Ординатура / Офтальмология / Английские материалы / Oxford American Handbook of Ophthalmology_Tsai, Denniston, Murray_2011

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Lens

Anatomy and physiology 228

Cataract: introduction 230

Cataract: types 232

Cataract surgery: assessment 234

Cataract surgery: consent and planning 236

Presbyopia correcting strategies 238

Cataract surgery: perioperative 239

Cataract surgery: postoperative 241

Phacoemulsification (1) 242

Phacoemulsification (2) 244

ECCE and ICCE 246

Intraocular lenses 247

Cataract surgery and concurrent eye disease 249

Cataract surgery: complications 251

Postoperative endophthalmitis 254

Postoperative cystoid macular edema 257

Abnormalities of lens size, shape, and position 258

Related pages:

Congenital cataracts b p. 625–627

228 CHAPTER 9 Lens

Anatomy and physiology

The lens is a transparent, biconvex structure with an outer acellular capsule. It provides one-third of the refractive power of the eye. In the unaccommodated state, the lens is around 4 mm thick, with a 10 mm anterior radius of curvature, a 6 mm posterior radius of curvature, a refractive index of 1.386 (1.406 centrally), and an overall diopter power of 18D.

Anatomy

Embryologically derived from surface ectoderm

Capsule

This unusually thick basement membrane is rich in type IV collagen; the anterior capsule arises from the epithelium, while the posterior capsule arises from the elongating fiber cells. The capsule is thicker at the equator than centrally and thicker anteriorly (8–14 μm, increasing with age) than posteriorly (2–3 μm).

Epithelium

The lens epithelium lies just deep to the anterior capsule. Centrally, the epithelium is cuboidal and nonmitotic; peripherally, the epithelium is columnar and mitotic, producing almost 2 million transparent lens fibers over an adult’s life.

Fibers

As the cells elongate (up to 10 mm long), transparency is attained by loss of organelles, a tight regular arrangement, and a 90% crystallin composition. The nucleus (comprising embryonic and fetal parts) consists of the fibers laid down before birth; however, the clinical “nucleus” observed at the slit lamp also includes deep cortex.

Lens sutures are formed by interdigitation of the ends of the fibers. The most visible example are the two Y-shaped sutures of the fetal nucleus— anterior Y, posterior Y. The cortex contains the more recently formed fibers and the nucleus contains the older nondividing cells.

Zonules

These comprise sheets of suspensory fibers composed of fibrillin (Ch15q) that arise at the ciliary body and attach to the lens pre-equatorially, equatorially, and post-equatorially.

Physiology

The lens has a low water (65%) and high protein (35%) content. It has a resting pH of 6.9 and a relatively low temperature and is relatively hypoxic. Most energy production and active transport occurs at the epithelium, but peripheral lens fibers demonstrate significant protein synthesis (mainly of crystallins), and even central lens fibers show limited carbohydrate metabolism.

Although oxidative phosphorylation occurs at the epithelium, most energy production is anaerobic (via glycolysis, pentose–phosphate pathway, and the A-glycerophosphate shuttle). Most glucose is thus converted to glucose-6-phosphate and, to a lesser degree, sorbitol.

230 CHAPTER 9 Lens

Cataract: introduction

Cataracts account for around 50% of global blindness, representing around 18 million people. While cataracts are ubiquitous, occurring in almost every aging population, the inequity of eye care means that 99% of bilateral blindness due to cataracts is seen in developing countries. Cataracts account for 50% of cases of reversible low vision in the United States.

Risk factors

The prevalence of cataract increases markedly with age. In the United States, a visually significant cataract (VA <20/40) is present in approximately 2.5% of those aged 40–49, 6.8% of those aged 50–59, 20% of those aged 60–69, 42.8% of those aged 70–79, and 68.3% of those aged >80.

Other risk factors include exposure to sunlight, smoking, alcohol, dehydration, radiation, corticosteroid use, and diabetes mellitus. Nutritional or vitamin supplementation has not consistently been shown to be useful in preventing cataract formation.

