Visual Acuity, Refractive Procedures, and Sudden Vision Loss

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Visual Acuity, Refractive Procedures, and Sudden Vision Loss

Refractive Error

Definition

The state of an eye in which light rays are not properly focused on the retina, resulting in image blur.

Ametropia

A refractive error (e.g., myopia, hyperopia, or astigmatism).

Anisometropia

A difference in refractive error between the two eyes; usually 2D or more.

Astigmatism

Light rays are unequally focused producing two lines rather than a single point because the curvature of the cornea or, less commonly, the curvature of the lens varies in different meridians. If the cornea is steeper in the vertical meridian, it is referred to as “with-the-rule” astigmatism; if it is steeper in the horizontal meridian, it is called “against-the-rule” astigmatism. Astigmatism can also be designated regular (symmetric or asymmetric) or irregular. A cylindrical lens corrects regular astigmatism.

Emmetropia

No refractive error; light rays are focused on the retina, and thus no corrective lens is required for distance vision.

Hyperopia (Farsightedness)

Light rays are focused at a point behind the retina. A “plus” spherical lens is used to correct this refractive error.

Myopia (Nearsightedness)

Light rays from a distant object are focused at a point in front of the retina and those from a near object are focused on the retina. A “minus” spherical lens is used to correct this refractive error.

Presbyopia

Loss of accommodation of the crystalline lens with age; average age of the onset of symptoms is early 40s and this process continues through the early 60s. A “plus” spherical lens (i.e., bifocal “add”) is used to correct this problem.

Etiology
Astigmatism

The curvature of the cornea or, less commonly, the curvature of the lens varies in different meridians. Acquired forms may be caused by disorders of the eyelids (tumor, chalazion, ptosis), cornea (pterygium, limbal dermoid, degenerations, ectasias, surgery), lens (cataract), and ciliary body (tumor).

Hyperopia

The refractive power of the cornea is too weak or the axial length of the eye is too short. Acquired forms may be caused by disorders that decrease the eye’s refractive power (alteration in the lens [posterior lens dislocation, aphakia, diabetes], drugs [chloroquine, phenothiazines, antihistamines, benzodiazepines], poor accommodation [tonic pupil, drugs, trauma], corneal flattening [contact-lens-induced], intraocular silicone oil) or effective length (central serous retinopathy, retrobulbar masses, choroidal tumors).

Myopia

The refractive power of the eye is too strong or the axial length of the eye is too long. Acquired forms may be caused by disorders that increase the eye’s refractive power (alteration in the lens [diabetes, galactosemia, uremia, cataracts, anterior lenticonus, anterior lens dislocation], drugs [sulfonamides, miotic agents], excessive accommodation, corneal steepening [keratoconus, congenital glaucoma, contact-lens-induced]) or effective length (congenital glaucoma, posterior staphyloma, retinopathy of prematurity, scleral buckle surgery). Myopia also increases in the dark (night myopia).

Presbyopia

Loss of lens elasticity or possibly reduced ciliary muscle effectivity. Premature presbyopia may occur with debilitating illness, diphtheria, botulism, mercury toxicity, head injury, cranial nerve III palsy, Adie’s tonic pupil, and tranquilizers.

Epidemiology

Hyperopia is normally present during infancy and early childhood and then declines between ages 8 and 13 years, resulting in emmetropia in most adults. The incidence of refractive errors in the US population is approximately 25% for myopia and 25% for hyperopia; 50% have some degree of astigmatism. After the age of 40 years old, 50% of people have hyperopia. Hyperopia is associated with shallow anterior chambers and narrow angles; myopia is associated with lattice degeneration and retinal detachment.

Symptoms

Decreased vision when not wearing corrective lenses; distant objects are blurry and near objects are clear (myopia), distant and near objects are blurry (hyperopia), or near objects that are blurry become clearer when held further away (presbyopia). May have asthenopia (eye strain) due to sustained accommodative effort (overcorrected myopia, or undercorrected or uncorrected hyperopia; asthenopia may also be produced by convergence insufficiency or accommodative insufficiency [hypothyroidism, anemia, pregnancy, nutritional deficiencies, and chronic illness]). Hyperopia may be asymptomatic in children and young adults, who can compensate enough with accommodation.

