Ординатура / Офтальмология / Английские материалы / Practical Ophthalmology A Manual for the Beginning Ophthalmology Residents 4th edition_Wilson_1996
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6.After noting the responses to the animals, direct the patient's attention to the squares with the four circles in each. Ask the patient to tell you which circle is coming forward in each square. Alternatively, you may ask the patient to point to the appropriate circle or, particularly with children, to push the button that is popping up.
7.Score the response as the last correctly identified before two consecutive circles are missed. If you suspect that the patient does not understand
the test, turn the book upside down and ask which circle is now behind the page. i
8. Record the stereopsis as seconds of arc as designated in the instruction booklet that is included with each test. It is advisable to copy the scoring
chart for each test and attach it to the back of the test chart for ready |
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reference; you will probably not remember the numbers for each target, |
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and if the test that you are using is not the standard one, the scoring |
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may vary a little. |
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9.Be sure that the patient has both eyes open while doing the test. Some patients are so accustomed to being tested monocularly that they automatically close one eye.
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5
Refraction
efraction is the process by which the patient is guided Trough the use of a variety of lenses so as to achieve the best possible acuity on distance and near vision tests. Refraction involves both objective, and subjective measurements. The objective portion of the process of refraction is called retinoscopy and can be accomplished by manual or automated methods. The measurements, obtained by retinoscopy can be refined by subjective methods to achieve a final prescription for eyeglasses or other optical aids. This chapter offers instruction in these basic techniques of refraction, including guidelines for spectacle lens prescription. Because refraction requires an understanding of basic refractive
.'&*: states of the eye and the basic characteristics of lenses used for optical correction, this chapter briefly reviews those topics and also provides instruction in lensometry, a technique for determining the prescription of existing corrective lenses.
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58 Chapter 5: Refraction
Overview of Refraction
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The physicist defines refraction as the bending of light rays as they |
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encounter interfaces between materials with differing refractive |
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indices. In clinical ophthalmology, the term refraction is employed to |
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describe the process of measuring a patient's refractive error and deter- |
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mining the optical correction needed to focus light rays from distant |
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and near objects onto the retina and provide the patient with clear and |
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comfortable vision. The clinical process of refraction comprises five |
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activities, which are discussed in greater detail later in this chapter: |
1.Retinoscopy (or objective refraction) is a clinical test used to determine the approximate nature and extent of a patient's refractive error (ie, nearsightedness, farsightedness, or astigmatism). It is sometimes called objective refraction because it does not require subjective responses from the patient. Retinoscopy is performed primarily with a retinoscope, a handheld instrument consisting of
•. a light source and a viewing component. Sometimes an automated machine is used for retinoscopy.
2.Cycloplegia (not always done) is the use of medication to temporarily paralyze accommodation, enabling the refractionist to determine the patient's baseline nonaccommodative refractive error.
3.Refinement (or subjective refraction) provides a precise measurement of refractive error and appropriate lens correction. Refinement utilizes patient participation and reaction ("I can see better with this lens than with that one") to obtain the refractive correction that gives the best visual acuity. Tools used in refinement include the refractor (also called a Phoroptor) or trial lenses and trial frame and a visual acuity chart. Because refinement requires subjective participation from the patient, it is not possible to perform this part of refraction with infants, most toddlers, and other patients who are unable to communicate adequately.
4.Binocular balancing is the final step in retraction that determines
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whether accommodation has been equally relaxed in the two eves. |
5.Prescription of spectacle lenses is the outcome of the clinical process of refraction. The patient is given an optical prescription (a writ-
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ten description of the optical requirements for correction of the |
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patient's refractive error) based on the results of the steps listed |
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above. |
Overview of Ophthalmic Optics |
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Overview of Ophtha Imic Optics
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Performing accurate retinoscopy and refinement and prescribing |
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appropriate optical correction require a fundamental understanding of |
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the properties of light rays, of the types and properties of optical lens- |
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es, and of the interaction between the two. This chapter can touch on |
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the principles of ophthalmic optics only briefly; more detailed infor- |
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mation is available in Section 3, Optics, Refraction, and Contact Lenses, of |
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the Basic and Clinical Science Course, published by the American |
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Academy of Ophthalmology. |
Principles of Vergence
As applied to li| ht rays, the term vergence describes the direction of a ray as it passes between some luminous point to a lens in question, Vergence is the reciprocal of the distance from the lens to the point of convergence of the light. Light rays that are moving away from each other are termed divergent. Light rays that are moving toward each other are termed convergent. Parallel light rays have zero vergence, ie, they do not move toward or away from each other. Figure 5.1 illustrates these thr te types of rays. Light rays emanating from a point source of light are divergent. Convergent light rays do not usually occur in nature 3ut are the result of the action of an optical system (eg, a lens). Light rays emanating from the sun are essentially parallel and
have zero vergience.
