Refractive errors By Dr. Khalil Al-Salem
Refractive errors definitions
Refractive errors lecture is intended to teach medical students important tips on Myopia, Hyperopia , astigmatism, and treatment regimes like glasses, contact lenses and refractive surgery.
In the Emmetropic (normally refracted) eye, entering light rays are focused on the retina by the cornea and the lens, creating a sharp image that is transmitted to the brain.
Ammetropia ( the presence of refractive error), the eye is unable to converge parallel rays from infinity directly to the retina without the aid of the elastic lens. The lens is elastic, more so in younger people.
During Accommodation, the ciliary muscles adjust lens shape to properly focus images.
Anisometropia is a significant difference between the refractive errors of the 2 eyes (usually > 3 diopters). When corrected with eyeglasses, a difference in image size (aniseikonia) is produced; it can lead to difficulties with fusion of the 2 differently sized images and even to suppression of one of the images.
Refractive errors are failure of the eye to focus images sharply on the retina, causing blurred vision
types of refractive errors
In myopia (nearsightedness), the point of focus is in front of the retina because the cornea is too steeply curved, the axial length of the eye is too long, or both. Distant objects are blurred, but near objects can be seen clearly. To correct myopia, a concave (minus) lens is used. Myopic refractive errors in children frequently increase until the child stops growing.
In hyperopia (farsightedness), the point of focus is behind the retina because the cornea is too flatly curved, the axial length is too short, or both. In adults, both near and distant objects are blurred. Children and young adults with mild hyperopia may be able to see clearly because of their ability to accommodate. To correct hyperopia, a convex (plus) lens is used.
In astigmatism, nonspherical (variable) curvature of the cornea or lens causes light rays of different orientations (eg, vertical, oblique, horizontal) to focus at different points. To correct astigmatism, a cylindrical lens (a segment cut from a cylinder) is used. Cylindric lenses have no refractive power along one axis and are concave or convex along the other axis.
Refractive errors with Emmetropia
presbyopia is loss of the lens’ ability to change shape to focus on near objects due to aging. Typically, presbyopia becomes noticeable by the time a person reaches the early or mid 40s. A convex (plus) lens is used for correction when viewing near objects. These lenses may be supplied as separate glasses or built into a lens as bifocals or variable focus lenses.
Symptoms and Signs
The primary symptom of refractive errors is blurred vision for distant objects, near objects, or both. Sometimes the excessive ciliary muscle tone can cause headaches. Prolonged squinting and frowning with ocular use can also lead to headaches. Occasionally, excessive staring can lead to ocular surface desiccation, causing eye irritation, itching, visual fatigue, foreign body sensation, and redness. Frowning when reading and excessive blinking or rubbing of the eyes are symptoms of refractive error in children.
Visual acuity testing, Refraction, Comprehensive eye examination. Visual acuity testing and refraction (determination of refractive error) as needed should be done every 1 or 2 yr. Screening children’s visual acuity helps detect refractive errors before they interfere with learning. A comprehensive eye examination, done by an ophthalmologist, should accompany refraction.
Treatments of refractive errors include corrective lenses, contact lenses, and refractive surgery. Myopia and hyperopia are corrected with spherical lenses. Concave lenses are used to treat myopia; they are minus or divergent. Convex lenses are used to treat hyperopia; they are plus or convergent. Astigmatism is treated with cylindrical lenses. Corrective lens prescriptions have 3 numbers. The first number is the power (magnitude) of spherical correction required (minus for myopia; plus for hyperopia). The second number is the power of cylindrical correction required (plus or minus). The third number is the axis of the cylinder. As an example, a prescription for a person with myopic astigmatism may read
-4.50/ + 2.50 × 90,
sphere cylinder/axis and a prescription for a person with hyperopic astigmatism may read
+3.00/ + 1.50 × 180.
Contact lenses in the treatment of refractive errors
Contact lenses often provide better visual acuity and peripheral vision than do eyeglasses and can be prescribed to correct the following:
Myopia, Hyperopia , Astigmatism, Anisometropia, Aniseikonia (a difference in image size), Aphakia (absence of the lens) after cataract removal, Keratoconus (a cone-shaped cornea)
Either soft or rigid lenses are used to correct myopia and hyperopia. Toric soft contact lenses (which have different curvatures molded onto the front lens surface) or rigid lenses are used to correct significant astigmatism; they are satisfactory in many cases but require expert fitting.
Contact lenses are also used to correct presbyopia. In one approach, termed monovision, the nondominant eye is corrected for near vision (reading) and the dominant eye is corrected for distant vision. Rigid and soft bifocal and multifocal contact lenses can also be successful, but the fitting procedure is time-consuming because precise alignment is essential. Neither rigid nor soft contact lenses offer the eyes the protection against blunt or sharp injury that eyeglasses do.
Contact lenses Care and Complications
Instructions for hygiene and handling lenses must be strictly observed. Poor contact lens hygiene may lead to infection of the cornea or persistent inflammation. Contact lenses occasionally cause painless superficial corneal changes. Contact lenses can be painful when
The corneal epithelium is abraded (see Corneal Abrasions and Foreign Bodies), the eye becomes red, and the cornea stains with fluorescein. The lenses fit poorly (eg, too tight, too loose, poorly centered). There is too little moisture to keep the lens floating above the cornea. The lenses are worn in a nonideal environment (eg, O2-poor, smoky, windy). A lens is improperly inserted or removed. A small foreign particle (eg, soot, dust) becomes trapped between the lens and the cornea. The lenses are worn for a long time (overwear syndrome).
