It is estimated that 80% of one’s sensory input is visual. Fig 16 Snellen chart measures the central eight degrees of vision. A patient should read the Snellen chart (Fig. 16) from 20 ft (6 m) with the left eye occluded first. Take the vision in each eye without and then with spectacles. Vision is expressed in a fraction‐like form. The top number (numerator; usually 20) is the distance in feet at which the patient reads the chart. The bottom number (denominator) is the size of the object seen at that distance. Whenever acuity is less than 20/20, determine the cause for the decreased vision. The most common cause is a refractive error; i.e., the need for lens correction. If visual acuity is less than 20/20, the patient may be examined with a pinhole. Improvement of vision while looking through a pinhole indicates that spectacles will improve vision. Use an “E” chart with a young child or an illiterate adult. Ask the patient which way the ∃ is pointing. Near vision is checked with a reading card held at 14 inches (36 cm). If a refraction for new spectacles is necessary, perform it prior to other tests that may disturb the eye. Your mission is to get them to see, while theirs is to have the vision to notice what’s invisible to others. In an emmetropic eye (Fig. 17), light from a distance is focused on the retina. The cornea contributes 43.50 diopters (D) (Fig. 73) and the lens adds another 15.00 diopters but can increase by +2.50 for near focus (Fig. 377). Fig 17 Emmetropic eye. In this disorder, light is not focused on the retina. Three types of refractive errors, hyperopia, myopia, and astigmatism, are often inherited and occur early in life. The fourth kind called presbyopia refers to the loss ability to focus up close and typically occurs in everyone in and around the age of 43. Fig 18 A hyperopic eye often has a shorter than normal axial length creating a crowded anterior segment with a smaller space between this iris and cornea. This is referred to as a narrow‐angle (compare Figs 328 and 329). This may obstruct the egress of aqueous from the eye resulting in elevated eye pressure (angle‐closure glaucoma. Figs 362‐364). An attack may be precipitated by dilation of the pupil caused by medication or stress. Parallel rays of light are focused behind the retina (Fig. 18). The patient is farsighted and sees more clearly at a distance than near, but still might require glasses for viewing objects at a distance. A convex lens is used to correct hyperopia (Fig. 19). The power of the lens needed to focus incoming light onto the retina is expressed in positive diopters (D). A positive 1 D lens converges parallel rays of light to focus at 1 m (Fig. 20). Fig 19 Hyperopic eye corrected with convex lens. Fig 20 Parallel rays focused by 1 D lens. Parallel rays are focused in front of the retina (Fig. 21). The patient is nearsighted and sees more clearly near than at a distance. Myopia often begins in the first decade and progresses until stabilization at the end of the second or third decade. A 2016 study—the largest ever done in America—showed that in the past 50 years, the prevalence of myopia in young Americans has more than doubled to about 40%. It has been reported to be as high as 90% in Asia, where, 60 years ago, there was an incidence of 10–20%. It is strongly linked to inheritance; higher levels of education; more near work and less outdoor activity, possibly related to not enough sunlight. A concave negative lens (Fig. 23), which diverges light rays, is used to correct this condition. Fig 21 Myopic eye. Myopia may be due to increased curvature of the cornea or the human lens, but is more often due to elongation of the eye. In axial myopia, the retina is sometimes stretched so much that it pulls away from the optic disk (see Figs 472A and 472B) and may cause retinal or scleral thinning (Fig. 317) that could result in retinal holes or detachments. This is more common in myopic eyes of –6.00 D (high myopia) and most common if greater than −10.00 D (pathologic myopia) (Fig. 22). Severe loss of vision could also be due to patches of complete atrophy of the retina or from choroidal neovascularization in 10% of those eyes (Figs 522 and 527) causing a condition similar to wet age‐related macular degeneration. Glaucoma and cataract are also more common in pathologic myopia. Fig 22 OCT of pathologic myopia showing: A—degeneration of macula; and B—increased axial length. The choroid may be thickened from neovascularization. Fig. 524. Source: Courtesy of University of Iowa, Eyerounds.org. Fig 23 Myopic eye corrected by concave lens. In this condition, which affects 85% of people, the eye, rather than having a spherical shape like a basketball, is instead shaped like a football. Rays