

Primitive vascular supply

•The hyaloid system (HA) extends from the developing retina to the posterior aspect of the lens where it divides into a capillary network that invests the lens, known as the tunica vasculosa lentis (TVL)

•The posterior part of the vascular system disappears some 2–3 weeks after birth, whereas the anterior part of the system has degenerated by 3–4 weeks after birth. For most of the animal’s life, therefore, the lens is avascular

•Any parts of the primitive vascular system remaining in situ eight weeks after birth will persist for life

FEATURES OF THE LENS

•Biconvex structure made up of lens fibres, contained within an elastic capsule. In the adult lens there is a single layer of epithelial cells beneath the anterior lens capsule

•The lens grows throughout life by the production of lens fibres from epithelial cells in the equatorial region. The epithelial cells turn inward and elongate to become secondary lens fibres. Old fibres are compressed centrally into a solid nucleus while younger ones form a softer cortex

•Avascular, but metabolically active

•Naturally-occurring lens tumours are unrecognised in the dog

•Consists of approximately 65% water and 35% protein. Soluble proteins (crystallins) predominate with lesser quantities of insoluble albuminoids. The insoluble proteins increase as the lens ages, with concomitant decrease in the soluble proteins

EXAMINATION

•In the normal animal, pen light and slit lamp examination of the lens following topical application of one drop of mydriatic (tropicamide) will reveal details of the anterior and posterior lens sutures, the corticonuclear junction and Mittendorf’s dot. Mittendorf’s dot is located the posterior lens capsule, beneath and between the ventral suture lines, which marks the original attachment of the hyaloid artery.

•In older animals, nuclear sclerosis, a normal feature of ageing that affects the lens nucleus, is apparent (Figures 3.52(a,b) and 3.53). The lens appears grey on naked eye examination and with distant direct ophthalmoscopy refractive rings at the nuclearcortical interface can be delineated clearly. Senile nuclear sclerosis should be differentiated from the true opacities of cataract in which lens opacities appear as black silhouettes with distant direct ophthalmoscopy.

(a)

(b)

Figure 3.52(a,b) Senile nuclear sclerosis in an aged Collie Cross. Gross appearance (a) and appearance with distant direct ophthalmoscopy (b). The distinction of the nuclearcortical interface and the lack of any true opacity are particularly clear with the latter technique. Note that benign melanosis of the iris is also present.

Figure 3.53 Senile nuclear sclerosis of the lens and equatorial cataract (peripheral vacuoles) in an old Miniature Long-Haired Dachshund.

CATARACT

Cataract is defined as any opacity of the lens and/or the lens capsule and is a common problem in the dog.

Classification (applies to all species)

•Time of onset (e.g. congenital, juvenile, senile)

•Location (e.g. capsular, subcapsular, equatorial, cortical, nuclear, axial, polar)

•Degree of maturation (e.g. incipient, immature, mature, hypermature, Morgagnian)

•Aetiology (e.g. primary and secondary inherited, traumatic, metabolic, nutritional, toxic, radiation-induced, electrocution, complicated). Complicated cataracts accompany or follow other ocular disease, such as neoplasia, or inflammations, such as uveitis

Developmental canine cataract

Developmental cataracts are almost certainly congenital (i.e. present from birth) but they will only be apparent when the eyelids open after birth. They may be inherited,

GENERAL AND CANINE OPHTHALMOLOGY

GENERAL AND CANINE OPHTHALMOLOGY

154 Notes on Veterinary Ophthalmology

or more commonly, non-inherited (as a reaction to intrauterine influences, e.g. teratogens).

Clinical findings

Can occur at any stage of lens formation, be unilateral or bilateral and may or may not be associated with other ocular anomalies (e.g. microphthalmos, nystagmus, persistent pupillary membrane, iris abnormalities, persistent hyperplastic primary vitreous, retinal dysplasia).

Inherited canine cataract

Primary

Primary inherited cataract is associated with no other ocular abnormality and is a relatively common cause of canine cataract; both congenital and acquired types are recognised (Figure 3.54). For an up-to-date list of affected breeds see In Practice Supplement on Hereditary Eye Disease.

Figure 3.54 Primary inherited cataract in a Golden Retriever. Posterior polar subcapsular cataract is a common type of inherited cataract in a number of breeds. The photograph approximates to the view obtained with distant direct ophthalmoscopy and the cataract appears as a black silhouette (compare this appearance with that in Figure 3.52(b)).

Secondary

Secondary inherited cataract is associated with other ocular abnormality, for example, generalised progressive retinal atrophy, retinal dysplasia, persistent pupillary membrane, persistent hyperplastic primary vitreous and multiple ocular defects. The luxated lens of animals with primary lens luxation can also become secondarily cataractous.

Acquired cataract

•Traumatic: blunt and penetrating injuries

•Metabolic: e.g. diabetes mellitus is a common cause of rapid-onset total bilateral cataract

•Nutritional: e.g. hand rearing on milk substitutes

•Toxic: e.g. drug-induced and teratogenic

•Radiation and electrocution

•Complicated: e.g. following severe ocular inflammation

Management of cataracts

Potential cataract patients should be referred as early as possible in order to allow accurate assessment and simplify the surgery.

