Primary lens luxation in dogs: A laymanís guide & a warning to owners of Minature Bull Terriers in particular.

It is not the primary lens luxation which causes a dogís eventual blindness, but the glaucoma with which it is associated and which requires immediate treatment if its consequences are to be minimised.

Description

What to look for

What happens

The treatment

Aftercare

Critical Advice (read here if you canít be bothered to read the rest!)

Other Stuff

Glossary (terms you just donít understand!)

Description:

This is a condition where the lens is partially or fully dislocated from the threads that hold it in position within the eye. These tiny threads, sometimes called ligaments are known as zonules and it is they that weaken and break causing this condition. How quickly and how many break determines the speed of onset of this condition. Using normal ophthalmic instruments (such as your own vet may possess) it is difficult to even see these small threads. What the experienced ophthamist (not your own vet) should also be looking for is a wobble in the lens as the dog moves his head around. This indicates that the zonules are becoming weak. Weak zonules is called subluxated, completely detached (luxated)

In our case, the regular twice yearly eye examination had occurred a month before the actual onset of the condition, therefore it is imperative that you conduct your own eye examinations as well.

Unfortunately, each eye can behave differently, so a rapid onset on one eye, may or may not lead to a gradual (and predictable) onset with the other. As this appears to be a genetic defect in dogs, then it is almost certain that eventually both eyes will be effected.

There are two directions in which the lens can go: forward and backward. Forward in called anterior luxation. Backwards is called posterior luxation. The lens can also detach at an angle which can cause bruising of the cornea (edema of the cornea). This causes the cornea to loose its transparency in the area of contact. This may or may not be permanent.

What to look for:

Forward is the usual way. In my dogís case, the results were obvious. The iris (the coloured part of the eye, surrounding the pupil (the normally dark spot in the centre of the eye)) was almost invisible: being fully open & unable to contract even in strong light. The pupil was consequently very large revealing the greenish hue (in fact, a reflection) from the back of the eye: the sort of colour exhibited by animal eyes when illuminated at night.

In addition, the eye might appear slightly swollen and the dog may have difficulty holding his eye open. These are VERY BAD signs. They indicate that pressure has already built up in the eye, possibly to damaging levels.

In the case of my dogís first eye, the only other symptom was shivering - due to the pain. Bull terriers have a habit of shivering for no reason in any case, but it is worth checking the eyes if he/she is shivering for no apparent reason. When the second eye went, I noticed that the dog was having trouble finding food items on the floor. I think I noticed this one almost immediately it luxated. Even so, by the time I had got the dog to the Veterinary Centre for treatment (about two hours later), the dog was just beginning to close his eye and the pressure (the IOP, intraoccular pressure) was already 51!

If the lens goes backwards, I donít know what the symptoms are (there may not be any) or what the prognosis is, however there is the possibility that the lens could go forward at some time and cause glaucoma and the other conditions mentioned.

What happens:

When the lens goes forward in the eye, it usually blocks the flow of fluid (aqueous humour) which is continually produced in the normal eye by a ring of tissue (the cilary body) between the iris and the lens. The majority of this fluid normally flows out of the eye. Because the fluid cannot drain away, a condition arises which can cause irreparable damage to the eye due to the build up of pressure. This condition is known as glaucoma.

Nowadays, pressure (tonometry, the measurement of eye pressure - impress your vet with that!) is easily measured with a small hand held digital (no moving parts!) pressure gauge (a tonopen) held against the front of the eye. . Normal pressure should be in the range of 15 to 25 torr (mm of mercury). But can reach as high as 60 (with glaucoma) That is over one pound per square inch!.

At the back of the eye is the light receptive area. These cells (rods and cones) are mounted in a structure called the retina, which is held on to the interior wall of the eye. The blood flow and information from the light sensitive cells pass out of a central area in the back of the eyeball, which contains the optic nerve (which transmits the information from the cells). Pressure build-up in the eyeball can cause permanent damage to the optic nerve itself and constrict the blood flow, which can lead to the death of the light sensitive cells. A consequence of this is that the retina detaches from the eyeball. This can be seen eventually by examination of the eye, though it might take some months to happen. Partial detachment can also occur. Detachment can be progressive. The amount of detachment relates to the level of vision available to that eye. This detachment is not in any way painful to the dog, however.

If glaucoma is allowed to remain untreated for as little as six hours, then your dog will be fully blind in that eye.It important that the dog is immediately seen by a vet who is fully knowledgeable in this subject. Most vets arenít, or donít appreciate the seriousness of the situation. This is an ACUTE condition (requires urgent attention), not CHRONIC (slow).

The treatment:

At first I had understood that the only treatment was an immediate operation to remove the lens. However, now I understand that there are drugs which, if given soon enough will reduce the pressure in the eye. By careful monitoring, a dog can be maintained (at the veterinary centre) until an operation is undertaken. This can be a period of some days or even weeks. Alternatively, some surgeons allow dogs to go home with the owner administering the pressure reducing treatment, until the lens has become almost completely detached and moved to a suitable position so it can be easily removed.

