Today, more than ever, the ophthalmologist’s clinical acumen is enhanced by the many instruments available that facilitate the determination of refractive errors of the eye, the detection of muscular imbalance, and the magnification and visualization of the interior structures of the eye. This chapter deals with ophthalmic instruments, their purpose and mode of use, and their advantages and limitations.
Equipment used for refraction
Determining the refractive error of an eye permits the ophthalmologist to prescribe lenses that enable the patient to obtain the best possible visual acuity.
Projector and projector slides
The projector provides a means of projecting, on a silver screen, test letters and characters that can be used in assessing visual acuity.
It consists of a housing for a bulb, an opening for introduction of different target slides, and a lens system that can be focused onto a silver screen. The housing for the bulb is made readily accessible for interchange of bulbs when bulbs darken or burn out. Rheostats may be introduced into the power circuits to lengthen the life of the bulb. The lens system and slides should be kept clean and dust free.
Projectors provide a means of illuminating: (1) a horizontal row of test letters or characters, (2) a vertical row of test letters or characters, and (3) a single-test letter or character, and may allow the introduction of red–green to illuminate the letters.
The use of red and green letters is the basis of the duochrome test and a means of fine-tuning the refraction. In this test, half the panel is illuminated in red and half the panel in green. Under the duochrome principle, green is normally focused in front of the retina, whereas red, having a longer wavelength, is focused behind the retina for the emmetrope. Therefore a patient seeing the letters on the green panel more clearly than those on the red panel is hyperopic, requiring more plus to bring the red wavelengths onto the retina. The patient seeing the letters more clearly on the red panel is myopic, requiring more minus to bring the green onto the retina. The emmetrope sees both equally blurred. The duochrome test thus provides a means to arrive at the final correcting lens for the refractive error present.
Available projector slides have a large variety of test targets and specialized tests for refraction. Commonly available slides include:
Snellen test letters
Landolt (split circle) rings
An astigmatic clock
A picture chart
A Worth four-dot test
Projectors may be controlled by remote control units. The use of remote control units is especially important if projectors are installed in inaccessible places or areas that are awkward for the examiner to control.
Trial case and lenses
The trial case is a tray of lenses and accessories used to determine the refractive error of an eye. These lenses are individually marked in the strengths of the dioptric power of each lens, as well as in the direction of axis of the cylindric lenses. The trial case consists of:
A pair of plus spheres ranging from +0.12 to +20.00 diopters
A pair of minus spheres ranging from −0.12 to −20.00 diopters
A pair of plus cylinders ranging from +0.12 to +8.00 diopters
A pair of minus cylinders ranging from −0.12 to −8.00 diopters
These lenses are designed to fit a standard trial frame. Each lens is encircled by a metal rim for protection. Handles are provided with spheres for ease of handling and are optional with cylinders. The choice is governed by the type of trial frames used. Cylinders with handles are used with the trial frames illustrated in Fig. 10.1 , but not with simple types having no revolving cylinder lens carriers. Handles would interfere with the free rotation of the cylinder in the latter type. The cylinder is marked with reference to its axis and not the meridian. Thus the position on the cylinder, as marked on the lens, is the axis of zero power and indicates the position of the image on the retina.
Accessory lenses available in a trial case are:
An occluder lens
A pinhole disc
A stenopeic slit
A Maddox rod lens
Prisms ranging from 0.50 to 6.00 prism diopters
A red glass filter lens
Use of trial lenses
Trial lenses are not used routinely because they have been eclipsed by the refractor or phoropter (see following text), which offers the ophthalmologist the speed of exchange of lenses in a completely enclosed housing. The trial lens, however, has a place in determining the refractive error of children who are intimidated by the massive bulk of the refractor, or whose narrowly set eyes cannot be positioned properly behind the openings in the refractor ( Fig. 10.2 ). Bifocals are prescribed by use of the trial frame with lenses because the patient can best judge a comfortable working and reading distance with the head bent and the eyes lowered in a natural reading position. Trial lenses are also used in refraction of aphakic and high-myopic eyes because it is expedient that the correcting lenses and the spectacles that the patient receives approximate each other with reference to their distance from the eye itself. Trial lenses must be used when low visual aids in the form of high-plus prescription lenses are used.
The trial frame is essentially a frame capable of holding a group of three or four trial lenses for each eye. It has adjustable earpieces and an adjustable bridge that alters the interpupillary distance. Some trial frames have an adjustment for tilting the frames toward the reading position. In high-minus and high-plus prescriptions, the proximity of the lens in the frame to the eye (vertex distance) must be measured. This aids the optician in duplicating the prescription. The calibration scale incorporated on the outer side of the frame can be used for this purpose, but is not really an accurate method of making this measurement. Modern trial frames have a thumbscrew mechanism on the side of the trial frame to rotate the front lens carrier, which is used to house the cylinder. This enables the cylinder to be rotated to the proper axis.