•Cataract surgery is the only means of treatment at present and is a specialist procedure

•It is wise to refer cataract cases early for specialist assessment (e.g. fundus examination, diagnostic imaging, electroretinography). Hypermature cataracts, for example, are not ideal for surgery as the leakage of lens protein produces low-grade uveitis (Figure 3.47, p. 137)

•Diabetic cataracts can rapidly become hypermature, therefore affected patients should be referred as early as possible, provided their diabetes is under effective control

•Certain types of cataract are likely to be unsuitable for surgery. For example, cataracts secondary to other inherited eye diseases such as generalised progressive retinal atrophy (GPRA), post-traumatic cataracts and some types of cataract secondary to, or associated with, uveitis

•Prior to referral, the patient should be examined carefully, especially for other adnexal and ocular abnormalities (e.g. eyelid infections, eyelid tumours, distichiasis, KCS, uveitis, GPRA) and for systemic disease (e.g. diabetes mellitus)

•Cataract surgery is an elective procedure, so the patient can be prepared thoroughly

•The patient should be of good temperament and easily handled

•The age of the patient is not a contraindication

VITREOUS

Embryologically, development of the vitreous occurs in three stages:

1The primary vitreous forms initially, mainly as a primitive mesodermal vascular hyaloid system, which gradually disappears as the eye matures

2The secondary vitreous forms the definitive adult vitreous

3The tertiary vitreous forms the lens zonule

In the normal adult eye, Cloquet’s canal is all that remains of the primary vitreous. The vitreous fills the posterior cavity of the eye; it is a transparent, semi-fluid hydro-

gel which is almost 99% water. Collagen fibrils make up the framework of the vitreous and the collagen content varies between species. Hyaluronic acid interacts with the collagen network, both to stabilise the system and to produce a type of molecular sieve. This means that the normal vitreous excludes large molecules and impedes others.

CONGENITAL ANOMALIES OF THE CANINE VITREOUS

Persistent primar y vitreous

Anterior remnants

•Persistent hyaloid remnants, sometimes with a network of branching ghost vessels, are normal findings in puppies of a few weeks of age. Hyaloid remnants may not be obvious without a slit lamp.

•A small spiral tag, or a focal lens opacity (Mittendorf’s dot), inferonasal to the confluence of the posterior suture lines, is usually all that is apparent in adults. These remnants may not be apparent without a slit lamp.

GENERAL AND CANINE OPHTHALMOLOGY

GENERAL AND CANINE OPHTHALMOLOGY

•Persistence of an extensive hyaloid system in the adult is abnormal and may be associated with congenital posterior capsular cataracts.

Posterior remnants

The posterior part of the hyaloid artery may persist, usually extending from the central optic disc into the posterior vitreous within Cloquet’s canal. It may or may not contain blood.

Persistent hyperplastic primar y vitreous (PHPV)

•In this condition there is failure of regression of the foetal vasculature and fibroblastic hyperplasia of mesodermal tissue within the posterior vascular tunic of the lens. The result is a plaque of fibrovascular tissue on the posterior surface of the lens, with an attached hyaloid stalk, presenting as leukocoria (a white pupil) and interfering with vision (Figure 3.55(a,b)).

•Hereditary PHPV has been described in the Dobermann and Staffordshire Bull Terrier. It is pleomorphic in both breeds and usually bilateral. For an up-to-date list of affected breeds see In Practice Supplement on Hereditary Eye Disease.

•Retinal detachment

•Severe vitritis (active inflammation)

•Endophthalmitis

•Chronic degenerative changes of the vitreous (e.g. asteroid hyalosis – see below)

•Neoplasia

ACQUIRED CONDITIONS OF THE CANINE VITREOUS

Syneresis

Vitreal liquefaction is known as syneresis. It is a degenerative process associated with ageing and disease.

Asteroid hyalosis

•Asteroid hyalosis is an occasional finding in dogs with no previous history of ocular or generalised disease. The condition usually affects only one eye, but can affect both, and is more frequently encountered in older animals

•The asteroid bodies are calcium soaps (calcium-lipid) attached to the vitreous fibrillar framework, so that they move with eye movement but return to their original position when the eyes are still

•The opacities usually have little or no effect on vision, and the majority are of no clinical significance

Synchysis scintillans (Figure 3.56)

•This condition is also known as cholesterolosis bulbi. The numerous scintillating crystals that are present in the vitreous are indeed cholesterol, but the condition is not associated with systemic dyslipoproteinaemia

•The vitreous is liquefied and the crystals swirl around the vitreous cavity when the eyes move and settle ventrally (inferiorly) when movement stops

•The condition is not uncommon and is seen in association with a variety of ocular insults and diseases (e.g. after intraocular haemorrhage and other inflammatory and degenerative eye conditions)

•There is no obvious effect on vision, the condition is non-progressive and treatment is not required

GENERAL AND CANINE OPHTHALMOLOGY