CANINE GLAUCOMA AND MANAGEMENTAL MODALITIES

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CANINE GLAUCOMA AND MANAGEMENTAL MODALITIES

Rai Somil1 & Chandrapuria V.P.2 1

PhD Scholar

2

Professor & Director Clinics

Department of Surgery and Radiology, College of Veterinary Science and A.H.,

Nanaji Deshmukh Veterinary Science University, Jabalpur (M.P.)

INTRODUCTION

Glaucoma is a syndrome in which progressive elevation in intraocular pressure (IOP)

of the eye results in damage to the optic disc and retina and characteristic changes of

glaucoma include disrupted axoplasmic flow in the optic nerve head, death of retinal ganglion

cells and their axons, cupping of the optic disc, and visual impairment or blindness. Normal

canine IOP is reported as approximately 10 to 20 mmHg. Glaucoma starts occurring at IOP

above 25 mmHg in dogs. The elevated IOP is due to a disharmony between production and

outflow of the aqueous humor. More specifically, a reduction in the outflow rate of the

aqueous humor is responsible for the elevated IOP, not the overproduction of the aqueous

humor (Magrane, 1977). The most commonly affected breeds are Labrador retriever, Toy

poodle, Chow chow, American cocker spaniel, Basset hound, Samoyed, Beagle, Siberian

husky (Maggs et al., 2013). Glaucoma is result of impaired aqueous humor outflow. The

mechanism of this impairment may be etiologically classified as Primary glaucoma have no

association with another ocular or systemic disorder, are typically bilateral, have a strong

breed predisposition, and hence are believed to have a genetic basis. Primary glaucoma is

subdivided into two main forms.

Primary open angle glaucoma (POAG), in which the drainage angle appears

gonioscopically normal. POAG is a bilateral disorder in which IOP tends to increase in a

slow, insidious fashion simultaneously in both eyes in young to middle aged dogs. Initially

the gonioscopically visible angle is open. Over time the angle closes, the globe becomes

buphthalmic, and the lens may subluxate (Curtis and Barnett, 1980). The precise mechanism

of POAG in dogs is unclear, but it most likely results from biochemical alterations in the

trabecular meshwork that ultimately leads to greater resistance to aqueous outflow and

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gonioscopically narrowed or closed. PACG is also a bilateral disorder but it tends to manifest

as an initially unilateral, rapid, marked increase in IOP in middle aged to older dogs. Again,

the precise mechanism by which PACG occurs is uncertain, but there is a clear association

with congenital pectinate ligament dysplasia. It is also associated with a female sex

predisposition, periods of stress or excitement, and dim light. In the dog, PACG is at least

eight times more common than POAG. Acute PACG also is two times more common in

female dogs than in male dogs (Ekesten et al., 1997)

Secondary glaucoma is at least twice as common as primary glaucoma in dogs and is

associated with other ocular or systemic disorders that alter aqueous humor dynamics

(Kennard, 2009). Secondary glaucoma may be unilateral or bilateral and may or may not be

inherited and the cause such as iridocyclitis, intraocular neoplasm, inflammation, lens

displacement, uveitis, intraocular hemorrhage and advance cataracts in the eye can be

identified (Crispin, 1988).

Glaucoma is one of the most commonly misdiagnosed eye conditions. The signs

present in a particular animal depend on the duration, intensity, and causes of the pressure

elevation and commonly includes Descemet’s streaks, Aphakic crescent, Luxated lens,

Corneal edema, Iris atrophy, Enlarged episcleral vessels, Fixed dilated pupil, Shallow

anterior chamber, Cupping of optic disc, Bupthalmos, Increase intraocular pressure, Partial or

complete loss of vision and Ocular pain (Allgoewer, 2006).

DIAGNOSTIC MODALITIES

Normal canine IOP is reported as approximately 10 to 20mmHg. However, significant

variation is noted among individuals as well as among techniques and the time of day at

which IOP is measured. Therefore, comparison of IOP between right and left eyes of the

same animal is critical to interpretation of results. A good rule of thumb is that IOP should

not vary between right and left eyes of the same patient by more than 20%. Diagnostic

procedures are invaluable for the diagnosis and treatment of the glaucoma that are

particularly useful include:

Tonometry

Tonometry measure intraocular pressure (IOP) and is perhaps one of the most important

diagnostic tests in veterinary practice. Before use, a drop of topical anesthetic is applied to

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using the nondominant hand. Care must be taken not to place pressure on the globe. Its three

types: (Maggs et al., 2013). Indentation Tonometry in which the Schiotz tonometer relies

upon indentation tonometry. In this method, a standard force is applied with a metal rod over

the anesthetized cornea. The distance of the rod indents the cornea is measured and is

inversely related to the IOP (i.e., the greater the tonometer scale reading, the lower the patient’s IOP). This requires that the head be elevated so that the corneal surface is

horizontal. The tonometer is placed on the eye and scale reading is recorded. The calibration

table supplied with the instrument is then used to convert the scale reading to the IOP via the

appropriate column.