Power (or ve"gence power) describes the ability of a curved lens to converge or div :rge light rays. By convention, divergence is expressed in minus power; convergence is expressed in plus power. A diopter
B
Figure 5.1 Ligtjt rays can be divergent (A), convergent (B), or parallel (zero vergence) (C).
• 60 Chapter 5: Refraction
(abbreviated D) is the unit of measurement of the refractive power of a lens. The focal length of a lens is the distance between the lens and the image formed by an object at infinity. Focal length (in meters) = 1/D.
Types of Lenses
Lenses may be spheres, cylinders, or spherocylinders. A. spherical lens has the same curvature over its entire surface, and thus die same refractive power in all meridians. Convex spherical lenses converge light rays and are called plus lenses; concave spherical lenses diverge light rays and are called minus lenses (Figure 5.2). The focal point of a plus lens is that position where parallel light rays that have passed through the lens converge to form an image. The focal point of a minus lens is the point from which parallel light rays entering the lens appear to diverge. Examples of plus and minus lenses illustrating the relationship of lens power to focal length are shown in Figure 5.3. In the case of convex, or plus, lenses, using the mathematical formula D = 1/f, 1 diopter of plus power converges parallel rays of light to focus at 1 m from the lens. A +0.25 D lens focuses parallel light rays 1/0.25 m, or 4 m, from the lens. A +4.00 D lens converges parallel light rays to a focus at 1/+4.00 m, or 0.25 m from the lens. In the case of concave, or minus, lenses, parallel light rays entering the lens diverge; a virtual image is considered to appear at a focal point in front of the lens. A minus lens with a focal length of 1 m has a power of-1.00 D; a minus lens with a focal length of-0.2 5 m has a power of-1/0.25 or -4.00 D.
Focal point
Focal point
B
Figure 5.2 Types of lenses include (A) converging (convex or plus) lenses, and
(B) diverging (concave or minus) lenses. The focal point of a plus lens occurs where parallel light rays that have passed through the lens converge to form an image.The focal point of a minus lens occurs where parallel light rays entering the lens appear to diverge.
Overview of Ophthalmic Optics |
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4.0 m
+ 1.0 D
B
+4.0 D |
0.25 m |
-0.25 D
4.0 m
!.0D
.0 m
4.0 D
.25 m
Figure 5.3 Relationship of lens power to focal length for plus lenses (A, B, C) and minus lenses (D, E, F). F = focal point.
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62 Chapter ">: Refraction
Cylindrical lenses have vergence power in only one meridian, the one perpendicular to the axis of the cylinder. They have no power in the meridian parallel to the axis (Figure 5.4). Cylindrical lenses focus light rays to a line (Figure 5.5). The orientation of the axis of cylindrical lenses is assigned by convention, when looking at a patient, as noted in Figure 5.6. The orientation of corrective cylindrical lenses is the same for the right and left eyes, ie, 0° to 90° is to the patients left, whereas 90° to 180° is to the patient's right.
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Focat line
Figure 5.4 Refracting (vergence) power of a cylindrical lens. Maximum refractive power occurs in the meridian perpendicular to the axis of the cylinder (curved undotted line a). The cylinder has no refractive power in the meridian that corresponds to the axis of the cylinder (vertical undotted lines b).
Figure 5.5 Because a cylindrical lens has refracting power in only one meridian (which is perpendicular to its axis), it focuses light rays to a focal line.
Figure 5.6 Conventional assignment of the orientation of the axis of cylindrical lenses, when looking at a patient. « • - .
