Cataracts - Posterior LenticonusLiaison: Anngharaad "Annie" Reid
Posterior Lenticonus is an inherited, congenital lens abnormality which may result in cataract formation in both eyes. This type of “cataract” affects both male and female Samoyeds normally between 1 and 3 years of age.
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Signs and Symptoms
In time, the lens protein becomes liquid and may leak through the lens capsule into the eyes. This will cause cataract-associated intraocular inflammation (or lens induced uveitis). This inflammation must be treated with anti-inflammatory drops. If left untreated, complications such as glaucoma, retinal detachment or the development of intraocular adhesions will result.
This lens abnormality is hereditary.
Pattern of Inheritance. In the Michigan State University research study (AKC Grant No. 1637) the data points to an autosomal recessive pattern of inheritance, indicating that affected dogs must receive a gene from each parent.
Dr. Vilma Yuzbasiyan-Gurkan, Michigan State University) has stated her belief that the pattern of inheritance is autosomal (non sex-linked) recessive.
(Please note: For purposes of the following discussion, the term "dog" refers to both males and females.)
Genetics in General: The information from which all life develops is in the form of DNA (deoxyribonucleic acid). A gene is a portion of a DNA molecule, carried on a chromosome. Chromosomes occur in pairs (a pair from each parent).
The dog has 78 chromosomes, in 39 pairs, on which approximately 100,000 genes are located. This makes up the dog’s genotype. Each gene in a chromosome pair has a partner at the same position (or locus) on the matching chromosome. Each member of a gene pair is called an allele. A gene has many alleles within a population but an individual animal will have only 2 alleles that influence a particular trait. If the 2 alleles are identical, the individual is homozygous at that locus; if the alleles are different, then heterozygous.
If the allele is dominant, only 1 copy is required to express a trait; if recessive then 2 copies are necessary. The heterozygote will be a carrier for that trait – clinically unaffected but able to pass the harmful allele to the offspring. The homozygote will be clinically affected.
Autosomal Recessive: Autosomal recessive is the most common mode of inheritance for genetic conditions in dogs. To be affected, the animal must inherit 2 copies of the gene, 1 from each parent. Dogs that are genotypically normal or dogs that are carriers for a trait will clinically be normal, but the carrier will pass the affected gene to approximately half the offspring. As long as carriers are mated to normal animals, the offspring will be unaffected but some will remain carriers. If 2 carriers are mated, approximately 25% will be affected, 50% will be carriers and 25% will be normal.
AA = normal
aa = affected with posterior lenticonus
Aa = carrier of gene for posterior lenticonus
As long as the frequency of a gene for a recessive disorder, like posterior lenticonus, remains low in the population, the particular gene may be passed along for many generations before by chance 2 carriers are mated and affected individuals are born. However, the gene frequency may become unusually high due to breeding of close family members, or because a sire with a harmful recessive gene is mated frequently because of desirable traits.
Because the recessive gene
is carried in the population in outwardly normal animals, it is very difficult
to eradicate this trait. However, the incidence can be reduced by
identification of carriers and affected dogs and by the conscientious use of
this information in breeding programs. We must all work together – for the
benefit of this breed – if progress is to be achieved.
Sire or dam who has posterior lenticonus or carries the gene for it.
Note: Treatment of animals should only be performed by a licensed veterinarian. Veterinarians should consult the current literature and current pharmacological formularies before initiating any treatment protocol.
Treatment: The only treatment if cataract development causes vision loss is surgical removal of the lenses. Eye ointments, pills, special diets, vitamins or eye exercises will not dissolve or reduce a cataract.
Candidate Selection: Any major health problems and ophthalmic abnormalities should be addressed before cataract surgery is considered.
A complete physical examination will be performed by your veterinarian, which may include the following preoperative tests:
Blood and urine analysis;
Chest X-rays; and
A complete ophthalmic examination should always be performed:
Schirmer tear test (to test for tear production);
Tonometry (to check for glaucoma);
Electroretinography (to check retinal function); and
Ocular ultrasound (to detect any evidence of retinal detachment).