The front surface of the trial frame is marked off in degrees from 0 to 180 ( Fig. 10.3 ). By convention, frames are labeled in a counterclockwise direction beginning on the right-hand side of the horizontal meridian.
Refractor or phoropter
The refractor consists of the entire trial set of lenses mounted on a circular wheel so that each lens can be brought before the aperture of the viewing system by merely turning a dial ( Figs. 10.4 and 10.5 ). In addition to the conventional spheres and cylinders, accessories are available including a polarizing lens, a pinhole, a Maddox rod, and a working lens for retinoscopy. There are many types of refractors on the market, varying in the number of accessories available and the mode of housing these accessories. Fundamentally, these refractors are of the same design and purpose.
We shall discuss the Bausch & Lomb Phoropter II (manufactured by Reichert) because it is a typical example of the devices available. (The terms phoropter and refractor are often used interchangeably.)
The body consists of two disc-like casings that house the lenses. The entire instrument is mounted on a pole or a hydraulic stand. A forehead rest ensures that the patient is correctly positioned and that the eyes are as close to the lens system as possible. The knobs at either end on top of the phoropter adjust the interpupillary distance for the individual patient. If the patient has a head tilt or a vertical muscle imbalance, the phoropter may have to be tilted so that one aperture is higher than the other. Leveling adjustment knobs are adjacent to the interpupillary distance knobs.
For each eye, there are large circular discs that contain spheres and cylinders. These lenses may be presented individually or in combination. A large dial on the back surface of the phoropter introduces spheres in units of 3.00 diopters (some older models may be 4.00 diopters). A side wheel can be rotated to introduce spheres in small jumps of a quarter of a sphere. The total spherical power is read on the front casing. Plus spheres are recorded in white and minus spheres in red. The range of spherical lenses is from +20.00 to −28.00 diopters.
Phoropters are available in either plus or minus cylinders, but never both. Cylinders are introduced by the top knob on the front surface of the phoropter in units of 0.25 to 2.50. If higher cylinders are required, auxiliary cylinders of 2.50 can be flipped in front of the lens system, extending the range to 5.00. Additional loose auxiliary cylinders may be added to extend the range to 7.50 diopters. Astigmatism may be corrected to 0.12 diopter by introducing an auxiliary cylinder of 0.12 diopter. All cylinders have an axis that is controlled by a small knob, about which the accessories rotate. Markings on the front surface are in degrees, from 0 to 180, with individual axis markings in 5-degree intervals.
Aperture control handle
A small handle on the side of each eye of the phoropter controls the aperture. By moving the handle up or down, one may introduce the following:
An occluder to block out one eye
A pinhole disc
A +0.12 sphere, which can raise the total spherical power of the combination of lenses by 0.12 diopter
A retinoscopy lens, which may be custom ordered according to the distance at which retinoscopy is performed (the usual retinoscopy lens ranges from +1.00 to +2.00 spheres and is introduced by the control handle for retinoscopy and removed for subjective testing)
Prisms, 6.00 diopters base-up before the right eye and 10.00 diopters base-in before the left eye
Maddox rods, vertical and horizontal
From time to time, the complete phoropter should be returned for cleaning of lenses.
Auxiliary lenses include cylinders of 0.12 and 5.00 diopters for each eye (the phoropter has lenses available in either plus or minus cylinders, but not both), and cross cylinders of 0.25, 0.37, and 0.50 for each eye.
Accessory equipment includes Risley’s prisms to measure muscle imbalance, a cross-cylinder holder, and a reading-card holder. The cross-cylinder holder is geared to follow the cylinder axis control. The cross cylinder is inserted in a double ring of metal, the outer ring being fixed, whereas the inner ring, which holds the cross cylinder, is capable of being flipped or turned to reverse its position. The reading-card holder , attached to the front of the phoropter, permits the holding of a reading card at a variable distance from the phoropter. The reading-card holder is a rod, calibrated in inches, centimeters, and diopters, and it is capable of presenting four test cards to the patient.
Aids in care of refractor/phoropter
If the lenses are dirty, they should be cleaned with a lint-free swab slightly moistened with either alcohol or ether. Ammonia or ammonia-containing cleansers should not be used. Lenses may be dried with a tissue.
