Canine glaucoma

The term glaucoma refers to a group of neurodegenerative diseases that can affect the dog and the catcharacterized by polymorphic clinical symptoms having as a common denominator an increase in the intraocular pressure (IOP); this may trigger a chain of events first in the optic nerve and secondly in the retina, causing a damage to the integrity and function of these ocular structures which may persist even if the pressure returns within the normal limits.

CLASSIFICATION AND PATHOGENETIC MECHANISMS

In the dog, the classification of glaucoma can be based on numerous criteria. Based on the modality of onset, the glaucoma can be either acute or chronic, while based on the presumed triggering factor it can be primary or secondary.From an anatomical viewpoint, based on the appearance of the iridocorneal angle, it can be classified as open, narrow/closed angle glaucoma or as goniodysgenesis.

The acute form manifests itself in 24-48 hours. Clinically, it is characterized by a sudden and pronounced decrease or loss of vision, associated with the appearance of intense pain.IOP is increased.In chronic glaucoma the pain is seemingly less intense.The eye can be normo- or even hypotensive, as a result of the ciliary body damage caused by the increase in IOP, with consequent reduction in the production of aqueous humour.In this form, vision is constantly absent.

In primary glaucoma, no primary triggering causes of ocular hypertension are present, and the aetiopathogenetic mechanisms of the hypertonus lie exclusively in the iridocorneal angle and in the trabecular outflow pathways.Both eyes may be affected, with an extremely variable onset between the two; this may be influenced by the use or not of a prophylactic treatment in the eye not yet affected by the disease.It is more frequently present in adult or elderly animals.A higher incidence has been reported in purebred dogs, as there is a hereditary predisposition to this phenomenon.

Secondary glaucoma has one or more identifiable ocular aetiological entities that trigger the hypertensive phenomenon.Among the predisposing causes: dislocation and subdislocation of the crystalline lens, cataract, traumas, inflammatory processes, and intraocular tumours.

Dislocation of the lens can be caused by an increase in its volume, by its excessive mobility secondary to fragility of the zonular suspensory apparatus on a hereditary basis or by the rupture of the ligaments themselves on a traumatic or post-inflammatory basis.The anterograde displacement of the dislocated lens, obstructing the pupillary aperture, prevents the passage of aqueous humour into the anterior chamber, causing pupillary block.In these cases, also the vitreous body herniates into the anterior chamber and occupies the pupillary opening, contributing to the obstruction of the outflow of aqueous humour.The result of these changes is the accumulation of aqueous humour in the posterior chamber and the anterograde displacement of the base of the iris, which causes the closure of the iridocorneal filtration angle.

In the presence of a mature or hypermature cortical cataract, proteins may be released into the aqueous humour through the capsule, causing a low-grade phacolytic chronic uveitis, which may contribute in triggering the hypertensive crisis. An intraocular neoplasm, whether primary or metastatic, may be the cause of intraocular hypertension. This occurs through numerous pathogenetic mechanisms. Some neoplasms located in the ciliary bodies may close the filtration angle either through anterograde displacement of the lens and iris or by directly infiltrating the iridocorneal area through their exophytic growth.Some tumours tend to exfoliate their cells into the anterior chamber, consequently causing blockage of the filtration angle.Conjunctival, corneal and scleral tumours may infiltrate the trabecular meshwork, causing an increase in intraocular pressure.Haemorrhages secondary to neoplastic growth may obstruct the outflow of aqueous humour (Fig. 1).

In the course of anterior and posterior uveitis, the passage of proteins, fibrin and cellular components from the bloodstream (haemato-aqueous and haemato-retinal barrier)  causes an increase in aqueous humour viscosity, obstructing its outflow. adhesions (synechiae) between the iris and the anterior capsule of the lens, or pupillary occlusions, may often develop as a result of uveitis. In these cases the accumulation of aqueous humour in the posterior chamber causes an anterograde dislocation of the iris, which takes on a “bulging” appearance (iris bombè); as a result, the anterior chamber becomes shallower and the iridocorneal angle tend to narrow or close (Fig. 2).

Open-angle glaucoma is very common in humans, while in dogs it does not occur very often, except in Beagles, a breed in which heritability has been documented.The aetiopathogenic mechanism is not well known and the relatively slow onset is characterized by an initially subtle increase in intraocular pressure.

