Ординатура / Офтальмология / Английские материалы / Clinical Ophthalmology A Systematic Approach 7th Edition_Kanski, Bowling_2011

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Fig. 9.5 Secondary cataract. (A) Early uveitic posterior subcapsular cataract; (B) uveitic anterior plaque opacities; (C) extensive posterior synechiae and anterior lens opacity; (D) glaukomflecken

Acute congestive angle-closure

Acute congestive angle-closure may cause small, grey-white, anterior, subcapsular or capsular opacities within the pupillary area (glaukomflecken – Fig. 9.5D). They represent focal infarcts of the lens epithelium and are almost pathognomonic of past acute angle-closure glaucoma.

High myopia

High (pathological) myopia is associated with posterior subcapsular lens opacities and early-onset nuclear sclerosis, which may ironically increase the myopic refractive error. Simple myopia, however, is not associated with such cataract formation.

Hereditary fundus dystrophies

Hereditary fundus dystrophies, such as retinitis pigmentosa, Leber congenital amaurosis, gyrate atrophy and Stickler syndrome, may be associated with posterior subcapsular lens opacities (see Ch. 15). Cataract surgery may occasionally improve visual acuity even in the presence of severe retinal changes.

Traumatic cataract

Trauma is the most common cause of unilateral cataract in young individuals and may include the following.

1Penetrating trauma (Fig. 9.6A).

2 Blunt trauma may cause a characteristic flower-shaped opacity (Fig. 9.6B).

3Electric shock and lightning strike are very rare causes that may result in anterior and posterior iridescent opacities that have a stellate pattern (Fig. 9.6C).

4Infrared radiation, if intense as in glassblowers, may rarely cause true exfoliation of the anterior lens capsule (Fig. 9.6D).

5Ionizing radiation for ocular tumours may cause posterior subcapsular opacities (Fig. 9.7E) that may develop months or years later.

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Fig. 9.6 Causes of traumatic cataract. (A) Penetrating trauma; (B) blunt trauma; (C) electric shock and lightning strike; (D) infrared radiation (glassblower's cataract);

(E) ionizing radiation

(Courtesy of J Schuman, V Christopoulos, D Dhaliwal, M Kahook and R Noecker, from Lens and Glaucoma, in Rapid Diagnosis in Ophthalmology, Mosby 2008 – figs C-E)

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Management of age-related cataract

Preoperative considerations

Indications for surgery

1Visual improvement is by far the most common indication for cataract surgery. Operation is indicated only if and when the opacity develops to a degree sufficient to cause difficulty in performing essential daily activities.

2Medical indications are those in which a cataract is adversely affecting the health of the eye, for example, phacolytic or phacomorphic glaucoma (see Ch. 10). Cataract surgery to improve the clarity of the ocular media may also be required in the context of fundal pathology (e.g. diabetic retinopathy) requiring monitoring or treatment.

Systemic preoperative assessment

For elective surgery, a general medical history is taken and any problems managed accordingly. Table 9.1 sets out suggested further enquiry and action in relation to a range of systemic diseases. Routine preoperative general medical examination, blood tests and ECG are not usually required for local anaesthesia.

Table 9.1 -- Management of general medical conditions prior to elective surgery

Ophthalmic preoperative assessment

A detailed and pertinent ophthalmic evaluation is required. Following a past ophthalmic history, the following should be considered:

3Pupillary responses. Because a cataract never produces an afferent pupillary defect, its presence implies substantial additional

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pathology likely to influence the final visual outcome and requires further investigation.

4Ocular adnexa. Dacryocystitis, blepharitis, chronic conjunctivitis, lagophthalmos, ectropion, entropion and tear film abnormalities may predispose to endophthalmitis and require effective preoperative resolution.

5Cornea. Eyes with decreased endothelial cell counts (e.g. substantial cornea guttata) have increased vulnerability to postoperative decompensation secondary to operative trauma. Specular microscopy and pachymetry may be helpful in assessing risk, and special precautions should be taken to protect the endothelium (see below).