Depending upon the state of cataract maturation, the experienced ophthalmologist may still be able to view the fundus; however, an electroretinagram and ocular ultrasound are the best means to confirm the absence of any retina disease or detachment. These tests are mandatory at most hospitals.
Selection of appropriate candidate eyes/animals for lens extraction is an important consideration for a successful outcome.
The Surgical Procedure. Surgical removal of the cataracts is performed under general anesthesia. A small (2-3mm) incision is made into the eye. A thick viscoelastic gel is injected into the anterior chamber to prevent the eye from collapsing during surgery and to protect the inside of the cornea. A special needle-like ultrasonic instrument is inserted into the lens. The needle tip vibrates at a very fast rate (about 40,000 vibrations/second), which ultrasonically fragments and removes the lens. This procedure is called phacoemulsification.
During surgery the lens is removed from the lens capsule. In most cataract surgery patients, an intraocular lens (“IOL”) is placed inside the emptied lens capsule to replace the eye’s natural lens. The IOL is a tiny, lightweight, clear, plastic or polyacrylic disc and restores normal vision to the eye.
In some patients, the lens attachments may be loose or the lens may be too hard to be broken up by phacoemulsification. In these patients, the entire lens (including the outer lens capsule) is removed in which case an IOL cannot routinely (there are lenses that do not require a lens capsule and may be sutured in the eye with greater risk) be placed inside the eye. These patients will still see after surgery, but their vision will not be as well focused as patients with IOL implants.
In the past, cataract surgery was often delayed until the patient was completely blind. However, cataract surgery is more successful if surgery is performed sooner, rather than later. First, mature cataracts are more likely to cause inflammation, which can cause vision-threatening complications before and after cataract surgery. Second, the surgery itself is more difficult when the cataract is mature or has been present for a long time.
Surgery is usually performed on both eyes at the same time.
“Secondary Cataracts.” Once a cataract is removed, it will not “regrow.” However, in all cataract surgeries, not all of the lens cells can be removed from within the capsule. Sometimes these lens cells will attempt to make new lens protein or migrate across the posterior lens capsule and cause mild cloudiness and wrinkling of the capsule. This is similar to frost covering a window. This lens protein regrowth usually occurs within 6 months of surgery performed on young patients (2 years old or younger). Fortunately, IOL implants usually prevent clouding of the lens capsule. Sometimes a small hole in the posterior lens capsule is made when cataract surgery is performed, so that the posterior capsule cannot become cloudy.
Preoperative Care: It is recommended that you train your dog to wear an E-collar and become used to confinement. Anti-inflammatory and antibiotic drops are used daily for several weeks before surgery. A few days before surgery, the drops may be increased.
Postoperative Care: Exercise and barking must be restricted for at least 3-4 weeks after surgery (excessive activity and barking increase the risk for postoperative retinal detachment and suture breaks). Oral medications (antibiotics and anti-inflammatory medications) are administered. Eye drops are continued. The patient should wear an E-collar for about 2-3 weeks after surgery (to prevent any self-inflicted trauma (e.g., scratching at the eyes)). Post surgical re-evaluations are generally performed 1-2 days, 5 days, 1 week, 2 weeks, 4 weeks, 3 months, 6 months following surgery. Periodic re-examinations every 6 months are recommended (based on patient’s progress).
If you are unable to perform the pre- and postoperative treatments or return for periodic re-examinations or if your dog will not accept limited activity or permit you to apply the eye medications, then surgery is not recommended.
Success Rates: 85 to 90% of patients who undergo cataract surgery regain useful vision. The success rates are better when cataracts are removed in non-inflamed eyes or in eyes in the early stages of cataract formation and if you follow all the pre-and postoperative medication instructions and treatments and return for postoperative re-examinations.
Cost: The cost is expensive, varies by hospital and may not be covered by insurance.
See pre- and post-operative care in the treatment section.
Diseases of the Lens and Cataract Surgery Mark P. Nasisse, DVM, Diplomate, ACVO
Conversations with Dr. Vilma Yuzbasiyan-Gurkan at Michigan State University
See references above.