Signs

Decreased uncorrected visual acuity that improves with pinhole testing, glasses, or contact lenses; hyperopes may have normal uncorrected visual acuity.

Differential Diagnosis

Normal eye exam except for decreased vision: amblyopia, retrobulbar optic neuropathy, other optic neuropathies (toxic, nutritional), nonorganic (functional) visual loss, rod monochromatism, cone degeneration, retinitis pigmentosa sine pigmento, cortical blindness.

Evaluation

 Complete ophthalmic history and eye exam with attention to pinhole visual acuity (corrects most low to moderate refractive errors to the 20 / 25 to 20 / 30 level), manifest (undilated) and cycloplegic (dilated) refraction, retinoscopy, pupils, keratometry, cornea, lens, and ophthalmoscopy.

 Consider potential acuity meter (PAM) testing, rigid contact lens overrefraction, and corneal topography (computerized videokeratography) if irregular astigmatism is suspected.

Management

 Glasses are the first line of treatment and can correct virtually all refractive errors, with the exception of irregular astigmatism.

 Contact lenses (soft or rigid); numerous styles of contact lenses are available to correct almost any refractive error; rigid lenses can correct irregular astigmatism.

 Consider refractive surgery (see below).

Prognosis

Good except for pathologic myopia (see Chapter 10).

Refractive Surgery Complications

Intraocular Refractive Procedures

Used to correct moderate to high degrees of myopia and hyperopia.

Refractive Lens Exchange

The noncataractous crystalline lens can be removed and replaced with an intraocular lens implant of appropriate power to correct the resulting aphakia. The main disadvantage of this method is loss of accommodation. This procedure is controversial for myopia because of the risk of retinal detachment in pseudophakic eyes; hyperopic eyes with nanophthalmos may develop choroidal effusions.

Phakic Intraocular Lens

A lens implant is placed in the anterior chamber, posterior chamber, or fixated to the iris with the optic centered over the pupil. Prophylactic peripheral iridotomies are performed prior to surgery to prevent postoperative pupillary block angle-closure glaucoma (see Chapter 6).

Symptoms

Postoperatively may have photophobia, pain, decreased vision, glare, and halos.

Signs and Complications

Corneal edema (endothelial cell loss), cataract, glaucoma, pupillary block, iridocyclitis, and endophthalmitis; may have residual refractive error. Additional complications of clear lens extraction include posterior capsular opacification, cystoid macular edema, retinal detachment, suprachoroidal hemorrhage, choroidal effusion, retained lens material, and iris damage.

Corneal Refractive Procedures

Incisional

Radial keratotomy (RK)

This procedure corrects low-to-moderate myopia. Deep, radial, corneal incisions are created with a diamond knife to flatten the central cornea. The surgical effect depends on various factors including depth and number of incisions, size of optical zone, patient age and gender, design of diamond blade, and surgeon experience. Radial keratotomy can be combined with astigmatic keratotomy for compound myopia (myopia with astigmatism). It is rarely performed anymore.

Astigmatic keratotomy

This procedure corrects corneal astigmatism. Deep, midperipheral arcuate or straight incisions (parallel to the limbus) are made (traditionally with a diamond knife and more recently with femtosecond lasers) on the steep corneal meridian to flatten it. As with RK, the surgical effect depends on depth and number of incisions, size of optical zone (usually 7 mm), patient age, and surgeon experience. The incisions must not be > 90° or intersect with RK incisions. This technique is useful after penetrating keratoplasty for high-to-moderate astigmatism.

Limbal relaxing incisions / peripheral corneal relaxing incisions

Similar to astigmatic keratotomy, this technique is used to correct low amounts of astigmatism, usually in association with cataract surgery. Arcuate incisions, 500–600 μm deep with a guarded blade, are placed at the 10–11 mm optical zone concentric to the limbus to correct 1–2D of astigmatism. Because ultrasound is not required, these relaxing incisions can be performed at the time of surgery or in the clinic. Femtosecond lasers allow these incisions to be created at greater depth, at smaller optical zones (8–10 mm), and also intrastromally, with more precise control of each parameter. Corneal topography is recommended prior to performing this procedure.

