Abnormalities of the lens are frequently detected during the ophthalmic examination. Cataract (opacity of the lens or its capsule) is the most common disorder of the lens that occurs in domesticated animals. Abnormalities of the zonular fibers that normally hold the lens in position may occur, resulting in luxation of the lens from its normal position into the anterior chamber or into the vitreous humor.

Anatomy

The lens is a transparent, biconvex structure that consists of a capsule surrounding the lens proper. The lens capsule consists of anterior and posterior parts that meet at the equator. The lens epithelium is located beneath the anterior lens capsule. Lens cortex apposes the lens epithelium anteriorly (anterior cortex) and the lens capsule posteriorly (posterior cortex). The nucleus represents the largest component of the lens and is centrally located. Lens sutures represent junctions between the tips of the lens fibers that make up the lens cortex. Suture lines usually form an upright "Y" anteriorly and an inverted "Y" posteriorly. Zonular fibers extend from the ciliary body to the anterior and posterior capsule circumferentially near the equator and suspend the lens in the pupillary axis. The lens is seated in the patellar fossa, a shallow depression in the vitreous body.

Congenital Lens Abnormalities

Congenital cataracts--often associated with other abnormalities

 Microphakia-small lens

 Posterior lenticonus

 Persistent pupillary membranes

 Persistent hyperplastic primary vitreous / persistent hyperplastic tunica vasculosa lentis (PHPV/PHTVL)

 Retention of hyaloid vasculature

 Fibrovascular tissue on posterior lens capsule

 Secondary cataract

 Possible intraocular hemorrhage

 Lens Coloboma

Lens Aging (Nuclear Sclerosis)

Due to the continued production of lens fibers throughout life, the more central portions of the lens become compressed. With age this results in hardening (sclerosis) of the lens nucleus. The refractive index of the denser portion of the lens is increased. The junction of greatest optical discontinuity occurs between the nucleus and the cortex and becomes apparent at ~5 years of age. This imparts a grayish appearance to the lens of an old dog (nuclear sclerosis) and is commonly confused with lens opacity (cataract). Differentiation of nuclear sclerosis from cataract can be made by viewing the eye from a distance with a direct ophthalmoscope (distant direct ophthalmoscopy): true lens opacities appear as a black shadow against the tapetal reflection, nuclear sclerosis does not obscure the reflection.

Cataracts

A cataract is an opacity of the lens or its capsule.

Classification of Cataracts

Cataracts are classified based on:

 Age of onset

 Congenital

 Juvenile

 Senile (>6yrs of age)

 Stage of maturation

 Incipient (<15% of lens involved)

 Immature (still see some tapetal reflection through lens)

 Mature (obscures tapetal reflection)

 Intumescent (imbibed water and is swollen)

 Hypermature (liquefaction of lens cortex occurs)

 Morgagnian (complete liquefaction of lens cortex, leaking of lens material leaving nucleus ventrally within a shrunken capsule)

 Location Within the Lens

 Capsular--opacity confined to the lens capsule.

 Subcapsular--most of opacity involves cortex directly beneath lens capsule.

 Cortical--opacity of the lens cortex (may be anterior or posterior cortical).

 Nuclear--opacity primarily in center of the lens (nucleus). Frequently congenital and nonprogressive.

 Equatorial--opacity is primarily in the area of the lens equator (near zonular fiber attachment).

 Polar/axial--within the pupillary axis.

 Clock-hour--the location of a lens opacity may be described by referring to its position relative to the face of a clock.

 Anterior/Posterior--used to describe opacities of the capsular, subcapsular, polar/axial, and cortical areas of the lens.

 Etiology

 Hereditary--A common causes of cataracts in dogs. Characteristic appearance/progression of breed-specific cataract.

 Metabolic--Diabetes mellitus is the most common cause of metabolic cataract in dogs and is related to abnormal metabolism of glucose by the lens (does not occur in cats). These cataracts are always bilateral, and often form rapidly.

 Hypocalcemia (e.g., due to hypoparathyroidism) can lead to cataracts with a characteristic appearance-multiple, punctuate subcapsular opacities.

 Associated with other ocular abnormalities

 Congenital disorders: Microphthalmos, PPM, PHPV/PHTVL, Vitreoretinal dysplasia

 Intraocular disease such as:

 Uveitis--Chronic iridocyclitis can induce cataract

 Glaucoma

 Lens luxation

 Retinal Disease-e.g., progressive retinal atrophy

 Senile

 Trauma--Perforating injury to the cornea and lens frequently induces cataract formation.

 Nutritional--Unsuitable canine and feline milk replacers (arginine, tryptophan deficiency)

 Electric shock

 Radiation

Treatment (Cataract Surgery)

The only treatment for cataracts is surgical removal. Other major health problems (diabetes mellitus, renal disease) and ophthalmic abnormalities should be addressed before cataract surgery is considered. A complete ophthalmic examination should always be performed, including Schirmer tear test and tonometry. Often electroretinography is performed to check retinal function, and ocular ultrasound used to detect any evidence of retinal detachment. Selection of appropriate candidate eyes/animals for lens extraction is an important consideration for a successful outcome.

Phacoemulsification with intraocular lens implantation is the commonest procedure performed. This is a skilled procedure requiring the correct instrumentation. With correct case selection and in experienced hands the success rate exceeds 90%. The ideal stage of cataract development for surgery is immature. Once cataracts become mature or hypermature they may induce an anterior uveitis. Prior inflammation in the eye reduce the success rate of cataract surgery.

Lens Luxation

This may be a primary or secondary condition. Secondary lens subluxation is commonly associated with glaucoma (due to stretching of the globe); it may also be seen secondary to anterior uveitis (particularly in cats). Primary lens luxation is a hereditary condition in terrier breeds (Jack Russell Terrier, Smooth and Wirehair Fox Terrier, Tibetan Terrier, Miniature Bull Terrier) and also the Shar pei and Border Collie. The typical presentation is an acute-onset of a painful eye with redness and corneal edema. The pain and inflammation is due to secondary glaucoma that usually develops if the luxated lens passes through the pupil into the anterior chamber of the eye. Prior to luxation the lens can be seen to be unstable (wobbles with eye movement--phacodonesis) and like-wise the reduced support for the iris results in iris wobble (iridodonesis). If the pupil is dilated an aphakic crescent may be seen. Most commonly the lens will luxate into the anterior chamber causing pupil block and secondary glaucoma. Less commonly it moves posteriorly into the vitreous were it causes less problems. Primary lens luxation is a bilateral disorder.

Management of primary lens luxation

Surgical removal of anteriorly luxated lenses (intracapsular extraction) is indicated. The intraocular pressure of the eye should be lowered pre-operatively (e.g., intravenous mannitol). Consideration should be given to the removal of subluxated lenses in the second eye of dogs with primary lens luxation. Surgery to remove a subluxated lens prior to complete luxation is associated with far fewer complications than once the lens has luxated and a secondary glaucoma is established. The implantation of sutured in intraocular lenses can be performed after intracapuslar lens extraction.

References

1.  Gelatt KN (Ed) Veterinary Ophthalmology, 3rd Edition, Lippincott, Williams & Wilkins, 1999, Chapters 23 and 24.

1.  Petersen-Jones SM & Crispin SM (Eds) BSAVA Manual of Small Animal Ophthalmology. 2^nd Edition, BSAVA Publications

2.  Veterinary Clinics of North America: Small Animal Practice. Surgical Management of Ocular Disease, 1997. Ed. Nasisse MP.