Article Date: 11/1/2003

fundus lens fundamentals
FUNDUS Lenses
How new technologies, strategies and lenses will impact your practice.
BY ANDREW S. GURWOOD, O.D., F.A.A.O.; C.A. RINEHART, O.D., M.S.; AND MARI ALZI, O.D., Philadelphia, Pa.

T he dilated fundus examination has earned its place in the annals of the standards of eye care. While many practitioners have become accustomed to some form of indirect ophthalmoscopy and biomicroscopy, I'll explain how new technologies, strategies and lenses have contributed to the evolution of the optical instruments we use to perform dilated fundus exams.

ILLUSTRATION BY SHARON AND JOEL HARRIS / CLINICAL PHOTOGRAPHY BY MARC BLOUIN - CHUM, UNIVERSITY OF MONTREAL, CANADA

Understanding the systems

For many years, eyecare practitioners have used external lens systems to facilitate the observation of fundus structures. These lenses may have plus power, they may have a net power of zero when in contact with the eye (fundus contact lenses) or even minus power, as seen with the Hruby lens.

Any fundus (Hruby, gonio, 90D, 78D or 20D) lens, when used with the refracting elements of the patient's eye, can produce an image of the fundus, the image isn't useful because it's typically small and dark. To create a clinically useful image of the patient's fundus, we need two additional systems: a viewing system to provide magnification, and an illumination system to brighten the image. When these additional systems are used in conjunction with the fundus lens/cornea system we can achieve excellent views of the patient's fundi. However, to understand the mechanics of the marriage of these three systems, we must understand them individually.

Illumination system. This is the most simplistic of the three systems. The light source is housed in the headset of the binocular indirect ophthalmoscope (BIO). The fundus lens acts to condense (hence the name "condensing lens") this light and focus most of the light through the patient's dilated pupil. The slit lamp's illumination system is housed on a rotating arm. The system traditionally is used "in click" or aligned confocally with the viewing system for optimal viewing of the fundus.

The fundus lens/cornea system. This is a combination of the patient's eye and the external fundus lens. For example, consider the most simplistic condition where the patient's eye is an emmetropic refracting surface put in combination with a high plus external viewing lens. In this case, light from an object on the retinal surface leaves the cornea parallel.

We can express the magnification for this part of the system as urDe where "ur" is the typical reference distance used for near objects by the observer, and "De" is the dioptric power of the eye. For many magnification problems we use 40 cm as ur.

Parallel rays from the patient's eye enter the condensing or fundus lens. The magnification for this second element is 1/urDfl (where "Dfl" is the dioptric power of the plus fundus lens). The magnification for the initial combination of eye and condensing lens is the product of the individual magnifications. The final image is inverted with respect to the initial retinal object yielding the classic equation for this initial system m1=-De/Dfl.

Viewing system. This is the optical system designed to view the image produced by the fundus lens/cornea system. With indirect ophthalmoscopy the viewing system involves a headset that typically houses +2.50 lenses. This headset is designed for viewing the real, aerial image of the fundus (produced by the patient's eye and condensing lens) at a distance of 40 cm from the headset. (This combination of working distance and headset lens power eliminates the need for doctor accommodation and facilitates conjugacy of pupils).

The magnification for the headset viewing system is m2= -(-0.4)(2.50)=1. Therefore, for binocular indirect ophthalmo-scopy (BIO), the magnification for the complete system of patient's eye, fundus lens and viewing systems is equal to mt=m1m2=-De/Dfl. This relationship demonstrates the clinical observation that as the condensing lens power increases the magnification decreases. The higher plus fundus lenses also yield a larger static as well as dynamic field of view.

When using a high plus fundus lens with a biomicroscope, the initial magnification of the patient's eye and fundus lens remains -De/Dfl. There is, however, a significant change in the viewing system. The biomicroscope is exactly as named, a compound microscope. The magnification of a compound microscope is a function of the optical tube length and the power of the oculars and objectives. A prism system is also built into the biomicroscope to re-invert the image. (This inversion ensures that your view of the anterior structures of the eye is erect. The complete magnification for slit lamp fundoscopy can be expressed as: mtotal= -(De/Dfl)(mbiomicroscope).