In the dog, narrow/closed-angle glaucoma is more common than open-angle glaucoma, and females are more frequently affected; surgery is often required to preserve eyesight.In this form, the base of the iris is displaced towards the cornea, the anterior chamber is shallower and the iridocorneal angle narrower or even closed.The condition is characterized by increasing IOP peaks that precede the actual acute glaucoma attack.

SYMPTOMS

Symptoms vary depending on whether the glaucoma is acute or chronic. Pain, sometimes sudden, appears during acute glaucoma, which manifests itself with dejection, anorexia, blepharospasm and third eyelid protrusion secondary to enophthalmos.Conjunctival hyperaemia and congested episcleral vessels (Fig. 3), epiphora and generalised or localised corneal oedema are present. The pupil is mydriatic or semi-mydriatic. From an ophthalmological viewpoint,in the initial phases of this condition the ocular fundus is usually normal,with theoptic disc appearing oedematous in some cases.High values of intraocular pressure are present.From the neuro-ophthalmological viewpoint, the menace response is almost always absent and the direct and consensual pupillary reflexes of the affected eye are altered.In this phase, no eyeball size variations are present.

In chronic glaucoma the pain is seemingly diminished.Conjunctival and episcleral congestion and epiphora are still present. The cornea is oedematous and with time neovascularisation and Haab’s striae may appear (Fig. 4). Retinal lesions characterised by vascular attenuation, peripapillary and tapetal hyperreflectivity, optic disc atrophy and excavation (cupping) are present. In the advanced forms,with the presence of buphthalmia (increase in eyeball size, more common in puppies due to the greater scleral and corneal elasticity) (Fig. 5), exposure keratitis with secondary corneal ulceration is often present. The menace response is absent, the pupil is mydriatic and the intraocular pressure may be continually high or normal or diminished in relation to the state of the ciliary bodies.In some cases, depending on the pathogenetic mechanism or on the duration of the symptoms, dislocation or subdislocation of the crystalline lens and cataract can be observed. In terminal cases bulbar phthisis (marked decrease in eyeball volume) may be present.

DIAGNOSIS

Goniodysgenesis, or dysplasia of the pectinate ligament, an anatomical structure that occupies the space between the base of the iris and the sclera, is a congenital disease linked to an altered differentiation of the iridocorneal angle during embryonic life. The latter initially appears as a strip of smooth, compact tissue and later, during gestation and in the immediate postnatal period, it becomes progressively rarefied (less dense, more porous and thin) until becoming a layer of tissue formed by thin fibres (pectinate ligaments) that delimit the aqueous humour outflow spaces (the spaces of Fontana). In subjects in whom the normal differentiation process stops after birth, in place of the pectinate ligament a compact and wide strip, with few holes that prevent the normal outflow of aqueous humour, is found. Other dogs may instead have long strings that often tend to intertwine with the adjacent ones until forming short, strong cords.

The diagnosis of glaucoma is based on the clinical appearance, on the measurement of intraocular pressure with tonometry and on the use of gonioscopy to evaluate the state of the iridocorneal angle.

The clinical appearance, for the purpose of evaluating the symptoms and clinical signs mentioned above, may be assessed by means of a slit lamp, which allows to highlight the depth of the anterior chamber and the position of the lens, especially in breeds prone to primary dislocation. Direct and indirect ophthalmoscopy, instead allows for a careful evaluation of the ocular fundus and of the optic disc.

Tonometry is ordinarily performed using four techniques:palpation (digital), indentation (impression), applanation and dynamic contour.The latter two techniques are much more accurate but also more expensive than the former two.

Transpalpebral digital palpation tonometry allows to determine the intraocular pressure by judging the resistance when exerting pressure on the eyeball with the index finger placed on the upper eyelid.The method is obviously  extremely empirical and subjective, and only allows to highlight extremely high pressure variations.

Applanation tonometry is undoubtedly more precise and easier to use.The tonometer is a small, lightweight, electronic, digital device shaped like a large pen that works based on the Imbert-Fick principle:“the force required to flatten a given area of a sphere is equal to the pressure inside the sphere”.In practice, with this instrument, a standard force (determined by the weight) is applied with a metal rod; the area is known and is equal to the dimension of the instrument’s plate.In this way, through the formula P = f/A, the force required to flatten (applanate) a small constant area is measured.The instrument allows precise measurement even in very reactive subjects; indeed, the vertical positioning of the head is not necessary and the need for animal restraint is minimal.The presence of corneal surface irregularities does not affect the resulting values and errors due to the different radii of curvature between one eye and the other are minimised (Fig. 6).