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Spherical and cylindrical lenses can be combined in one lens to form a spherocylindrical lens, also known as a compound lens, or toric lens. A spherocylindrical lens focuses die light in two line foci. The shape of the light rays as they are focused by the spherocylindrical lens is called the conoid of Sturm (Figure 5.7). Between the two line foci produced by the conoid of Sturm is a point called the circle of least confusion, which represents the point of best overall focus for a spherocylindrical lens.
Prisms are technically not lenses, but prismatic effects are inherent characteristics of lenses. A prism is a wedge of refracting material with a triangular cross section that deviates light toward its base. Objects viewed tli rough a prism appear to be displaced toward the apex of the prism (Figure 5.8). Spherical lenses can be thought of as paired prisms, with convergent (plus) lenses made of prisms that are base to base, and divergent (minus) lenses made of prisms that are apex to apex. Thus, a spherical lens has prismatic power at every point on its surface except at the optical center of the lens.
Conoid of Sturm
Figure 5.7 Because its two radii of curvature (x,y) are not equal, a spherocylinder does not focus light to a point, but to two lines (y focal line,xfocal line) in different places.The clearest image is formed between the two, at the circle of least confusion.The conoid of Sturm is the name given to the shape the light rays take as they are focused by a spherocylindrical lens.
Figure 5.8 (A) Because of its shape, a prism refracts light rays toward its base.
(B) If an object is viewed through a prism, the object appears in space as if it were displaced toward the prism apex. , • • -.
ijtit& Chapter 5: Refraction
1 prism-diopter prism
1 m
Figure 5.9 Measurement of prism power. A prism measuring 1 PD deflec ts a light ray 1 cm at a distance of 1 m.
The power of a prism to deviate light rays is expressed in pris-m diopters (abbreviated PD or with a superior delta A). A prism measuring 1 PD (1A) deviates parallel rays of light 1 cm when measured at a distance of 1 m from the prism (Figure 5.9). A prism that deviates light rays 1 cm at a distance of 2 m measures 0.5 PD; a prism that deviates light rays 1 cm from l/i meter (0.5 m) measures 2 PD.
Refractive States of the Eye
In the normal eve, parallel light rays are focused sharply on the retina, a condition known as emmetropia. When the relaxed, or nonaccommodating, eve is unable to bring parallel light rays from a distant object into focus, the condition is referred to as ametropia. The three basicconditions that may produce ametropia are
1. Myopia (nearsightedness)
2. Hyperopia (farsightedness, also called hypermetropia)
3. Astigmatism
A myopic (nearsighted) eye has excessive convergent power; the light ravs focus anterior to the retina. A minus (divergent) lens is used to correct myopia (Figure 5.10 A, B). A hyperopic (farsighted) eye has insufficient convergence power to focus light rays on the retina; the
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rays focus posterior to the retina. A plus (convergent) lens is used to |
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correct hyperopia (Figure 5.10 C, D). |
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The cornea (and sometimes the eye's crystalline lens) may not have |
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the same radius of curvature in all meridians. Aberration of the corneal |
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or lenticular surfaces that produces differing radii of curvature is called |
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ustio-mntisin. A cvlindrical lens is used to neutralize astigmatism (Figure |
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5.10 K, V). In most patients, the axis of plus cylinder needed to correct |
Lens Notation |
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B
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Figure 5.10 (A, B) A concave (minus) lens is used to correct myopia, in which parallel rays are focused anterior to the macula. (C, D) A convex (plus) lens is used to correct hyperopia, in which parallel rays are focused posterior to the macula.
(E, F) A cylindrical (or spherocylindrical) lens is used to correct astigmatism, in which parallel rays are not focused uniformly in all meridians.
the astigmatism is either close to 90° (with-the-rule astigmatism) or close to 180° (against-the-rule astigmatism). In clinical practice, many myopic patients and hyperopic patients also have astigmatism. A spherocylindrical lens is used to correct myopic and hyperopic astigmatism.
Presbyopia is a progressive loss of accommodative ability of the crystalline lens caused by the natural process of aging. It generally evidences itself as difficulty witli near visual work, such as reading. Presbyopia occurs in the presence of myopia, hyperopia, and astigmatism. It can be remedied optically with plus lenses.
Lens Notation
A written spectacle lens prescription follows a standard format. The power of the sphere (abbreviated sph) is recorded first, along with its sign (+ or -). This is followed by the power of the cylinder, if a cylin-