Diagnostic Ophthalmology

History and clinical signs

A 5-year-old fox terrier was examined at a Canine Eye Registration Foundation Clinic (CERF); a photograph of the left eye is provided for your assessment (Figure 1). The right eye was similar in appearance. The owners had noticed visual impairment for several months and the referring veterinarian had diagnosed cataracts. Findings from physical examination, complete blood cell count, serum biochemical profile, and urinalysis revealed no additional abnormalities. The neuroophthalmic examination was normal. The Schirmer Tear Test Strips, Alcon Canada, Mississauga, Ontario) were 30 mm and 25 mm per min in the left and right eye, respectively. Both corneas were anesthetized with topical proparacaine (Alcaine, Alcon Canada). Intraocular pressures, estimated with an applanation tonometer (Tonopen XL; Biorad Ophthalmic Division, Santa Clara, California, USA) were 35 mm Hg and 40 mm Hg in the left and right eye, respectively. Biomicroscopy (Osram 64222; Carl Zeiss Canada, Don Mills, Ontario) and indirect ophthalmoscopy (Heine Omega 200; Heine Instruments Canada, Kitchener, Ontario) were completed; abnormalities in both eyes were limited to those evident in figure 1.

Figure 1. A photograph of the left eye of a 5-year-old fox terrier. The right eye was similar in appearance.

Discussion

Our clinical diagnoses were subluxated lenses, mature cataracts, and secondary glaucoma. Note the aphakic crescent on the lateral aspect of the pupil where the tapetal and nontapetal fundus can be seen. An aphakic crescent is pathognomic for zonular disruption and subluxation of the lens. Luxations of the lens may be primary and develop from an inherited zonular degeneration; secondary and develop as a consequence of zonular degeneration secondary to chronic glaucoma and buphthalmos, uveitis, and hypermature or intumescent cataracts; or congenital and develop secondary to multiple ocular anomalies, including zonular aplasia, microphakia, and congenital glaucoma (1).

Congenital lens luxations are diagnosed on the basis of signalment and clinical manifestations. Primary lens luxations usually lead to secondary glaucoma, so differentiation of primary and secondary lens luxations is important and usually challenging for the clinician. All terriers are at risk of developing primary lens luxations; however, these dogs may also develop primary glaucoma. We advised and completed gonioscopy, which revealed normal filtration angles in this dog. In addition, we completed a thorough anterior and posterior examination, which identified only mature cortical cataracts but no other apparent cause of glaucoma. In conclusion, we measured the vertical and horizontal corneal diameters of both eyes and compared them with each other and with another adult fox terrier. The corneal diameters were similar, which ruled out buphthalmos. These examinations supported our diagnosis of primary lens luxation and secondary glaucoma.

We advised bilateral intracapsular lens extractions. In addition, the eyes were medicated topically with 1% prednisolone acetate (Prednisolone 1%; Sabex, Boucherville, Quebec) to suppress the anterior uveitis that develops secondary to the lens movement within the globe. We also treated both eyes topically with 0.005% latanaprost (Xalatan Pharmacia & Upjohn, Mississauga, Ontario). Latanaprost induces miosis, increases uveal-scleral aqueous humor outflow, and reduces the intraocular pressure. It may also reduce secondary corneal endothelial damage, if the iris and miotic pupil maintain the lens in the posterior segment.

Primary lens luxations in terriers develop secondary to zonular degeneration (2). The genetic mutation has not been identified. The mode of inheritance is likely autosomal recessive in the Tibetan terrier (3). The mode of inheritance in the fox terrier and other terriers is unknown (1). The clinical manifestations of primary lens subluxation usually develop between 3 and 6 y of age in affected dogs (1). The initial clinical manifestations are subtle and include mild conjunctival hyperemia, vitreous degeneration and vitreous prolapse into the anterior chamber, iridodonesis, and decreased or increased anterior chamber depth that develops secondary to the movement of the subluxated lens. Aphakic crescents and zonular degeneration may be observed if the pupil is dilated and the equator of the lens and pars plicata are examined. Although most primary lens luxations become bilateral, the clinical manifestations of luxation often develop at varied times in each eye. Complete lens luxation into the posterior or anterior segment, vitreous degeneration, secondary glaucoma, buphthalmos, and retinal and optic nerve degeneration are common manifestations of chronic lens luxation. The diagnosis is confirmed by ruling out primary and other secondary glaucomas with gonioscopy, thorough eye examinations, and globe measurements to rule out buphthalmos. In the case described, the mature cataracts were thought to be unusual and a significant differential diagnosis and etiology for secondary glaucoma. Cataracts may precede and induce zonular degeneration and lens luxation. Intumescent cataracts induce inflammation, zonular degeneration, and zonular rupture, secondary to stretching as the lens swells. Similarly hypermature cataracts shrink and increase zonular tension and inflammation, which may induce zonular degeneration.

The vitreous degeneration usually develops secondary to the lens motion during luxation. When the vitreous liquefies, it often flows through the pupil into the anterior chamber. Vitreous prolapse within the anterior chamber manifests as white cotton-like fibers suspended within the aqueous humor.

The etiology of glaucoma secondary to lens luxation remains idiopathic (1). Most ophthalmologists speculate that the glaucoma develops secondary to inflammation and vitreous prolapse, and, occasionally, to pre-iridal fibrovascular membranes. We have noted with some interest that despite anterior chamber vitreous prolapse, the filtration angles are usually normal in appearance and not obstructed with cellular, vitreous, or fibrovascular debris. We speculate that ciliary cleft or the endothelium of the collection veins is somehow affected by the lens luxation, and the lack of fluid exit at this level induces the secondary glaucoma.

Lens luxations should be referred to a veterinary ophthalmologist. Acute primary lens luxations are usually treated with prompt intracapsular lens extraction, coupled with appropriate topical medications to control inflammation and secondary glaucoma (1,4). Topical prednisone or dexamethasone will suppress the anterior uveal inflammation and is required pre and post lens removal, provided that corneal ulceration is not present. Topical antiglaucoma medications will reduce the intraocular pressure initially. Prostaglandin antiglaucomas induce miosis and may reduce the anterior chamber lens luxation and the corneal endothelial damage secondary to lens contact; however, the miosis must be reversed prior to surgery to allow extraction of the lens. Mydriasis may be accomplished with iris retractors intraoperatively, or by discontinuing the prostaglandins and applying topical mydriatics, such as atropine, preoperatively. Glaucoma, chronic uveitis, and retinal detachments are common postoperative complications of lens removal. If lens luxations are left untreated, these complications are also common. Most blind and buphthalmic globes with secondary glaucoma are treated by enucleation or evisceration and an intrascleral prosthesis.