The lens is also attached to the vitreous humour (the jelly-like substance) in the eyeball itself. In order to remove the lens, it has to be severed from the vitreous humour. This is a delicate operation, as it is possible that the any outward pressure on the lens can move the whole of the vitreous humour forward which can detach the retina with consequent loss of vision. Some vitreous humour is inevitably removed with the lens, though in a sucessful operation the surgeon takes as little as possible because the space left by the lens and the removed vitreous humour causes an imbalance within the eye. In this situation, there is a tendency for the vitreous humour to move forward causing partial or complete retinal detachment. Some surgeons remove the lens prematurely by forcible suction, the majority hold (I believe) that this is not advisable that because the danger of retinal detachment by this method. In addition, there is usually more chance of hemorrhage (internal bleeding), more scar tissue, again increasing the likelyhood of retinal detachment.

Because stabilty of the eye has been upset, surgeons recommend that the dog does nothing that might cause the movement of the retina. For instance Ďrat killingí (violent toy shaking). Even straining against a collar is considered by some to be inadvisable, so they recommend a body harness (which does not bear on the neck).

After successful treatment, the eye has no focusing ability and the light image which would (at least in humans) be inverted by the lens now falls directly on the retina. However, in dogs, apparently, the cornea (the structure in front of the lens) provides an appreciable focusing (though fixed) element to the eye and although focusing is absent, some resolution of an image should be possible. Presumably, in time, the brain will eventually learn to interpret what falls on the retina, which should give the dog the so called Ďguidance visioní.

Sometimes, the short term results are good, particularly if both eyes have retained their retinas. I have seen a dog playing and catching! the thin cord attached to another dogs lead. She obviously knew where it was, despite having both her lens removed.

However, as mentioned, prolonged glaucoma may have already caused irreparable damage to the retina. In addition, this major alteration to the structure of the eye can lead to glaucoma again! (for reasons which are not entirely understood) The eventual consequences of glaucoma require removal of the eye.

Aftercare:

Our surgeon recommends that the dog should generally be kept quiet for a number of days, and stressed the importance of the regular and continued use of the eye drops supplied.

At least one specialist recommends that the dog be seen at intervals of several months to check that the IOP has not risen to dangerous levels. Personally, in view of the rapid rate of onset of glaucoma, and the speed of consequential damage to the retina and the apparent natural variation in the IOP on a daily basis, I canít think that this would be useful.

One ophthalmic surgeon states that further surgery is essential to maintain vision. He recommends laser treatment to reduce the capability of the eye to produce aqueous humour.

In human eye conditions where the retina is detaching from the eye, lasers can be used to Ďpiní the retina back against the eye.

Critical Advice:

Be prepared. A normal vet will NOT be able to carry out this treatment, so he will refer you to a specialist at a Veterinary Centre.

Is there one within a reasonable distance of you, that has suitably qualified staff for this operation?

Do these staff maintain a 24 hour rota and will they see a luxated dog immediately and at ANY time/day?

Do you have their telephone number?

Does the phone number you have been given actually work at any time? Why not try it some bank holiday?

If you think your dog is at risk, and he probably is if he is a minature bull terrier, find a centre that is prepared to take action, when necessary, quickly .

Keep looking at your dog and notice any particular change in behavour that might be eye related

Other stuff:

In the States other aftercare treatments have been tried, for instance cyclocryothermy: freezing of some of the ciliary body to prevent the production of fluid, however this can apparently cause complications and may not be successful.

It is possible to insert (permanently) what is called a shunt, a small valve, into the eye which can relieve excess pressure, though this tube can sometimes become blocked.

If glaucoma necessitates removal of the eye (enucleation), this is usually a straightforward operation with no complications. It is possible to insert a silicone implant within the eye after removing the contents (called evisceration). This apparently results in a normal looking pain free eye, but with no vision of course. In my opinion, having had a pet already go through some surgery for this painful condition in any case, this is unnecessary and benefits only the owner.

In the case of the minature bullterrier operations are made more difficult by the shape and the small size of the eye.

Maybe you think all vets should have a tonopen? I agree, at least then they would be in a position to administer the pressure reducing drugs; however the cost of one of these instruments approaches £2000.

Drugs used to control the production of aqueous humour include (under various brand names): Pilocarpine (pye-loe-KAR-peen), Dorzolamide (dor-ZOLE-a-mide), Timolol (TYE-moe-lole), Epinephrine (ep-i-NEF-rin), Epinephryl Borate (ep-i-NEF-rill BOR-ate), Acetazolamide (a-set-a-ZOLE-a-mide), Dichlorphenamide (dye-klor-FEN-a-mide), Methazolamide (meth-a-ZOLE-a-mide) and combinations of these drugs, some of which my be directly applied to the eye or taken by mouth or injection.

I acknowledge with thanks the assistance provided by an opthalmic surgeon, who has checked this document at a fairly early stage for accuracy, however I cannot say that it has been endorsed by her as it has been revised a number of times since then. I have tried to be as accurate as I can, without getting too technical, I have used (I think) the correct medical terms so you should be able to understand what the medical staff you speak to are meaning. I myself am a layman, so I welcome any constructive criticism from the medical profession. Please email jc@ch-w.demon.co.uk

Glossary: (click on the word for information: keywords are coloured green) acute, anterior luxation, aqueous humour, cilary body, cyclocryothermy, chronic, edema, enucleation, evisceration, glaucoma, hemorrhage, iris, intraoccular pressure, ligaments, posterior luxation, pupil, retina, subluxated, tonometry, tonopen, torr, vitreous humour, zonules

E.& O.E.

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