Applanation tonometry using the Tono-Pen is suitable for large animals, is unaffected

by variations in ocular rigidity, requires no conversion tables, and needs no sterilization. The

principle of applanation tonometry is that the force required to flatten a given area of a

sphere is equal to the pressure within the sphere (the Imbert-Fick law). Therefore, if the area

is known and the force is measured, the pressure can be calculated (Barnett, 2006).

Rebound tonometer eject a small probe (such as a metal pin with a rounded end) at a

fixed distance from the cornea and assess the motion of the probe as it strikes the cornea and

is returned to the instrument. Eyes with higher IOP cause a more rapid deceleration of the

probe and shorter return time to the instrument. This technique is affected by ocular surface

tension and so should be performed before application of any topical medications.

Ophthalmoscopy

Direct or indirect ophthalmoscope may be used to examine the optic nerve head for

evaluate the ocular fundus and cupping of the optic disc, which is the hallmark of glaucoma.

The red-free filter (green light) of these instruments facilitates examination of the optic nerve

and retinal nerve fiber layer. The ocular fundi of both eyes of the canine glaucomatous

patients must be evaluated and compared. In the event IOP is elevated above normalcy,

critical evaluation of the ocular fundus must be delayed until IOP is decreased. Corneal

transparency may be reduced by IOP, which tends to obscure interpretation of the optic disc.

Frequently, elevated IOP also cause reversible compression of the retinal vessels, Cupping of

the optic disc, loss of myelin in the lateral aspects of the optic disc as well as enlargement of

the optic disc. Its two types (Gelatt, 1981) direct and indirect opthalmoscopy. The Direct

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may be valuable in the evaluation of details of the optic disc. The indirect ophthalmoscopy

can also be performed through a small pupil provided. The special monocular or binocular

small pupil indirect ophthalmoscope is use. The 2 to 4 X upside down and backward

magnification of the ocular fundus with the indirect method is useful in the presence of

corneal and lens opacities.

Gonioscopy

Gonioscopy describes examination of the iridocorneal or “drainage” angle (the

junction between the iris and cornea). In the normal eye of most species light rays that are

reflected from the drainage angle strike the posterior cornea and undergo total internal

reflection as in a prism. This occurs because of the difference in refractive index between the

cornea and the surrounding air, and the high angle of incidence of the light rays from the

drainage angle. By replacing the air surrounding the cornea with a goniolens that has an index

of refraction close to that of the cornea, total internal reflection is avoided and light rays from

the drainage angle can be viewed directly through the goniolens. Gonioscopy is used

primarily to determine whether the angle is open, narrow, closed, or obstructed by

mesodermal remnants, and to check for the presence of foreign bodies, tumors, and

inflammatory exudates (Bedford, 1985).

MANAGEMENT

In most canine glaucomatous patients, the condition is presented in an advanced stage

often requiring intensive medical treatment as well as combination with a surgical

antiglaucoma procedure. Medical treatment of glaucoma often becomes a long term

administration of single or a combination of drugs to maintain IOP within normal limits, and

to prolong vision for as long as possible (Reinstein et al.,2009). Medications, often in the

form of eye drops, work to decrease intraocular fluid production, increase fluid drainage, or

both. These medications are associated with side effects, including burning sensations in the

eye, headaches, cardiac fluctuations and blurred vision. The administration of medications for

the treatment of animals is difficult and ineffective (Abrams, 2001).

Iridencleisis

A fixation suture is placed through the conjunctiva and tenon’s capsule close to the

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grooved incision is made on the sclera side of the limbus with a 64 beaver blade. Pin point

coagulation of episcleral vessels is necessary for hemostasis. The anterior chamber is entered at the 12 o’clock position with a 65 beaver blade. Entrance into the anterior chamber is

shallow to avoid damage to the iris, and the blade is positioned parallel to the surface of the

iris. Corneoscleral scissors are used to extend the incision for the entire length of the groove.

