UPDATE: Minimum Intensity Photocoagulation
Four surgeons share their experience using these techniques to treat diabetic retinopathy and AMD.
By Christopher Kent, Senior Associate Editor

Since the late 1960s, argon laser photocoagulation has been used to treat a variety of retinal diseases. During that time, many surgeons accepted the idea that the collateral tissue damage caused by this procedure was a necessary side-effect of treatment -- perhaps even the reason for its success.

With the introduction of semiconductor-based lasers in the '90s, however, it became possible to fine-tune laser treatment, both in terms of power and wavelength. This has made it possible to apply lower energy laser treatments that don't cause visible damage and scarring. Studies are showing that therapeutic results still occur.

To find out what's been happening in this area of investigation, we interviewed a number of surgeons who are currently involved with using and/or testing minimum intensity photocoagulation (MIP).

Treating diabetic retinopathy

Dr. Jeffrey K. Luttrull, a vitreoretinal specialist practicing in Ventura County, Calif., uses the MIP MicroPulse technique -- a train of repetitive, very short, low energy laser pulses -- to treat symptoms of diabetic retinopathy and retinal vascular disorders such as branch and central retinal vein occlusion.

"I've used MIP exclusively for treating retinal vascular disease for the last 2 and a half years," says Dr. Luttrull. "In my experience, MIP works great. My impression is that the micropulse diode laser is as effective as thermal laser in treating proliferative retinopathies, and the visual results are better than with a standard thermal laser. I look forward to these clinical impressions being more rigorously tested by prospective multicenter clinical studies.

"Using the IRIDEX laser and MicroPulse technique also has other advantages. For one thing, I haven't observed any significant side-effects. There's no scarring, and I hear no complaints of night or peripheral vision loss. If problems persist, I can treat a patient repeatedly without any apparent adverse cumulative effect. And I can be a bit more aggressive about treating in the macula, because there's no apparent damage being caused.

"Another very important advantage is patient compliance. The procedure is very well tolerated, because the micropulse mode is essentially painless, even at very high energies. Pa-tients aren't reluctant to undergo treatment, which they often are with treatments that are painful or require anesthesia."

When asked what aspect of the protocol he'd like to see improved, Dr. Luttrull shared an experience mentioned by several practitioners who use MIP: Because the treatment is "subthreshold," no visible change occurs during the procedure. (Even afterward, it's hard to detect a change using fundus fluorescein angiography.) This makes it hard to be sure when the desired effect has been achieved.

"The absence of visible indications during treatment is disconcerting when you first start using it," he says. "It's like the emperor's new clothes. You think, I haven't done anything! But after a while you see that it works remarkably well.

"I've developed working parameters based on my experience, but studies should confirm my clinical impressions and firm up treatment parameters for different situations. It's difficult to titrate treatment exactly because you don't see the endpoint at the time of treatment."

Prognosis: promising

Dr. Luttrull believes that the lack of visual feedback during surgery and the expense of buying another laser probably prevent many doctors from trying this technique. "I bought the laser because my old laser was failing and I wanted more options. The IRIDEX laser was attractive because it has a lot of capabilities, including continuous wave thermal diode laser and TTT. In fact, buying this laser turned out to be one of the most positive additions to my practice I've ever made.

"I expect this treatment will eventually become standard for these patients, unless research identifies problems I haven't observed. On the face of it, it seems to offer every utility of thermal laser treatment for appropriate indications, with the added advantages of reduced or eliminated side-effects and improved patient compliance. In short, it seems to offer all of the advantages and suffer from none of the disadvantages of thermal lasers for treating retinal vascular disease."

TTT and AMD

Transpupillary thermotherapy (TTT) is a subthreshold, low irradiance, long exposure duration, large spot size MIP protocol. The increase in tissue temperature caused by this treatment for choroidal neovascularization (CNV) is much less than that required for conventional short-pulse photocoagulation. During TTT the low-intensity laser is applied for 60 seconds. Treatment power is adjusted for retinal lesion size, chorioretinal pigmentation, macular elevation and media clarity. A parfocal laser delivery system is used to ensure uniformity of irradiance across large diameter treatment spots.

A number of small studies have shown that treatment with TTT causes partial reabsorption of subretinal fluid and stabilization of vision in a majority of patients. A larger, randomized clinical trial -- TTT4CNV -- is currently underway.

Thomas Friberg, M.D., who practices in Pittsburgh, Pa., and is the study director for the Prophylactic Treatment of AMD Trial, uses TTT in his practice. He says it shows promise for treating AMD patients with neovascular membranes.

"I frequently see patients with extensive fluid overlying the neovascular complex. Most treatment alternatives for these patients are not very effective. However, I've been impressed by TTT. It seems to work relatively well in making that fluid go away, and that should be good for the function of the retina. Whether it affects occult neovascularization remains to be seen. But I'm encouraged by what I've seen so far.

"TTT has another big advantage going for it: It's inexpensive, especially compared to PDT. We all want these treatments to work, but if proven to be useful, TTT would also be cost-effective. I'm sure the insurance carriers would like to see a less expensive treatment. In fact, some insurers now reimburse when TTT is used to treat occult membranes."

Treating lesions large and small

Jack Sipperley, M.D., and Allen Thach, M.D., and their associates offer MIP treatments to patients at Retinal Consul-tants of Arizona, located in Phoenix. "We use TTT to treat occult AMD," says Dr. Sipperly. "Nothing else seems to help.

"We started using the infrared laser about the same time as we tried PDT -- about 2 years ago. We weren't happy with the PDT results, but we've used TTT on more than 240 patients, and we've been able to stop visual loss or improve their vision 70% of the time." When asked how the TTT procedure could have such a beneficial effect, Dr. Sipperly refers to the leakage abnormal vessels tend to produce. "I suspect we're gently sealing lining tissue, which stops the retinal swelling." He adds that, in their experience, TTT works equally well on both occult and classic conditions.

Dr. Thach described the results of a study they completed involving 69 AMD patients, all of whom had predominantly occult neovascular membranes. "We used large spot TTT and treated patients with lesions up to 6,000 microns in size.

"We found that 1 year after treatment using TTT, 71% of the patients had stable or improved vision." (Drs. Thach and Sipperly define stable as meaning that no more than 1 line of acuity was lost.) "Only 29% of the patients lost 2 or more lines. Left untreated, about 62% of patients with occult neovascular membranes lose two or more lines of vision in a year. Even using PDT, 65% to 66% lose two or more lines."

Dr. Thach believes that lesion size may not be a deciding factor in the success of the treatment. "Dr. Reichel at Tufts University, who is currently running the TTT4CNV trial, did a pilot study involving patients with smaller lesions -- up to 3,000 microns in diameter. He found that about 75% of these patients had stable vision following treatment. The fact that our results are very similar suggest that TTT may be effective regardless of the size of the lesion.

"I think TTT has great potential for treating predominantly occult neovascular membranes."

Dr. Sipperly concurs. "TTT is good because we can treat earlier when patients are still 20/30 or 20/40 and stop them from getting worse. And some patients get great results, which never happens with PDT or the argon laser. We just can't bring back the vision of patients who have really deteriorated before we treat them.

"Overall, it's the best treatment option we've found so far."

More data to come

Although clinical data is still in the early stages, it seems clear that MIP procedures hold tremendous promise, at least for some patients. And considering the devastating nature of diseases such as diabetic retinopathy and AMD, that can only be a good thing.