Primary lens luxation usually affects middle-aged terriers or Shar-Peis. 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 and horses, the most common cause of lens luxation is chronic anterior uveitis.
Anterior lens luxation often presents with:
The elevated IOP often results from pupillary blockage, with vitreous adherent to the posterior lens capsule and/or secondary iridocorneal angle closure. Applanation tonometry should be directed away from the lens because IOPs measured from the central cornea may yield erroneously high measurements. Direct examination of the posterior segment is often not possible because of corneal edema, and B-scan ultrasonography may be used to evaluate the integrity of the retina and vitreous.
Treatment of anterior lens luxation consists of lowering IOP usually with mannitol, 1–2 g/kg, IV (this drug must be administered very slowly), and topical and/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 PLR), 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 may be reestablished with dilation with 1% atropine and/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 antibiotics 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 twice daily 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.