The TonoVet is used for dynamic tonometry; it measures the effect given by a very small magnetic probe that lightly touches the eye and bounces back (rebound effect). One of its advantages is that it does not require the use of a topical anaesthetic.Among its disadvantages there is the positioning of the device, which forces the operator to keep the instrument straight during the measurement operation, unlike the Tonopen, which allows greater positioning freedom.

The Schiotz tonometer is used for indentation tonometry (Fig.7). It is composed of a cup-shaped base that has to be rested on the cornea, a plunger, a support rod or arm, a graduated scale and known weights (5.5-7.5-10-15 g) that are put on the plunger.

The examination is based on the principle according to which the length of the plunger that comes out of the base is in relation to the deformation of the cornea indented by the plunger itself, which, in its turn, depends on the endocular pressure. The plunger is connected to a scale, so that one unit on the reading scale corresponds to 0.05 mm of corneal indentation. After having instilled one drop of anaesthetic eye drops, the exam is performed by putting the animal’s head perpendicular to the measurement cup, taking care not to compress the jugular veins, which would cause the intraocular pressure to increase, and resting the base centrally on the cornea. At least 3-5 readings are necessary; the average value is calculated and then converted with a species-specific table, since the instrument is calibrated for measurement on the human species.Some variables in the measurement are to be taken in consideration, including the eyeball size:in larger than normal eyes, the IPO reading tends to be lower, while the opposite occurs in smaller eyes.The last recordings tend to be lower than the first and if the exam is performed near the limbal region of the cornea, which is more rigid, the IOP appears higher.

Gonioscopic examination allows viewing the iridocorneal angle for the purpose of identifying any structural or amplitude anomalies. In view of a refractive index problem, the iridocorneal angle cannot be viewed directly through the cornea and for this reason a gonioscopic contact lens has to be placed over the cornea.This allows the deviation of light so that it comes out through the cornea towards the observer.Several types of lenses are available on the market, to be used according to the operator’s preference; among these the Koeppe, Barkan, Troncoso, Lovac, Cardona, Swan-Jacob and Franklin lenses may be mentioned.

The use of ocular ultrasonography through ultrasound biomicroscopy (UBM) has recently become an important diagnostic tool for studying the state of the iridocorneal angle, of the ciliary slit and of the peripheral iris.The examination is minimally invasive, even if it requires general anaesthesia.

TREATMENT

The main purpose of glaucoma treatment is to maintain vision by preserving the optic nerve and retinal function.This goal is reached by decreasing IOP to values such as to prevent further damage to the nerve structures involved.The obtained result is, however, closely correlated to the stage of the disease, being the prognosis from extremely guarded to negative in the chronic forms and guarded in the acute forms.In fact, in case of an especially elevated rise in pressure, the neurological damage may be irreversible even after just 24-48 hours.The treatment available to the clinician can be medical, surgical or a combination of the two.

Medical treatment
The goals of medical treatment (Table 1) may be pursued through:

• dehydration of the vitreous body
• reduction in aqueous humour production
• increase in aqueous humour outflow.

Reduced hydration of the vitreous body is obtained by using hyperosmotic drugs which, causing an increase in blood osmolarity, draw water from the various organic tissues, including the vitreous body.Their action at intraocular level, by causing a reduction in vitreous body and aqueous humour volume, is accompanied by a rapid drop in IOP.The active substances belonging to this category are mainly used in emergency situations (acute glaucoma attacks) and for short periods of time.They require an intact haemato-aqueous barrier and are thus not suitable in cases of glaucoma secondary to inflammation.To this end, 50% glycerine (glycerol) administered orally at a dose of 1-2 g/Kg (may cause vomiting) and 18-20% mannitol by slow intravenous infusion at a dose of 1-2 g/Kg in around 2 hours are normally used.The hypotensive effect of the latter manifests itself within approximately 30 minutes from the time of administration and lasts for around 5 hours.Mannitol is preferred over glycerol because it is faster-acting and has fewer side effects.Administrations of these agents can be repeated as needed, and no water should be given for a few hours.In heart patients or nephropathic subjects these drugs should necessarily be avoided.