Do not put fingers, pens, or pencils in the front aperture to see if a lens is in place because the marks left are extremely difficult to remove. The rear apertures are often protected by a cover glass.
The instrument should not be lubricated because the design of the instrument is such that no interior oiling is necessary. It may be helpful at times to oil the bearings on which the cross cylinder and the Maddox rod ride.
Cleaning material should not be used on the numbers and workings that indicate lens power. These should be cleaned with a dry, soft cloth.
The forehead and cheek rests are removable and should be cleaned periodically with cotton moistened in 70% alcohol solution.
The instrument should be covered with a plastic cover when not in use.
The retinoscope is the most valuable instrument in determining the refractive error of an eye ( Figs. 10.6 and 10.7 ). It is useful in determining the total objective refractive error of an eye and may be the only means of assessing refractive error in infants and small children. It is also useful for the objective estimation of the refractive error in people who cannot read, are learning disabled, are debilitated and uncooperative, and have speech loss. There are two basic types: the spot retinoscope and the streak retinoscope.
The spot retinoscope is designed so that the refractionist can look down the center of a slightly diverging beam of light through the pupil of the patient’s eye. The modern retinoscope has a light source in the handle of the instrument, shining upward, which strikes a mirror set at 45 degrees. The beam is therefore turned through 90 degrees.
The mirror may be semisilvered or may have a hole through its center through which the refractionist can look. Therefore an area of the patient’s retina is illuminated and the refractionist sees this as a red-reflected glow. This is termed a reflex .
In the eye with no refractive error, the rays of light come to a focus on a point on the retina and the refractionist sees the whole pupillary area lit with a red glow. This is analogous to an automobile’s headlight, in which the whole 6-inch (15-cm) circular diameter of the headlight appears to be illuminated, whereas the source of this illumination is a small filament in the bulb, about 5 mm long, positioned correctly at the point of focus of the optical system of the headlight. Moving the retinoscope away from the pupil extinguishes the red reflex.
If the patient is myopic, the rays of light from the retinoscope will come to a focus in front of the retina, cross at this point, and illuminate a relatively larger area of the retina behind the focal point. If the light source is moved across the pupil, the rays of light from the retinoscope, pivoting on the focal point, will move the illuminated area on the fundus in a direction opposite to that of the retinoscope. This apparent shift of the illuminated area is termed an against motion . The refractionist therefore adds minus lenses before the patient’s eyes to move the focal point back onto the retina. When he or she has the correct combination of lenses, the movement of the light across the pupil causes no movement of the reflex, it merely turns on and off.
If the patient is hyperopic, the ray of light from the retinoscope, when going through the eye, would focus at a point behind the retina (if the retina does not block the rays of light). Lateral movement of the retinoscope across the pupil causes the area illuminated on the retina (pivoting about the focal point) to move in the same direction as the retinoscope, indicating that the eye is hyperopic or far-sighted. This shift is termed with motion . The refractionist then adds plus lenses to bring the focusing point up to the retina until the on and off light reflex appears without any apparent movement.
In summary, if a retinoscope beam produces with motion of the red reflex, the patient’s eye is hyperopic or far-sighted and needs plus lenses to correct the condition. If the retinoscope beam produces against motion , the patient is myopic or near-sighted and needs minus lenses to correct the refractive error.
If the eye is astigmatic, it will exhibit two powers on axes at 90 degrees to one another. The retinoscope is then used to correct the power on one axis and then on the other. A cylindric prescription can be obtained in this manner with use of spheres alone, but generally cylinders are added, as well as the spheres, until the on and off reflex is observed on all axes.
All the aforementioned theory depends on the patient’s relaxed accommodation (i.e., the patient’s looking at some object 20 feet [6 m] away) and on parallel rays of light entering the eye and coming to a focus on the retina. The light source of the retinoscope, held about 18 inches (0.5 m) from the patient during retinoscopy, produces diverging, not parallel, rays of light from the retinoscope. Therefore a +2.00 diopter lens (in the refractor, known as the retinoscopy lens) is put in the trial frame so that the divergent rays from the retinoscope are in fact parallel when they enter the pupil. The power of this lens depends on the working distance of the refractionist; for example, if he or she works at 0.5 m (18 inches), this would be a +2.00 diopter lens.
Some refractionists prefer a working distance that requires a +1.50 diopter retinoscopy lens.
The final prescription, taken from the lenses in the trial frame or on the phoropter, is reduced by the working distance power to determine the distance prescription of the patient.
