In all the realm of modern medicine there has probably been no one discovery of greater beneficence to humanity than the invention of the ophthalmoscope by Helmholtz in 1851. Through its use the mysteries of the interior of the eye stand revealed and many conditions that previously resulted in blindness are now made remediable. With it we are able to study changes in the circulatory system as exhibited in the retinal vessels; and in the optic nerve and retina we have, under the eye of the surgeon, direct communication with the brain and spinal system. The ophthalmoscope, therefore, has become of the greatest value in general medicine as an aid to diagnosis for in the fundus of the eye are found many characteristic changes of disease of the various organs. Helmholtz`s discovery was not a matter of chance but resulted from a careful study of the laws of optics, one of which is that light follows the same lines in returning through a lens (in case it can return) as when entering.
The rays of light returning from the eye must go direct to the luminous source from which they emanated, and in order to fall upon the retina of the observer his eye must be in the path formed by the source of the illumination and the eye under examination. The device used by Helmholtz consisted of a transparent mirror formed of three slips of plane glass. The present principle of a perforated metallic mirror was first proposed by Ruete, in 1852. In examining the interior of an eye, light is thrown into eye by the mirror, and in order to see the fundus we must receive in our own eye the light reflected from the fundus and unite its rays to form a sharp image. The mirrors used may be either plane or concave. The concave mirror by converging the rays from the source of light gives a stronger illumination and is therefore generally used. The modern ophthalmoscope, of which Loring’s is one of the best consists then of a concave mirror, silvered on the back, for illuminating the eye and a series of lenses for measuring the refraction, and for diagnosing pathological changes by the direct method.
The art of using the ophthalmoscope is one much more difficult to acquire than that of any other instrument of precision and is only accomplished after long and persistent practice. Every physician realizes the months or years of practice required to detect with the stethoscope the finer shades of sounds due to varying diseases of the heart and lungs. In one case the ear and in the other the eye has to be trained by long experience before the examiner can become expert. The beginner is apt to think that after he has acquired a few details of the nerve and vessels that he can see all that is to be seen. At this stage we have often told our students that they have as yet not crossed the threshold of that vast storehouse of beautiful pictures formed by diseases within the eye. Even after years of daily use this little instrument reveals significant and often important variations of pathological states not heretofore seen, the meaning of which the observer is often at a loss to understand.
The first and most essential point in order to become a skilled ophthalmoscopist, and which is often neglected, is familiarity with the healthy fundus. The student should first practice over and over again upon every healthy eye-ground he can before attempting to study diseased states. This necessity becomes apparent from the fact that the normal fundus in health varies with the age, condition and complexion. What a large range of physiological pigmentation may be found from the negro to the albino. The skilled use of the ophthalmoscope is in the determination of the very slightest changes from normal, as the detection of gross pathological conditions does not present the importance that does the recognition of the incipient stages of disease.
In making an ophthalmoscopic examination artificial light is generally used and is preferable to daylight. We therefore darken the room and use a single-light, the best being that from an Argand burner or a student`s lamp. The eye is first illuminated from a distance of about eighteen inches, and as the light plays over the cornea we note any opacities that may be present in the cornea or lens. Occasionally when there is a marked error of the refraction the retinal blood-vessels will be seen. If the eye is highly far-sighted the vessels will move in the same direction as the head of the observer, while if it is a very near-sighted eye the vessels will move in an opposite direction.
There are two methods of examining the fundus of the eye: First, the direct method, so called because the eye-ground is studied by rays coming directly from it, and by this method we have an upright image; and second, the indirect, because the rays are received from an aerial image, or indirectly from the observed eye, and the image seen is inverted. The latter method will first be considered because it is more frequently employed and because it is the more natural order after the preliminary examination of the cornea and lens.
The indirect method, or the method of examination by the inverted image, is made as shown in Figure 6. The patient is seated in a darkened room with the light from an Argand burner about eighteen inches behind, on the same side, and level with the eye to be examined. He should be instructed to fixate the unused eye upon some distant object. The observer sits about eighteen inches in front of the patient and holds the ophthalmoscope in the hand corresponding to the eye to be examined. A convex lens, about thirteen to eighteen diopters, is held between the thumb and forefinger of the unused hand, before the eye of the patient. By resting the middle, third and little fingers upon the outer part of the supra-orbital ridge of the patient`s eye the lens is held steadily and focused upon any part of the fundus desired, and the middle finger may also be used if necessary to raise the upper lid for a better view. In all ophthalmoscopic work the student should learn to keep both eyes open, as the effort to close one eye tires the eye and prevents the complete relaxing of the accommodation.
He should also accustom himself to using the right eye and holding the ophthalmoscope in the right hand when examining the right eye of the patient, and the left eye and hand when examining the left eye. The first objective point is the optic nerve head, and this is brought into view by having the patient look at the right ear of the observer, and vice-versa, when examining the left eye, the patient should be told to look at the left ear of the surgeon. From this point he may be told to look directly at the centre of the observer`s forehead, which gives a view of the macula lutea, and then, up and down, to the right and left, in order to examine all parts of the fundus. If the image of the disc when first brought into view appears dim and ill-defined, the lens and the ophthalmoscope should be moved slightly forward or backward until the image is as clear and distinct as possible.
The student must always remember that by the indirect method he sees the aerial picture of the fundus and that it is inverted and reversed. The image by the indirect method is magnified about four or five times, while by the direct method we get a picture magnified about fourteen times. The extent of the field of vision on the contrary is about four times greater in the indirect than it is by the direct method. The intensity of the illumination is also greater with the indirect than with the direct, hence a view of the fundus can often be had by the indirect method when, owing to haziness of the refracting media, it is no longer visible by the direct. The indirect method gives then a larger view and better general relation of the fundus, while the direct method is particularly adapted for the recognition of the finer details.
The direct method, or the examination with the erect image, is shown in Figure 7. The patient and light are placed in the same positions as the in the indirect examination. The surgeon seats himself by the side of the patient and again uses his right eye in examining the right eye of the patient, and vice versa. The ophthalmoscope is held in the same hand as the eye to be examined and brought up to about one inch from the eye of the patient. Both eyes are to be kept open so as to as avoid as much as possible the impulse to accommodate. As the field is enlarged, and the examination by this method greatly facilitated by a dilatation of the patient`s pupil, the use of a mydriatic is to be recommended to the student when first learning to use the direct method. The dilatation of the pupil can be increased also by having the room as dark as possible, by closing the other eye, and lowering the light from which the illumination is received. If a still larger pupil be required for an examination of the fundus a 4 per cent. solution of cocaine should be used, as it will give the necessary dilatation in from twenty to thirty minutes and its effect passes away in a few hours.
Professor of Ophthalmology in the College of the New York Ophthalmic Hospital; Surgeon to the New York Ophthalmic Hospital. Visiting Oculist to the Laura Franklin Free Hospital for Children; Ex-President American Homoeopathic Ophthalmological, Otological and Laryngological Society. First Vice-President American Institute of Homoeopathy : President Homoeopathic Medical Society of the State of New York ; Editor Homoeopathic Eye. Ear and Throat Journal : Associate Editor. Department of Ophthalmology, North American Journal of Homoeopathy, etc.