The ophthalmoscope also makes it possible to examine the fundus of the eye, which is generally done in two ways:
Indirect method; reverse image examination
- When the fundus eye illuminated by a strongly myopic eye is observed at a distance large enough for the punctum remotum to fall between the subject’s eye and the observer, the details of the background can be seen, provided that the observer can accommodate the image formed by the punctum remotum.
- The principle of the indirect method of ophthalmoscopy is precisely to make the eye observed highly shortsighted, placing in front of it a convex lens of sufficient force that a real image of the background is formed between the lens and the observer, in position to be observed.
- This image of the retina is enlarged about five times by the condensing lens. Of 13 diopters.
- The concave mirror of the ophthalmoscope and a convex lens of 13 or 15 diopters are used.
- The subject sits in the darkroom on a seat a little lower than the observers
- The light source is located next to him, a little behind, at eye level.
- The observer, placed in front of him at 50 cm, recommends to the patient to look away, in the direction of the flag of the right ear of the observer if one examines the right eye, in the direction of the left ear if one examines the left eye.
- Using the concave mirror, held in front of our eye, slightly inclined to receive the light of the light source, the light is projected on the observing eye and we see, through the hole of the mirror, the pupillary glow.
- The lens, grasped between the thumb and forefinger of the left hand, is worn in front of the eye to observe, in the frontal plane, 6 or 7 cm in front of it (about its focal distance).
- The other fingers take a point of support on the forehead.
- In this position of the eye (confirmed by the fact that the image of the lamp, reflected by the mirror, falls on the outer edge of the pupil) the papilla appears.
- Just bring the lens a little closer or farther to focus the image.
- It is a reversed image that must be imagined located a short distance ahead of the lens, at its foyer (about 8 cm) between it and the observer.
- The beginner is often embarrassed by the corneal reflection which he will gradually learn to disregard.
- He is disoriented above all by the reversed image, to which he must become accustomed after long trial and error.
- It must be imagined that the image of the fundus moves in the same direction as the lens, in the direction opposite to that which takes its head.
- If he sees only the upper half of the disc, he is tempted to lower the lens to see the lower half; the opposite is the case.
Direct method; exam in the right image
- The fundus of the observed, illuminated by the mirror, if emmetropic, returns parallel rays.
- If the observer places himself on the path of these reflected rays (looking through the hole of the mirror), these rays come to form their focus on his retina (if he is emmetropic) and give a clear image.
- It suffices to illuminate the fundus of the observed with a simple concave mirror, without the need of interposing a convex lens as one is obliged to do for the examination with the reversed image; the exam is “live.”
- The image is formed clear that if the eye of the observed and the observer’s eye are two emmetropes.
- If either or both are ametropic, the image will not be clear.
- In the same way, the image can be disturbed by the fact that the observer, in the exam in the right image which is close to his eye, always accommodates a little and behaves like myopic, so he almost always, in practice, interpose behind the hole of the mirror a curved glass.
- At present, refractive ophthalmoscopes are so-called electrical ophthalmoscopes; the light source is enclosed in the very handle of the device.
- We will take as an example, because of more representative, the examination with the ordinary refractive ophthalmoscope of Morton.
- The examination is performed in a dark room, but it does not require darkness as complete as the examination with the reversed image.
- The light source is placed to the right of the subject when one examines the right eye, on the left when one examines the left eye, a little further away from it than for the reversed image (to illuminate the mirror inclined at 45 ° ).
- The observer stands in front of the patient, holding the ophthalmoscope in the frontal plane and looking through the orifice.
- It is placed close to the observing eye, a few centimeters away, examining the right eye of the observer in the right eye, examining the left eye of the observer in the left eye.
- In this close position, illumination of the pupillary field is only made possible by the inclination of the 45 ° mirror, whose surface is “turned” towards the light source.
- The pupillary field being illuminated, it is recommended to the patient to look at the lion in front of him, a little inward, in the direction of the observer ‘s ear.
- We can see the image of the papilla.
- To make it clear, we act on the index finger on the lens- carrying disc and we thus bring to the mirror, diopter, the glass that ensures the best focus.
- The observer can not, so long as he is himself emmetropic (or made emmetropic by the corrective lenses), and has relaxed his accommodation, determined the refraction of the observed eye, it is sufficient for him to note which disc glass had to be brought in to ensure focus.
- He can also, by observing the deformation of the disc and appreciating that the vessels are not as clear in a meridian as in the meridian which is perpendicular to him, to get an idea of the astigmatism of the eye.
- To see the macular region, it is recommended that the patient look straight ahead.
- To observe the periphery, one is asked to look in the direction corresponding to the field of the retina which one wishes to examine.
- The technique is infinitely more straightforward and more flexible when using an electric ophthalmoscope.
Comparison of the reversed image and the right image
The reversed image
- It offers a wider field of observation; it allows to have an overview of the field of the retina.
- It allows examining the peripheral parts of the retina better.
- The refractive errors of the subject do not hinder it.
- The reversed image is magnified only about five times.
- It does not allow to observe the fine details of the vessels.
- On the other hand, it requires laborious learning that discourages beginners.
The right image
- The right image is an enlarged image about 14 times.
- It allows the very minute exploration of the details of the fundus.
- It gives an enlarged right image, which can be quickly developed, from which we can appreciate and measure the relief.
- It is within reach of the beginner and any doctor.
- It makes it easy to identify the iconography of the fundus.
- It allows us to examine at the same time a limited field of the fundus.
- It makes it difficult to examine the peripheral parts of the retina.
Choosing the method to use
- The ease of the exam in the right image especially with the help of the electric ophthalmoscopes whose maneuverability allows the examination to the bed of the patient is precisely what makes the defect.
- Examination with the reversed image is gradually abandoned.
- The young ophthalmologists do not train to practice; they deprive themselves of an indispensable mode of examination.
- One should always start with the reverse image examination and specify the signs observed if necessary by the examination in the right image.
- It is in the examination of the detachment of the retina that the reversed image reveals its real advantages: first, to settle for the examination with the reversed image leads to not forgetting to use the plane mirror, to examine the pupillary glow.
- Above all, only the examination with the reversed image makes it possible not to disregard a disinsertion of the retina in the vicinity of the ora serrata.
- It also allows for easier tracking during the procedure.
- It is thus in the case of strong myopia that the examination with the reversed image makes it possibly better to observe the fundus.
- The ophthalmologist’s beginners should be warned against the tendency to choose the “easy way out.”