Reduced aqueous humour production can be obtained through the topical or systemic use of carbonic anhydrase inhibitors (CAI), β-blockers and α₂-agonists through local application.In some cases these active substances can also be used in combination with each other.In veterinary medicine there is a preference for drugs in the first two classes, especially for the CAIs.In the non-pigmented epithelium of the ciliary body, the carbonic anhydrase enzyme catalyses the reaction between a molecule of water and one of carbon dioxide with formation of bicarbonate, which in its turn causes the entry of sodium and then of water inside the ocular structure, promoting the formation of aqueous humour.Dichlorphenamide, acetazolamide and methazolamide belong to the category of CAIs for systemic use.Their maximum action is carried out in around 4-8 hours and the intraocular pressure is lowered by approximately 20-30%.These drugs induce metabolic acidosis, and for this reason their use may lead to side effects such as vomiting, diarrhoea, dejection, poor appetite, hypotension, tachypnea and shock.They are also a cause of hypokalaemia, so their administration should always be combined with potassium supplementation.Among topical CAIs, the most used are 2% dorzolamide (2-3 times per day) and 1% brinzolamide (2-3 times per day).A local burning sensation may be present at the time of application but they have none of the systemic side effects caused by the analogues for systemic use.

The action of adrenergic β-blockers is limited to the β receptors only (β₁ for the most recent active substances), through the blocking of the adrenergic stimulation on the β receptors of the ciliary body, causing a reduced production of c-AMP and thus of aqueous humour.Since beta-adrenergic receptors are widespread in the body, side effects involving the cardiovascular system (bradycardia and decrease in blood pressure) and lungs (broncospasm) may appear. Among the β-blockers used most in glaucoma there are imolol maleate (0.25-0.5% 2-3 times per day) and betaxolol.These active substances are often used in combination with topical CAIs.

An increased aqueous humour drainage may be obtained by using direct-acting miotic drugs that increase the ease of outflow towards the trabecular meshwork through the contraction of the longitudinal ciliary muscle.Parasympathomimetics, such as 1-2% pilocarpine administered every 8-12 hours, are among these drugs.

Other drugs have an indirect miotic action through the potentiation of the activity of acetylcholine (acetylcholinesteraseinhibitors), such as 0.125-0.25% demecarium bromide(1-3 times per day).They should not be administered in case of intraocular inflammation and tend to act locally as an irritant.

Non-selective sympathomimetics, such as 0.5-2% epinephrine or dipiverine, acting on both α and β receptors, cause a reduced production and a greater outflow of aqueous humour.

Topical prostaglandins (latanoprost, travoprost, bimatoprost, unoprostone) carry out their hypotensive action by increasing the outflow towards the unconventional (uveoscleral) pathway, which in the dog is responsible for around 15% of the entire outflow.However, in the case of glaucoma secondary to inflammation they should be used with extreme caution, due to the possible exacerbation of the inflammation itself.

Table 1. Anti-glaucoma drugs

Pharmacological maintenance and prophylactic treatment in glaucoma
In a subject with a unilateral disease, should the healthy eye show predisposition to a possible glaucoma attack similar to the contralateral eye a preventive treatment is advisable, in the attempt of limiting the risks of a hypertensive crisis over time.To this end, and depending on the clinician's choice, a topical carbonic anhydrase inhibitor or a beta-blocker, in combination or not with a topical corticosteroid, may be used. In some canine breeds subject to this disease it has been reported that this treatment considerably increases the interval between the diagnosis of primary glaucoma in one eye and the development of the glaucomatous process in thecontralateral eye.

Surgical treatment
The selection of the surgical treatment is extremely different depending on the state of vision of the examined eye.Indeed, if the patient can still see, the short- and medium-term control of IOP with medical treatment alone is often extremely complex and, frequently, the control of the pressure state is unsatisfactory, as it tends to fluctuate and rise in spite of a correct pharmacological approach.For this reason, in selected cases, conservative surgical techniques that reduce the production of aqueous humour can be used, or techniques that increase its drainage, or a combination of both.

On the contrary, if the glaucoma is in a chronic state and the patient’s eyesight has been irreparably lost, more invasive but resolving operations on the eyeball are preferred, in order to prevent discomfort to the animal and not to oblige the owner to face long, difficult and often expensive treatments that do not achieve tangible results.