Again, the higher the power lens used (with the magnification systems of the biomicroscope being constant), the smaller the magnification and larger the viewable visual field. Practically speaking, you can see more fundus at less magnification with a 90D lens than with a 60D lens.

When the observer uses a fundus contact lens instead of a condensing lens for the fundus lens/cornea system, then the image of the fundus becomes virtual and erect with magnification approaching +1. Therefore the slit lamp provides the bulk of the magnification alone. When viewed through the central portion of the contact fundus lens, the final image is virtual and erect with respect to the original fundus object.

Noncontact fundus lenses of any kind (used as the fundus lens/cornea system) induce prismatic power so you can use it to your advantage to scan the off-axis fundus details. The amount of prismatic power induced is governed by Prentice's Law, which states that deviation in prism diopters equals decentration in centimeters from the optical center of the lens (the dioptric power of the lens).

If you were to simplify all lenses to their prismatic components, then you could represent plus lenses as two prisms placed base to base. You can present minus lenses as two prisms placed apex to apex. Light rays entering a prism deviate toward the base of a prism causing the image to deviate in the direction of the apex.

Plus lenses, such as the 90D lens, produce an against-motion effect, while minus lenses produce a with-motion effect. For example, if you used a 90D lens to view the superior periphery of a retina, then you should direct the patient to look up while you direct the slit lamp beam to a location below the optical center of the lens.

The more peripheral the view desired, the greater the decentration necessary (inferior decentration, in this example). Under standing these relationships will help you move about in the fundus while examining the eye and to locate lesions of interest.

A lesson in MIOs and BIOs

You can perform monocular indirect ophthalmoscopy (MIO) with a prefabricated Welsh Allyn Panoptic ophthalmoscope or by constructing one using a handheld direct ophthalmoscope in conjunction with either a high plus trial case sphere or standard binocular indirect viewing lens. Other MIOs do exist, such as the one formally built and marketed by American Optical (AO)/Reichert, but they're out of production now.

The Panoptic MIO uses a plus-powered ocular in tandem with a plus-powered objective (the condensing lens is incorporated into the head), to provide you with a real, erect image at approximately 19X. The image appears erect because of an inverting lens that's built into the system to provide for image correction.

The advantage of this system is that, when used properly, it will provide a larger field of view than direct ophthalmoscopy. We can achieve this increased field of view because this system allows the doctor to observe the image of the fundus from the entrance pupil of the patient's eye. (The entrance pupil is the image of the patient's pupil as viewed through the patient's cornea. This opening restricts the doctor's view of the internal structures, hence the closer we get to the pupil, the larger the field of view we're able to achieve.)

The advantage of using the Panoptic is that it contains an easy-to-use focusing mechanism -- an eye cup, which helps get the instrument into the ideal viewing position while allowing for one-handed operation. You can assemble a crude version of an MIO by combining a direct ophthalmoscope with the correct working distance lens dialed into the head (typically +2D or ±3D), while holding a high plus powered trial case sphere (+18D, +20D, etc.) or BIO condensing lens in front of the patient's eye (always silver side/white side toward the patient, as these lenses aren't double aspheric). This allows you a wider view of the posterior pole, a greater perspective to the juxtaposition of findings and far better views of the retinal periphery than direct ophthalmoscopy.

The Panoptic works optimally with a dilated pupil but is can still provide adequate fundus images with normal size undilated pupils.

An in-depth lesson in BIOs

The traditional BIO combines an illumination source with a headset or a spectacle-mounted viewing system designed for use in combination with a handheld condensing lens. The system makes use of plus-powered oculars to provide accommodation-free viewing of the real, inverted image. The system provides magnification that is -3X in magnitude.

Many practitioners consider BIO with scleral depression the gold standard for detecting and monitoring peripheral retinal pathology because of its ability to expand the boundaries of ophthalmoscopic examination, generating images that you can augment with scleral depression to view peripheral retina as far as the ora serata.

The advantages of higher-powered condensing lenses are that they increase the field of view. The disadvantages include an inverse relationship of field to magnification -- that is the higher the power of the condensing lens, the greater the field of view but the smaller the image magnification. You can purchase the condensing lenses in clear or tinted yellow (for both patient comfort [given the brightness] and safety during extended viewing).

