Getting Started

Having done all the talk to get the patient ready, it is now time to do the walk. First, you have to choose your weapon, or at least its color. At the beginning of the book it was assumed that you have a green laser, but it is worth noting that the other colors can come in handy when doing a PRP. No study has shown that any wavelength is superior in terms of the ultimate outcome, but just like it is nice to have different wrenches to work on a car, it is nice to know that different colors can be helpful in certain situations. Different wrenches don't cost $40,000, though, so don't feel like you have to run out and get another laser— this is simply an option to be aware of.

An example of a situation in which a different wavelength can be useful is when there are a lot of media opacities, such as a hemorrhage or a nuclear sclerotic cataract. In this case, you may want to use a redder wavelength because there is less scattering relative to green. An infrared laser can be even more effective at getting through such opacities, but it is trickier to use. (You should check the Appendix if you are going to use this wavelength.) Fortunately, you can get most routine jobs done with a green laser. If you really need to go to sky-high powers, though, you may want to refer the patient out to someone who has other wavelengths in order to avoid some of the complications discussed in upcoming chapters.

Next, remember that the spot size you set on the laser may end up being very different from what you get on the retina, especially if you are using a wide-field indirect lens. Refer to back Chapter 4 for the details—but the important thing is to know the magnification factor for the lens you are using. For instance, a very wide-field lens can double the spot size: If you are set at 500 microns on the slit lamp, you can end up placing 1,000-micron spots on the retina—and these are really huge, painful spots. (It is likely you would be using equally massive powers, and between the power and the pain, you would quickly realize your settings were unnecessarily harsh.) You should also remember that the number of spots you need to use is very dependent on the spot size. For instance, if you want to do a "standard" Diabetic Retinopathy Study PRP of 1,500 spots that are 500 microns in size—but you are using a smaller spot size—you can't just put in 1,500 spots and quit. You have to increase the number of spots you place so that you cover the same area that would have been covered by 500-micron burns (Figure 3).

Figure 3. It takes at least four 200-micron spots to fill a 500-micron spot (the n-r-squared thing). If you want to place a certain "dose" of PRP based on the 500-micron burn size, then you have to increase the number of spots you place, depending on how many of your burns it takes to equal the area of a 500-micron spot.

Figure 3. It takes at least four 200-micron spots to fill a 500-micron spot (the n-r-squared thing). If you want to place a certain "dose" of PRP based on the 500-micron burn size, then you have to increase the number of spots you place, depending on how many of your burns it takes to equal the area of a 500-micron spot.

How hot do you make the burns? It is hard to photograph the kind of burn that you actually see when you do a laser, because burns tend to soften rapidly after treatment—they also spread out a bit. The ETDRS asked for medium-white burns as seen in Figure 4, and this is fairly standard. Note the whitening in the center of most burns—if you are getting a much whiter burn, you are probably running too hot, and you should turn down the power.

Figure 5 shows a spread of different intensities. Many specialists feel that you can use lighter intensities (note, in the figure, the milder, grayish burns without any white center). One problem is that over time you will see that milder burns may not form scars as large as the original burn—so you can end up with less area treated than you had planned at the time of treatment. There are no long-term studies that prove you can get DRS results with burns that are less intense than the DRS standard, but in a patient with early, slowly progressive disease, it may be reasonable to use a lighter burn intensity and follow the patient closely to be sure their disease remains quiescent.

Figure 4 is also the standard density for a full PRP—the spots are about one-half burn width apart. The ETDRS had a "mild scatter" treatment arm, with fewer burns that were spaced further apart, and it is shown in Figure 6 for comparison. This milder pattern resulted in the middle (yellow) line in Figure 2. It is clearly not as effective as a full PRP, and most experts would not go this lightly—or at least not for the full treatment. Still, it gives you an idea of what has been tried and what to expect so you don't have to reinvent the wheel.

Figure 4. This is the ETDRS gold standard for burn intensity and density for a full scatter PRR If you do less than this, you could get, uh, burned—but some patients may do quite well with less. Note that laser burns tend to spread out shortly after treatment, so the actual treatment density was a bit less than this. (Courtesy of the Early Treatment Diabetic Retinopathy Study Group)

Figure 5. Variable-intensity burns with the upper burns being similar to ETDRS standards and the lower, grayer burns representing a milder approach Note that you can see some of the choroidal detail through the lighter burns, but the whiteness of the heavier burns obscures the underlying choroid. This can help you decide how hot your burns are. Truth in advertising moment: This image was Photoshop'd to simulate the burns. It is hard to justify doing this to a patient for didactic reasons.

Figure 6. The ETDRS photo showing the pattern used for the "mild scatter" treatment arm. This is a very light pattern, but it may work in older patients with mild, slow disease. It is the pattern that resulted in the middle line in Figure 2 —not as effective as a full scatter pattern, but it still had a treatment effect. (Courtesy of the Early Treatment Diabetic Retinopathy Study Group)

Incidentally, when this chapter suggests a given number of spots, it is assumed that these are equivalent to 500-micron spots. You will likely prefer to use smaller spots (for instance, for patient comfort), so you will need to adjust the actual number of spots you place, as outlined above. Also, when this chapter refers to a "standard DRS PRP" it means about 1,500 500-micron spots.

