Within the eye, the chief focusing devise is the crystalline lens, which is most often referred to as just “the lens”. It is located behind the iris (the colored portion inside the eye) within a recess of the ciliary body (the structure that produces the fluid “aqueous humor” that fills the front portion of the eye). Behind the lens is a gelatinous material known as the vitreous body. The lens is inside a sac call the lens capsule that is normally held in place by small fibres called zonules. The zonules are attached to the lens and to the ciliary body to keep the lens in position.
WHAT IS A LUXATED LENS? If the zonules break, for any reason, the lens can either become partially loose (subluxated) or completely detached (luxated). When the lens is luxated to the rear (into the vitreous body), it is known as being posteriorly luxated. If the lens is completely loose and falls forward (into the anterior chamber), it is known as being anteriorly luxated.
WHAT CAUSES THE LENS TO LOOSEN? There are several causes of zonular rupture and lens luxation:
- In many terrier breeds, lens luxation occurs due to a hereditary rupture of the lens zonules. The condition is most common in the Jack Russell, Sealyham, and Bedlington Terriers. This is referred to as zonular dysgenesis.
- Serious trauma or inflammation (“uveitis”) within the eye can cause zonule ruptures. In cats, subluxated lenses are due to unknown cause and lens luxation occurs most commonly due to uveitis. When uveitis is present, several blood tests are necessary to determine the exact cause of the uveitis in the dog or cat.
- Lens luxations and subluxations may be congenital (present at birth) due to genetic defects. These are seen in very young patients.
- In any patient with glaucoma (increased pressure with the eye) that has been present for some time, the eye will enlarge, rupturing the zonules.
WHAT ARE THE CONSEQUENCES OF A LUXATED LENS? The fluid inside the front portion of the eye (“aqueous humor”) flows forward through the pupil and out of the eye through the “filtration angle” where the iris attaches to the inside of the eye. When a lens is anteriorly luxated into the front portion of the eye (in front of the iris), it physically blocks the pupil, obstructing the flow of fluid out of the eye. This condition called “glaucoma” (increased pressure inside the eye) is painful and extremely serious. Immediate treatment is required. Within 72 hours, glaucoma causes blindness by irreversibly damaging the optic nerve and retina.
WHAT IS THE TREATMENT FOR A LUXATED LENS? The treatment for a luxated lens depends on whether the patient is able to see with the affected eye or if there is the possibility of vision. In many cases with long-standing luxated lens, the lens becomes a cataract, which obstructs the view of the retina and optic nerve. A procedure called an electroretinogram (ERG) is performed to measure the function of the retina, allowing the ophthalmologist to determine if the patient has the potential to see after surgery is performed.
If the chance of vision is good, lens removal surgery (called lensectomy) is performed. One of two procedures is performed to remove a lens. Phacoemulsification is a procedure, which uses high frequency ultrasonic waves emitted from the tip of a small probe to break the solid lens into a liquid that can be vacuumed out of the eye through a 2.5mm incision. This method can only be used if the lens capsule (the sac the lens is inside) is still partially attached to the inside of the eye. If the lens capsule is too loose or completely detached, the entire structure is removed in one piece. This requires a much larger incision – about half the diameter of the cornea. If, however, irreversible damage has been caused and the eye will not be visual again, then one of two surgeries can be performed:
- Enucleation – the eye is removed, an infraorbital implant is placed and the lids are sewn shut.
- Silicone implant (Intrascleral Prosthesis)– the contents of the inside of the eyeball are removed, leaving the cornea and sclera (the white of the eye and the clear part at the front), and a silicone ball is placed inside. The shape of the eye is maintained and the eye moves normally. This procedure is for cosmetic purposes only. The clear cornea usually turns a hazy bluish color with time.
WHAT ABOUT A SUBLUXATED LENS? Treatment for lenses that are subluxated depends on whether the pressure inside the eye is normal or elevated. If the pressure is normal, then ophthalmic drops might be used to ensure that the pressure remains normal. In this situation, frequent re-examinations are performed over time to assess whether the lens is becoming looser or is staying in place.
WHAT ABOUT THE OTHER EYE? The other eye is susceptible to partial if not complete lens luxation or dislocation at any time in your pet’s life. This can occur suddenly, without warning. During the initial examination, the opposite eye will be checked to ascertain if the zonules are intact. If the lens appears normal, regular exams should be performed to monitor its position. Even if there is a subluxation no medication or surgery will prevent the eventual dislocation of the lens. Intraocular pressure (IOP) will be monitored to ascertain if there is the potential for glaucoma in that eye. Medications may be prescribed to constrict the pupil in case there is a subluxation. Eventual removal of the lens from the subluxated eye is needed.
WHAT WILL I NEED TO DO AFTER SURGERY?
- Keep the elizabethan collar on at all times (even at night). The stitches in the eye are very fine and can easily be damaged by a rubbing paw.
- Restrict exercise to an absolute minimum. Your pet should be kept very quiet.
- Administer the medications at the appropriate times.
- When using eye drops, be sure to hold the lids open and the nose pointed towards the ceiling so the medication is placed directly onto the eyeball. Wait 10 minutes between different drops so as not to flush out the previous medication before it has been absorbed. If you are required to use drops and ointment, use the ointment last as it is thick and forms a barrier, preventing any following drops from being absorbed.
Gently wipe away any discharge from the eye with a clean, moist Kleenex or face cloth.