The incision is opened after cauterizing the corneal and sclera lips with electrocautery. An

iris hook is used to withdraw the iris through the incision. The superior papillary border of

the iris is grasped with two iris forceps and a radial iridotomy is performed with an

electroscalpel. Each iris pillar is sutured to the sclera with a simple interrupted 6/0 gut suture. The conjunctiva and tenon’s capsule have been closed with a simple continuous suture

pattern using 6/0 gut. Closure of the lateral canthotomy has been completed (Lew and Lew,

2009).

Sclerectomy and Iridocyclectomy

A 20 mm long incision parallel to and 8 mm from the limbus has been made through

the conjunctiva utilizing a stevens tenotomy scissors. The conjunctiva and tenon’s capsule are

bluntly dissected and reflected at the limbus. A no. 15 blade is used to create a sclera window

and thermal cautery is used to control haemorrhage. The sclera window as outlined is

removed exposing the suprachoroidal space. A cyclodialysis spatula is passed into the

anterior chamber from the suprachoroidal space to separate the ciliary body from the sclera

for approximately 120 degree superiorly. The arrow indicates the direction of motion of the

cyclodialysis spatula. The iris hook has grasped the iris root and withdrawn the iris and

ciliary body through the sclera window. An electroscalpel is used to create an iridocyclectomy. Closure of the conjunctiva and tenon’s capsule is done with a simple

continuous 6/0 suture(Gelatt, 1981).

Anterior chamber implants

A conjunctival-Tenon’s capsule incision makes parallel to and approximately 5mm

posterior to the limbus. The dissection continues down to the sclera and extended both

anteriorly toward the limbus and posteriorly between the extraocular muscle to form a

subconjuctival-Tenon’s capsule pocket After preparing pocket the Ahmed’s gonioimplant

valve (AGV) verify to be patent by flashing it with 0.9% normal saline and blunt tipped 27

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technique 8 mm posterior to limbus. After using 23-gauge needle to enter the anterior

chamber and make a tunnel. The Ahmed's gonioimplant tube was inserted through the 23

gauge needle track. The tube ideally protrudes into the anterior chamber 2 mm so that not to

touch the cornea, and not to extended into the papillary aperture. To secure the tube, Tenone’s capsule and conjunctiva close with a simple continuous suture using 7-0 polyglactin

910 (Fattahian et al., 2008).

Cullen frontal sinus shunt

A long incision made over the zygomatic process of the frontal bone. The frontal sinus

end of the shunt is placed within the trephined hole of the sinus and can be held in place by

slight compression of the sealing device. A conjunctival incision is made from the limbus to

the fornix and extended through to the episclera. A small forceps is used to bluntly dissect a

tunnel from the conjunctival incision to the frontal sinus incision. When the tip of such

instrument can be visualized it can be used to grasp the free end (anterior chamber end) of the

shunt. The anterior chamber end of shunt enter in the anterior chamber at 10-25° anterior to

the limbal plane to ensure that the shunt is positioned equal distance between the iris and the

cornea to avoid contact. The shunt is anchored to the sclera with nonabsorbable suture in a

simple interrupted pattern. The Tenon's capsule and conjunctiva are closed with absorbable

7-0 suture in a simple interrupted suture pattern. The frontal sinus end of the shunt is withdrawn

from the frontal sinus and monitored for flow of aqueous humor through the shunt. When

fluid flow through the shunt is confirmed, such flow is adjusted by valve device.

Subcutaneous tissues and skin are closed with routinely manner (Cullen et al., 1998).

Cyclocryothermy

Cyclocryothermy is the application of intense cold directly through the bulbar

conjunctiva and sclera to the ciliary body to reduce the rate of aqueous humor formation. The

advantage of this procedure is that it may be applied repeatedly because the bulbar

conjunctiva and sclera are not adversely affected. The cryoprobe is applied 5 mm from the

limbus directly on the dorsal bulbar conjunctiva and 3 to 5 sites are frozen to - 60ºc for 60

seconds each (Benson and Nalson,1990).