In overwear syndrome, spontaneous healing may occur in a day or so if lenses are not worn. In some cases, active treatment is required (eg, topical antibiotic eye drops or ointments). Dilating the eye with mydriatic drops can ease photophobia. Mydriatics work by temporarily paralyzing the muscles of the iris and ciliary body (movement of the inflamed muscles causes pain). In overwear syndrome or any other condition in which pain does not quickly resolve when lenses are removed, an ophthalmologist should be consulted before lenses are worn again.
Risk factors for contact lens–related corneal infection (keratitis) include the following:
Poor lens hygiene. Overnight or extended wear. Use of tap water in the cleaning regimen. Eyes with a compromised ocular surface (eg, dryness, poor corneal sensation). Infections require rapid treatment by an ophthalmologist.
A corneal ulcer, which is a potentially vision-threatening infection of the cornea, is suspected when a contact lens wearer has intense eye pain (both foreign body sensation and ache), redness, photophobia, and tearing. Use of contact lenses increases risk of corneal ulcer. The risk increases about 15 times if contact lenses are worn overnight. Corneal ulcers can be caused by bacteria, viruses, fungi, or amebas.
Diagnosis is by slit-lamp examination and fluorescein staining. A corneal epithelial defect (which stains with fluorescein) and a corneal infiltrate (collection of WBCs in the corneal stroma) are present. At times, the corneal defect is large and dense enough to be seen with handheld magnification or even with the naked eye as a white spot on the cornea. Microbiologic analysis of cultures and smears of the corneal infiltrate, contact lens, and contact lens case is indicated.
Contact lens use is stopped. Antibiotic eye drops are given empirically for possible bacterial infection. Initial therapy is broad-spectrum, using a fluoroquinolone antibiotic eye drop every 15 to 60 minutes around the clock for the first 24 to 72 h, then at gradually longer intervals. Additional antibiotic eye drops, such as cefazolin, vancomycin, or concentrated tobramycin, are used if the ulcer is large, deep, or close to the visual axis. The antibiotic may be changed or stopped later based on culture results. Neglected cases may respond poorly or not at all to treatment, and severe vision loss may result.
Rigid Corneal Contact Lenses
A rigid lens is able to revise the natural shape of the cornea into a new, better refracting surface than a soft lens and thus tends to provide more consistent improvement in refraction for people who have astigmatism or an irregular corneal surface. Older polymethyl methacrylate rigid contact lenses have been replaced by gas-permeable contact lenses (GPCLs) made of fluorocarbon and polymethyl methacrylate admixtures. GPCLs are 6.5 to 10 mm in diameter and cover part of the cornea, floating on the tear layer overlying it.
Rigid contact lenses can improve vision in people with myopia, hyperopia, and astigmatism. Rigid contact lenses can also correct corneal irregularities, such as keratoconus. In most cases, patients with keratoconus see better with rigid contact lenses than glasses.
GPCLs can be designed to fit the eye exactly. For complete wearing comfort, they require an adaptation period, typically about 4 to 7 days. During this time, the wearer gradually increases the number of hours the lenses are worn each day. Importantly, no pain should occur at any time. Pain is a sign of an ill-fitting contact lens or corneal irritation. Wearers usually have temporary (< 2 h) blurred vision (spectacle blur) when wearing eyeglasses after removing rigid contact lenses.
Soft Hydrophilic Contact Lenses
Soft contact lenses are made of poly-2-hydroxyethyl methacrylate and other flexible plastics (such as silicone hydrogels) and are 30 to 79% water. They are 13 to 15 mm in diameter and cover the entire cornea. Soft contact lenses are often replaced daily (disposable single-use), every 2 wk, or monthly.
Soft contact lenses can improve vision in people with myopia and hyperopia. Because soft contact lenses mold to the existing corneal curvature, anything greater than minimal astigmatism cannot be treated unless a special toric lens, which has different curvatures molded onto the front lens surface, is used. Weighting the lower aspect of the toric lens maintains its orientation by reducing lens spinning.
Soft contact lenses are also prescribed for treatment of corneal abrasion, recurrent erosions, or other corneal disorders (called bandage or therapeutic contact lenses). Prophylactic antibiotic eye drops (eg, fluoroquinolone qid) may be advisable with a bandage lens. Extended wearing of soft contact lenses, especially in aphakia after cataract surgery, is practical, but an ophthalmologist should examine the patient regularly. The patient should clean the lenses once/wk.
Because of their larger size, soft contact lenses are not as likely as rigid contact lenses to eject spontaneously and are less likely to allow foreign bodies to lodge beneath them. Immediate wearing comfort allows for a brief adaptation period.
Soft contact lenses have a higher incidence of corneal infections than GPCLs, particularly when soft lenses are worn overnight. When dry, soft contact lenses are brittle and break easily. They absorb a certain amount of moisture (based on the water content) from the tear film to retain adequate shape and pliability. Therefore, patients with dry eye are usually more comfortable wearing lenses that have a low water content.