6Anterior chamber. A shallow anterior chamber can render cataract surgery difficult. Recognition of a poorly dilating pupil allows intensive preoperative mydriatic drops, planned mechanical dilatation prior to capsulorhexis and/or intracameral injection of mydriatic. A poor red reflex compromises the creation of an adequate capsulorhexis, but can be largely overcome by staining the capsule with a dye such as trypan blue 0.06% (VisionBlue®).

7Lens. Nuclear cataracts tend to be harder and may require more power for phacoemulsification, while cortical opacities tend to be softer. Black nuclear opacities are extremely dense and extracapsular cataract extraction rather than phacoemulsification may be the superior option. Pseudoexfoliation indicates a likelihood of weak zonules (look for phakodonesis), a fragile capsule and poor mydriasis.

8Fundus examination. Pathology such as age-related macular degeneration may affect the visual outcome. Ultrasonography may be required, principally to exclude retinal detachment and staphyloma, in eyes with very dense opacity that precludes fundoscopy.

9Current refractive status. It is critical to obtain details of the patient's pre-operative refractive error in order to guide intraocular lens implant (IOL) selection. The keratometry readings (obtained during biometry – see below) should be noted in relation to the refraction, particularly if it is planned to address astigmatism by means of targeted wound placement or a specific adjunctive procedure. It is particularly important to obtain a postoperative refractive result from an eye previously operated upon so that any ‘refractive surprise’, even if minor, can be analysed and taken into account.

Biometry

Biometry facilitates calculation of the lens power likely to result in the desired postoperative refractive outcome; in its basic form this involves the measurement of two ocular parameters, keratometry and axial (anteroposterior) length.

1Keratometry involves determination of the curvature of the anterior corneal surface (steepest and flattest meridians), expressed in dioptres or in mm of radius of curvature. This is commonly carried out with the interferometry apparatus used to determine axial length (see below), but if this is unavailable or unsuitable manual keratometry (e.g. Javal–Schiøtz keratometer) can be performed.

2Optical coherence biometry is a non-contact method of axial measurement that utilizes two coaxial low-energy laser beams which are partially coherent and produce an interference pattern (partial coherence interferometry). The Zeiss IOLMaster (Fig. 9.7A) is a complete biometry system which also readily performs keratometry, anterior chamber depth and corneal white-to-white measurement, and is able to calculate IOL power using a range of formulae. Measurements (Fig. 9.7B) have high reproducibility and generally require less skill than ultrasonic biometry (see below). Data storage and A-constant validation are other useful features. Aphakic, pseudophakic and silicone-filled eyes can be measured, with variable tailored settings.

3A-scan ultrasonography is a generally slightly less accurate method of determining the axial dimension and can be acquired either by direct contact (Fig. 9.7C) or more accurately but with greater technical difficulty by using a water bath. The sound beam must be aligned with the visual axis for maximal precision; each reflecting surface shows up as a spike on the oscilloscope screen (Fig. 9.7D).

4IOL power calculation formulae. Numerous formulae have been developed which utilize keratometry and axial length to calculate the IOL power required to achieve a given refractive outcome. Some formulae incorporate additional parameters such as anterior chamber depth to optimize the accuracy of prediction. The SRK-T is an example of a commonly used formula for eyes of axial length greater than 22.0 mm. Specific formulae may be superior for very short (generally the Hoffer Q) or long eyes, but research results and opinions vary and it is always wise to take the time to plan individually for an unusual eye, consulting the latest research and recommendations.

5Previous refractive surgery. Any form of corneal refractive surgery is likely to make a significant difference to the IOL power required for a given refractive outcome, and standard IOL calculations are unsuitable.

6Contact lenses. If the patient wears soft contact lenses, these may need to be left out for up to a week prior to biometry to allow corneal stabilization; hard/gas permeable lenses may need to be left out for three weeks.

7Personalized A-constant. If a consistent postoperative refractive deviation is found in most of an individual surgeon's cases, it is assumed that some aspects of personal surgical (or possibly biometric) technique consistently and similarly influence outcome, and a personalized A-constant can be programmed into biometry apparatus to take this into account.