While some have unofficially touted certain fundus camera systems and noncontact indirect biomicroscopic techniques as suitable substitutes for BIO, the literature has remained steadfast in reporting that for detecting peripheral pathology.

Biomicroscopic Systems

Historically, practitioners have used the biomicroscope to view the fundus of the eye in conjunction with a high minus lens (-55D to -58D) known as the Hruby lens. This indirect system provides a virtual, erect image of the fundus of approximately +1X. The total magnification is therefore the product of the Hruby lens (the magnification of the biomicroscope).

When using a high-plus lens for fundus evaluation, the condensing lens forms a real image of the doctor's pupils within the patient's pupil (conjugacy of pupils). This provides optimal field of view. When using a high minus lens (Hruby), the doctor's pupils are not imaged at the plane of the patient's entrance pupil. This causes a much reduced field of view. Even though Prentice's rule of prismatic displacement applies equally in plus and minus, the significant reduction in the field of view with the Hruby only allows fundusopic evaluation of the posterior pole. This same principle applies to the images formed by the fundus contact lenses (three mirror gonio lenses).

In the past, the Hruby lens was included in the purchase price of many slit lamps. The older model Zeiss lamps had a unit that was positioned underneath the mounted Goldmann tonometer and when rotated down and released from its resting position, the lens moved along a sliding track forward until it contacted a focusing bar mounted on the inside (examiner) surface of the patient forehead rest. The mechanism kept the unit from contacting the eye. It positioned the lens a constant distance from the cornea, allowing you to maintain focus easily.

Haag-Streit's and Topcon's lamps placed the lens on top of a metal rod. These Hruby lenses were fitted with a focusing handle to allow you to manipulate fine focus. The bottom of the mounting rod had a tongue that would slip into a groove of a footplate. This would hold the lens in position, but allow the slit lamp to move forward and backward while maintaining a constant distance from the cornea. As you moved the slit lamp in and out, the rod would slide inside the grooved track, thus maintaining the preset focus.

The Hruby lens was convenient and easy to use. It took little dexterity to get started and was always available because it was attached to the viewing instrument. It can still be useful when used for stereoscopic viewing of the optic cup and macula. It remains useful for viewing the vitreous. When used with red-free illumination in the hands of a skilled observer, the Hruby lens can demonstrate the optically empty space between the posterior vitreous face and the retina in the case of a posterior vitreous detachment or the absence of nerve fiber layer seen in glaucomatous defects. It's also adept at viewing the vitreous humor itself.

The +90D, +78D, +60D lenses. The high plus handheld fundus lenses were the first in a line of successors to the Hruby lens. This grouping of double-aspheric lenses (both sides having the same sphericity so that it doesn't matter which side of the lens faces the patient) are used in combination with the biomicroscope to produce a system that provides the examiner with a real, inverted, stereoscopic image of the fundus. The aerial image produced by these lenses isn't degraded by media opacities nearly as much as the virtual image produced by the Hruby lens.

Many new high-plus fundus lenses are available for retinal evaluation, each with its own attributes. These lenses may be clear, have a yellow tint or clear with a yellow filter designed to clip on the rim. Just as with BIO, the higher the dioptric power of the viewing lens, the less the magnification and the greater the field of view. The major advantage between this technique and BIO is that here you can adjust the microscope when necessary for detailed viewing to compensate for the reduced magnification of the higher power lenses.

When these lenses were first introduced, the prototypes were relatively low power lenses, on the order of +60D. The real, aerial image of the fundus produced by the lens was located in an area such that many sit lamps could not focus on the image. The original solution was for the manufacturer to cut out a notch in the lens's metallic rim so that clinicians could position the lens closer to the patient's eye.

The final solution has evolved into the development of higher power lenses (+78D, +90D) with reduced working distances. For a 90D lens, using a slit lamp with 10X magnification, the overall magnification would be 6.6X (see section on magnification calculations in viewing systems parts one and two). This compares to the 3X magnification of BIO.