Surgical treatment of eyes with preserved eyesight
Surgical techniques that reduce the production of aqueous humour

• Cyclocryosurgery
• Transscleral cyclophotocoagulation (TSCP)
• Endoscopic cyclophotocoagulation

Surgical techniques that increase aqueous humour outflow

Gonioimplantation

Cyclocryosurgery
The purpose of this technique, described for the first time in the dog in 1980, is to reduce, through targeted destruction, the secretory capacity of the epithelium of the ciliary body using cold.Since it has been demonstrated that a temperature of -20°C causes cryonecrosis, temperatures fluctuating between -12 and -15°C are usually used.This type of surgery makes use of equipment that works with liquid nitrogen or nitrogen protoxide.

The operating principle of this method derives from the observation that intracellular and extracellular crystallisation, drawing water from the cell, causes dehydration.The resulting thermal shock and thrombosis are followed by ischaemic necrosis.The liquid nitrogen system is considered the most effective, as intracellular crystallisation causes more localised effects and results from a rapid reduction in temperature in the tissues.This system has the advantage of reaching the desired degree of cooling more rapidly, with a consequent more controllable cryonecrosis; indeed, in the treatment with liquid nitrogen using a cryoprobe temperature of -185°C, the ciliary body reaches a temperature of -23°C already after 30 seconds.

The freezing technique is performed using a 2-3 mm diameter probe applied externally to the eyeball, 5 mm behind the limbus, exerting a slight pressure on it.From 4 to 8 points are treated on the upper half of the eyeball, avoiding the 3 o’clock and the 9 o’clock positions, around the course of the long posterior ciliary arteries, to prevent infarction and anterior segment ischaemia.When using liquid nitrogen, the freezing time is determined by the dimensions of the “ice ball” that develops on the periphery of the cornea and that normally becomes 1 mm in size in 15-25 seconds, with a ciliary body temperature of -10°C.The results of freezing are noticeable after 2-4 weeks.Damage to the ciliary epithelial cells and iris necrosis have been reported one week after the operation, but are no longer present after 6 months, which proves that partial regeneration of the epithelium occurs over time, even though the intraocular pressure remains at lower values than those assessed before the operation.

Cyclocryotherapy causes a marked uveitis, with all the correlated symptoms.For this reason, treatment with NSAIDS or topical and systemic corticosteroids should be carried out before and after the operation.The complications of this technique include hyphaema, iris depigmentation, cataract, retinal detachment and bulbar phthisis. A temporary increase in intraocular pressure is often present, requiring the use of anti-glaucoma therapy for a few weeks after surgery.Some authors have reported a success rate of the technique in controlling intraocular pressure of approximately 90%, and of 60% in maintaining eyesight 6 months after surgery.

Transscleral cyclophotocoagulation (TSCP)
This surgical technique is performed using laser (Light Amplification by Stimulated Emission of Radiation).The energy produced reduces the production of aqueous humour primarily by the direct action on the pigmented epithelium of the ciliary body, and secondarily by the indirect damage caused on the same cells by the ischaemia and by the inflammation which result from the treatment. Compared to the previously described cyclocryosurgery, TSCP has the advantage of causing lesions which are more focal, thus causing lesser damage to neighbouring tissues.

Lasers are classified according to the physical state of the active material with which they operate and are differentiated into:

• solid state lasers (neodymium:yttrium-aluminium-garnet)
• liquid dye lasers
• gas lasers (argon, helium-neon, CO₂)

Lasers have different wavelengths and can operate in pulsed or continuous mode.Nd:YAG (neodymium:yttrium-aluminium-garnet) lasers and semiconductor diode lasers are the ones used most often in veterinary ophthalmology.The Nd:YAG laser combines the advantages of a wavelength characterized by high affinity for blood pigments with those deriving from it being a solid state laser, more stable and reliable than ion lasers.The neodymium YAG laser is composed of an inert, highly stable crystal formed by yttrium, aluminium and garnet, used as support for the neodymium atoms.The produced energy is absorbed only in minimal part by the transparent tissues, while the energy emitted with the continuous wave module produces a thermal effect which is absorbed by the pigmented tissues containing melanin, with consequent coagulative necrosis of these tissues.The energy of the Nd:YAG laser can also be released through the Q-switching process, which releases an intense ray of energy for an extremely short period of time; in this case the effect obtained is the photo laceration of the tissue in the site of impact.

Diode (810 nm) lasers are preferable to Nd:YAG (1064 nm) lasers as they are smaller, more reliable, less expensive and easier to transport; they are also efficient, since they penetrate deeper into the tissues compared to the Nd:YAG laser, allowing greater depth of action in the uveal tissue; in addition, since melanin absorbs the energy released by the diode laser to a greater degree, this laser is especially useful in veterinary medicine, where the eyes have a greater pigmentation.