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Fig. 9.7 Biometry (A) IOLmaster; (B) ideal scan; (C) contact A-scan biometry; (D) ultrasonic A-scan display

(Courtesy of D Michalik and J Bolger)

Postoperative refraction

1Emmetropia is typically the ideal postoperative refraction, though with spectacles needed for close work since a conventional IOL cannot accommodate. Many surgeons aim for a small degree of myopia (about −0.25 D) to offset possible errors in biometry.

2Contralateral eye. If this has a significant refractive error but is unlikely to require cataract surgery within a few years, the postoperative target for the operated eye might be set for within less than 2.0 D of its fellow, to avoid problems with binocular fusion. In some cases, such as when there is minimal lens opacity in the fellow eye or when ametropia is extreme, the patient can be offered lens surgery to the other eye to facilitate targeting both at emmetropia.

3‘Monovision’ is a concept in which the (usually) non-dominant eye is left at or just less than –2.0 D myopic to allow reading whilst emmetropia is targeted in the dominant eye. This is attractive to some patients, generally those who have previously been using contact lenses or spectacles to achieve monovision.

4Multifocal lens options use a variety of optical means to attempt to achieve satisfactory near, distance and intermediate vision. Many patients are very satisfied with the results but a significant minority are unhappy, complaining of phenomena such as glare. Highly accurate refractive outcomes, including very limited astigmatism, are necessary for optimal function and a greater likelihood of tolerance.

5Younger patients. With a conventional monofocal IOL, patients younger than about 50 need to be aware that they will experience the sudden loss of active focusing and that it will often take some time to adjust.

Intraocular lenses

Positioning

An IOL consists of an optic and the haptics. The optic is the central refracting element, and the haptics the arms or loops which sit in contact with the ocular structures (capsular bag, ciliary sulcus or anterior chamber angle) for stable optimal positioning (centration) of the optic. Modern cataract surgery, with preservation of the capsular ‘bag’, affords positioning of the IOL in the ideal location – ‘in the bag’. Complicated surgery, with rupture of the posterior capsule, may necessitate alternative positioning in the posterior chamber with the haptics in the ciliary sulcus (a 3-piece IOL only, not 1-piece including those with plate haptics, as these may not be stable), or in the anterior chamber (AC) with the haptics supported in the angle – AC positioning requires a specific lens type, an ‘AC-IOL’.

Design

1Flexible IOLs are now in general use and allow introduction into the eye through a very small incision. For insertion they may be folded in half with special forceps or loaded into an injector delivery system, then unfolded or unrolled inside the eye. Injector-based delivery has become increasingly popular, as it allows introduction without lens contact with the ocular surface, so reducing the risk of bacterial contamination. Injection also allows insertion through a slightly smaller incision than folding. Flexible materials available are discussed below; there seems to be no distinct superiority of one over another, and a combination IOL can also be used.

aSilicone IOLs are available in both loop haptic (1- or 3-piece) and plate haptic (1-piece) conformations, the latter consisting of a roughly rectangular leaf with the optic sited centrally. Silicone IOLs may exhibit greater biocompatibility, exciting less inflammatory reaction, than hydrophobic acrylic IOLs. They may be particularly prone to significant silicone deposition in

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silicone oil-filled eyes.

bAcrylic IOLs, 3-piece or 1-piece, may be hydrophobic (water content <1%) or hydrophilic, with much higher water content.

•Hydrophobic acrylic materials have a greater refractive index than hydrophilic lenses and are consequently thinner. They tend to produce a greater reaction in uveitic eyes, and some surgeons prefer not to use them in this scenario.

•Hydrophilic acrylic (hydrogel), in theory, offers superior biocompatibility and so should be better tolerated by uveitic eyes. Posterior capsular opacification (PCO) rates are probably higher than with other materials.

cCollamer is composed of collagen, a poly-HEMA based copolymer and a UV-absorbing chromophore. It is marketed principally on the basis of high biocompatibility and a favourable track record.

2Rigid IOLs are made entirely from polymethylmethacrylate (PMMA). They cannot be folded or injected so require an incision larger than the diameter of the optic, typically 5 mm, for insertion. For economic reasons, they continue to be widely used in developing countries. PCO rates are higher with PMMA lenses than silicone and acrylic. Some surgeons favour heparin-coated (see below) IOLs in uveitic eyes, particularly in children.

