The lens

image of The lens
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The chapter looks at the lens, its anatomy and physiology; investigation of disease; canine and feline conditions.

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16.1 Embryology of the lens. The optic vesicle develops as an outbudding of neurectoderm in the region of the developing forebrain. It induces a thickening of the overlying surface ectoderm to form the lens placode. As the optic vesicle invaginates to form the optic cup, the lens placode invaginates from the surface ectoderm. The lens vesicle consists of a hollow sphere with a single layer of cells. The posterior cells of the lens vesicle elongate to form primary lens fibres, which obliterate the cavity forming the lens vesicle. Drawn by S.J. Elmhurst BA Hons (www.livingart.org.uk) and reproduced with her permission.
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16.2 Zonular attachments to the lens. Note that zonular fibres arising from the peaks of the ciliary processes insert posterior to the lens equator, whilst those arising from the valleys between the processes insert anterior to the lens equator. CB = Ciliary body; I = Iris; L = Lens.
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16.3 Different areas of the adult lens and the lens sutures. The anterior lens suture lines form a Y shape, whilst the posterior suture lines form an inverted Y shape. Cloquet’s canal can often be seen as an optically clear space within the vitreous arising from the posterior lens surface.
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16.4 Early immature cortical cataracts at the lens equator with some minor cataract at the posterior suture lines. The majority of cataractous change in this lens would not be visible without pupil dilation.
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16.5 Technique for localizing lens opacities using parallax. Illustrations showing three axial opacities at different levels in the lens that are (a) superimposed when viewed by an observer in front of the patient along the visual axis (blue line). (b) If the patient’s eye remains stationary in a forward gaze and the observer moves to the side, opacities posterior to the lens nucleus will appear to move in the same direction as the observer, nuclear opacities will remain stationary and anterior opacities will appear to move in the opposite direction to the observer. (c) If the observer remains stationary and the patient shifts its gaze to the side, posterior opacities will appear to move in the opposite direction to the eye movement and anterior opacities will appear to move in the same direction. This phenomenon is illustrated by the position of the dense white posterior lens opacity, which in (d) is seen at the centre of the visual axis, in (e) appears to move to the right as the observer moves to the right, and in (f) appears to move to the left when the animal directs its gaze to the right whilst the observer remains stationary. Other equatorial opacities in this patient are also located within the posterior lens in this patient. (a–c courtesy of G McLellan)
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16.6 Remnants of a persistent pupillary membrane (PPM) in a dog. Note how the strands arise from the iris collarette (mid-portion of the iris; arrowed). In this case, they insert on to the anterior lens capsule. Posterior synechiae (adhesions from the posterior aspect of the iris to the anterior lens capsule; arrowed) and iris rests (pigment deposited on the anterior lens capsule from the posterior iris; arrowheads) secondary to penetrating corneal trauma in a dog. (Courtesy of D Gould)
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16.7 A 2-year-old Hungarian Vizla with mesenchymal remnants on the anterior lens capsule (arrowed). Compare the lighter colour and axial location with the appearance of the iris rests in Figure 16.6 .
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16.8 Nuclear cataract in a Miniature Schnauzer. (Courtesy of D Gould)
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16.9 Nuclear cataract in an English Springer Spaniel with a feather-like appearance, which is unlikely to be progressive because of its nuclear position.
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16.10 Pulverulent nuclear cataract in a 4-year-old Cocker Spaniel.
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16.11 PHPV/PHTVL in a 2-year-old Cocker Spaniel. As the lesion was imaged from a lateral position, the posterior lens capsule opacity appears lateral rather than central. The presence of a vascular network indicates that this is more than a simple cataract.
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16.12 Microphakia and immature cataract in a 9-year-old Cavalier King Charles Spaniel. Note the presence of the stretched zonules medially (arrowed).
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16.13 Lens/zonule coloboma. Deficit in the lens with a flat edge to the lens equator and a lack of zonules (arrowheads). Some stretched zonules (arrowed) can be seen laterally. (Courtesy of K Wendlandt)
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16.15 Incipient senile cortical cataract nasally (arrowed) with early nuclear sclerosis in an 8-year-old Miniature Poodle.
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16.16 Early water clefting (arrowed) of an immature diabetic cataract in a 4-year-old crossbred dog.
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16.17 Corneal and lens penetration, which occurred 2 hours previously, in the right eye of a 6-year-old German Shepherd Dog. Note the corneal penetration temporally (arrowed), the lens laceration and iris rests inferiorly (arrowhead) and the free-floating clump of fibrin nasally (*). The aqueous humour has cellular debris throughout.
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16.18 Mature cataracts secondary to generalized PRA in a Miniature Poodle. The PLRs were sluggish and the pupils were dilated, implying underlying retinal disease (cataracts in the absence of concurrent intraocular disease should be associated with normal pupil size and PLRs). (Courtesy of D Gould)
