How to avoid bubble formation during SLT

Take home

The adjustment of treatment parameters may be an effective technique for surgeons to avoid bubble formation during selective laser trabeculoplasty.

By Brian A. Francis, MD, MS, Special to Ophthalmology Times

Los Angeles-Bubble formation-formerly a treatment endpoint of selective laser trabeculoplasty (SLT)-is now a marker of the tissue’s reaction to an unnecessarily high-energy pulse, which can result in complications like elevated IOP and long-term inflammation, discomfort, and scarring.

Cavitation energy bubbles that form in the aqueous during SLT indicate that tissue is absorbing the laser. However, once these bubbles develop, it may be an indicator that too much energy is being transferred to the tissue.

A new approach

Following research published by the Journal of Glaucoma, there is a new tendency to adjust laser (Selecta II SLT, Lumenis) parameters so that the energy and the spot size have decreased.

Previously, where recommendations were to perform 100 spots over 360° of the meshwork with higher energy, clinicians are now performing about 150 spots that are slightly overlapping but with much less energy. The hope is that more of the trabecular meshwork is reached while keeping the energy level low to prevent IOP spikes.^1-2

Overall, the same amount of total energy is applied to the eye but decreasing the amount per burst and increasing the number of spots. Clinicians are applying more energy to the right tissue at more appropriate energy levels.

In the past, it would not be unusual to use a 1.0 energy setting, but now clinicians are starting at 0.5. Highly pigmented eyes with pigmentary discersion glaucoma or exfoliation glaucoma are more likely to develop bubbles because the tissue absorbs energy more easily. In these patients, consider starting at an even lower energy level.

Risk factors for SLT

It is critical to assess the angle structures and utilize techniques to locate the trabecular meshwork properly. Other tissues may be mistaken for a pigmented trabecular meshwork. If the angle is narrow, then a pigmented Schwalbe’s line may be mistaken for the trabecular meshwork. Treating the cornea by mistake may cause inflammation in the cornea or corneal edema. If a patient has a narrow angle, use compression gonioscopy at the slit lamp to determine the angles structures. Compression gonioscopy opens up the angle further to view the full width of the angle.

If the angle is deep or wide open without much pigmentation, a ciliary body band may be mistaken for the trabecular meshwork. Treating the ciliary body causes inflammation and pain, neither of which is effective clinically in terms of lowering IOP.

A technique can be used by having the patient look toward the mirror of the gonio lens, which is referred to as “looking over the hill”-in other words, looking over the iris into the angle to see the trabecular meshwork more easily.

Viewing the angle inferiorly can avoid mistaking the ciliary body band for the trabecular meshwork, because it will often reveal pigment in trabecular meshwork that is not evident when looking at the superior angle.

Resolution

In the event too much energy is applied, several steps can be taken to decrease inflammation. Begin the patient on nonsteroidal anti-inflammatory drops or a short dose of topical steroids.

Also, be careful to detect IOP spikes in patients who might have received too much energy. Check IOP an hour later and, perhaps, even check it again that day, and at 1 day and 1 week or more frequently if necessary.

Conclusion

SLT is a safe and effective initial therapy for patients with open-angle glaucoma or ocular hypertension. However, excessive energy-indicated by bubble formation-causes a higher incidence of elevated IOP, as well as long-term inflammation and discomfort and scarring of the angles.

Adjusting treatment parameters by lowering energy levels is an effective solution.

References

1. Katz LJ, Steinmann WC, Kabir A, et al; SLT/Med Study Group. Selective laser trabeculoplasty versus medical therapy as initial treatment of glaucoma: a prospective, randomized trial. J Glaucoma. 2012;21:460-468.

2. Alvarado JA, Katz LJ, Trivedi S, et al. Arch Ophthalmol. 2010;128:731-737.

Brian A. Francis, MD, MS, is the holder of the Ralph and Angelyn Riffenburgh Professorship in Glaucoma and an associate professor of ophthalmology at USC Eye Institute, Keck School of Medicine in Los Angeles. He did not indicate any proprietary interest in the subject matter. Dr. Francis may be reached at 323/442-6415 or bfrancis@doheny.org.