3Sharp/square-edged optics are significantly associated with a lower rate of PCO compared with round-edged optics, and the former is now the predominant design. Lens material seems to have a less important effect than shape on PCO.

4Blue light filters. Although essentially all IOLs contain ultraviolet light filters, a number also include filters for blue wavelengths, in order to reduce the possibility of damage to the retina.

5Aspheric optics counteract spherical aberration and improve contrast, particularly in mesopic conditions, and are available in some newer IOLs.

6Heparin coating reduces the attraction and adhesion of inflammatory cells, and this may have particular application in eyes with uveitis. However, there is no clear evidence about whether heparin-surface modification is clinically beneficial, and indeed about which IOL material is superior for use in cataract surgery on eyes with uveitis.

7Multifocal IOLs aim to provide clear vision over a range of focal distances. So-called accommodative IOLs attempt to flex and thereby alter focal length but in practice the amplitude of accommodation is slight. Pseudoaccommodative IOLs achieve their purpose by refractive or diffractive means.

8Toric IOLs have an integral cylindrical refractive component to compensate for pre-existing corneal astigmatism. The main potential problem is rotation within the capsular bag, which occurs in 10–20%, following which surgical repositioning may be carried out.

9Adjustable IOLs allow the alteration of refractive power following implantation. One version uses low-level ultraviolet irradiation at the slit-lamp about a week after surgery to induce polymerization of its constituent molecules in specific patterns with precise spherical and cylindrical (astigmatism) correction.

Anaesthesia

The vast majority of cataract surgery is performed under local anaesthesia (LA) although general anaesthesia is required in some circumstances such as children and many young adults, very anxious patients, some patients with learning difficulties, epilepsy, dementia and those with a head tremor.

1Sub-Tenon block involves inserting a blunt-tipped cannula through an incision in the conjunctiva and Tenon capsule 5 mm from the limbus inferonasally, and passing it through the sub-Tenon space (Fig. 9.8A). The anaesthetic is injected beyond the equator of the globe (Fig. 9.8B). Although anaesthesia is good and complications minimal, akinesia is variable. Chemosis and subconjunctival haemorrhage are common but penetration of the globe is extremely rare.

2Peribulbar block is given through the skin or conjunctiva with a 1-inch (25-mm) needle (Fig. 9.9A and B). It generally provides effective anaesthesia and akinesia. Penetration of the globe is a very severe, though extremely rare, complication, and for this reason peribulbar is avoided, or approached with great caution, in longer eyes (which also tend to have a larger equatorial diameter).

3Topical anaesthesia involves drops or gel (proxymetacaine 0.5%, tetracaine 1% drops, lidocaine 2% gel) which can be augmented with intracameral preservative-free lidocaine 0.2%–1%, usually during hydrodissection; combined viscoelastic/lidocaine preparations are also commercially available. Although analgesia is generally adequate, it tends to be less effective than peribulbar or sub-Tenon blocks. Despite the absence of akinesia most patients can cooperate adequately.

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Fig. 9.8 Sub-Tenon anaesthesia. (A) Dissection; (B) infiltration

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Fig. 9.9 Peribulbar anaesthesia. (A) Insertion of needle; (B) injection

Phacoemulsification

Introduction

Phacoemulsification (‘phaco’) has become the preferred method of cataract extraction over the last 15 years. The smaller incision of phacoemulsification is associated with little induced postoperative astigmatism and early stabilization of refraction (usually 3 weeks for 3.0 mm incisions but less for sub-2.5 mm incisions). Postoperative wound-related problems such as iris prolapse have been almost eliminated. One disadvantage of phaco is that it requires complex machinery to break up the lens nucleus and remove it through a small incision. Considerable training and practice is required to learn the techniques adequately.

Phacodynamics

The surgeon must understand the machine dynamics and the interaction of fluidics in treating different forms of cataract. The various machines behave differently but the basic mechanism is similar. Choosing appropriate settings makes surgery safer and easier.

