Subcapsular cataract

Anterior subcapsular cataract lies directly under the lens capsule and is associated with fibrous metaplasia of the lens epithelium. Posterior subcapsular opacity lies just in front of the posterior capsule and has a granular or plaque-like appearance on oblique slit lamp biomicroscopy ( Fig. 9.1A ), but typically appears black and vacuolated ( Fig. 9.1B ) on retroillumination; the vacuoles are swollen migratory lens epithelial cells (bladder or Wedl), similar to those commonly seen postoperatively in posterior capsular opacification (see Fig. 9.21A ). Due to its location at the nodal point of the eye, a posterior subcapsular opacity often has a particularly profound effect on vision. Patients are characteristically troubled by glare, for instance from the headlights of oncoming cars, and symptoms are increased by miosis, such as occurs during near visual activity and in bright sunlight.

Age-related cataract. (A) Posterior subcapsular; (B) posterior subcapsular on retroillumination, showing Wedl cells; (C) minimal and (D) moderate nuclear sclerosis

Nuclear sclerotic cataract

Nuclear cataract is an exaggeration of normal ageing change. It is often associated with myopia due to an increase in the refractive index of the nucleus, resulting in some elderly patients being able to read without spectacles again (‘second sight of the aged’); in contrast, in the healthy ageing eye (and in occasional cases of cortical and subcapsular cataract) there is mild hypermetropic shift. Nuclear sclerotic cataract is characterized by a yellowish hue due to the deposition of urochrome pigment, and is best assessed with an oblique slit lamp beam ( Fig. 9.1C ). When advanced, the nucleus appears brown ( Fig. 9.1D ) or even black, the latter being typical of marked post-vitrectomy opacity.

Cortical cataract

Cortical cataract may involve the anterior, posterior or equatorial cortex. The opacities start as clefts and vacuoles between lens fibres due to cortical hydration. Subsequent opacification results in typical cuneiform (wedge-shaped) or radial spoke-like opacities ( Figs 9.2A and B ), often initially in the inferonasal quadrant. As with posterior subcapsular opacity, glare is a common symptom.

Age-related cataract. (A) Cortical; (B) cortical on retroillumination; (C) Christmas tree

Christmas tree cataract

Christmas tree cataract, which is uncommon, is characterized by polychromatic needle-like formations in the deep cortex and nucleus ( Fig. 9.2C ).

Cataract maturity

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  Immature cataract is one in which the lens is partially opaque.

  
  
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  Mature cataract is one in which the lens is completely opaque ( Fig. 9.3A ).

  
  

  
  

  
  

  

  
  

  
  

  Cataract maturity. (A) Mature cataract – secondary divergent squint is present; (B) hypermature cataract with wrinkling of the anterior capsule; (C) Morgagnian cataract with liquefaction of the cortex and inferior sinking of the nucleus

  
  

  (Courtesy of S Chen – fig. A; C Barry – figs B and C)

  
  

  
  

  

  
  

  
  

  

  
  
  
  
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  Hypermature cataract has a shrunken and wrinkled anterior capsule ( Fig. 9.3B ) due to leakage of water out of the lens.

  
  
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  Morgagnian cataract is a hypermature cataract in which liquefaction of the cortex has allowed the nucleus to sink inferiorly ( Fig. 9.3C ).

Diabetes mellitus

Hyperglycaemia is reflected in a high level of glucose in the aqueous humour, which diffuses into the lens. Here glucose is metabolized into sorbitol, which accumulates within the lens, resulting in secondary osmotic overhydration. In mild degree, this may affect the refractive index of the lens with consequent fluctuation of refraction in line with the plasma glucose level, hyperglycaemia resulting in myopia and vice versa. Cortical fluid vacuoles develop and later evolve into frank opacities. Classic diabetic cataract, which is actually rare, consists of snowflake cortical opacities ( Fig. 9.4A ) occurring in the young diabetic; it may mature within a few days or resolve spontaneously. Age-related cataract occurs earlier in diabetes mellitus. Nuclear opacities are common and tend to progress rapidly.

