LUMINOUS RETINA

Luminous Retina

As we have seen in the previous material, we will illuminate the fundus with the retinoscope and observe rays coming from the retina, as if it were luminous. When light leaves the retina, the optical system of the eye applies vergence to the rays. If we illuminate the retina with parallel rays (plane mirror), the reflected rays leave the eye according to the refractive error.* That is: In emmetropia, rays leave parallel. In hyperopia, rays leave diverging. In myopia, rays leave converging. This backward approach may seem confusing at first, but simplifies understanding of the retinoscopic reflex. Things happen differently when we illuminate the retina with rays that are not parallel, but we will ignore these for now. In visualizing the situation, we will use a graphic presentation to illustrate luminous retina optics in three basic situations: emmetropia (plano), hyperopia (+1 D), and myopia (–1 D). Since the rays entering the eye remain parallel in all cases, we will ignore them entirely and simply look at what emerges. This is a very graphic presentation, so study and compare the diagrams carefully. Figure 4-1 offers one more way of looking at the FP optics . We see the emmetropic FP at infinity, the hyperope’s FP beyond infinity, and the myope’s FP at less than infinity. Figure 4-1. Rays from the illuminated retina. Correction refers to the lens desired to correct the refractive error. Now, picture yourself sitting before each of the eyes in Figure 4-2. Looking through the peephole in your retinoscope, you see these emerging rays as a red reflex in the patient’s pupil. If you sweep the streak across the eye, the reflex you see will also move. If the emerging rays have not converged to a point (the FP), the retinal reflex will move in the same direction as you move the streak; this is called the with motion reflex (WITH). If the rays have come to the FP and diverged, the reflex will move opposite to your movements; this is the against motion reflex (AGAINST) (see Figure 4-2).† Figure 4-2. Retinal reflex movement. Note movement of the streak from face and from retina in WITH versus AGAINST motion. Now picture yourself sitting almost at infinity looking through your retinoscope. This is what you would see in each of the three cases (Figure 4-3). In the emmetrope and hyperope, the emerging rays have not converged to the FP, so you see WITH motion. In the –1 myope, the rays have come to a focus at the FP (1 meter) and have diverged; thus, you would see AGAINST motion. Figure 4-3. Retinoscopy at infinity. Note that within the FP, you see WITH motion; beyond the FP, you see AGAINST motion. Consider this situation another way: if you see AGAINST, you are beyond the FP; if you see WITH, the FP is beyond you! So much for what you would see if you sat at infinity. Optical infinity is anywhere beyond 6 meters (20 feet), but you cannot reasonably sit that far away: the reflex would be too dull, and you cannot place correcting lenses before the eye. But if you sit at 1 meter, the reflex appears brighter, and you can (almost) reach the patient conveniently (Figure 4-4).  Retinoscopy at one meter. Note that the FP of the 1 D myope (–1) is at 1 meter. With your scope 1 meter from the patient, you would still see WITH, but in the case of the emmetrope and hyperope, their FPs are beyond you. However, in the case of the 1 D myope (FP at 1 meter), you would see a different reflex: if you leaned forward, you would now see WITH; if you tilted backward, you would see AGAINST. But when you sit with your retinoscope right at the FP of the eye, you see the neutrality reflex (Figure 4-5). . Neutrality reflex (NEUT) FP conjugate with the peephole of the retinoscope. When you are at the FP, the pupil floods with light. There is no streak reflex and no movement WITH or AGAINST. The retina of the eye is conjugate with the peephole of the retinoscope. Since the reflex reverses itself (ie, changes from WITH to AGAINST motion) at the FP, some call neutrality the reversal point.

OPTICS AND REFRACTION

The Schematic Eye : Auxiliary Lenses and Vertex Distance

We will use auxiliary trial lenses to create refractive errors greater than those obtainable by changing the length of the schematic eye. We call these phantom lenses, and they sit in the rack in front of the eye. These lenses create an ametropia of equal power but opposite sign; that is, plus lenses create a myopicerror and minus lenses a hyperopic error. The power of the phantom is added to the setting of the schematic eye. For example, if you set the scale at –2 D and put a +10 D phantom before the eye, the combination would simulate a myopia of –12 D. The +10 D phantom simulates a 10 D myopia by converging retina rays to the myopic FP of 10 cm. We also use auxiliary lenses to correct refractive errors, as you have already seen. To correct an ametropia, we add lenses of appropriate sign and power, with allowance for our working distance. Problems arise, however, in creating or correcting errors when you stack several lenses together. Thenominal power of a lens (marked on the handle) presumes a short vertex distance (the distance between the back of the lens and the front of the cornea). The effective power changes as the vertex distance (VD) increases. The difference is slight with weak lenses, but increases dramatically as the lenses become stronger. As a rule of thumb, the effective power of a strong lens (±10 D) changes about 1% for each millimeter it is moved. The power of a 10 D lens moved 10 mm changes by about 1 D.† When you increase the VD, pluslenses get stronger (effective power increases) and minus lenses become weaker (effective power decreases). Since effective power relates to VD, make it a habit to keep the strongest lenses closest to the eye. It is especially important to be aware of this change in effective lens power when working with the schematic eye, where the distance between the front and rear cells may be as much as 25 mm. Thus, on some models, the effective power of a strong lens changes by 25% when it is moved from the rear to the front cell. Obviously, you will want to keep the lenses close together. However, errors will still creep in when using strong lenses, and failure to appreciate this leads to a lot of unnecessary weeping. Since the distance between cells is at least 5 mm, a –10 D phantom in the rear cell is slightly overcorrected by a +10 D lens in the next cell. With lens power less than +3 D, the effect between adjoining cells is insignificant. Thankfully, refracting machines have vertex compensation to prevent all these errors. You will only need to consider the VD of the machine itself (compared to the VD of the spectacles) in writing the final prescription. As if this were not enough, each additional lens gives two more surface reflections for you to cope with. Each lens also reduces the brightness of the reflex, since the light has been refracted four times in the two-way path through the lens. Always use the fewest lenses to achieve a desired result; for example, replace a stack consisting of +1.50 D, +0.75 D, and +0.25 D lenses with a +2.50 D lens. You will have problems with your schematic eye in the pages ahead; everyone does, but you will catch on. Be a little forgiving of its calibration and inconsistencies; it is imperfect, but it will teach you fundamentals. By the time you are truly thwarted by the model eye’s limitations, you will be ready to retinoscope patients. Since many of you may already be employed in an ophthalmic office, or have access to an eye refracting lane, I’ve asked Rich Reffner to explain his teaching method. He uses a refractor in retinoscopy of the training eye.

                   Phthisical globe:

A 29 year-old sustained a rupture right globe in a motor bike accident 2 years ago and despite repair, the globe became severely phthisical with a significant loss of orbital volume. Despite an ocular prosthesis, the right eye appears enophthalmos. She wishes to improve her appearance. Surgery to increase the orbital volume is planned by performing an enucleation and simultaneous orbital implant. The prosthesis also needs to be changed to complement the iris colour of the good eye.