Symptoms

Postoperatively may have decreased vision (due to undercorrection, overcorrection, or irregular astigmatism), fluctuating vision, difficulty with night vision, halos, glare, starbursts, ghost images, double vision, and foreign body sensation.

Signs and Complications

Corneal scarring, infection, or perforation; may have residual refractive error, regression, or progression over time.

Excimer Laser

This ultraviolet laser (193 nm) ablates corneal tissue to correct myopia (central ablation), hyperopia (peripheral ablation), regular astigmatism (eliptical ablation), irregular astigmatism (topography-guided ablation), or reduce higher-order aberrations (wavefront ablation).

Photorefractive keratectomy (PRK)

This procedure uses excimer laser ablation to reshape the corneal stroma after epithelial removal (mechanical, alcohol, or laser). The visual recovery period is longer than with LASIK, but final visual outcomes are similar in low and moderate myopia. Patients may experience initial pain due to the epithelial defect, and decreased vision due to corneal haze.

Laser-assisted subepithelial keratectomy and epithelial-laser in-situ keratomileusis

These procedures combine PRK and laser in-situ keratomileusis (LASIK) techniques. A flap composed of only epithelium is created with topical alcohol (LASEK) or a mechanical epithelial separator (epi-LASIK), the epithelial flap is carefully retracted, laser energy is applied to the stromal bed, and the epithelial flap is replaced or excised. This procedure may combine the advantages of PRK and LASIK by decreasing the incidence of pain and haze associated with PRK and risk of flap complications associated with LASIK.

Laser in-situ keratomileusis

This procedure combines automated lamellar keratoplasty and PRK techniques. A mechanical microkeratome or femtosecond laser is first used to cut a partial-thickness hinged corneal flap, which is then folded back. The programmed excimer laser energy is applied to the underlying stromal bed, and the flap is replaced. LASIK allows for faster visual recovery and minimal pain after surgery, but there is a higher risk of complications due to flap-related problems than with PRK.

Symptoms

Postoperatively may have decreased vision (due to undercorrection, overcorrection, irregular astigmatism, or haze), fluctuating vision, difficulty with night vision, halos, glare, starbursts, ghost images, double vision, and variable discomfort (ranging from foreign body sensation to moderate pain).

Signs and Complications

Corneal scarring, infection, keratectasia, central toxic keratopathy, decentration (apparent on corneal topography), or dry eyes; may have residual refractive error, regression over time. Additional LASIK complications include flap striae, epithelial ingrowth, flap trauma, and diffuse lamellar keratitis (DLK, “sands of the Sahara”). Haze from surface ablation may be early (due to delayed epithelial healing) or late (≥ 3 months, due to deeper ablations and UV exposure).

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Figure 12-10 Laser in-situ keratomileusis flap striae demonstrating vertically curved wrinkles in the flap.

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Figure 12-11 Same patient as Figure 12-10 with laser in-situ keratomileusis flap striae demonstrating the curved striae that are enhanced with topical fluorescein and viewed with a blue light.

Implants

Intracorneal inlays

This procedure combines a partial-thickness corneal flap (similar to LASIK) with a thin, contact-lens-like, intrastromal implant that is placed in the central optical zone to correct refractive errors without removing corneal tissue. It may be most useful for treating presbyopia.

Intrastromal corneal ring segments (Intacs)

Polymethylmethacrylate (PMMA) ring segment implants are placed into peripheral corneal channels at two-thirds depth outside the visual axis to correct low-to-moderate myopia by flattening the cornea without cutting or removing tissue from the central optical zone. These channels can be made manually or with a femtosecond laser. Also used to treat mild-to-moderate keratoconus by shifting and stabilizing the cone, allowing for improved contact lens tolerance and best spectacle-corrected visual acuity.

Symptoms
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