Ординатура / Офтальмология / Английские материалы / Moorfields Manual of Ophthalmology_Jackson_2007

■Past medical and surgical history : especially deafness, confusion, neck disease, orthopnoea, difficulty lying flat, and seizures.

■Drug history : especially anticoagulants, tamsulosin (flomax), and allergies to iodine or antibiotics (some surgeons avoid subconjunctival cephalosporins in those with penicillin allergy due to the risk of cross-sensitivity).

■Social history : identify preand postoperative social support and any occupational visual requirements.

Examination

■Best corrected VA (BCVA): for near and distance and pinhole VA (PHVA, may indicate potential postoperative vision better than BCVA). Contrast sensitivity and glare disability are useful if symptoms are disproportionate to the VA loss, or the degree of cataract.

■Binocular balance : cover test and ocular motility.

■Lids and adnexae : blepharitis, ectropion, entropion, orbital, or lacrimal disease may need treatment prior to cataract surgery.

■RAPD : Check RAPD before dilating pupils.

■Slit lamp examination : assess the anterior segment including corneal clarity, guttatae, anterior chamber (AC) depth, pupil size, evidence of previous intraocular inflammation or trauma, pseudoexfoliation, iridoor phakodonesis, IOP, and gonioscopy if angle disease is suspected.

■Cataract : note the type (nuclear, cortical, posterior subcapsular, mixed) and grade (1+ to 5+) of cataract. Cortical cataracts are characterized by spokes, water clefts and vacuoles due to osmotic imbalances in lens epithelial cells. Nuclear cataracts result from accumulation of protein aggregates in the centre of the lens, becoming progressively harder and more brunescent with time. Posterior subcapsular cataracts occur at the posterior pole immediately beneath the lens capsule and are associated with the highest rates of cataract surgery.

■Fundoscopy : note posterior vitreous detachment (reduced risk of postoperative retinal detachment) or vitreous pigment cells, and exclude optic nerve or macula disease, retinopathy and peripheral retinal breaks or degenerations (especially in high

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Preoperative assessment

Investigations

■Blood pressure : should be controlled preoperatively – there is

an increased risk of suprachoroidal haemorrhage with systolic BP ≥180 mmHg or a diastolic >100 mmHg.

■BM : diabetics may present with cataracts.

■Other blood tests and ECG : only if clinically indicated – routine testing does not reduce morbidity and mortality.

If taking warfarin ensure the INR is in the therapeutic range (discontinuing warfarin increases the risk of stroke 1:100).

■Special investigations : as indicated, including B-scan ultrasound to exclude retinal detachment or mass lesion if there is no fundal view; corneal topography (particularly if high or irregular astigmatism) to plan astigmatic surgery; specular microscopy and pachymetry to assess endothelial compromise.

Consent

■Benefits : These may include

1.Improved quality of vision.

2.Improved fundal view to monitor or treat posterior segment disease.

3.Reduced spectacle dependence (a reading correction is usually required)

More than 90% of patients (without coexisting ocular disease) achieve a BCVA of 6/12 or better.

■Risks : Complications are infrequent and most can be treated. The overall risk of blindness is 1 in 1000, with 1 : 10 000 chance of losing the eye as a result of surgery. There is almost no risk to the fellow eye. Potentially sight-threatening

complications include: infectious endophthalmitis (0.1%), retinal detachment (<1%), suprachoroidal haemorrhage

(0.1%), cystoid macular oedema (1–2%), and corneal decompensation (<0.3%). Other specific complications

include: ecchymosis (common), posterior capsule rupture and/ or vitreous loss (<5%), dropped nucleus (<1%), postoperative IOP rise (common), subluxation of intraocular lens implant, iris prolapse/wound leak, uveitis, refractive surprise, spectacle correction for best vision, drop allergy, diplopia, ptosis, further surgery, and posterior capsule opacification. There may be a

Biometry

Biometry

Selecting the correct power of intraocular lens (IOL) for the desired refractive outcome relies on careful biometry. Measure the corneal curvature, axial length (AL), focimetry (or refraction) and, if keratometry is impossible, corneal topography.

Keratometry, K (Dioptres) Central corneal curvature is measured by manual or automated keratometry with paired readings taken in two orthogonal meridia. Take the average of 3 pairs of readings including axes. Corneal power in dioptres

(D) = 337.5/keratometry in mm, (where 337.5 is the hypothetical refractive index of the cornea).

Axial length of the eye (mm) Usually measured using either optical interferometery (e.g. IOLMASTER, Carl Zeiss) or A-Mode ultrasound. Ideally, contact lenses should be removed for as long as possible before measurements are taken. Tightly fitting contact lenses can induce a reversible corneal flattening which can lead to a postoperative gradual myopic shift.

■Optical interferometery : noncontact and measures AL to the

RPE (rather than the vitreo-retinal interface as in ultrasound). A trace with a signal-to-noise ratio (SNR) of ≥2.0 is acceptable; however, the higher the SNR, the more accurate the reading. AL readings are typically 0.1 mm longer with the IOLMASTER compared to A-scan. As formulae for IOL power

calculations have been tested with A-scan AL measurements, targets of − 0.3 D to − 0.5 D are recommended to achieve emmetropia when using optical biometry. Improved refractive targeting can be achieved by optimising IOL A-constants for optical biometry. The IOLMASTER has a built-in keratometer, facilities to measure preoperative ‘anterior chamber depth’ (ACD, the distance from the anterior corneal vertex to the anterior lens along the visual axis), horizontal white-to-white measurement (HWTW), optional settings, e.g. ‘pseudophakia’ (PMMA, silicone, and acrylic) or ‘silicone oil filled vitreous’, and software to calculate IOL implant powers using a selection of formulae.