Streak retinoscope (see Fig. 10.7 )
The same principles that apply to the spot retinoscope also apply to the streak retinoscope. In the streak retinoscope the light source is a straight-line filament of the bulb. There is a condensing lens between the mirror and the bulb so that the filament itself may be focused onto the patient’s eye as a straight line. By means of a movable sleeve that envelops the whole retinoscope, the bulb may be rotated and moved up and down to adjust its focus. Because the light source is a streak of light rather than a cone, if the eye is not emmetropic, the area of the retina illuminated becomes a straight line rather than a spot. If astigmatism is present, it is very easy to determine its axis because the streak, playing externally across the patient’s face and trial frame with its axis graduation, will not be at the same angle as the streak seen on the retina. The retinoscope streak is rotated until it parallels the streak on the retina and the axis is thereby established (see Ch. 12 and Fig. 12.3 ).
From this point, the procedure is basically the same as with a spot retinoscope and lenses are added until the reflexes on both axes exhibit no with or against motion when the streak is passed across the pupil.
Accessories used in refraction
The cross cylinder consists of a plus and a minus cylinder set at right angles to each other, with a handle set midway between the two axes ( Fig. 10.8 ). The axis of the plus cylinder is marked in white and the axis of the minus cylinder marked in red. Cross cylinders are available in dioptric strengths of 0.12, 0.25, 0.50, and 1.00.
The cross cylinder is a refining instrument that determines the exact axis of the astigmatic error and the exact power of the cylinder. The method of use is described in Chapter 12 .
The pinhole disc is a small disc with a small central opening that eliminates peripheral rays of light, permitting only the central rays to pass through. The pinhole disc permits the examiner to differentiate poor vision caused by refractive errors from poor vision resulting from disease of the eye. In general, vision that can be improved with a pinhole disc usually can be improved by spectacle lenses.
A multiple pinhole disc serves the same purpose as the pinhole disc, but it is an easier device to use because the patient does not have to search for a solitary tiny central hole ( Fig. 10.9 ). The patient is asked to view a small line of print with one eye occluded and with the pinhole disc placed before the other eye. If looking through the pinhole disc improves the patient’s visual acuity above the uncorrected vision, the findings are recorded as vision with PH (pinhole). Corrective lenses often improve vision to the level obtained with the pinhole disc.
The distometer is a caliper used to measure the vertex distance ( Fig. 10.10 ). The vertex distance is the distance from the cornea of the patient’s eye to the back surface of the lens inserted in the trial frame, refractor, or glasses. The distometer consists of a scale in millimeters, an indicator, a movable arm, and a fixed arm.
To use the distometer, the examiner places the fixed arm of the caliper on the closed lid of the eye and the other arm against the back surface of the lens. The separation between the posterior surface of the lens and the eyelid is recorded on the millimeter scale. One millimeter is incorporated in the calibration of the distometer to allow for the thickness of the eyelid to arrive at the correct vertex distance. It is important to measure the vertex distance on all high-plus or high-minus lenses; the power of the lens in the trial frame or refractor may change when the lens is moved to a new location in the spectacle frames.
For example, if a +12.00 diopter lens in the trial frame is 10 mm from the cornea, but the correcting lens in the spectacle frame is 13 mm from the cornea, a +11.50 lens will be required at this position to give the patient the same visual acuity. The vertex measurement by the distometer permits the dispensing optician to calculate the effective power of a lens required in the final prescription when there is disparity between the distance of the position of the trial frame lens to the cornea and the final spectacles. To calculate the change in the power of a lens, one may refer to small disc or vertex conversion tables (see Appendix 13 ).
In such a case, if the optician has adjusted the prescription to compensate for this closer vertex distance fitting (an ideal for comfortable vision), the lensmeter reading of the patient’s glasses will not correspond with the prescription on the patient’s records, either in sphere or in cylinder. The only part of the prescription that will remain the same is the axis.
Halberg and Janelli clips
The Halberg or Janelli trial clip ( Fig. 10.11 ) eliminates the need for measuring the vertex distance. The clip is designed to accommodate two trial case rings (or, in the case of the Janelli, three case rings), a sphere, and a cylinder. The trial clips are placed on the patient’s glasses; by overrefracting, one can arrive at the prescription with proper effective power for that particular frame. This is extremely important in the high-minus or high-plus prescriptions.
The clips are also useful for small children wearing glasses, when it is not possible to use a conventional trial frame or refractor.
Of major importance in preliminary assessment of refractive errors is the autorefractor. Some have subjective components, some have vision assessment capability, and others have keratometer assessments. (See the section on autorefractors in Ch. 12 .)
