Lasers Used for Glaucoma Treatments
Chat Highlights
March 23, 2005

On Wednesday, March 23, 2005, Dr. Rick Wilson a glaucoma specialist at Wills, and the glaucoma chat group discussed "Lasers Used for Glaucoma Treatments."

Moderator:  Lasers have been used for decades to treat glaucoma.  The types of lasers and what they are used for, however, can be confusing for glaucoma patients. For instance, early in my treatment I had a peripheral iridotomy (PI).  What type of laser is used for that procedure?

Dr. Rick Wilson:  First, let me explain that PIs are usually performed if the angle is narrow and the iris is being pushed forward by the pressure of aqueous fluid behind it. Making a hole in the iris equalizes the pressure behind and in front of the iris, so the iris settles back against the lens and away from the drain (trabecular meshwork) in the angle.

[Editor's note: The Microruptor Nd:  YAG laser is now manufactured by Meridian.]

P:  Is Nd:YAG an acronym?

Dr. Rick Wilson:  Yes:  Neodymium (Nd), yitrium (Y), aluminum (A), and garnet (G). Some lasers, such as the YAG crystal, use a solid medium for lasing; others, like argon, use gas. When a uniform medium is used, the wavelength of the light is all the same, as opposed to the white light surrounding us that consists of all colors.  The lasing medium allows energy to be put into it (Light Amplification through Stimulated Emission of Radiation = Laser). The light gathers energy, until it is let out of the crystal as a parallel, collimated beam.

P:  How many years have iridotomies been performed?

Dr. Rick Wilson:  Laser PIs have been performed since about 1978 or 1979.  We burned a hole through the iris first with the argon laser.  About 1984, if I remember correctly, we started cutting a hole with the Nd:YAG.

Moderator:  When are PIs usually performed?

Dr. Rick Wilson:  PIs are usually performed if the angle is narrow and the iris is being pushed forward by the pressure of aqueous fluid behind it.  Making a hole in the iris equalizes the pressure behind and in front of the iris, so the iris settles back against the lens and away from the drain (trabecular meshwork) in the angle.

P:  Please explain the difference between argon laser trabeculoplasty (ALT) and selective laser trabeculoplasty (SLT).

Dr. Rick Wilson:  An argon laser is a green or blue-green laser.  The selective laser is a frequency doubled YAG at a different wavelength.  The argon leaves little burns where it hits, if there is enough pigment to absorb the energy.  The SLT laser wavelength is only absorbed by the pigment inside the cells, and does not leave a burn that is visible, even using a microscope.

P:  How long might it be before studies show that SLT is better than ALT in that it can be repeated and is just as effective, or more effective, in lowering IOP?

Dr. Rick Wilson:  If that were true, I would think that would have been proven by now.  I am increasingly pessimistic that it is not true.

P:  What is the difference in the number of burns in the meshwork created by ALT and SLT?

Dr. Rick Wilson:  ALT makes tiny, 50-micron burns in the trabecular meshwork. SLT makes 400-micron burns that are not visible and no scar develops.

P:  How can the burns created by ALT be seen?

Dr. Rick Wilson:  When we do a trabeculectomy, we can send the small rectangle of tissue containing the trabecular meshwork to pathologists to examine with high-powered microscopes.  The pathologists can see the old, contracted scars from previous ALT burns.

Moderator:  Why are the burns 50 microns?

Dr. Rick Wilson:  Fifty microns is just the smallest aperture on most lasers, i.e., the smallest spot size they produce.

P:  In what percentage of patients does SLT lower intraocular (IOP)?

Dr. Rick Wilson:  Over 90% of my patients have a good effect, because I don't use laser on those I don't think will do well.  We know that the laser works better the older you are and the more pigment you have in the trabecular meshwork.  For a good effect, the diagnosis must be pseudoexfoliative, pigmentary, normal-tension or primary-open angle glaucoma.  The angle must also be open so the laser can get to the trabecular meshwork.

