Ординатура / Офтальмология / Английские материалы / Practical Ophthalmology A Manual for the Beginning Ophthalmology Residents 4th edition_Wilson_1996

der is required, with its sign and axis. Sometimes the cylinder designation is preceded by the symbol C, which means "combined with." The axis of the cylinder is designated by x, followed by the degree of the orientation of the cylinder axis. The degree symbol is commonly omitted and only the numbers written. A prescription is recorded for each eye, using the abbreviations OD (oculus dexter) for the right eye and OS (oculus sinister) for the left eye. Table 5.1 displays examples of typical spectacle prescriptions. In writing spectacle prescriptions, many practitioners omit the "combined with" sign.

Lens Transposition

In a written spectacle lens prescription, cylinder power can be recorded in either plus form or minus form. Many ophthalmologists customarily record cylinder with plus notation, except for contact lenses, whereas opticians and optometrists generally use minus notation. A plus cylinder is ground onto the anterior surface of a lens, and the minus cylinder is ground onto the posterior surface of the lens. Unless specifically stated by the practitioner, the lenses will be filled as minus cylinders. This generally allows for a more cosmetically acceptable lens, because plus cylinders produce more magnification than the equivalent minus-cylinder form. A conversion of a prescription from one form to the other is called lens transposition and is achieved in three steps:

1.Add algebraically the cylinder power to the sphere power.

2.Reverse the sign of the cylinder.

In writing spectacle prescriptions, some practitioners omit the "combined withT'C: sign.The myopic astigmatism example then might be recorded as follows:

OD -0.75 +0.50x150 or -0.25-0.50x60.

OS -1.00 +0.50x120 or -0.50-0.50x30

Lensometry 67

3.Add 90° to the cylinder axis. If the resulting number exceeds 180°, subtract 180.

Examples of lens transposition:

-1.00 O+1.50 x 95 is equivalent to +0.50 C-1.50 x 05

+3.00 O +2.00 x 20 is equivalent to +5.00 C -2.00 x 110

-2.00 C+1.00 x 160 is equivalent to -1.00 C-1.00 x 70

Spherical Equivalent

Lensometry

Lensometry is a procedure used to measure the prescription of a patient's present spectacle lenses. Lensometry is performed with an instrument called a lensometer, or lensmeter. Both manual and automated lensometers are available (Figure 5.11). Lensometry measures four principal properties of spectacle lenses:

1.Spherical and cylindrical pozra-

2.Cylindrical axis if cylinder is present

3.Presence and orientation of prism,

4.Optical centration

Clinical Protocol 5.1 describes the steps in standard manual lensometry for single-vision spectacles. Clinical Protocol 5.2 describes lensometry for multifocal spectacles. Clinical Protocol 5.3 lists steps in measuring prism power and orientation, and Clinical Protocol 5.4 describes how to determine optical centration of lenses.

pter 5: Refraction

frames—eyeglasses that can hold a variety of lenses from a trial set of spheres, cylinders, and prisms (Figure 5.13). Trial frames have adjustable eyepieces, temple pieces, and nose pieces, and the examiner should become proficient in adjusting these elements to align the frame properly on the patient's face.

Halberg clips, which can be affixed onto the patient's own eyeglasses, also hold trial lenses and can be used to allow for modification of the patient's glasses with them in place, a procedure referred to as overrefraction.

Refractor

The refractor, or Phoroptor, provides an alternative to a trial frame and loose lenses. It consists of a face plate that can be suspended before the patient's eyes. The plate contains a wide range of spherical and cylindrical lenses that the examiner can dial into position (Figure 5.14). Most refractors have a variable setting for the interpupillary distance and have a tilt feature to allow the eyes to converge for determining near vision correction. Other accessories vary widely among makes and models of refractors. Table 5.2 lists the major refractor controls and their uses.

Figure 5.13 A trial frame can be adjusted to conform to the patient's anatomy and allows manual insertion of multiple lenses selected from a trial set.

Jackson cross cylinder

This instrument is a special lens used as part of the refinement process to confirm first the axis and then the power of a correcting cylindrical lens for astigmatism (Figure 5.15). The handheld device consists of a handle attached to a lens containing two cylinders of equal power, one minus and one plus, set at right angles to each other. A cross cylinder often is built into the refractor.

Distometer

A distometer is a small handheld device used for determining vertex distance (that is, the distance between the patient's eye and the back of the corrective lens). A distometer is illustrated in Figure 5.16. A vertex distance of 13.5 mm is considered average but can vary among patients. It is important to keep the vertex distance for the patient's eyeglasses constant during refractometry. If the vertex distances used

Figure 5.14 Refractor, or Phoroptor, examiner's view.The most frequently used controls are listed in Table 5.2.

74 Chapter 5: Refraction

reflex will not be in the visual axis but rather in the axis of the deviation. The measurement of the eye's refractive error made in this axis will not be accurate. If the eye is not aligned with the retinoscopic reflex, the examiner may sit more directly in front of the patient and can occlude the eye not being examined, either with a hand or an occluding device. If cycloplegia has been used, the patient is instructed to look directly into the light.

Working distance

The distance between the examiner and the patient's eye must be measured and converted into diopters. For the examination, the patient may be given a "working lens," the power of which equals the dioptric distance between examiner and patient. The power of this lens is then subtracted from the final dioptric amount that is measured. If a working lens is not used, simply subtract the dioptric equivalent of the working distance from the neutralization point reached in retinoscopy.

For convenience in changing lenses in the refractor or trial frame, most examiners use a working distance of arm's length, usually about 66 cm (approximately 2/i m or 26 inches), and assign the patient a corresponding working lens of +1.50 D. A working distance of x/i m would require a +2.00 D working lens, and a working distance of 1 m would require a working lens of+1.00 D. The working distance must remain constant throughout the examination, although the examiner does move forward and backward slightly from this position to evalu-

• -, ate the movement of the patient's light reflex.

Neutralization With a Retinoscope

The term neutralization refers to the achievement of the point at which

,, a lens placed before the patient's eye effectively "neutralizes" the

retinoscopic reflex and the patient's pupil fills with reflected light. As mentioned before, because of its subtleties and complexities, retinoscopy is best learned by hands-on instruction. Nevertheless, the basic steps in retinoscopic neutralization are outlined below.