1Level of irrigating bottle is measured from the level of the patient's eye. The purpose of setting the bottle at a specific height is to maintain a stable eye at a reasonable intraocular pressure. The infusion flow is proportional to the height of the bottle and is dependent on gravity.

2Aspiration flow rate (AFR) refers to the volume of fluid removed from the eye in cc/minute. For a higher AFR the bottle must be elevated to compensate for increased fluid loss. High AFR results in attraction of lens material towards the phaco tip, with faster vacuum build-up and swifter removal of lens matter but with less power. Adjustment to a high AFR should be avoided by an inexperienced surgeon to reduce the chance of mishap.

3Vacuum, measured in mmHg, is generated during occlusion when the pump is attempting to aspirate fluid. Vacuum helps to hold nuclear material and provides the ability to manipulate lens fragments. High vacuum can also decrease the total power required to remove the lens.

4Surge. When occlusion is broken pent up energy in the system results in surge. This is undesirable as it may result in collapse of the anterior chamber and capsular rupture.

Pumps

1Peristaltic flow pumps pull liquid and lens material into the phaco tip by milking fluid-filled tubing over rollers inside the cassette attached to the phaco machine. The speed at which this is performed is determined by the speed of rotation of the rollers. However, in order for the pump to generate vacuum, occlusion of the tip is required. As vacuum builds to the pre-set level, the pump slows down until it stops when the required vacuum is achieved.

2The Venturi pump creates a negative pressure in a vessel by passing compressed gas across its entrance, generating vacuum. This has the practical effect of synchronizing vacuum and AFR. Depression of the foot pedal increases vacuum towards the preset

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level independent of occlusion, and tip vacuum is therefore always available.

Handpiece

The phaco handpiece (Fig. 9.10A) contains a series of piezoelectric crystals which act as rapid switching devices, causing the tip to vibrate at ultrasonic frequencies. The tip itself consists of a hollow titanium needle 0.7–1.1 mm in diameter with an enclosing sleeve (Fig. 9.10B) to protect the cornea from thermal and mechanical damage. Differently-shaped phaco needles have various characteristics in terms of cutting and holding nuclear material. Emulsification of the lens is the result of the following phenomena:

1Jackhammer pneumatic drill effect is probably the most important.

2Cavitation resulting from the swift movement of solid in a liquid. At the end of each oscillation backstroke, the tip retracts and creates a vacuum which causes cavitation bubbles. The bubbles implode and release large amounts of energy.

3Acoustic shock wave generated by the excursion of the phaco tip.

4Impact of the fluid particle wave as the tip impacts on aqueous. In softer cataracts it is possible to see this in action by removing tissue without direct contact.

Fig. 9.10 (A) Phaco handpiece with tip; (B) phaco tip with sleeve

Viscoelastics

Viscoelastics are biopolymers whose main constituents are glycosaminoglycans and hydroxypropylmethylcellulose. All have the propensity to cause raised intraocular pressure unless carefully removed at the end of surgery.

The main types are:

1Cohesive (e.g. Healon®, Healon GV® and Provisc®)

•Long chains and high molecular weight.

•Easy to remove.

•Used to create and maintain intraocular spaces, for example to maintain the AC during capsulorhexis and inflation of the capsular bag to facilitate introduction of the IOL.

2Dispersive (e.g. Viscoat®)

•Low molecular weight and a tendency to break up.

•Used to coat and protect the endothelium.

•Can also be used to create and maintain space, forming compartments.

•More difficult to remove than cohesive viscoelastics.

3 Adaptive (e.g. Healon 5®) displays properties of both cohesive and dispersive agents.

4Clinical use may also include:

•The ‘soft shell’ technique involves the injection of a dispersive followed by a cohesive viscoelastic underneath. The former adheres to and protects the endothelium. Some surgeons use this routinely for all eyes, others for eyes at higher risk of

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corneal decompensation such as those with a guttate cornea.

•In small pupils a high molecular weight cohesive viscoelastic (e.g. Healon GV) will push the iris away from the lens and help to induce mydriasis.

•Can be used to break posterior synechiae with minimal trauma.