There are basically three ocular pigments responsible for the absorption of laser energy:melanin, xanthophyll and haemoglobin. Melanin, the most important pigment for the absorption of laser light, absorbs the visible and infrared wavelengths (400-1400 nm).Absorption is reduced as the wavelength decreases.Since melanin is highly concentrated in the uveal tissue, in the retinal pigment epithelium and in the choroid, the former is the main site of absorption of laser energy for the anterior ocular segment, while the last two for the posterior segment.Xanthophyll, found in the macula, absorbs blue light.Finally, haemoglobin absorbs blue, green and yellow light well and red light weakly.The absorption of laser radiation by the ocular tissues causes an increase in the temperature and thus photocoagulation.The photocoagulation effect depends on the extent of the impacts, on the exposure time and on the power used.

TSCP is a non-invasive procedure with low probabilities of complications.As a matter of fact, the diode laser is capable of preserving eyesight in approximately 50% of treated dogs, with a significant advantage in the treatment of primary glaucoma in dogs.Generally speaking, animals with less pigmented uvea require higher laser energies and repeated treatments, with a higher possibility of failure in controlling intraocular pressure.Although both the diode laser and the Nd:YAG laser are capable of reducing intraocular pressure, comparative studies performed on dogs with glaucoma have demonstrated that the control of intraocular pressure up to 25 mmHg was accomplished more quickly with the diode laser (hours) than with the Nd:YAG laser (weeks).In both, the control of intraocular pressure was maintained for six months.Intraocular complications, presumably caused by the effects of the laser energy or by secondary inflammations, were similar for both methods, although the incidence of cataract was higher when using the Nd:YAG laser.

Since in ophthalmology lasers with different wavelengths are used, it is important to be aware of the damage that each laser can cause to the patient’s eye and to operating room staff.The damage depends on the wavelength, on the intensity of the radiations and on the absorption typical of each tissue.The diode laser wavelength requires the use of protective safety goggles with an optical density ranging from a minimum of 4.5 and up to 980-1064 nm.

Animals undergoing TSCP must be pretreated with antibiotic and anti-inflammatory therapy and require a mild general anaesthesia.The treatment protocol for the diode laser envisages the activation of the laser on 30-40 points, distributed above and below, 3 mm behind the limbus and avoiding the 3 o’clock and 9 o’clock positions, near the posterior ciliary arteries, using 1250 mwatts of energy for 1500 msec.A possible increase in intraocular pressure in the immediate period following surgery is treated using a 30G needle inserted into the limbus to allow the passive outflow of aqueous humour until an intraocular pressure of 10 mmHg is obtained.In the 7-10 days after the operation, drugs are to be used for pressure control, as well as anti-inflammatory agents to limit uveitis caused by the operation.Some authors also recommend the use of subconjunctival steroid injections (e.g. 2-4 mg of triamcinolone).The side effects are intraocular haemorrhages, retinal detachment, cataract and corneal ulcerations, most likely of neurotrophic origin.

Endoscopic cyclophotocoagulation
The recent development of the video endoscope has decreased the use of the diode laser in cyclophotocoagulation, thanks to the possibility of direct visualisation of the laser probe and of its action on the ciliary processes.In both external and endoscopic cyclophotocoagulation operations, drainage systems or gonioimplants are often used in combination with laser surgery.