Pathogenesis

The way in which these factors cause cataracts is unclear, although a common pathway appears to be protein denaturation (e.g., by oxidative stress). Metabolic disturbance (hyperglycemia in diabetes mellitus or hyperuremia in dehydration or renal failure), toxins (e.g., smoking, alcohol), loss of anti-oxidant enzymes (e.g., superoxide dismutase), membrane disruption, reduced metabolism, failure of active transport, and loss of ionic–osmotic balance may all contribute to this process.

Clinical presentations

Common

•Change in vision—reduced acuity, contrast sensitivity, or color appreciation, glare, monocular diplopia, or ghosting.

•Change in refraction—typically a myopic shift due to nuclear sclerosis.

•Change in fundus view—clinicians may have difficulty “looking in” long before the patients feel they have difficulties “looking out.” This may be a problem when trying to monitor or treat posterior segment disease such as diabetic retinopathy or macular degeneration.

Uncommon

Phacomorphic glaucoma

The large cataractous lens may cause anterior bowing of the iris with secondary angle closure. Presentation may occur as acute angle closure with high IOP, shallow AC, and fixed semidilated pupil.

Phacomorphic glaucoma can be distinguished from primary angle closure glaucoma by the presence of an ipsilateral swollen cataractous lens and contralateral open angle with deep AC.

232 CHAPTER 9 Lens

Cataract: types

Cataracts may be classified according to age of onset, morphology, grade of opacification, and maturity.

Age of onset

Cataracts may be congenital (p. 625), juvenile or presenile (p. 626), or age related (senile) (p. 234).

Morphology

Cataract morphology may be divided into fiber based (pattern relates to anatomical structure of the lens) or nonfiber based (a more random distribution). Fiber-based cataracts may be divided into sutural (pattern relates to lens sutures) and nonsutural types (Table 9.1).

Grade

Grading systems have been designed that aim to quantify the degree of opacification. These vary from simple assessment by direct ophthalmoscopy to more sophisticated methods such as the Lens Opacities Classification System III (LOCS III), where slit-lamp examination is compared to a standard set of photographs (separate set for nuclear, cortical, and posterior subcapsular).

Maturity of cataract

•Immature: opacification is incomplete.

•Mature: opacification is total.

•Hypermature: lysis of cortex results in shrinkage, seen clinically as wrinkling of the capsule.

•Morgagnian: liquefaction of cortex allows the harder nucleus to drop inferiorly (but remaining still within the capsule).

Table 9.1 Classification of cataract morphology

234 CHAPTER 9 Lens

Cataract surgery: assessment

Surgical removal of cataracts is effective and safe. In the United States, 85 to 95% of patients attain best-corrected visual acuity (BCVA) 20/40 within 3 months of surgery, 58% had BCVA of 20/25 or better, and 75% are within 1D of predicted refraction. Sight-threatening complications are rare. However, this is in part due to careful preoperative preparation and postoperative assessment.

1) Referral

Referrals may be made by the PCP, optometrist, or ophthalmologist.

Appropriate referral

•The cataract is likely responsible for the patient’s visual complaint.

•The cataract is compromising the patient’s lifestyle.

•The risks and benefits of cataract surgery have been discussed with the patient.

•The patient wishes to have the operation.

2) Outpatient appointment

Table 9.2 Initial assessment for cataract surgery

CATARACT SURGERY: ASSESSMENT 235

Appropriate scheduling for cataract surgery

•There is a visually significant cataract responsible for the patient’s complaint and compromising his/her lifestyle.

•There is no coexisting ocular disease precluding surgery. Any disease that may affect surgery (e.g., PXF) or outcome (e.g., AMD) has been discussed with the patient and an appropriately guarded prognosis given.

•The patient wants to proceed and understands the risks.

•Informed consent is taken and a surgical plan is formulated (p. 236).

The younger patient

In the younger patients, also consider why they might have developed presenile cataracts (e.g., trauma, steroids, etc., p. 626).

3) Preoperative assessment

For patient convenience, this is often on the same day as the initial assessment. Aspects of the workup may be performed by a suitably trained assistant or technician according to local protocol.

History

•General health—past medical history, drugs, allergies.

•Social history—support, telephone, ability to manage topical medication.

•Education—surgery, postoperative care, information leaflet.

Investigation

• Biometry/IOL power calculations.

Treatment

•Prescription of preoperative treatments—e.g., topical antibiotics for blepharitis, or atropine for poor dilation.