By having patients change their gaze, in combination with larger field of view captured by the lens's optics and techniques of lens/slit beam decentration (Prentice's rule/prismatic deviation), significant peripheral views are possible. You may also evaluate the vitreous using handheld lenses. You must pull the slit lamp back toward you more than when viewing the fundus when you want to focus on the vitreous. You can use these lenses under undilated conditions for varied, limited posterior pole viewing.

The introduction of other noncontact fundus lenses such as the Volk Super field NC and the Volk Super Pupil XL have made peripheral fundus observation/examination even easier. You won't experience any disadvantages in using these lenses and in adding them to your armamentarium of fundus exam tools. However, don't limit your thinking to make one tool fit all purposes.

The Super Pupil XL, touted for its wide field of view, has applications for examination of both the posterior pole and peripheral retina. Even through an undilated pupil, this lens provides a 124º or 25 disc diameter field of view. Similarly, Volk's Super Vitreo Fundus lens is another "small pupil lens" that provides a 124º field of view. Although the aforementioned lenses share field of view as a commonality, the Super Vitreo fundus provides a slightly larger magnification factor (0.57X versus 0.45X).

The Super 66 Stereo fundus lens (Volk) is touted to provide an enhanced stereoscopic view of the posterior pole. This may allow observers to have special discrimination of subtle changes of optic nerve detail, serous detachments and macular edema. It provides a 96º field of view and a 1X magnification factor.

Procedurally, the easiest way to obtain an image is to focus a slit beam onto the cornea to bisect the pupil. After placing the lens 2 mm to 3 mm in front of the patient's eye, with the beam now bisecting the lens and pupil, pull the slit lamp straight back. The image will fall into focus with little effort. Moving the patient's gaze, the slit lamp or lens or all, as aforementioned, will permit navigation.

Fundus contact lenses

One of the traditional fundus contact lenses is the Goldmann 3 Mirror lens, which uses a circular plastic housing to mount three mirrors:

1. a thumbnail, or semi-lunar mirror, mounted at 50º for ora and pars plan viewing

2. a rectangular mirror mounted at a 64º angle for midperipheral viewing

3. a trapezoidal-shaped mirror mounted at a 72º angle for equatorial viewing.

The center concavity impacts the cornea to create a negligible minus system, which you can use with the scope to examine the posterior pole with a magnification that you can vary with lenses inside the biomicroscope. The virtual images you see in the mirrors are erect and perpendicular (180º) to the mirror's surface.

If a peripheral fundus pathology requires additional magnified study, then the Goldmann lens is ideal. The obtrusive instrument itself acts as a lid speculum to prevent blinking and you can rotate and tilt it for custom navigation inside the eye. Its disadvantage, however, is that it requires skill and practice for expert use and despite topical anesthesia, is often considered inconvenient, if not frankly uncomfortable by patients.

New coupling solutions such as Refresh Tears (Allergan), Celluvisc (Allergan) and GenTeal (Novartis), that lubricate the fundus contact lens as it impacts the cornea have all but eliminated the need for lavage following the procedure, however, old coupling gels such as Gonioscopic (Alcon) and Gonak (Akorn) require rinsing.

Procedurally, most students learn that they should place the beam source perpendicular to the mirror and that they should accomplish the lens-instrument rotation outside of the slit lamp.

Three-mirror technique

These procedures aren't necessarily set in stone. In fact, most experienced clinicians, once they get the lens on the eye, use a technique where they support the lens with their thumb, first and middle fingers, such that they can rotate it while viewing it without coming out of the biomicroscope.

Using this method, the examiner, rarely, if ever, needs to alter the vertical orientation of the beam no matter what mirror is in use or where it is located. Alternatively, the use of a circular aperture can eliminate the need for considering beam to mirror perpendicularity.

Wisdom to live by

Hopefully this articles has helped you understand the roles that the three systems play in creating a clinically useful image of a patient's fundus. Use the information on technologies, strategies and lenses to impact your patients and the bottom line of your practice.

References are available on request.

Drs. Gurwood and Rinehart are associate professors of clinical sciences in Module 1 of the Eye Institute of the Pennsylvania College of Optometry (PCO). Dr. Alzi is completing her residency in primary care optometry in Module 2 of the Eye Institute of PCO.

 



Optometric Management, Issue: November 2003