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16.19 Iris rests and mature cataract secondary to prior uveitis in a Bearded Collie. The iris rests are distributed in a circular pattern, indicating previous miosis and pigment deposition. The eye also has secondary glaucoma.
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16.20 Typical appearance of an inherited posterior polar subcapsular cataract (arrowed) at the confluence of the posterior suture lines in a 4-year-old Labrador Retriever. (Courtesy of G McLellan)
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16.21 Immature (incomplete) cataract in a Labrador Retriever. The cataract involves the majority of the lens but a tapetal reflex is still present. (Courtesy of D Gould)
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16.22 Mature (complete) cataract in a Springer Spaniel. The tapetal reflex is absent. Note the relatively normal iris surface architecture and colour, implying minimal lens-induced uveitis (compare with Figure 16.23 ). (Courtesy of D Gould)
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16.23 Hypermature (resorbing) cataract in a 4-year-old Tibetan Terrier. There is hyperpigmentation of the iris, indicating a lens-induced uveitis and some early fibrosis and wrinkling of the lens capsule.
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16.24 Morgagnian cataract in a Shih Tzu. In this case, resorption of the liquefied cortex of a hypermature cataract has led to partial clearing of the peripheral visual axis, but a dense nuclear cataract persists. The uveitis associated with the lens resorption has contributed to partial retinal detachment in this eye. (Courtesy of D Gould)
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16.25 Dense nuclear cataract (N) with small areas of equatorial cortical cataract formation (arrowed) in a Labrador Retriever. Small iris cysts at the pupil margin (C) were an incidental finding.
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16.26 Nuclear sclerosis with a senile equatorial cortical cataract (arrowed) in a 12-year-old Chow Chow. The tapetal reflection is still visible through the lens nucleus, which has a circular outline.
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16.27 Vitreous prolapse associated with lens subluxation. The wisp-like strands of vitreous (white arrows) can be seen protruding within the aphakic crescent between the pupil margin and the lens equator. There is also some pigment dispersion from the posterior iris into the anterior chamber (black arrow). (Courtesy of D Gould)
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16.28 Lens subluxation in a 5-year-old Tibetan Terrier. Note the aphakic crescent and the lack of lens zonules (arrowed).
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16.29 Anterior lens luxation in a 4-year-old terrier crossbreed. Note the edge of the pupil behind the lens (arrowed), the refractile edge of the visible lens equator (LE) and the subtle corneal oedema giving a steamy appearance to the cornea (arrowheads).
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16.30 Persistent pupillary membrane (PPM) in a cat. The membrane remnants arise from the iris collarette (black arrows). Note the focal cataract (white arrow) at the attachment of the membrane to the anterior lens capsule.
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16.31 Hypermature primary cataract in a 4-year-old Domestic Shorthaired cat. Note the crystalline appearance associated with subcapsular plaques (arrowed).
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16.32 Immature cataract, lens subluxation and extensive posterior synechiae secondary to uveitis in a 12-year-old Domestic Shorthaired cat. Note the fibrovascular membrane covering the majority of the anterior lens capsule (arrowed).
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16.33 Anterior lens luxation in an 11-year-old Domestic Shorthaired cat secondary to chronic uveitis. Note the iris rubeosis/neovascularization (black arrow) and pigment deposition on the lens (white arrow).
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16.34 Phacoemulsification technique. The main entry port is created with a slit knife through clear cornea adjacent to the limbus. Injection of viscoelastic through a corneal entry port to maintain the anterior chamber and to protect the corneal endothelium. Anterior capsulorrhexis (arrowed) is performed, creating a circular hole in the capsule. The lens nucleus is divided into manageable sections. In this case, the lens is divided into two halves (arrowed) using a chop technique. Lens material is phacofragmented and aspirated from the eye using a phacoemulsification needle (arrowed). The instrument on the left-hand side helps to position the material against the phacoemulsification needle. The remaining lens cortex is aspirated from the eye, in this case using irrigation/aspiration handpieces. The instrument on the right-hand side is aspirating cortex from the lens equator inside the capsular bag (arrowed). The intraocular lens (arrowed) is folded and injected through the main entry port into the lens capsule. The intraocular lens within the capsular bag at the end of surgery. Both entry ports have been sutured.
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16.35 Appearance of the intraocular lens in a cat 1 week after cataract surgery. Note the opacities in the peripheral anterior lens capsule (arrowed).
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16.36 Appearance of an intraocular lens in a 9-year-old Labrador Retriever 3 years after surgery. Note the peripheral opacification of the lens capsule (white arrows) and some minor wrinkling of the posterior capsule deep to the lens optic (black arrow).
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16.37 Suture-fixed intraocular lens in a 9-year-old English Springer Spaniel. Note the lack of lens capsule and unrelated minor iris atrophy (arrowed).

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