P:  Before you decide whether to do ALT or SLT, how do you examine the meshwork for pigment? Can you actually see the pigment inside cells?

Dr. Rick Wilson:  The trabecular meshwork is easily seen on gonioscopy (the little mirror we rest on the cornea to direct the light of the slit lamp and the view of the doctor into the angle) and the amount of pigment judged.  We can't see the cellular level; we can only see the amount of gross pigment.

Moderator:  Are steriods used with SLT and ALT?

Dr. Rick Wilson:  Yes, usually for about five days afterward.

P:  If medication is no longer lowering my IOP, how will it be determined whether ALT or SLT is best for me?

Dr. Rick Wilson:  Since there does not seem to be any difference in the effectiveness of the two lasers, it usually comes down to which laser is readily available.  If both lasers are available, most glaucoma specialists would choose the SLT, since it seems to be more benign.

P:  Is ALT ever associated with worsening astigmatism?

Dr. Rick Wilson:  I can't think of a way an ALT would worsen astigmatism.

P:  When was it determined that ALT doesn't work in young people?  I had ALT on my eyes in 1992 at age 17.  Did most glaucoma doctors know then that the effort was futile?

Dr. Rick Wilson:  We knew that at Wills in the mid to early 1980s.  Perhaps your doctors were so desperate they were clutching at any straw they could get their hands on.

P:  How does zapping the pigment cells with SLT lower IOP?

Dr. Rick Wilson:  No one knows for sure.  The theory is that the laser injures the cells, which causes them to divide, and the new cells are much more effective in engulfing the debris in the drain and cleaning it out.

P:  Why are SLT and ALT sometimes ineffective in lowering pressure for certain patients? And if no one knows why, is anyone trying to find out why?

Dr. Rick Wilson:  Laser is light energy, so it needs pigment to absorb it.  People have different amounts of pigment in the trabecular meshwork, have slightly different causes for their glaucoma, and their trabecular meshwork may be functioning at a level typically found in an older or younger person.  All these factors affect how well the laser works in ALT or SLT.

P:  Is laser surgery a temporary measure for lowering IOP?

Dr. Rick Wilson:  ALT or SLT can last for five or more years, but in most of my patients it lasts two to four years.

P:  Can lasers (e.g., ALT) cause PAS (peripheral anterior synechiae)?

Dr. Rick Wilson:  A PAS is a spot where the iris is pulled up onto or over the trabecular meshwork, covering it and rendering it ineffective.  That is a common side effect, especially if too much power is used, causing a burn that is bigger than needed.

P:  If a patient with ICE (irido-corneal syndrome) already has PAS, would laser cause more harm than good?

[Editor’s note:  The ICE syndrome is caused by the diseased lining of the cornea, which grows over the drain in the eye, blocking it, and over the iris, causing stretching and a lack of blood supply.]

Dr. Rick Wilson:  ALT or SLT does no good in ICE syndrome, and may stir up the endothelial membrane causing the problem.

P:  A clinician once told me that the amount of iris pigment released as a result of a peripheral iridotomy is "a lifetime's worth of pigment."  Is that hyperbole?  I have pigmentary glaucoma (PG), so obviously that's scary, but even in a patient without PG or PDS (pigment dispersion syndrome), doesn't that much pigment release damage the trabecular meshwork?

Dr. Rick Wilson:  I would think that it would be a lifetime of pigment for many people who have little pigment knocked off the back of their iris naturally, but not that much for people who have PG or PDS.

P:  What determines how much pigment you have?  Is it eye color?

Dr. Rick Wilson:  No, it is probably related more to how tightly the pigment granules lining the back of the iris are stuck to the structure of the iris.

P:  Could SLT cause dry-eye syndrome?

Dr. Rick Wilson:  No. It can exacerbate dry eye for a week or two, and the steroids that are used with it can make dry eye worse while the patient is on them.

On March 30, Dr. Werner discussed "What is Normal-Tension Glaucoma?" in the Chat room. Click here for highlights of that meeting.

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