Ординатура / Офтальмология / Английские материалы / Manual for Eye Examination and Diagnosis_Leitman_2007

Plastic lenses are typically prescribed because they are lighter and have less chance of shattering, especially in children. Glass is heavier, but has the advantage of not scratching as easily. For photophobia, grey tints are often prescribed because they distort all colors equally.

Photochromic glass lenses and Transitions plastic lenses darken in sunlight. Ultraviolet filters reduce the incidence of skin cancer, cataracts, and macular degeneration.

Contact lenses

Plastic contact lenses, invented in 1947, are now worn by 34 million Americans, usually as an alternative to spectacles, to correct myopia, hyperopia, astigmatism, and presbyopia (Fig. 25)

Fig. 25 Plastic contact lens.

Candidates for contact lenses

Soft lenses make up the vast majority of lenses fit. However, hard and rigid gas permeable contacts may be preferred for certain cases of dry eye, astigmatism, and irregularly shaped corneas such as occur in keratoconus (see Figs 168 and 169). Because harder lenses are used less frequently, and because they are difficult to fit, this beginner's manual will limit the discussion to soft lenses.

The patient usually helps to determine whether the individual should be fitted with contact lenses, which type of lens should be prescribed, the type of care products to prescribe, and whether difficulties with lens wear may be expected. It is important to know if the patient has ever failed at wearing

Fig. 26 Wake up and see with extended wear lenses.

contacts and why. Their occupation, hobbies, and unusual visual requirements may also foretell potential problems. Allergies may reduce contacts lens success due to discomfort and excess mucus production. Similarly many medications that dry the eye surface may limit success.

Most wearers enjoy their contacts during the day only ("daily wear") or less often on an overnight schedule ("extended wear") (Fig. 26). Others enjoy lens wear on a part-time basis for sports or social occasions (Fig. 27). Lenses may be replaced yearly but are more commonly disposed of every two weeks to three months ("frequent replacement") or on a daily basis ("disposable").

The frequency of replacement often depends on comfort and the rate of mucus accumulation (Fig. 28).

Fig. 27 Contacts are great for almost every sport.

Fig. 28 Mucus deposits on contact lenses.

Tests for fitting contact lenses

Keratometry (Fig. 29)

After the refraction for spectacles, the corneal curvature is measured with a keratometer or corneal topographer. This determines whether to fit a flatter or steeper lens (see Fig. 39). The keratometer also reveals distortion of the cornea from unhealthy contact lens wear (Fig. 30).

Fig. 29 Keratometer.

Fig. 30 Circular images projected on a damaged cornea have distorted keratometric readings.

Slit lamp examination

1. Integrity of cornea and tear film

Fluorescein dye when placed in the eye and illuminated with the cobalt blue light normally reveals an even green fluorescence. Damaged corneal epithelial cells take up the dye and appear brighter (Fig. 31).

Fluorescein dye is also used in the tear film break-up time test to evaluate for dry spots. If the tear film breaks up quickly after a blink, it could foretell a potential problem with drying of the contact lens surface, deposit formation, and discomfort.

2. Conjunctival examination

The upper and lower lids should be everted to examine the conjunctiva covering the underside of the lids (palpebral conjunctiva) (see Fig. 189). The upper palpebral conjunctiva is the area most often irritated by contact lenses. Called papillary conjunctivitis (Fig. 32), it is often due to excessive contact lens deposits, especially in allergic individuals. It responds well to more frequent lens replacements.

The bulbar conjunctiva surrounding the cornea reddens when the cornea is being compromised or with tight-fitting lenses (Fig. 33). Another sign of a tight-fitting lens is an air bubble getting trapped beneath it.

Fig. 31 Fluorescein staining of the cornea.

Fig. 32 Papillary conjunctivitis with characteristic whitish elevations of conjunctiva.

Fig. 33 Limbal injection from a tightfitting lens.

Fig. 34 Colored contact lenses.

Iris

Fig. 35 Typical package markings of contact lenses.

Fig. 36 Place contact lens directly on the cornea using the tip of the index finger for the contact, the middle finger to hold lower lid down, and the finger of the other hand to lift upper lid.

Determination of diameter and curvature of lens

1. Centering

A trial lens of approximate power is placed on the eye and allowed to settle for at least 5 minutes (Fig. 36).

The lens should be initially comfortable to wear. If discomfort is present, the lens may have a speck of debris and should be removed and rinsed (Fig. 37).

The lens should completely cover the cornea and extend just beyond the entire limbus (corneal-conjunctival junction) (Fig. 38).

If adequate centration is not achieved, a different base curve may be tried. Base curves generally range from 8.2 to 9.1 mm (Fig. 39).

If a change in base curve does not solve the centration problem, a larger lens diameter, if available, may be tried. Typical diameters are in the range of 13.5 to 14.5 mm, and many brands are available in only one fixed diameter (Fig. 40).

If changing the base curve, diameter, or both does not solve the lens centration problem, a different lens design (brand) should be evaluated.

Fig. 37 Remove lens by sliding it off cornea onto sclera and then gently pulling it off using thumb and index finger.

Fig. 38 Contact lens properly overlapping limbus.

Fig. 39 (a) Steep base curve 8.2 mm.

(b) Flat base curve 9.1 mm.

Fig. 40 (a) 13.5 mm diameter.

(b) 14.5 mm diameter.

2. Movement

A lens should move 0.5–1.0 mm on each blink.

If adequate lens movement is not achieved, a flatter base curve may be tried. If a lens moves too much, a steeper base curve is placed.

If a change in base curve does not solve the movement problem, a larger lens diameter will usually tighten this lens and a small diameter will loosen a lens on the eye.

