◊ 11-Lens & Orbit ◊
Lens is a transparent, avascular, biconvex structure derived solely from epithelial cells. The function of the lens is to refract light rays onto a small area of retina to produce a sharp visual image. The lens is maintained in position by ciliary zonules (suspensory ligament). It consists of about 120 fibers that arise from the surface of the ciliary epithelium to the lens capsule at the equator. The shape of the lens changes by contraction or relaxation of ciliary musculature. Diseases of the lens can be catagorized into 4 groups:
1- Loss of accomodative power. 2- Congenital defects.
3- Decreased transparency (cataract). 4- Abnormal position (subluxation and luxation).
I- Loss of Accommodation: -
Loss of accommodation has not been clinically estimated in domestic animals, but the condition does occur. Lack of clinical symptoms may not be due to poor accommodations but due to depending of these animals on other sensory systems as smell, touch, and hearing. Hence, total removal of the lens does not result in a great degree of functional disability and many animals exhibit normal clinical vision with aphakia.
II- Congenital Defects: -
1- Aphakia: -
Congenital absence of the lens.
2- Microphakia: -
Abnormal small size of the lens.
3-Lenticonus and lentiglobus: -
It is a congenital anomaly in the shape of lens. Lenticonus" is anterior or posterior protrusion of the lens in a conical c6ntour and lentiglobus in a spherical contour.
4-Coloboma of the lens: -
Segment of lens is absent.
5-Persistent pupillary membrane: -
Is the presence of remnants of the pupillary membrane. The strands extend to the anterior surface of the lens.
6-Persistent hyaloid vasculature: -
Remnants are attached to the posterior lens capsule.
Ill-Cataracts (loss of transparency, or opacity of the lens and/or its capsule)
1-According to the degree of maturation: -
A-Incipient cataracts: -
This is the stage of beginning opacity. Incipient cataracts characterized by early cortical vacuolar changes, prominent lens sutures, and minute subcapsular alterations. Sight is still normal.
2-Immature cataracts: -
The lens is markedly increased in size due to imbibition of fluid (intumescent) that results in complete opacification within days or weeks. The enlargement of the lens leads to splitting and separation of the lens suture lines and formation of Y-shaped fissures. The sight is impaired but the fundic light reflex is still present.
3-Mature cataracts: -
The lens becomes totally opaque and the vision is lost. The fundic light reflex is disappeared and the fundus cannot be visualized with an ophthalmoscope.
4-Hypermature cataracts: -
Some cases of mature cataracts progress to a hypermature stage and undergo additional changes;
a-Shrunken cataracts: -
The lens becomes small, hard and with rough surface. It may pull away from the zonules.
b-Morgagnian cataracts: -
The cortex undergoes liquefaction leading to a milky texture and the lens nucleus gravitates downward. When the head is moved abruptly, the nucleus will float up and then settle down when the movement stopped.
c-Resorption cataracts: -
The liquefied cortical material may leak through the lens capsule and lenticular fragments remain. The escaped liquefied material may results in iridocyclitis because lens protein is foreign to the animal's immune system. This is exhibited by aqueous flare, miosis, lowered intraocular pressure, episcleral congestion and chronic conjunctivitis.
II-According to the age of onset: -1-Congenital cataracts: -
Begin during fetal life and present at birth and may be stationary or progressive. They are noticed before 2 weeks of age in calves and colts and by 8 to 12 weeks in dogs. Congenital cataracts are observed secondary to or associated with other ocular developmental abnormalities such as; persistent pupillary membrane, persistent hyaloid artery, microphthalmia and multiple ocular abnormalities.
2-Juvenile cataracts (developmental cataracts): -
Usually develop during early years of life between 1-6 years age. They may be genetic, nutritional, inflammatory, toxic or secondary to radiation.
3-Senile cataracts: -
They are part of the aging process and occur in all species of domestic animals in advanced age. They are of great clinical importance in dogs and horses and then cats and bovine. Nuclear sclerosis is a common lenticular changes in senile cataracts. It is considered a normal aging change associated with compression of peripheral lens epithelial fibers towards the nucleus of the lens. This process is analogous to keratinization of the skin except that in the lens, cells are retained rather than exfoliated.
Ill- According to the part of the lens involved (location): -1-Capsular cataracts: -
The lens capsule is a homogeneous membrane surrounding the lens and consists of an outer zonular lamella or pericapsular membrane and deep cuticular lamellated layer. The capsule provides insertion for zonular fibers and owing to its elastic nature, it plays a major role in accommodation. The capsule maintains the lens integrity and regulate the movement of nutrients and waste products between the lens, the aqueous humour and vitreous body. It varies in thickness, increasing anteriorly and at the equator. The anterior capsule is subjected to laceration and tearing due to penetrating foreign body or severe trauma. The capsule usually curl when it is torn and the underlying lens fibers swell with fluids resulting in focal cataracts. More lens fibers may be involved and complete cataractous lens will result. Capsular tear may be associated with posterior synechia which may close the wound. Lens contusion without tearing may affect the permeability of the capsule resulting in increased fluid uptake and cataractous formation. Deposition of pigments on the anterior capsule is associated with persistent pupillary membrane, surgical manipulation of the iris and intraocular inflammation. Opacities involving the posterior lens capsule are usually associated with persistent hyaloid artery (Mittendorf's dot).
