Lens luxation occurs in all species due to rupture of the zonular fibres. The fibres can be abnormally developed, degenerate, or they can rupture directly due to trauma.
Lens luxation occurs much more frequently in dogs than in cats.
Primary lens luxation is seen more commonly in dogs. The luxation is due to weakened zonule fibres that rupture early in life (before 5 years of age). The condition is inherited in small terrier breeds such as: Dandie Dinmont, Fox, Jack Russell, Tibetan and Welsh. The Border Collie and Shar Pei are also affected. Studies have shown that the insertion of the zonule fibres into the lens capsule are abnormal. In such cases, minor trauma will rupture the weakened fibres and lead to lens luxation.
Cats do not show a genetic predisposition for luxation.
Secondary luxation can be due to any of the following conditions:
Blunt trauma: can lead to traumatic luxation and may lead to other ocular lesions.
Glaucoma: when the globe enlarges in chronic glaucoma the zonules may break. Glaucoma is also caused by lens luxation and it may be difficult to decide which condition arose first if both are present in the eye.
Uveitis: inflammatory mediators in the posterior chamber may weaken the zonules. This is the most common cause of luxation in cats and horses.
Cataract: if the lens swells due to cataract the zonules may break.
If lens fibres are broken, vitreous can leak via the posterior chamber over the edge of the pupil into the anterior chamber. If the fibres are ruptured over a greater area, subluxation will occur. If the lens becomes completely loose, it can remain in a normal position or it can be displaced anteriorly or posteriorly.
The lens and/or the vitreous can thus block the drainage of the aqueous in the pupil or at the level of the drainage angle, resulting in secondary glaucoma.
Secondary glaucoma occurs usually less acutely and less rapidly in cats than in dogs. Cats can have a lens luxation for a longer time without developing glaucoma.
The earliest sign of lens subluxation is leakage of vitreous, which hangs over the edge of the pupil into the anterior chamber like very thin white clouds.
If the zonule fibres loosen, rapid eye movement causes the lens to oscillate back and forth, which causes iris vibration, or iridodenesis. This is a sign of impending lens displacement and becomes increasingly evident as the condition progresses.
As subluxation progresses to luxation, the lens usually sinks ventrally due to gravity, and the dorsal edge becomes visible in the pupil. The dorsal area of the pupil where the lens is missing is called the aphakic crescent.
If the lens is displaced anteriorly, the iris is pressed anteriorly, the anterior chamber becomes shallow, and the angle of the chamber becomes narrower. If the lens is displaced posteriorly, the anterior chamber is deeper and the angle of the chamber is noticeably wider. The iris loses its support and becomes a flat disc instead of curving with the form of the lens.
If the lens is completely displaced, the fundus can be seen with the naked eye.
If the lens is completely in the anterior chamber it is recognisable as a glassy disc lying directly behind the cornea.
The corneal endothelium can be damaged by contact with the lens. Central corneal oedema can develop and eventually lead to the formation of deep vessels.
If secondary glaucoma has developed, a diffuse and dense corneal oedema will be present, the luxated lens is then hardly recognisable.
There will also be acute onset severe pain in the affected eye and it will usually be blind due to the optic neuropraxia. There will be blepharospasm and possibly hyphema.
The condition is suspected in cases of acute onset severe pain, corneal oedema and glaucoma, especially in unilateral cases in the susceptible breeds.
Ophthalmic examination is usually all that is needed to diagnose the condition, and will help determine if the lens has luxated anteriorly or posteriorly.
In cases where the lens is obscured by corneal oedema and other signs, ultrasonography may be useful to visualise the lens and its position.
It is important to measure the intraocular pressure in order to determine treatment priorities.
Anterior lens luxation is an ophthalmic emergency due to the oedema, pain and glaucoma that develop.
Intracapsular lens extraction should be performed as soon as possible in dogs and horses. The condition is less urgent in cats as glaucoma develops more slowly. The lens should be removed in the intact capsule. Phacoemulsification is another technique which can be used.
Pilocarpine can be used post-operatively to prevent a rise in intra-ocular pressure and vitreous prolapse.
A subluxated or posteriorly displaced lens can be managed medically to ensure that the lens does not displace anteriorly. Clients will then be informed of the signs of an anterior displacement and glaucoma.
Medical treatment involves: pilocarpine or latanoprost to keep the pupil closed.
The lens can also be removed using intracapsular lens extraction and a vitrectomy to prevent glaucoma.
Affected animals should be excluded from breeding. Parents and siblings of the affected animals should also be excluded if possible as they are significantly more likely to be carriers of the condition.
The condition is usually bilateral in affected animals, and owners should be educated about the signs to look out for if the other eye is involved.
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Maggs, D. (2008) Slatter's Fundamentals of Veterinary Ophthalmology Elsevier Health Sciences
Stades, F. (2007) Ophthalmology for the veterinary practitioner Wiley-Blackwell
Crispin, S. (2005) Notes on veterinary ophthalmology Wiley-Blackwell
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