Gonioimplantation
Gonioimplantation consists in the positioning of a drainage system, which can be fitted or not with a valve sensitive to intraocular pressure variations, in the sclera under the conjunctiva and Tenon’s capsule, usually in the superotemporal quadrant.The device consists of a small tube (made of silicone or sylastic), with an outer diameter of approximately 0.7 mm and an inner diameter of 0.3 mm, which is inserted about 5 mm into the anterior chamber through a thin scleral incision.This measurement is very important because an excessive length can cause a progressive and persistent oedema, due to contact with the corneal endothelium, while an insufficient length can cause contact with the iris and consequently obstruction.The tube is sutured with non-absorbable thread to the sclera, 4 mm behind the limbus, and runs under the conjunctiva up to a drainage plate made of rigid material (polypropylene or methyl methacrylate), which is also sutured to the sclera 10-12 mm behind the limbus at the level of the equator, with 6/0-8/0 non-absorbable material. The conjunctiva above is closed with a continuous suture with absorbable thread.In order to inhibit fibroblast activity in the implant site and prevent its occlusion, during surgery the authors of this article resort to the subconjunctival application of an antifibroproliferative agent, e.g. 5-fluorouracil or mitomycin C (0.5 mg/ml).The products are applied on the exposed sclera with a surgical sponge, for 5 minutes, before positioning the drainage; a thorough washings with lactated Ringer's solution or Balanced Salt Solution (BSS) is then performed to remove the excess product.A 25 µg dose of TPA (Tissue Plasminogen Activator) can also be used at intrachamber level to limit the production of fibrin.For valveless implants, the operating principle envisages the bidirectional passage of the aqueous humour through the tube, up to the posterior implant surface, around which a fibrous capsule forms during the first 3-4 weeks; this functions as an internal filtering bleb with passage and reabsorption of the liquid in the episcleral spaces and through the capillary and lymphatic pathways.Implants fitted with a valve only allow one-way circulation of the aqueous humour; the valve is set so that if the IOP falls below 10 mm/Hg the flow stops, to prevent possible hypotonia of the eyeball.When valveless implants are used, an extremely low IOP is usually observed in the immediate postoperative period.As mentioned beforehand, this method may also be used in combination with the previously cited cycloablation techniques.

Antibiotics and corticosteroids or NSAIDS should be administered topically and by systemic route after the operation.Some authors suggest the use of subconjunctival injections of corticosteroids, which should be given weekly for 8-12 weeks in the implant site.The implant models used most often in veterinary ophthalmology are:Molteno, Schocket, Baerveldt, White and T-implant among the valveless ones and the Krupin, Ahmed, Hitchins and Joseph among the valve implants.

The complications associated with the grafting of artificial drainage devices include implant extrusion but especially, even after a few months, their obstruction due to fibrin deposits inside the tube, with reduced filtration and a new increase in pressure.

Surgical treatment of eyes that have not preserved eyesight
In cases of chronic glaucoma, with an irreversible loss of sight, in order to prevent the subject from experiencing persistent pain, which is not always easy for the owner to evaluate, or the onset of possible complications, more invasive surgical treatments are envisaged, such as chemical ablation of the ciliary body or the removal of the eyeball or of its content (enucleation and intrascleral implant).

Chemical ablation of the ciliary body
The chemical ablation of the ciliary body may be accomplished by aspirating 0.5-1.5 cc of the vitreous body, which in the chronic forms is often deteriorated and thus liquefied, using a 20-22G needle and subsequently injecting gentamicin or cidofovir, an antiviral drug, into it.The injection through the superotemporal sclera, 5-6 mm behind the limbus, at the 12 o’clock position, of strong doses of gentamicin (25 mg) mixed with 1 mg of dexamethasone, taking advantage of the toxic action of the antibiotic towards the retina and the ciliary processes, causes the destruction of the ciliary body epithelial cells that produce aqueous humour and thus the decrease in IOP.The gentamicin dosage should never be below 20 mg per subject, being careful never to exceed the maximum daily dose of 4.4 mg/kg.

The needle should be inserted gently and inclined so as to avoid the crystalline lens and thus the risk of an endophthalmitisresulting from the laceration of its capsule.Possible complications include bulbar phthisis and cataract formation.As an alternative we can resort to the injection of cidofovir which, compared to gentamicin, is difficult to find and is expensive; however, these factors are compensated for by the possibility of administering a very low dosage (300-500 μg), which avoids aspiration of the vitreous body, and by the lesser postoperative side effects, as it does not damage the lens or retina but only the ciliary body cells.The failure rate of this drug is of around 25%.If an intraocular neoplasm is suspected, chemical ablation of the ciliary body is not recommended.

Evisceration with insertion of intraocular prosthesis
For this procedure the subject should be positioned in sternal recumbency. A lateral canthotomy is first performed, to enlarge the operating field, after which a limbus-based or fornix-based conjunctival flap is prepared.A dorsal incision is then made in the sclera, in the paralimbal region, to remove the contents of the eyeball, leaving only the fibrous tunic (consisting of the cornea and sclera) in place. The subsequent steps involve insertion of the prosthesis (Fig. 8), suturing of the scleral wound, suturing of the conjunctival flap, suturing of the canthotomy and, finally, a temporary tarsorrhaphy to protect the eyeball.The size of the black silicone prosthesis is selected using a gauge that calculates the horizontal diameter of the cornea of the healthy contralateral eye.Depending on the surgeon’s experience, this measurement is considered valid for the size of the prosthesis, otherwise 1-2 millimetres are added to or subtracted from it.The implant is positioned with a Carter silicone prosthesis introducer (Fig. 9).