■A-Mode ultrasound : essential if cataract is dense or other ocular opacity prevents optical measurement of AL, but more user-dependent. Immersion A-scan methods offer a reduced chance of foreshortening of eye by indentation (small potential

transducer is placed on the anaesthetized cornea (usually supported in spring-loaded assembly such as a tonometer holder). Between pulses, echoes are received from the ocular tissues and plotted as spikes on a display (Fig. 6.1). The height of the spike indicates the amplitude of the echo and the position along x-axis indicates time delay (converted to distance using assumed velocities in ocular tissue). A major source of error is poor alignment of the ultrasound probe; ensure central fixation with a perpendicular incidence (indicated by strongest echo with steepest rise time from baseline). The AL of silicone oil filled eye = anterior cornea to posterior lens plus [measured vitreal length × 0.64]. The length of the vitreous must be scaled to allow for the reduced velocity of sound in oil (both A & B-scan).

■B-Mode ultrasound : invaluable in babies, poorly cooperative patients, and to exclude staphylomata. Provides a crosssectional image of the eye (Fig. 6.2) and orbit through closed eyelid (with a good system the anterior cornea and foveal pit can be imaged). Assumes a single velocity in ocular tissues. A skilled operator is required for accurate AL measurement; care must be taken not to indent globe as the probe is hand-held and considerably heavier than the A-scan probe.

Focimetry of old (pre-cataract) spectacles (or refraction) Assess spherical and astigmatic error in each eye and ensure this is consistent with other biometry data. Aim to minimize anisometropia to 2 D or less, unless mono vision is desired (e.g. one eye focussed for distance; one for near). However, if the patient is adapted to longstanding anisometropia,

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optical interferometery IOLMASTER. Do not target Plano ‘0’ but choose − 0.3 D as ‘emmetropic correction’.

IOL power calculation after refractive

surgery Standard biometry will be inaccurate in patients who have undergone previous corneal refractive surgery. IOL power can be determined using three alternative methods:

■1. Refractive history technique : Must have prerefractive surgery K values and refraction. Effective corneal power, K (D) for use in IOL power calculations is derived as follows:

a.Calculate preand postprocedure refraction spherical equivalent at the corneal plane using:

RC = Rs / (1 − BVD × Rs)

where RC = Refraction at corneal plane (D), Rs = Refraction at spectacle plane (D) and BVD = Back vertex distance (m).

b.Determine change in refraction at the corneal plane by subtracting adjusted postprocedure refraction from adjusted preprocedure refraction. Take care with plus and minus signs.

c.Effective corneal power,

K(D) = Prerefractive surgery average K-value (D) + (Change in refraction at corneal plane).

Take care with signs.

■2. Contact lens technique : Useful when no pretreatment data exist. Spectacle refraction is performed followed by overrefraction with a hard CL of known base curve and plano (or low) power in situ:

a.Calculate preand post-contact lens refraction spherical equivalent at corneal plane, as above for refractive history technique.

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Biometry

b. Effective corneal power:

K (D) = (Base curve of CL) + (Contact lens power) + (Spherical equivalent at corneal plane with CL in situ) − (Spherical equivalent at corneal plane without CL).

■ 3. Nomograms : See Table 6.2 (Vahid Feiz, et al. 2001).

Unexpected postoperative refractive outcome In the event of a confirmed refractive surprise:

■Re-check IOL choice, A-constant, formula used, and target refraction.

■Re-measure AL and keratometry optically to exclude a biometry error.

■Measure distance from anterior cornea to anterior surface of the IOL with either A- or B-scan (be careful not to measure to the posterior implant surface): too shallow or too deep will lead to myopic or hyperopic surprises, respectively (usually due to excess anterior or posterior vaulting of implant, often due to crimping of haptics).

■In cases of myopic surprise, also consider retained visco elastic behind the implant – a distended bag may be visible on examination or with B-scan.

■Beware manufacturing/pack errors with mislabelled implant power (implant thickness can be measured in situ on B-scan and compared to a correctly labelled implant of the same model).

Table 6.2: Nomogram for IOL power adjustment according to

degree of previous refractive error correction

Management of astigmatism

Management of Astigmatism

Background Visually significant keratometric astigmatism of ≥2.0 Dioptres (D) is present in up to 15% of patients undergoing cataract surgery. Cylindrical targeting using incision placement, limbal relaxing incisions, toric intraocular lenses (IOLs) or a combination of techniques can reduce or eliminate preexisting astigmatism and improve unaided distance vision (Fig. 6.3).

Incision placement Placing clear corneal incisions (CCI) on the steep meridian causes wound-induced flattening sufficient to control 0.75–1.50 D. Temporal CCIs are almost (0.25 D) astigmatically neutral if ≤3.4 mm in size and are recommended if there is negligible corneal astigmatism. Superior CCIs induce up to 1.50 D flattening. It is not always possible to operate on the steep meridian (e.g. if superonasal in the right eye or inferotemporal in the left eye, unless left handed or ambidextrous).

Limbal relaxing incisions (LRI) Safe, rapid, and reliably corrects up to 3.00 D keratometric astigmatism (Box 6.1). Paired arcuate limbal incisions centred on the steep meridian flattens the central cornea, and is usually combined with neutral