Spot Power

and a Little About Spot Size

There are a number of different approaches to the laser treatment for diabetic macular edema. The traditional ETDRS approach was to start with a 50-micron spot and .1-second duration. Then treatment was initiated with a power of 50 milliwatts, and this was gradually increased in 10- to 20-mW increments until a color change occurred in the offending microaneurysm. (This direct treatment of a microaneurysm is referred to as focal treatment.) If there was diffuse leakage without any obvious focal source, then grid treatment was applied, consisting of light burns of 50 to 100 microns spaced >1 burn width apart.

For this discussion it is assumed you are using a duration of .1 second, although as long as you are careful with the other parameters it does not make that much difference—most doctors use something between .05 to .2 seconds. The longer durations use lower powers to get the same effect so the burn builds up slowly and can be titrated easily (recall the energy density equation in Chapter 3). On the other hand, a longer burn gives the patient more time to move and mess up your shot. Shorter durations need more power but some doctors prefer to obtain a quick hit that is relatively independent of patient movement. However, you have to be very careful to avoid a rapid buildup of the burn given the small spot and the higher power, especially given the variable pigmentation and retinal thickness that can be present in the macula.

Depending on the laser and the eye, you will usually start getting some sort of uptake around 80 to 110 mW, although the actual power requirements may vary a great deal depending on the thickness of the retina, media opacity and fundus pigmentation. For instance, black or Hispanic patients with clear media may need very little power, perhaps as little as 60 to 70 mW. Paler fundi may need frighteningly large powers, especially if there is any sort of cataract or grubby capsule. Clean pseudophakia, on the other hand, can let you get a hot burn very easily, whatever the pigmentation—be very careful and start with a very low power, especially if you have been learning your trade by treating through typical diabetic nuclear and cortical opacities.

Exactly what kind of burn are you looking for? No one knows. Even though the ETDRS called for "light" burns, you can see from Figure 1 that they could still be fairly white and hot (Figure 1 is from a teaching series from the 1980s). There are those who feel that if you want ETDRS results, you have to treat like the ETDRS, but most folks nowadays think "less is more" and will use much lighter burns.

Here are just a few reasons why: First of all, Figure 2 reminds you that the spots you put in are like gifts that keep on giving—they can slowly enlarge long after you have moved on. Second, it makes sense to treat with milder burns—simply because you can always go back and apply more treatment, but you cannot undo overly aggressive treatment. Finally, diabetics live a lot longer than they did back when the ETDRS was performed, and they tend to have better control. Better control means that they will respond better to milder treatment, and a longer life means that there is more time for any spots you put in to expand.

Figure 1. This is a training slide from over 25 years ago. These burns would be considered a bit hot nowadays, although one could argue that this eye is in trouble and should be hit hard (note the hard exudates building up in the fovea). Also note the extremely satisfying bombs dropped squarely on some of the microaneurysms (arrows). Ideally you would want to use a smaller spot to try and treat only the microaneuysm and minimize collateral damage. Also notice the classic pattern that occurs when the patient moves just enough to keep you from hitting a microaneurysm dead on and you end up peppering the entire area around it in frustration (arrowhead). This shows why you should not keep firing away at a moving target—you can take out a lot of retina with multiple spots trying to get one little microaneurysm. (Courtesy of the Early Treatment Diabetic Retinopathy Research Group)

Figure 1. This is a training slide from over 25 years ago. These burns would be considered a bit hot nowadays, although one could argue that this eye is in trouble and should be hit hard (note the hard exudates building up in the fovea). Also note the extremely satisfying bombs dropped squarely on some of the microaneurysms (arrows). Ideally you would want to use a smaller spot to try and treat only the microaneuysm and minimize collateral damage. Also notice the classic pattern that occurs when the patient moves just enough to keep you from hitting a microaneurysm dead on and you end up peppering the entire area around it in frustration (arrowhead). This shows why you should not keep firing away at a moving target—you can take out a lot of retina with multiple spots trying to get one little microaneurysm. (Courtesy of the Early Treatment Diabetic Retinopathy Research Group)

Figure 2. Note enlargement of laser scars—especially the confluence of the scars around the fovea. This is why you need to tread lightly.

Also, diabetic maculas don't tend to fall apart quickly, and you don't need to feel like your first treatment is the only thing standing between your patient and a white cane—especially if the patient has reasonable diabetic control and the disease is away from the fovea.

Practically speaking, this means that the goal is often a very subtle, small burn— something that just begins to show some lightening of the RPE—if you are doing a grid. If there is a lot of diffuse, thick edema, it may be worthwhile to go for a bit more whitening beyond this level, although heavier burns should be done only in areas that are farther from the fovea.

If you are trying to get a specific microaneurysm, the ETDRS wanted you to get some sort of color change within the lesion, either lighter or darker. This is still a nice thing to aim for, but recently there has been more emphasis on just getting the microaneurysm treated and not hammering away until you see a color change.1 Basically, if you can get a color change, great, but don't go postal trying to get it.

Figure 3. An example of a milder grid. The angiogram shows some microaneurysms but there is also a lot of diffuse leakage in the entire temporal half of the posterior pole. Aggressive white laser spots would create a large scar and likely shove edema right into the fovea. A very light grid can be seen in the area of leakage—this is a good degree of uptake to start with, although in retrospect some of the burns are a bit too close together.

Figure 3. An example of a milder grid. The angiogram shows some microaneurysms but there is also a lot of diffuse leakage in the entire temporal half of the posterior pole. Aggressive white laser spots would create a large scar and likely shove edema right into the fovea. A very light grid can be seen in the area of leakage—this is a good degree of uptake to start with, although in retrospect some of the burns are a bit too close together.

Figure 4. An example of light treatment to areas of focal leakage. These are light burns and if you are worried that they are insufficient you can bring the patient back in six to eight weeks and add more if necessary.

Ultimately, the subtleties of this are learned from clinical experience and not just clinical trials, so understand that these are, at best, guidelines—there is no proven "perfect" burn. Survey the retina people around you and take advantage of any hands-on teaching you can get, and then try to develop a treatment pattern that works best for you. Figures 3 and 4 give examples of milder treatment approaches, and Figure 5 shows the appearance of a mild grid after a number of years.

Whatever burn you are trying for, the first step is to get the tightest focus you can with your aiming beam—if you don't get this first bit right, you will be punching marshmallows and your settings and uptake will be changing all over the place. And don't worry if at first it seems like you are spending hours getting a tight aiming beam and determining an effective power. Review Chapter 7, practice like crazy, and try some of the tricks discussed later in the text. As you develop experience you will be able to rapidly factor in all of the variables and quickly dial in safe and effective settings.

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Figure 5. This gives you an idea of how a light grid such as in Figure 4 can look years later. The red free on the left shows that you can barely see the spots that were placed about nine years prior to these photographs. The angiogram on the right lights up the spots. This patient went from 20/200 to 20/60 with laser combined with better systemic control.

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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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