Preoperative treatment envisages the use of tobramycin-based and/or fluoroquinolone-based topical antibiotic prophylaxis, a systemic antiobiotic prophylaxis with amoxicillin and clavulanic acid (15-20 mg/kg/bid) or cephalexin (10 mg/kg/bid), and an anti-inflammatory therapy with corticosteroids:prednisone (0.5-1 mg/kg/day) or NSAIDs, carprofen (4 mg/kg/day).Post-surgical therapy, besides using the above-mentioned active substances, also envisages the use of pain medications such as opiates and morphine derivatives, in particular:

• Butorphanol                 0.11-0.22 mg/kg
• Tramadol                     2-4 mg/kg
• Fentanyl                       1 µg/kg i.v.
• Buprenorphine             10 µg/kg
• Morphine                     0.5-1 mg/kg

Intraoperative complications include the onset of significant haemorrhages, scleral rupture due to prostheses larger than the scleral incision and difficulty in suturing the sclera because of a too large prosthesis.The appearance of corneal oedema, neovascularisation, formation of corneal erosions or ulcers, temporary keratoconjunctivitis sicca, dehiscence of the canthotomy sutures, onset of pigmentary keratitis, dehiscence of the conjunctival suture, secondary entropion and corneal mineralisation can be considered minor forms of postoperative complications.The appearance of refractory corneal ulcers, chronic keratoconjunctivitis sicca, onset of endophthalmitis or panophthalmitis, recurrence of a possible intraocular neoplasm, implant rejection, dehiscence of the corneal or scleral suture are considered to be among the severe complications.A histopathological examination should always be performed on the eviscerated tissue to rule out the presence of an intraocular neoplastic form.

Enucleation
This is the least complex procedure and is the surgery of choice if intraocular tumours or widespread infections are present.It consists in the removal of the eyeball and suturing of the eyelids (permanent tarsorrhaphy).To this end, both the transconjunctival and the transpalpebral approach may be performed.This operation has an extremely low percentage of postoperative complications.An additional technique may be the positioning of a silicone prosthesis in the empty socket once the enucleation procedure has been performed.With regard to the pre- and postoperative therapy for these operations, please refer to the description in the section on evisceration with insertion of an intraocular prosthesis.

SSuggested readings

1. Bentley E, Miller PE, Murphy CJ et al. Combined cycloablation and gonioimplantation for treatment of glaucoma in dogs: 18 cases (1992-1998). J Am Vet Med Assoc. 215: 1469-1472, 1999.
2. Bingaman DP, Lindley DM, Glickman NW et al. Intraocular gentamicin and glaucoma: a retrospective study of 60 dog and cat eyes (1985-1993). Veterinary and Comparative Ophthalmology 4: 113-119, 1994.
3. Brightman A , Vestre W, Helper L: Cryosurgery for the treatment of canine glaucoma. J Am Anim Hosp Assoc 18: 319-322, 1982.
4. Brooks DE, Dziezyc J. The canine glaucomas: Pathogenesis, diagnosis, and treatment. Compend Contin Educ Prac Vet 5: 2992-301, 1983.
5. Brooks DE. Glaucoma in the dog and cat. Vet Clin North Am Small Anim  Pract 20 (3): 775-797, 1990 .
6. Cook C, Davidson M, Brinkmann M et al. Diode laser transscleral cyclophotocoagulation for the treatment of glaucoma in dogs: results of six and twelve months follow–up. Veterinary and Comparative Ophthalmology 7: 148-154, 1997.
7. Sapienza JS, van der Woerdt A.Combined transscleral diode laser cyclophotocoagulation and Ahmed gonioimplantation in dogs with primary glaucoma: 51 cases (1996-2004). Vet Ophthalmol. 2005 Mar-Apr;8(2):121-7.
8. Hardman C, Stanley RG. Diode laser transscleral cyclophotocoagulation for the treatment of primary glaucoma in 18 dogs: a retrospective study.Vet Ophthalmol. 2001 Sep;4(3):209-15.