Cataract in systemic disease. (A) Diabetic snowflake cataract; (B) posterior subcapsular cataract spokes assuming a stellate morphology in myotonic dystrophy; (C) shield-like anterior subcapsular cataract in atopic dermatitis

Myotonic dystrophy

About 90% of patients with myotonic dystrophy (see Ch. 19 ) develop fine iridescent cortical opacities in the third decade, sometimes resembling Christmas tree cataract; these evolve into visually disabling wedge-shaped cortical and subcapsular opacities, often star-like in conformation ( Fig. 9.4B ) by the fifth decade. Later, the opacities may become indistinguishable from typical cortical cataract.

Atopic dermatitis

About 10% of patients with severe atopic dermatitis develop cataracts in the second to fourth decades; these are often bilateral and may mature quickly. Shield-like dense anterior subcapsular plaque that wrinkles the anterior capsule ( Fig. 9.4C ) is characteristic. Posterior subcapsular opacities may also occur.

Neurofibromatosis type 2

Neurofibromatosis type 2 (NF2; see Ch. 19 ) is associated with early cataract in more than 60% of patients. Opacities are posterior subcapsular or capsular, cortical or mixed, and tend to develop in early adulthood.

Chronic anterior uveitis

Chronic anterior uveitis is the most common cause of secondary cataract, the incidence being related to the duration and intensity of inflammation. Topical and systemic steroids used in treatment are also causative. The earliest finding is often a polychromatic lustre at the posterior pole of the lens ( Fig. 9.5A ). If inflammation persists, posterior and anterior opacities ( Fig. 9.5B ) develop. Cataract appears to progress more rapidly in the presence of posterior synechiae ( Fig. 9.5C ).

Secondary cataract. (A) Early uveitic posterior subcapsular cataract; (B) uveitic anterior plaque opacities; (C) extensive posterior synechiae and anterior lens opacity; (D) glaukomflecken; (E) anterior subcapsular cataract in retinitis pigmentosa

(Courtesy of S Chen – fig. E)

Acute congestive angle closure

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

High myopia

High (pathological) myopia can be associated with posterior subcapsular lens opacities and early-onset nuclear sclerosis, which ironically may increase the myopic refractive error.

Hereditary fundus dystrophies

Hereditary fundus dystrophies (see Ch. 15 ) such as retinitis pigmentosa, Leber congenital amaurosis, gyrate atrophy and Stickler syndrome, may be associated with posterior and, less commonly, anterior subcapsular lens opacities ( Fig. 9.5E ). Cataract surgery may improve visual function even in the presence of severe retinal changes.

Indications for surgery

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  Visual improvement is by far the most common indication for cataract surgery. Operation is indicated when the opacity develops to a degree sufficient to cause difficulty in performing essential daily activities. Clear lens exchange (replacement of the healthy lens with an artificial implant) is an option for the management of refractive error.

  
  
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  Medical indications are those in which a cataract is adversely affecting the health of the eye, for example phacolytic or phacomorphic glaucoma; clear lens exchange usually definitively addresses primary angle closure, but less invasive options are generally preferred (see Ch. 10 ). Cataract surgery to improve the clarity of the ocular media may also be required in the context of monitoring or treatment of fundus pathology.

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 electrocardiogram (ECG) are not usually required for local anaesthesia. If general anaesthesia is planned, assessment is according to local protocol, e.g. general examination, urea and electrolytes, random blood glucose, full blood count and ECG; an anaesthetic opinion may be considered for chronically unwell or medically complex patients.

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  Current medication should be recorded. This will often guide general medical assessment. Medications relevant to eye surgery include:

  
  
  
  
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    Systemic alpha-blockers (e.g. tamsulosin) are commonly associated with intraoperative floppy iris syndrome (IFIS).

    
    
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    Management of anticoagulant therapy or an antiplatelet agent should follow local protocol. Most surgeons do not stop antiplatelet drugs for cataract surgery, though this may be preferred for larger oculoplastic procedures. Anticoagulation status, usually expressed as the international normalized ratio (INR) level, should be within the therapeutic range appropriate for the individual indication (e.g. usually higher for heart valve thrombosis prophylaxis than following deep vein thrombosis); a common approach is to check the INR within the 24 hours prior to surgery in stable patients.

