Dislocation of the Lens in Small Animals

Lens luxation can be primary or secondary to underlying causes such as hypermature cataracts, chronic anterior uveitis, or chronic glaucoma. With anterior lens luxation, it is critical to assess the potential for vision as well as measure intraocular pressure. With anterior lens luxation, immediate referral is recommended so that the intraocular pressure can be controlled and the lens or globe can be removed in visual or nonvisual eyes, respectively. The prognosis for longterm vision and comfort after lens removal is guarded because secondary glaucoma is common postoperatively.

Lens luxation occurs when there is loss of all or most zonular attachments and the crystalline lens becomes dislocated from its normal anatomical position.

Primary lens luxation usually affects middle-aged terriers or Shar-Pei. It is associated with zonular defects due to a genetic mutation in adamts17.

Secondary lens luxation can occur in dogs because of hypermature cataracts, chronic anterior uveitis, chronic glaucoma, and microphakia. In cats, the most common cause of lens luxation is chronic anterior uveitis.

With anterior lens luxation, the lens becomes dislocated into the anterior chamber. With posterior lens luxation, the lens becomes dislocated into the vitreous chamber.

With lens subluxation, there is partial loss of zonular attachments and the lens is only partially dislocated. An aphakic crescent (crescent-shaped area of the pupil where the lens is absent, defined by the margin of the iris and the equator of the displaced lens) is the classic sign of lens subluxation.

Anterior lens luxation is often associated with the following clinical signs:

• elevated intraocular pressure (IOP)

• concomitant diffuse corneal edema

• blepharospasm

• tearing

• episcleral and conjunctival hyperemia

The elevated IOP often results from pupillary blockage, with vitreous adherent to the posterior lens capsule or secondary iridocorneal angle closure. Applanation tonometry should be directed away from the lens because IOPs measured from the central cornea can yield erroneously high measurements. Direct examination of the posterior segment is often not possible because of corneal edema, and B-scan ultrasonography can be used to evaluate the integrity of the retina and vitreous (see anterior and posterior lens luxation images).

Anterior lens luxation, dog

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Courtesy of UC Davis Comparative Ophthalmology Service.

Posterior lens subluxation with hypermature cataract, dog

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Courtesy of UC Davis Comparative Ophthalmology Service.

Treatment of anterior lens luxation consists of lowering IOP, usually with mannitol (1–2 g/kg, IV, administered very slowly), and topical or systemic carbonic anhydrase inhibitors; topical prostaglandin analogs are contraindicated with anterior lens luxation because the intense miosis that occurs can trap the vitreous attached to the posterior lens and further increase IOP.

If the eye has the potential for vision (typically assessed with a dazzle reflex and consensual pupillary light reflex), then lens removal, typically by intracapsular lens extraction, should be performed as soon as possible; eyes that are blind should be enucleated because this condition will result in a chronically painful globe. If surgery is declined, transpupillary aqueous humor flow can be reestablished with dilation with 1% atropine or 10% phenylephrine, and the dog's head can be positioned so the lens can move back to the vitreous chamber.

Postoperative treatment consists of topical and systemic corticosteroids and antimicrobials and topical antiglaucoma medications. IOP is closely monitored in the postoperative period, and additional antiglaucoma medications are prescribed as necessary. Longterm postoperative complications are common and include secondary glaucoma, retinal detachment, and uncontrolled anterior uveitis; thus, a guarded prognosis should be given. In dogs with early primary lens instability or a posteriorly luxated lens, demecarium bromide every 12 hours can be prescribed to delay the onset of anterior lens luxation.

Posterior lens luxations can also cause secondary glaucoma, retinal detachment, and chronic anterior uveitis and thus must be regularly monitored and treated with topical anti-inflammatory and antiglaucoma medications.