LASER SURGERY

Lasers are routinely used in veterinary ophthalmology for the treatment of a number of

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aim is to damage the cells, which secrete aqueous in the eye:

Trans Scleral CycloPhotocoagulation (TSCP)

The delivery of laser energy through the sclera may be performed by either the

non-contact or non-contact method. In the non-non-contact approach, a slit lamp is employed to apply laser

energy through the conjunctival/ scleral surface. A contact lens may be used to keep the

eyelids open. The focus of 4-8 Joule energy for 10-20 second delivers 1–1.5 mm behind the

limbus so that maximal therapeutic effect is at the level of the ciliary body. The total number

of laser applications is usually about 32 (eight per quadrant), for 360˚ circumference but avoiding the 3 and 9 o’clock positions in order to preserve the long posterior ciliary arteries

(Martin and Broadway, 2001). While in contact cyclophotocoagulation,

Neodymium:Yttrium-Aluminium-Garnet (Nd:YAG) laser was more effective in penetrating

the sclera and optimizing energy absorption by the ciliary epithelium. In the Nd:YAG laser, a

hand held sapphire tipped quartz probe is placed on the conjunctiva and sclera, centered 1–2

mm behind the limbus, for the transmission of 4-9 watts energy for 0.5-0.7 sec. The total

number of laser applications is usually about 32, for 360˚ circumference but also avoiding the 3 and 9 o’clock positions. Energy levels are titrated to avoid an audible “pop” sound which

indicates overtreatment of the ciliary body tissue (Nasisse et al.,1990).

Endolaser Cyclophotocoagulation (ECP)

In the limbal approach cyclophotocoagulation is done through a temporal and nasal

limbal site. After dilatation of the pupil, paracentesis is created and the anterior chamber is

filled with viscoelastic agent. A 2.2 mm keratome is then used to enter into the anterior

chamber at the temporal limbus. After orientation of the probe image outside of the eye, the

20 gauge probe is inserted through the incision and into the posterior sulcus (Ofri and

Narfstrom, 2007). At this time, the ciliary processes are viewed on the monitor and treatment

can begin. The laser is set at continuous wave and energy settings are 60–90 mW.

Approximately 180 degree span of ciliary processes is photocoagulated. Laser energy is

applied to each process until shrinkage and whitening occur. If excessive energy is used, the process explodes (or “pops”) with bubble formation, leading to excessive inflammation and

breakdown of the blood-aqueous barrier. After the temporal ciliary processes are treated, a

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chamber (Lin, 2002).

Cyclodialysis

A lateral canthotomy is made to exposure of the globe. A fixation suture has been placed through the conjunctiva at the limbus, and the conjunctiva and tenon’s capsule are

incised to exposed the sclera. The sclera is grooved (between the 10 and 1 o’clock position)

with a 15 no. blade. The incision is made 6 mm from and parallel to the limbus. Alternating

incisions with cauterization helps to deepen the groove, retract the wound edges, and

maintain haemostasis. Once the sclera groove has been extended down to the suprachoroidal

space (appearance of heavily pigmented tissue), a cyclodialysis spatula is introduced into this

space. Careful sweeping motion of the spatula separates the ciliary body from the sclera and

permits entrance into the anterior chamber through the ciliary cleft. Sweeping of the spatula

to either side now breaks the iris root attachments at the limbus. Spatulate anterior chamber

irrigator now replaces the cyclodialysis spatula. The anterior chamber is reformed with

balanced salt solution. The conjunctiva and tenon’s capsule are closed with a simple

continuous 6/0 gut suture. Closure of the lateral canthotomy has begun(Bistner et al., 1977).

Iridectomy for Iris Bombe

The posterior chamber is distended and drainage angle occluded by the forword

displacement of the iris. A lateral canthotomy is performed. A limbal based conjunctival flap

has been made. The anterior chamber has been entered at the limbus and iris tissue bulges out

through the incision. An iris hook fixes the iris base and a basal iridectomy is made with and

electroscalpel. The corneoscleral incision has been closed with simple interrupted 7/0 gut

sutures buried beneath the conjunctiva. The conjunctiva has been closed with a simple

continuous 6/0 gut suture. Closure of the lateral canthotomy has begun(Magrane, 1977).

CONCLUSION

Glaucoma remains a leading cause of blindness in veterinary patients. Because of the

nature of the disease, many pets are presented at a time when it is not possible to restore

vision to the first eye affected. Glaucoma is very difficult to treat in pets. Unlike humans

where medication resolves over 80% of the cases of glaucoma, surgery is almost always

required in veterinary patients. The goal of the Veterinary Ophthalmologist in treating a pet

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possible.

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