•May be useful to dissect cortex away from the lens capsule to minimize traction on fragile zonular ligaments.

•If a capsulorhexis shows signs of running out to the periphery, injecting a cohesive viscoelastic will flatten the anterior capsule, aiding the exertion of a centrally-directed vector (and expanding the pupil).

•In small posterior capsular tears a dispersive viscoelastic will push the vitreous back into the posterior chamber and plug the capsular defect, facilitating cortex removal.

•Higher molecular weight viscoelastics tend to promote iris prolapse in shallow anterior chambers.

Technique

It is beyond the scope of this book to describe the technique in detail; the following are the basic steps:

1Preparation

aTopical anaesthetic is instilled into the conjunctival sac prior to antiseptic application.

bPovidone-iodine 5% or chlorhexidine is instilled into the conjunctival sac (Fig. 9.11A) and is also used to paint the skin of the eyelids prior to draping (Fig. 9.11B), ensuring thorough eyelash application; the antiseptic should be left to work for a minimum of 3 minutes.

cCareful draping is performed, ensuring that the lashes and lid margins are isolated from the surgical field, and a speculum is inserted (Fig. 9.11C).

2Incisions

aA side port incision is made around 60° to the left (in right-handed surgeons) of the main incision; some surgeons prefer two side ports approximately 180° apart.

bThe main corneal incision may be clear corneal or limbal (Fig. 9.12A); many surgeons locate the incision on the steepest corneal axis, though others prefer consistent siting. Temporal incisions may be associated with a slightly higher risk of endophthalmitis.

cViscoelastic is injected into the anterior chamber.

3Continuous curvilinear capsulorhexis (Fig. 9.12B) is performed with a cystotome, a bent hypodermic needle and/or capsule forceps and involves two movements:

a Shearing in which a tangential vector force is applied along the direction of the tear. b Ripping in which a centripetal vector force strains and tears the capsule.

4 Hydrodissection is performed to separate the nucleus and cortex from the capsule so that the nucleus can be more easily and safely rotated.

a A 26-gauge blunt cannula with fluid is inserted just beneath the edge of the rhexis and fluid is injected gently under the capsule (Fig. 9.12C).

b A hydrodissection wave should be seen provided there is a good red reflex.

cThe phaco probe is inserted and superficial cortex and epinucleus are aspirated.

5 Four quadrant ('divide and conquer’) technique for removal of the nucleus is a very widely used, safe technique.

a'Sculpting’ is performed with the probe to create a groove (Fig. 9.12D).

bThe nucleus is rotated and a second groove is made at a right angle to the first.

cThe probe and second instrument are engaged in opposite walls of the groove and the nucleus is cracked by applying force in opposite directions (Fig. 9.12E).

d The nucleus is rotated 90° and a crack made in the perpendicular groove in a similar manner.

eEach of the four quadrants is emulsified and aspirated in turn (Fig. 9.12F).

6Nuclear phaco chop takes greater experience, but has the advantage of generally requiring lower total phaco energy.

aIn horizontal chopping a blunt-tipped chopper is placed horizontally underneath the capsule and rotated vertically as the equator is reached.

b Vertical chopping is performed with a pointed-tip chopper which does not need to pass beyond the capsulorhexis.

cThe nucleus is chopped into several pieces each of which is emulsified and aspirated.

7Cortical clean up. The cortical fragments are engaged by vacuum, pulled centrally and aspirated (Fig. 9.13A). Some surgeons prefer a manual aspiration method, producing vacuum with a hand-held syringe (e.g. Simcoe cannula), or a bimanual automated method.

8Insertion of IOL

a The capsular bag is filled with viscoelastic (Fig. 9.13B).

bThe corneal incision is enlarged (Fig. 9.13C).

cThe IOL is inserted into a cartridge for injection which is loaded into an injector handset. The tip of the cartridge is introduced through the section (Fig. 9.13D) and the IOL slowly injected into the eye, with careful unrolling (Fig. 9.13E). Alternatively, the IOL is folded and inserted directly into the eye.

dIf necessary the IOL is centred by dialling (Fig. 9.13F).

9Completion

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