Fig. 41 Lens properly aligned on eye with center marking at 180°.

Fitting spherical, astigmatic, bifocal, and extended-wear contact lenses

Spherical soft lenses correct myopia and hyperopia with small amounts of astigmatism of 0.75 diopters or less.

Astigmatic lenses are preferred for refractive errors of 0.75 or more. They are elliptical in shape with markings at 90° or 180° axis (depending on the brand) (Fig. 41). When placed on the eye, these lines should line up close to those axes. If the lens settles on the eye off the desired axis (Fig. 42), for example rotated 10° counterclockwise, 10° must be subtracted from the spectacle astigmatic correction. If rotated 10° clockwise, add 10°. Companies use different methods to prevent rotation of the lenses. The lower portion of the lens could be made thicker or the top and the bottom could be thinner.

Presbyopic bifocal contact lenses may be very successful for motivated patients—often over age 40—who have problems focusing up close (Figs 43 and 44). In fitting this lens, allow 20 minutes for the patient to adapt before testing vision. An alternative to a bifocal contact lens in correcting a presbyopic patient is to use a standard spherical contact lens, making one eye focused for near and the other focused for distance. This is called monovision. Usually, the eye with the clearest vision is chosen for distance. Caution the patient about driving and possibly give a distance glass to wear over the contacts when driving.

Fig. 42 Lens settled on to eye rotated 10° counterclockwise.

Fig. 43 Acuvue bifocal contact lens with concentric zones of alternating near and far vision.

Fig. 44 Vision with bifocal contact lens.

Refractive surgery

The refractive power of the eye may be altered by surgically reshaping the cornea, thereby eliminating the need for distance correction.

Refractive surgery began twenty-five years ago in Russia with the radial keratotomy technique (Fig. 47).

In this procedure, the cornea is flattened with 4-8 radial incisions through 90% of the corneal depth. It has lost popularity due to slow healing, the inability to accurately predict the amount of correction, variable vision throughout the day, glare, halos, infection, and corneal perforation with secondary cataract formation.

The three newest surgical procedures used to correct myopia, hyperopia, and astigmatism utilize an excimer laser to remove corneal stroma. The three techniques vary in the way the surface epithelium is removed prior to laser ablation.

1. Laser in situ keratomileusis (LASIK—Figs 48–52) is the most commonly used technique with over 1 million procedures done in the USA in 2005. A flap of epithelium, Bowman’s membrane, and stroma is created with a blade or femtosecond laser. Then, a different laser called an excimer is used to ablate the underlying stromal bed.

Fig. 51 Excimer laser used to remove a layer of central corneal stroma. Courtesy of Summit Technology, Inc.

Fig. 47 Rare instance of traumatic rupture of radial keratotomy wound. Courtesy of Leo Bores.

Fig. 48 Normal cornea.

Excimer laser beam

Fig. 49 LASIK—Flap of epithelium, Bowman’s membrane, and stroma is created with blade or laser. Then, an excimer laser ablates the stroma.

Fig. 50 Sculpted cornea after LASIK with remaining Bowman’s membrane.

Fig. 52 LASIK surgery showing flap being lifted with spatula and laser beam on central cornea ablating stroma.

A disadvantage of LASIK is a resulting decrease in ocular rigidity. This is due to loss of ablated stromal bed and decreased effectiveness of stroma remaining in the flap since it never completely heals. Up to six years later, the flap can still be lifted with a forceps. In eyes with over 8 diopters of myopia that require a lot of stromal ablation, this combined thinning becomes excessive and could result in irregular bulging of the cornea (ectasia) called keratoconus. Rarely, it may necessitate a corneal transplant.

Also, LASIK cuts through more corneal stroma due to the flap thus destroying more nerve tissue resulting in increased dry eye complaints.

2.An alternative procedure, photorefractive keratectomy (PRK— Figs 53 and 54), removes the epithelium by mechanically creating a central corneal abrasion. The advantage is it leaves more functioning stroma. The disadvantage is significant post-operative pain and prolonged visual haze.

3.The newest technique, called epi-LASIK (Figs 54 and 55) creates an epithelial flap that includes no stroma.

Excimer laser beam

Fig. 53 PRK laser ablation of Bowman’s membrane and stroma after mechanical debridement of epithelium.

Fig. 54 Sculpted cornea after PRK or epi-LASIK.

Fig. 55 Epi-LASIK. Creation of epithelial flap with blade followed by laser ablation of stroma.

Therefore, there will be more stroma remaining to contribute to ocular rigidity. However, the epithelial flap heals more slowly than the LASIK flap so that vision takes longer to recover. It heals faster and has less pain than PRK where there is a total corneal abrasion after surgery.

All three laser techniques usually yield good results, but may be complicated by infection, glare, halos, dry eye, overor under-correction of refractive error, and unknown long-term effects. Although LASIK is still by far the most popular technique because of its quick healing, there is a movement toward epiLASIK because it minimizes dry eye and stromal flap complications.

Intac is a less used technique for correcting small amounts of myopia and keratoconus. It involves the placement of a plastic ring in the peripheral cornea (Fig. 56). Proponents argue that unlike LASIK, it is safer because it doesn’t involve surgery on the central visual axis.

Large amounts of hyperopia (over 4 diopters) and myopia (over 8 diopters) are difficult to correct with reshaping the cornea because it becomes too thin and unstable. Intraocular lenses can be inserted inside the eye (Fig. 57) to correct these larger refractive errors, but have all the inherent risks associated with intraocular surgery.

Fig. 56 Intac ring. Courtesy of Dimitri Azar, MD.

Fig. 57 Phakic 6H2 anterior chamber intraocular lens to correct refractive errors. Courtesy of Oii Inc.