FUNDUS EXAMINATION TECHNIQUES

Dr. Sriram Gopal

Introduction

Fundus examination is an integral part of the routine ophthalmic examination and should not both eyes considered as an ancillary test. Since the time Charles Babbage invented the direct ophthalmoscope more than a century ago, this has indeed come a long way. With time newer modalities with greater ease and resolution have made it to the ophthalmologists’ armory. With examination of the fundus becoming increasingly important, sophisticated methods available today provide great accuracy and significant information. Retinal examinations may reveal the presence of vitreous and retinal diseases and even contribute to the detection of non-ocular diseases. Fundus exam techniques can be divided broadly into direct and indirect ophthalmoscopy.

Direct Ophthalmoscopy

When the fundus is viewed without any special lenses to neutralize the power of the eye it can be called direct ophthalmoscopy. In this case instead of neutralizing the power of the eye, it is used as a magnifying loupe and viewing is made possible. The virtues of direct ophthalmoscopy are as follows
1. Ease of use to the untrained hand
2. Portability, by the bedside &c
3. Good for screening

However it also has a few drawbacks such as

1. Poor visibility in case of media opacities
2. Very limited field of view
3. Lack of depth perception
4. Need to get too close to the subject

Indirect Ophthalmoscopy

An important era began in the late 1940’s, when Charles Schepens introduced the binocular indirect ophthalmoscope. It is, by far, the most valuable instrument available for evaluation of the detached retina and other pathologies of the retina and vitreous. The current high rate of reattachment is due not only to improvements in surgical technique but also to improved methods of ocular examination. Here high powered plus lenses are used to view the retina. The image formed is inverted, real and magnified. Since the optics of the system virtually creates a light source inside the eye media opacities are of a lesser problem.

The magnification of the image produced by the indirect lens used can be easily derived from the formula

Magnification = power of eye (+60D)/ power of lens
For example if using a 20D lens in an emmetropic eye it would be 60/20=3

Hence the higher the lens power smaller the image and vice versa. The lower the power the condensing lens the farther from the eye it must be held. The stronger the power the condensing lens the closer it must be held toward the eye. The relationship is very simple, for it is based mainly on the focal length of the condensing lens. Ideally one wants the light from the condensing lens to focus perfectly within the pupil of the patient's eye. This placement will result in the condensing lens forming an image of the retina, in front of the condensing lens, which fills the whole lens. The retinal image seen by the examiner will appear larger if the examiner moves closer to the patient. However, the condensing lens must be moved to maintain the conjugation of the pupils. The patient's refractive error really has very little effect on the size of image seen.
Most indirect ophthalmoscopes have convex lenses built into the eyepieces ranging from +1.75 to
+2.50D's depending on the manufacture. These lenses enable the examiner to relax their accommodation and view the aerial image of the retina.

The advantages of IO are many but the prime would be
1. Wide field of view including the fundus periphery
2. Binocular viewing hence stereopsis
3. Media opacities are less of an issue
4. Dynamic viewing combined with scleral depression is a boon

Hruby Lens

The non-contact Hruby lens is a high powered plano concave minus 55 diopter lens (-55D) which is available for most slit lamp biomicroscopes as an attachment from either above or below that can be rotated into the line of sight. The retinal focus is achieved by effectively neutralizing the optical power of the eye and extending the focus of the slit lamp biomicroscope back to the retina.
The view is erect, but is subject to reflections and a restricted field of 5-8 degrees or just larger than one disc diameter. This, plus the need for precise patient fixations and cooperation makes the Hruby lens a difficult procedure for screening the posterior pole.

Slit lamp Biomicroscopy

For all practical purposes this can both eyes imagined as an indirect ophthalmoscopy with the illumination and magnification of a slit lamp. Thus the magnification achieved is more than that of indirect ophthalmoscopy and stereopsis is much improved. The magnification on slit lamp
biomicroscopy can be derived from the same formula as for indirect ophthalmoscopy but the magnification of the slit lamp will have to be added. Thus for a +78D it will be as follows
60/78 = 0.76 X 10 (S/L magnification) = 7.6

These lenses have an advantage over the non-contact Hruby lens allowing a better view around cataracts. With these indirect fundus lenses magnification increases as power of the lens decreases, similar to that found with condensing lenses utilized for binocular indirect ophthalmoscopy. The +60 D lens gives greater magnification than a +90D lens which would provide a larger field of view. However the slit lamp biomicroscope permits variable magnification which neutralizes this magnification problem.

The +78 D lens obviously falls in between the +60 D and +90 D lenses in terms of magnification and field view. It is slightly smaller in overall size than the +60 D and noticeably larger than the
+90 D lens. The +78 D lens is usually preferred by the novice who feels it is easier to hold and manipulate.

The indirect slit lamp biomicroscopy is not intended to take the place of the binocular indirect ophthalmoscopy, but allows you to view an area stereoscopically and with higher magnification than with the binocular indirect ophthalmoscope.

Recommended method

1. Low-to-moderate intensity of light is recommended, and the beam should be adjusted to a width between 3mm and 4 mm. The light beam will be perpendicular and with an inclination between the grades of 0 – 10.
2. Hold the lens at a distance of approximately 8 mm from the patient’s eye, almost touching the eyelashes. Once the retina is in focus, then the light beam intensity can be increased, or slit-lamp filters can be changed for a better contrast observation.
3. If the physician is interested in a specific area of the retina, the patient can be asked to follow the fixation target. The hand can be moved to look for the desired point.
4. The less lens potency in diopters, the greater the distance between the lens and the patient’s eye needed to observe the retina. For a comfortable distance and a good observation of the retina most times the +78 D lens is recommended.
5. The vitreous of the eye can also be examined with these lenses. Remember, the vitreous is anatomically located in front of the retina, therefore, you have to pull the slit lamp farther back towards you to view the vitreous body.