entering the eye are not refracted uniformly in all meridians. Regular astigmatism occurs when the corneal curvature is uniformly different in meridians at right angles to each other. It is corrected with spectacles. For example, take the case of astigmatism in the horizontal (180°) meridian (Fig. 24). A slit beam of vertical light (AB) is focused on the retina, and (CD) anterior to the retina. To correct this regular astigmatism, a myopic cylindrical lens (Figs 25 and 26) is used that diverges only CD. Fig 24 Myopic astigmatism. For explanation, see text. Irregular astigmatism is caused by a distorted cornea, usually resulting from an injury or a disease called keratoconus (see Figs 43, 284, and 285). Fig 25 Myopic astigmatism corrected with a myopic cylinder, axis 90°. This is a decrease in near vision, which occurs in all people at about age 43. The normal eye has to adjust +2.50 D to change focus from distance to near. This is called accommodation (Fig. 377) and is accomplished when the shape of the lens becomes more convex. The eye’s ability to accommodate decreases from +14 D at age 14 to +2 D at age 50. Fig 26 Tomographic image of corneal astigmatism with the steepest power +47.70 D at axis 120° and the flattest +44.51 D at 30°. To correct this myopic astigmatic error, a –3.00 D myopic cylindrical lens would be placed in the spectacle at 30°. Source: Courtesy of Richard Witlin, MD. Middle‐aged persons are given reading glasses with plus lenses that require updating with age. The additional plus lens in a full reading glass (Fig. 27) blurs distance vision. Half glasses (Fig. 28) and bifocals (Fig. 29) are options that allow for clear distance vision when looking up. No‐line progressive bifocals are more attractive, and also allows for vision at the middle distance of 1 meter from the eye, but is more expensive. Fig 27 Full reading glass blurs distance vision. Fig 28 Half glasses. Fig 29 Bifocals. Refraction is the technique of determining the lenses necessary to correct the optical defects of the eye. The lens case (Fig. 30) contains convex and concave spherical and cylindrical lenses. The diopter power of spherical lenses and the axis of cylindrical lenses are recorded on the lens frames. Fig 30 Lens case with red concave and black convex lenses. The trial frame (Fig. 31) holds the trial lenses. Place the strongest spherical lenses in the compartment closest to the eye because the effective power of the lens varies with its distance from the eye. Place the cylindrical lenses in the compartment farthest from the eye so that the axis can be measured on the scale of the trial frame (0–180°). Some prefer using a phoropter (Fig. 32) that dials lenses in front of the patient instead of manually exchanging lenses from the lens case. Fig 31 Trial frame. This is the objective means of determining the refractive error in all patients before beginning a subjective refraction. It is the primary means to determine eyeglass prescriptions in infants and illiterate persons who cannot give adequate subjective responses. In these two instances, cyclogel 1% (Table 15, p. 147) may be instilled to prevent lens changes due to accommodation that can alter the findings. Hold the retinoscope (Fig. 33
Chapter 2
Measurement of vision and refraction
Visual acuity
Examples of visual acuity
Measurement in feet (meters in parentheses)
Meaning
20/20 (6/6)
Normal. At 20 ft (6 m), patient reads a line that a normal eye sees at 20 ft.
20/30–2 (6/9–2)
Missed two letters of 20/30 line.
20/50 (6/15)
Vision required in at least one eye for driver’s license in most states.
20/200 (6/60)
Legally blind. At 20 ft, patient reads line that a normal eye could see at 200 ft (60 m).
10/400 (3/120)
If patient cannot read top line at 20 ft, walk him or her to the chart. Record as the numerator the distance at which the top line first becomes clear.
CF/2 ft (counts fingers at 2 ft, 0.6 m)
If patient is unable to read top line, have the patient count fingers at maximal distance.
HM/3 ft (hand motion at 3 ft, 0.9 m)
If at 1 ft (0.3 m) patient cannot count fingers, ask if they see the direction of hand motion.
LP/Proj. (light perception with projection)
Light perception with ability to determine position of light.
NLP
No light perception: totally blind.
Record vision as follows
Key
OD
20/70 + 1
V
Vision
OS
LP/Proj.
Without spectacles
With spectacles
OD
Right eye
OD
20/20
OS
Left eye
OS
LP/Proj.
OU
Both eyes
Optics
Emmetropia (no refractive error)
Ametropia
Hyperopia
Myopia
Astigmatism
Presbyopia
40–45 years
+1.00
to
+1.50 D
50 years
+1.50
to
+2.00 D
Over 55 years
+2.00
to
+2.50 D 
Refraction
Trial case and lenses
Trial frame
Streak retinoscopy (“flash”)
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