Equipment used to detect muscle imbalance
The Maddox rod is a group of either red or colorless parallel glass rods that together act as a cylinder ( Fig. 10.12 ). The purpose of the Maddox rod is to dissociate the eyes and prevent them from fusing. The Maddox rod accomplishes this by changing the size, shape, and color of a point of light to a line, or streak, of light so that fusion is impossible. The relaxed fusion-free position of the eyes can then be measured easily.
The Maddox rod lens is useful in detecting: (1) the presence and amount of a heterophoria, which is the fusion-free position of the eyes, and (2) the presence and amount of a heterotropia, which is a manifest deviation of the eyes not held in check by fusion.
The grouped, red cylindric rods of the Maddox lens convert a white point source of light into a red line running perpendicular to the axes of the Maddox rod. To detect vertical heterophoria, the examiner holds the Maddox rod before one eye with the rods in a vertical position before the eye. The eye behind the Maddox lens perceives a point source of light as a horizontal red line. If the patient, with both eyes open, sees the red line passing directly through the white point source of light (which is viewed by the other eye), a vertical muscle imbalance is not present. With the Maddox rod lens before the right eye, if the patient sees the red line lower than the point source of light, then the patient has a right hypertropia or a right hyperphoria ( Fig. 10.13 ). If the red line is above the point source of light, a right hypotropia or a right hypophoria is present. By convention, vertical deviations of the eye are always designated in terms of the higher eye, so that in the latter example a right hypotropia would be designated as a left hypertropia ( Fig. 10.14 ). The Maddox rod test should be carried out both at near (16 inches [40 cm]) and distance (20 feet [6 m]).
If the Maddox rod is rotated so that the rods are placed horizontally before the eye, the patient will perceive the red line in the vertical direction. If the Maddox rod is held before the right eye of the patient and the line appears on the right side of the light, then esophoria or esotropia is present ( Fig. 10.15 ). If the line appears on the left side of the light, then exophoria or exotropia is said to exist ( Fig. 10.16 ).
The Maddox rod may also be used to detect torsion cyclophoria and cyclotropia . Torsion is the result of those ocular muscular anomalies that cause the eyes to rotate in a clockwise or counterclockwise fashion. To detect torsion, a red Maddox lens is placed before one eye and a white Maddox lens before the other eye, with the rods of both lenses held in the same direction. If the patient sees that the red line and white line are not parallel, then torsion is present, as well as cyclophoria or cyclotropia.
A prism is needed to measure a phoria or a tropia with a Maddox rod. For example, if the patient has a right hypertropia and reports seeing the red line below the point source of light, base-down prisms are placed before the Maddox rod in increasing strengths until the patient states that the red line runs through the light. The amount of prism required to center the red line on the small light is then a measure of the vertical muscle imbalance in prism diopters.
A prism is a triangular, or wedge-shaped, piece of plastic or glass that has the property of displacing a bundle of light toward the base of the prism ( Fig. 10.17 ). If the prism is placed before an eye, an object viewed in front of the prism will appear to be displaced toward its apex.
Prisms are used in measuring the presence and the amount of any tropias or phorias. The tests most commonly used to measure ocular muscle imbalance with prisms are the Krimsky test, the Maddox rod prism test (discussed previously), and the prism cover test.
In the Krimsky test , the observer notes the position of the corneal reflexes when a small light is shone into the eyes. The examiner notes where this reflex is centered in the fixating eye. Prisms are then placed before the deviating eye until the position of the reflex in the pupil of the deviating eye is located in the same position as that of the fixating eye. For example, if the patient’s right eye is turned in, the light reflex may be found overlying the temporal margin of the pupil and base-out prisms would be required to displace this reflex to a more central position.
The basis of the prism cover test is to displace the image of the deviating eye by the use of prisms so that it falls on the macula of that eye. Thus each eye projects to the same point in space, and covering one eye does not require any movement of the other eye to take up fixation. The amount of prism diopters required to achieve this endpoint is a measure of the deviation (see Fig. 10.17 ). The mechanics of this test are discussed in Chapter 29 .
Types of prisms available are the loose prism, horizontal and vertical prism bars, and Risley’s rotary prism.
The loose or individual prism is made of plastic or glass. These prisms are supplied in low powers in standard trial lens sets and in a full range of powers in individual prism boxes ( Fig. 10.18 ).
Horizontal and vertical prism bars ( Fig. 10.19 ) are fused prisms amalgamated into a single bar of gradually increasing strengths. These prisms may be set in a horizontal direction (base in or out) or in a vertical direction (base up or down). The prism bar is principally used to measure the amplitude or power of fusion. It is sometimes used in the cover–uncover test for measuring strabismus in children because it permits rapid examination in a patient whose patience and attention may be limited.