  
  
  
  
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  Allergy. True allergy rather than intolerance should be confirmed.

  
  
  
  
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    Medication, including sulfonamides and antibiotics commonly used following cataract surgery.

    
    
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    Iodine or shellfish – the latter may indicate an iodine allergy. If allergy to iodine is present an alternative skin and conjunctival antiseptic such as chlorhexidine should be used.

    
    
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    Others: latex (latex-free gloves may be necessary), sticking plaster, local anaesthetics, insect bites (cross-reaction with hyaluronidase that is often used with local anaesthesia).

  
  
  
  
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  Methicillin-resistant Staphylococcus aureus (MRSA) carriage. Relevant national and local protocols for the identification and management of patients at high risk for MRSA carriage should be followed.

  
  
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  Transport (to hospital and to the operating theatre within hospital): special arrangements may be needed for patients with poor mobility or exceptionally high body mass.

Ophthalmic preoperative assessment

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

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  Visual acuity is usually tested using a Snellen chart, despite its limitations (see Ch. 14 ).

  
  
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  Cover test. A heterotropia may indicate amblyopia, which carries a guarded visual prognosis, or the possibility of diplopia if the vision is improved. A squint, usually a divergence, may develop in an eye with poor vision due to cataract, and lens surgery alone may straighten the eye.

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

  
  
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  Ocular adnexa. Dacryocystitis, blepharitis, chronic conjunctivitis, lagophthalmos, ectropion, entropion and tear film abnormalities may predispose to endophthalmitis and in most cases optimization should be achieved prior to intraocular surgery.

  
  
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  Cornea. 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 precautions should be taken to protect the endothelium (see below). A prominent arcus senilis is often associated with a surgical view of decreased clarity, as are stromal opacities.

  
  
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  Anterior 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 a capsulorhexis, but can be largely overcome by staining the capsule with trypan blue.

  
  
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  Lens. Nuclear cataracts tend to be harder and may require more power for phacoemulsification, while cortical and subcapsular 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 (phakodonesis – lens wobble – may be present), a fragile capsule and poor mydriasis.

  
  
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  Fundus 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 fundus examination.

  
  
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  Sclera. If a prominent explant/encircling band has been placed during prior retinal detachment surgery, the eye is particularly large or the sclera thin (e.g. high myopia), peri- and retrobulbar local anaesthesia may be avoided and special care taken with sub-Tenon local anaesthetic infiltration.

  
  
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  Current refractive status. It is critical to obtain details of the patient’s preoperative refractive error in order to guide intraocular lens (IOL) implant 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, a toric IOL 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 taken into account.

Informed consent

It is essential that the patient has arrived at a fully informed decision to proceed with cataract surgery. As well as discussing the benefits, risks should be conveyed at a level appropriate to each patient’s level of understanding, with an explanation of the more common and severe potential problems. Points for discussion with the patient may include:

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  Most cataract operations are straightforward, with the patient achieving good vision.

  
  
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  Most complications can be dealt with effectively and cause no long-term difficulties, but some rare problems can be very serious.

  
  
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  In about 1 in 1000 cataract operations the eye will be left with little or no sight; in about 1 in 10 000 the patient will lose the eye.

  
  
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  Some complications mean that a second operation will be necessary.

  
  
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  Relatively mild and usually easily treatable but common complications include: periocular ecchymosis, allergy to eye drops, intraocular pressure (IOP) spike, iridocyclitis, posterior capsular opacification (currently in decline) and wound leak.

  
  
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  Moderate to severe but less common complications: posterior capsular rupture/vitreous loss (1% or less for experienced surgeons, higher for trainees dependent on experience), zonular dehiscence, cystoid macular oedema (CMO), dropped nucleus (about 0.2%), corneal decompensation sufficient to need corneal graft, intolerable refractive outcome (may need contact lens wear, lens implant exchange or corneal surgery), retinal detachment (<1%), IOL dislocation, persistent ptosis and diplopia.

