Ординатура / Офтальмология / Английские материалы / Moorfields Manual of Ophthalmology_Jackson_2007

■Presbyopia : For those aged ≥40, single vision lenses seldom provide adequate depth of focus. The usual solution is a contact lens distance correction and reading spectacles, varifocal, or bifocal over-correction. Alternatives include monovision (dominant eye focused for distance, other for near

– but this can interfere with binocular vision) and multifocal contact lens (these tend to reduce low contrast vision).

Choosing a therapeutic contact lens

■Choice: First → Last

Pain relief: Hydrogel → SiH → Limbal RGP → Scleral Epithelial healing: SiH → Hydrogel → Limbal RGP → Scleral Perforation: SiH → Hydrogel → Limbal RGP → Scleral Sensitive eye: Hydrogel → SiH → Limbal RGP → Scleral Ease of fit: Hydrogel → SiH → Limbal RGP → Scleral

■Severity: Mild → Severe

Hydrogel → SiH → Limbal RGP → Scleral

(Suitable for exposure, dry eye, corneal protection, irregular astigmatism)

Useful lenses First choice contacts lenses include:

■Irregular corneas, mild to moderate dry eye: Proclear Biocompatables (Coopervison) 8.60:14.20

■Steep corneas (Proclear too loose): D75 (e.g. Cantor & Nissel) 8.00:15.00 or 7.80:13.50

■Large corneas, unstable lens fit, limbal or scleral defects: D75 9.50:16.50, M&L 75 (e.g. Cantor & Nissel) 8.60:18.00 or 8.80:20.00

■Persistent epithelial defect, leaking wounds: Purevision (Bausch & Lomb) 8.60:14.00 or Night & Day (CIBA Vision) 8.40 & 8.60:13.80

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Laser refractive surgery

Laser Refractive Surgery

Background Excimer laser refractive surgery procedures are commonly used to correct myopia, hypermetropia and astigmatism. In the UK, this is predominantly undertaken in the private sector, but treatment can influence the management of conditions seen in the NHS, e.g. cataract and glaucoma. Some post operative complications may present acutely to the accident and emergency department e.g. dislocated LASIK flaps or deep lamellar keratitis. A basic understanding is therefore worthwhile.

Basic principles Excimer (excited dimer) laser photoablation can be used to remove corneal tissue with submicron accuracy without thermal build up or adjacent tissue damage. Myopia can be corrected by removing tissue from the

central cornea resulting in central flattening and reduced refractive power. In hyperopia tissue is removed from the peripheral cornea resulting in steepening of the central cornea and increased refractive power.

Contraindications Relative ocular contraindications include severe dry eyes, severe atopy, keratoconus, previous herpetic keratitis, neurotrophic corneas and nystagmus.

Relative systemic contraindications include collagen vascular diseases, pregnancy, hormone replacement therapy, and immunocompromise e.g. HIV infection.

Surgical techniques There are three main procedures (Table 5.4): Photorefractive keratectomy (PRK), laser assisted in situ keratomileusis (LASIK) and laser subepthelial keratomileusis (LASEK). All three are performed under topical anaestheasia.

■PRK

PRK involves removal of the corneal epithelium followed by photoablation of Bowman’s layer and the anterior stroma. After treatment a bandage contact lens (BCL) is inserted. Mild to severe foreign body (FB) sensation is normal for 3–4 days post operatively until the corneal surface has re-epithelialised. PRK is safe and effective for correcting low myopic refractive errors. The use of PRK is limited by the corneal wound healing response causing corneal haze, with subsequent reduced contrast sensitivity and Snellen acuity, and unpredictable refractive regression.

■LASIK

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Laser refractive surgery

flap is usually 120–180 μm thick and 8–9 mm in diameter. The hinge may be superior or nasal. The flap is lifted and the excimer laser treatment applied to the exposed stromal bed. The flap is then repositioned. Visual recovery is rapid and stable. There is minimal damage to the epithelium so post operative FB sensation is mild and short lived. Haze is only seen at the periphery of the flap where Bowman’s layer has been cut. However, the procedure is technically more challenging with the risk of intraoperative and post operative flap related complications and post operative ectasia if a residual stromal bed of ≤250 μm has not been left, or the patient had forme fruste keratoconus pre operatively.

