lensluxation

Canine Lens Luxation

The lens of the eye is the clear structure which focuses the image onto the retina. When the lens pathologically loses it's clarity, we call it a cataract.

The lens is located behind the iris, the central portion being exposed by the pupillary opening. The lens is normally held in position by small fibers called zonules, or the suspensory ligaments. The zonules are attached to the equatorial perimeter of the lens and to the ciliary body to keep it in position. Aqueous fluid (aqueous humor) fills the anterior chamber of the eye, and the vitreous, a jelly like material fills the vitreous chamber behind the lens. The aqueous fluid is manufactured in the ciliary body and flows through the pupil into the anterior chamber and exits the eye through the ciliary cleft or drainage angle where the cornea and the root of the iris meet in the periphery of the anterior chamber. Here, the aqueous fluid re-enters the general circulation of the body. The aqueous humor maintains the normal pressure of the eye known as intraocular pressure (IOP). A disruption or blockage of the flow of the aqueous fluid often results in glaucoma.

What is a Luxated Lens?
Should the zonules break the lens can either become loosened (subluxated) or completely detached (luxated). When the lens completely tears free of its zonular attachments and falls forward into the anterior chamber, we call this an anterior luxation. It is also possible for the lens to luxate posteriorly into the vitreous body.


(a) normal lens position	(b) anterior luxation forcing the iris forward. This results in a very shallow anterior chamber	(c) lens is partially through the pupil. If the lens touches the cornea, edema of the cornea will result	(d) complete anterior luxation. The anterior chamber is very deep as it contain the whole lens. Pupillary block is present.

Several causes of zonular rupture are recognized.
Primary (heritable) lens luxation seen in many Terrier breeds.
Secondary to trauma
Secondary to inflammation (uveitis)
Secondary to glaucoma
Congenital due to abnormal development
Idiopathic

Breeds with heritable lens luxation
Australian Cattle Dog Bedlington Terrier Border Collie Brittany Spaniel Deutsche Jadgterrier Fox Terrier German Shepherd Jack Russel Terrier Manchester Terrier Miniature Bull Terrier Norwegian Elkhound Scottish Terrier Shar-Pei Tibetan Terrier Welsh Terrier West Highland White Terrier Whippet . .

Since lens luxation may cause glaucoma, and since glaucoma may cause lens luxation it is important to determine which disease came first. When lens luxation occurs secondarily to glaucoma, it usually occurs late in the disease once the elevated pressure within the eye has caused the sclera to stretch, and the zonular ligaments to tear. This does not occur until long after vision has been lost. In such a case, attention must be given to resolving the pain associated with glaucoma.

What happens when the lens luxates?
An anteriorly luxated lens is extremely serious, because it blocks the flow of the aqueous fluid in the eye. This often results in the acute onset of glaucoma. We often use the term pupillary block glaucoma since the luxated lens itself and some displaced vitreous obstructs the flow of aqueous through the pupil. There are, however, other causes of pupillary block glaucoma. In dogs, it is generally accepted that within 72 hours, the elevated pressure in the eye will cause irreversible damage to the optic nerve and retina. In addition, the anteriorly luxated lens may cause corneal damage by injuring the endothelial layer of cells which help keep the cornea clear. Corneal edema of varying severity may be the result.

A posteriorly luxated lens can also cause glaucoma since the vitreous is displaced forward and can block the drainage angle.

Treatment
The first step in planning treatment for a dog or cat with a lens luxation is a careful assessment of the prospect for vision in the eye. If the lens luxation is longstanding and if there is glaucoma greater than 72 hours in duration, or if there is hemorrhage in the eye the chances of saving vision is reduced. If the lens luxation is recent, and if the glaucoma is not severe, and the retina and optic disc still look healthy, then there may be a reasonable chance of saving vision with surgery. In this case the surgery done is called an intracapsular lens extraction where the lens is removed with its capsule or covering intact. This requires a larger incision into the eye than traditional cataract surgery, and since the lens capsule is being removed, it is difficult, but not impossible, to replace the lens with an artificial lens (IOL). In many cases, it is also necessary to remove some of the vitreous which has also herniated forward. This is called a vitrectomy.

In some cases the patient is presented with the lens subluxated (partially luxated). If there is no pupillary block or glaucoma present, then medications may be used in an effort to keep the pressure low, and to keep the pupil relatively constricted to reduce the chance of anterior luxation. In some cases, where mild or intermittent glaucoma is present, laser surgery may help stabilize the intraocular pressure. Frequent re-examinations are required as the situation may change to true luxation in some cases.

If the eye has been blinded as a result of the glaucoma caused by the lens luxation, then emergency lens removal surgery will not benefit the situation. If the eye is painful, something must be done to relieve the pain. The two main solutions (also discussed on the glaucoma page), are enucleation (removal) of the eye, or an intrascleral prosthesis procedure where the contents of the eye are removed and replaced with a silicon ball, in many cases resulting in a comfortable blind eye with a very reasonable cosmetic appearance.

What about the other eye?
Examination of the fellow eye, especially in the terrier breeds predisposed to lens luxation may reveal a looseness or wobble to the lens as the head moves. This is due to weakness in the zonular ligaments and in such a case future luxation is likely. In these cases, preventative lens removal may be best, in an effort to prevent a crisis. Medical management by an observant owner is also an option, but should lens luxation occur, emergency surgery will be required.

What about future generations?
Dogs affected with primary lens luxation should not be used for breeding. Since this is a late onset disease, the breeder may encounter a situation where a dog who has already produced a number of litters of pups develops a lens luxation. This is disconcerting since it would be advisable not to use any of the second generation for breeding either. Routine eye certification examinations will not, in most cases, detect a dog predisposed to lens luxation, unless the ligaments have already started to weaken and the slight wobble of the lens is detected by the ophthalmologist during the examination.