  
  
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  Rare but invariably very serious complications: endophthalmitis (0.1%) and suprachoroidal haemorrhage (0.04%).

  
  
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  The risks of anaesthesia should be conveyed by the person administering it. Local anaesthesia carries only a low risk of problems, though some rare complications have the potential to be very serious including loss of the eye and even death: allergy to the anaesthetic agent, retrobulbar haemorrhage (see Ch. 21 ), perforation of the globe, and inadvertent infusion of anaesthetic agent into the cerebrospinal fluid via the optic nerve sheath causing brainstem anaesthesia.

  
  
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  There is virtually no risk to the other eye; sympathetic ophthalmitis is vanishingly rare following modern cataract surgery.

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.

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  Keratometry involves determination of the curvature of the anterior corneal surface (steepest and flattest meridians), expressed in dioptres or in millimetres 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) or corneal topography can be performed.

  
  
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  Optical coherence biometry ( Figs 9.7A and B ) is a non-contact method of axial length measurement that utilizes two coaxial partially coherent low-energy laser beams to produce an interference pattern (partial coherence interferometry). Modern biometry devices also perform keratometry, anterior chamber depth and corneal white-to-white measurement, and are able to calculate IOL power using a range of formulae. Measurements have high reproducibility and generally require less skill than ultrasonic biometry (see below).

  
  

  
  

  
  

  

  
  

  
  

  Biometry. (A) Optical coherence biometry; (B) optical biometry monitor display; (C) contact ultrasonic biometry; (D) ultrasonographic monitor display (A/C, anterior chamber depth; L, lens thickness)

  
  

  (Courtesy of D Michalik and J Bolger)

  
  

  
  

  

  
  

  
  

  

  
  

  
  

  

  
  
  
  
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  A-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 over the eye (immersion ultrasonography). The sound beam must be aligned with the visual axis for maximal precision; each reflecting surface is represented by a spike on an oscilloscope display monitor ( Fig. 9.7D ).

  
  
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  IOL power calculation formulae. Numerous formulae have been developed that 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 and lens thickness to try to optimize accuracy. The SRK-T, Haigis, Hoffer Q and Holladay 1 and 2 are commonly used. Specific formulae may be superior for very short (possibly the Hoffer Q) or long eyes, but opinions vary and it is always wise to plan individually for an unusual eye, consulting the latest research and recommendations. Short eyes in particular are prone to unexpected mean spherical and astigmatic errors following surgery.

  
  
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  Previous refractive surgery. Any form of corneal refractive surgery is likely to make a significant difference to the IOL power required, and standard IOL calculations are unsuitable. Several different methods have been described to address this situation. Most involve the calculation of the post-refractive procedure ‘true’ corneal power using a special process (refractive history method, contact lens method) and insertion of this into a standard (e.g. Hoffer Q) or specific (e.g. Masket) formula, but the Haigis-L regression formula uses statistical data to facilitate calculation on post-refractive surgery eyes using only standard inputs. It may prudent to utilize more than one method of IOL calculation.

  
  
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  Contact lenses. If the patient wears soft contact lenses, these should not be worn for up to a week prior to biometry to allow corneal stabilization; hard/gas permeable lenses may need to be left out for 3 weeks.

  
  
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  Personalized 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.

Postoperative refraction

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  Emmetropia is typically the desired postoperative refraction, though usually spectacles will be needed for near vision 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; postoperative hypermetropia, which necessitates correction for clear vision at all distances, is typically less well tolerated than myopia.

  
  
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  Contralateral eye. Postoperative refractive planning must take account of the contralateral 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 an early 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.

  
  
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  ‘Monovision’ is a concept in which the (usually) non-dominant eye is left with between 1 and 2 dioptres of myopia 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.

  
  
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  Multifocal 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.

  
  
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  Younger 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.