■LASEK (epi LASIK)

LASEK/epiLASIK are modifications of PRK that preserve the epithelial sheet. The epithelium is peeled back with a modified mechanical keratome (epiLASIK) or manually after application of 20% ethanol (LASEK). The excimer laser treatment is applied to the stromal bed. The epithelium is then repositioned and a bandage lens applied to protect the epithelium while it heals. The retention of the epithelium is said to reduce post operative pain and increase the rate of visual recovery. The problem of haze formation with surface ablations is thought to be reduced, at least in the short term, by the intraoperative application of Mitomycin C.

Outcomes

■Results obtained with different lasers and microkeratomes are not directly comparable

■Excimer laser refractive procedures are safe and effective for

the correction of mild to moderate myopia. The predictablility decreses in high myopia (> 8D), hypermetropia >4D, astigmatism >3D, and therapeutic treatments e.g. correction of post peretrating keratoplasty astigmatism.

■Better understanding and treatment of corneal wound healing, the use of wavefront guided treatments and iris recognition are leading to improved clinical outcomes.

■Dislocated or wrinkled flaps of short duration can be floated back into position in the operating theatre. If this is not possible referral to a corneal surgeon for suturing is advised. Amputation of the flap may be necessary.

■Following LASIK the interface between the flap and the stroma

triggering event should be treated and frequent topical steroids applied unless otherwise contraindicated e.g. G prednisolone 1.0% 6 times daily. As the inflammation settles, steroids can be tapered off.

■Interface infection is rare and often caused by low virulence organisms e.g. Mycobacteria. Emergency treatment is required. It may be necessary to lift the flap to obtain material for culture and sensitivity testing (p. 172). Frequent, fortified broad spectrum topical antibiotics should be applied. Amikacin is particularly useful for its activity against low virulence organisms e.g. Nocardia, Mycobacteria. Amputation of the flap may be necessary.

■Epithelial ingrowth is unusual unless the flap has been lifted for retreatment. Refer back to the treating surgeon.

■Most keratometers and topography systems assume that the cornea has a spherical profile with a constant relationship between the anterior and posterior corneal surfaces. Following excimer refractive surgery these assumptions no longer hold true. As a consequence conventional biometry is inaccurate. Following treatment for myopia biometry tends to underestimate IOL power required, resulting in post operative hypermetropia. Following refractive surgery advanced biometric calculations are required.

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Optometry and general practice guidelines

Optometry and General

Practice Guidelines

General comments

The most urgent corneal referral is microbial keratitis. A lesion close to the centre of the cornea is especially dangerous. Compared to conjunctivitis, there is less likely to be a history of contact with eye infection or systemic viral features such as a sore throat, and more likely to be history of pain, reduced vision, contact lens wear, or preexisting corneal disease. Look for a white infiltrate and focal fluorescein staining. Corneal scrapes taken prior to antiobiotic therapy have a higher yield, so if microbial keratitis is suspected refer without starting treatment. Conjunctival swabs have a low yield in corneal infection. Advise contact lens wearers to bring their lenses, solutions, and cases for testing.

Immediately refer patients with corneal grafts if they notice blurred vision, inflammation, or pain, as the risk of rejection lasts for life.

Optometrists

Patients with corneal diseases and poor spectacle-corrected acuity may have irregular astigmatism. A marked improvement may be achieved by fitting a contact lens but also consider early keratoconus. Tear film abnormalities may cause visual changes when the patient concentrates and the blink rate reduces, such as when driving or reading. Tear substitutes such as hypromellose are available over the counter but treat any accompanying blepharitis. Compared to bacterial keratitis, the sterile corneal infiltrates associated with contact lenses tend to be smaller, more discrete, with less inflammation and little or no discharge. This distinction can, however, be difficult, so most patients require urgent hospital review. Marginal keratitis is another common cause of corneal infiltrates in patients with blepharitis. Urgent referral may be required to rule out bacterial keratitis.

General practice

conditions such as herpes simplex keratitis, Thygeson’s keratitis, and recurrent erosion syndrome run a relapsing and remitting course, with recurrence even while the patient is being treated. It is preferable to refer back urgently rather than to alter medication. The use of topical steroids should be supervised by the treating ophthalmologist.

The following guide to referral urgency is not prescriptive as clinical situations vary.

Immediate

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