2-Subcapsular cataracts: -
They occur beneath the anterior lens capsule, a single layer of cuboidal epithelial cells is present. These cells may proliferate due to trauma or toxic insults resulting in focal subcapsular cataracts.
3-Cortical cataracts: -
Cortical changes are usually present at the anterior, posterior or equatorial cortex. A variety of cataractous changes including vacuoles, water clefts and spoke-like opacities are present.
4-Nuclear cataracts: -
They are most frequently congenital and bilaterally recorded. Significant visual impairment may occur if the opacity is dense.
5- Total cataracts: -
IV-According to the cause: -
1-Inherited cataracts: -
These types of cataracts may be not associated with other ocular disease, associated with progressive retinal atrophy, or accompanied with multiple ocular defects.
Trauma to the globe may result from crushing against hard object. A faring of the lens capsule and/or disinsertion of the suspensory ligaments may then occur. Foreign bodies (glass, shotgun pellets and thorns) may penetrate the lens capsule and disrubt cortical material. Escaped lens material usually results in endophthalmitis which is exhibited by severe pain,epiphora, conjunctival congestion, miosis, aqueous flare, uveitis, posterior synechiae and low intraocular pressure.
3-Metabolic cataracts (Diabetic cataracts): -
Diabetes mellitus is responsible for cataract formation in dogs. Development of cataractous changes is usually associated with alteration of certain osmotic factors. Cataracts in animals can be induced experimentally by the ingestion of excessive levels of lactose, galactose, and xylose.
4-Toxic cataracts: -
Several toxic substances can produce cataractous changes when administered systematically such as naphthaline and dinitrophenol.
5-Parasitic cataracts: -
Ocular filariasis may be the cause of cataract in some species of animals with other ocular manifistation.
Treatment:I-Medical treatment: -
Several medical treatment regimes have been proposed for cataract resolution in dogs:
3-Horse serium extract injected intramuscularly following the injection of a cytolyzed culture of Actinomyces bovis
The results of medical treatment are highly subjective due to the problems of interpretation of the results.
II- Surgical treatment: -
Numerous surgical techniques for treatment of cataract have been proposed; intracapsular extraction, extracapsniar extraction; discission/aspiration; and phacoemulsification.
1-Extracapsular extraction: -
This method is the standard and most popular method of lens removal in dogs. In this technique the anterior lens capsule, the lens cortex and the 'nuclear material are extracted and only the posterior lens capsule remains intact. The presence of posterior capsule prevent the occurrence of anterior vitreous presentation. There is a strong attachment between the posterior capsule and the hyaloid membrane. The only main disadvantage of this technique is the escape of the liquified lens material into the aqueous humour resulting, in postoperative iridocyclitis. Also posterior capsule may be opaque or may lose its transparency following surgery.
-Lateral canthotomy is usually performed to enlarge the small palpebral fissure.
-Lid speculum is used to retract the eyelids for proper surgical exposure. Sometimes stay sutures are used to stabilize the globe during surgery (4-0 to 6-0 silk, or nylon).
Two types of conjunctival flaps are used; limbal based flap and fornix based flap
-Entry into the anterior chamber is performed through one of the three basic anatomic incisions; limbal, corneal or scleral. Corneal and limbal ones are better than scleral one, which is usually accompanied by profuse hemorrhage.
-Removal of anterior lens capsule.
-Removal of the cortex and nucleus.
-Closure of the corneal or limbal wound. Preplaced sutures with limbal-based conjunctival flap are not uncommon in dogs.
-Conjunctival incision closure.
2-Intracapsular extraction: -
It is the same as extracapsular except that the lens is completely removed. The lens is delivered within an intact lens capsule. Before lens removal, zonulolysis of the suspensory ligament is tried using alphachymotrypsin as a zonulolytic enzyme. The lens can be removed using forceps either by tumbling or sliding and can also be removed by cryoextraction.
3-Phacoemulsification (phacofragmentation): -
It is the use of high frequency vibrations to fragment and emulsify the lens into a solution or emulsion and its aspiration through the anterior chamber. Ultra-sonic energy has been used to break down the lens material.
4-Discission and aspiration: -
It is performed in small animals when the lens material is soft enough to allow aspiration. The anterior lens capsule is penetrated and the content is aspirated. A small limbal incision is sufficient to introduce the aspiration needle.
1-Corneal ulceration 2-Corneal oedema 3-Surgical wound leakage 4-Secondary glaucoma
5-Uveitis 6-Anterior vitreous presentation 7-Bacterial endophthalmitis 8-Retinal detachment.
IV- Lens displacement: -Displacement of the lens results from partial or complete breakdown of the suspensory ligaments.
3-Secondary to intraocular tumours, glaucoma and hyper-mature cataracts.
1-Subluxation of the lens:
The displaced lens remains in the patellar fossa behind the iris.
2-Luxation of the lens:
Means complete displacement of the lens from the patellar fossa. It may be anterior luxation or posterior luxation. In anterior luxation the lens may force the iris forward or pass through the pupil to the anterior chamber of the eyeball. Posterior lens luxation is usually tolerated and may result in secondary vitreous absorption.
Removal of the lens especially in non-complicated cases