Positioning

An IOL ( Fig. 9.8A ) consists of an optic and haptics. The optic is the central refracting element, and the haptics the arms or loops that sit in contact with peripheral ocular structures to centralize the optic. Modern cataract surgery, with preservation of the lens capsule, affords positioning of the IOL in the ideal location – ‘in the bag’ ( Fig. 9.8B ). Complicated surgery, with rupture of the posterior capsule, may necessitate alternative positioning in the posterior chamber with the haptics in the ciliary sulcus (a three-piece IOL only, not one-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. In some circumstances a supplementary IOL may be placed in the sulcus in addition to an IOL in the capsular bag, for instance to address a residual refractive error following primary surgery (secondary pseudopolyphakia), and thin-profile IOLs are available for this purpose. It is preferable to avoid a secondary sulcus IOL (primary pseudopolyphakia) in very short (e.g. nanophthalmic) eyes due to the risk of angle closure; off-the-shelf IOLs of power up to 40 D are available, and custom IOLs can be produced in even higher powers.

Intraocular lens (IOL). (A) One-piece flexible IOL – note the square-edged optic; (B) IOL in situ in the capsular bag; (C) implanted toric IOL showing diametrically opposite sets of three dots marking the lens axis

(Courtesy of C Barry – fig. A)

Design

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  Flexible IOLs introduced into the eye via an injector and subsequently unrolled inside the eye are now in general use. Injector-based delivery utilizes a very small incision and also allows avoidance of lens contact with the ocular surface, so reducing the risk of bacterial contamination; simple folding of the IOL is an alternative, but requires a larger incision. Flexible materials available are discussed below; there seems to be no distinct superiority of one material over another.

  
  
  
  
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    Acrylic IOLs. Hydrophobic (water content <1%) acrylic materials have a greater refractive index than hydrophilic lenses and are consequently thinner, though this can result in dysphotopsia (troublesome glare and reflections). They have been reported to produce a greater reaction in uveitic eyes, but outcomes do not seem to be materially affected. Hydrophilic acrylic (hydrogel) in theory offers superior biocompatibility, but the image of hydrogel IOLs has been marred by the occurrence of severe calcification requiring IOL removal in some types, and inflammation in others; these problems have now been resolved by lens manufacturers. Posterior capsular opacification (PCO) rates may be higher with hydrogel IOLs than with other materials.

    
    
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    Silicone IOLs are available in both loop haptic (one- or three-piece) and plate haptic (one-piece) conformations, the latter consisting of a roughly rectangular leaf with the optic sited centrally. Silicone IOLs may exhibit greater biocompatibility than hydrophobic acrylic IOLs, but may be prone to significant silicone deposition in silicone oil-filled eyes. First-generation silicone materials were associated with a higher rate of PCO than acrylic, but there is no clear difference in rates with later IOLs.

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

  
  
  
  
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  Rigid IOLs are made entirely from polymethylmethacrylate (PMMA). They cannot be injected or folded so require an incision larger than the diameter of the optic, typically 5 or 6 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 IOLs (see below) in uveitic eyes, particularly in children.

  
  
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  Sharp/square-edged optics (see Fig. 9.8A ) are associated with a significantly lower rate of PCO compared with round-edged optics, and the former is now the predominant design. However, square edges may be associated with a higher rate of dysphotopsia (see below).

  
  
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  Blue 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 by this higher-energy visible light. The blue filters impart a slight yellow tint to the IOL, though similar only to that of the physiological young adult lens. Some evidence suggests slightly poorer visual function in scotopic conditions of illumination.

  
  
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  Aspheric optics to counteract corneal spherical aberration are widely available, and have been shown to improve contrast, particularly in mesopic conditions. A potential drawback is that the aspheric element of the manufactured IOLs is set at a single standardized level (this differs between manufacturers), but the extent of spherical aberration varies between individuals; some will be over- and some under-compensated.

  
  
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  Heparin 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.

  
  
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  Multifocal IOLs (see also Ch. 7 ) utilizing refractive or diffractive mechanisms 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.

  
  
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  Toric IOLs ( Fig. 9.8C ) 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 only a small percentage, and may be corrected by early surgical repositioning.

  
  
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  Adjustable 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.