AMOUNT, STAGING, & FREQUENCY OF TREATMENT

There is a tendency to feel as though it is some sort of emergency when a patient presents with mild asymptomatic neovascularization. Do not surrender to the temptation to slag the retina simply because you see some vessels that look disturbing. Such vessels are unlikely to change over a few weeks, and most of the acute complications from PRP occur with aggressive treatment delivered all at once. In some ways, the situation is analogous to a patient who presents with long-standing severe hypertension: If you aggressively bring the blood pressure down far and fast you can give them a stroke, but if you gently work it down you can save them safely. It seems as though diabetic eyes function in a similar fashion. Eyes that present with early proliferative disease have been living in an ischemic milieu for quite some time. They can be rather fragile, and if you jump with 1,500 spots all at once, you are very likely to create permanent changes in the macula—as well as an extremely unhappy patient. As will be discussed below, it is often best to start slowly with a few hundred spots if you can.

Make no mistake; there are some situations in which time is not your friend. Patients with lots of very angry-looking vessels, very ischemic retinas and active hemorrhaging can go bad fast. The concern is that the extensive neovascular-ization evolves into dense fibrous tissue, causing severe traction that no amount of complex vitrectomy can undo. Wasting time doing a slow gentle PRP in such a patient can be disastrous. These patients are often younger Type 1 diabetics with a history of poor compliance, but any demographic can be affected. Such patients do need a lot of laser, even if it means risking side effects, because just about any laser complication is better than where the eye is heading. In this type of situation, you may want to bite the bullet and put in a thousand spots at first, and then repeat weekly until the patient is controlled or hemorrhages and needs surgery. In fact, if you have a patient who looks this bad and you are considering this type of treatment, you should strongly consider just referring them to a retina specialist. ASAP. This is not to strip you of your prerogatives as a Renaissance Comprehensive Ophthalmologist—but you should recognize that these patients can do abysmally, with even specialist care. There will be lots of bread-and-butter diabetics for you to treat who don't need subspecialty intervention, so consider sending these scary-bad patients off if you can. If you really, really want to treat such misery, you may need to find yourself a retina fellowship.

Figure 7. This is a very sick eye—most commonly seen in younger patients with a history of poor control. Note the swollen, beaded veins and all of the smaller intraretinal vessels that seem to start and stop in no clear pattern. Also, note the neovascularization —it is not large, but the vessels are thick and succulent in a way that bodes poorly for the patient. The blotchy hemorrhages in the macula suggest a lot of ischemia, and you are caught between the Scylla and Charybdis of doing a fast, aggressive PRP, yet trying not to blow out the fragile macula. Consider referring if you can-skillfully balancing all the treatment variables in this eye may be very important to the final outcome.

Also, are you waiting for a little chart on spot size, duration and power for a PRP? It would be nice to have a standard setting for all of your PRPs, but hopefully you are realizing that there are so many factors that using one setting for all PRPs is like telling Rembrandt that he could only use one paintbrush. With experience you will automatically come up with tailor-made settings for each patient based on a number of variables. These variables will include things such as how aggressively you want to treat the patient, their degree of pigmentation, the presence of media opacities, the location of the treatment and even your estimate of their pain threshold. All of these are discussed at various points in this chapter and throughout the book.

However, you do need to begin somewhere, so here are some basic numbers to try:

Most feel it is best to start with a duration of 0.1 second—this is a nice middling length of time that will not be too likely to let you get in trouble. A good starting spot size is around 350 to 500 microns at the retina (remember the effect of the contact lens on spot size). For the "average" patient, this duration and spot size usually means that you will want to start with a power of about 200 milliwatts and slowly increase it until you get burns similar to those seen in the photos. Don't hesitate to start at an even lower power if the media is crystal clear and/or there is a lot of pigment—you can always turn it up but you can't undo a spot that is too hot.

You can then gradually increase the power—say, by 20- to 50-milliwatt increments—until you start to see a slight graying of the retina. Once you begin to see a change it is likely that you only need to turn it up about 50 milliwatts more in order to get a standard burn. Another clue that you are getting close to the right power is that the patient will often tell you that they are beginning to feel it. You should probably not stray very much from settings like this until you are starting to get an intuitive feel for how the retina responds to your treatment, but once you have some experience you will find these settings way too restrictive.

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Diabetes 2

Diabetes 2

Diabetes is a disease that affects the way your body uses food. Normally, your body converts sugars, starches and other foods into a form of sugar called glucose. Your body uses glucose for fuel. The cells receive the glucose through the bloodstream. They then use insulin a hormone made by the pancreas to absorb the glucose, convert it into energy, and either use it or store it for later use. Learn more...

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