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Laser ablation used to treat chronic open-angle glaucoma in pilot study
Excimer laser trabecular ablation ab interno could become a valuable alternative surgical treatment.
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The excimer laser trabecular ablation ab interno procedure is designed to remove corneoscleral trabecular meshwork to open Schlemm’s canal and thus facilitate outflow. The object of this pilot study on 14 eyes was to show the IOP-lowering effect of ELT in chronic open-angle glaucoma.
We used an excimer laser (
= 308 nm) with a 200-µm fiber. During each excimer laser trabecular ablation ab interno (ELT) procedure, two to eight laser spots were applied to the corneoscleral meshwork using the ab interno technique. Good visibility was achieved using a gonioscope.
The average IOP decrease was approximately 40% of preoperative IOP. The average number of antiglaucoma drugs needed dropped from 1.8 preoperatively to 1 at 6 months after surgery. Thus, ELT could become a valuable alternative low-risk measure in the surgical treatment of glaucoma.
Lasers in glaucoma surgery
The introduction of laser technology into glaucoma surgery opens up new possibilities for iridocorneal microsurgery. As numerous experimental and clinical studies have shown that permanent perforation of the trabecular meshwork cannot be achieved with photodisruptive lasers, the further development of certain infrared lasers offered a useful alternative. Numerous results are available.
The classic photoablative UV lasers (ArF and XeCl excimer lasers), which from the theoretical point of view seem most promising, have not been available in the past because of their high cost and high maintenance requirements. The energy absorption of the XeCl excimer laser evaporates human tissue and denaturalizes organic structures without producing undesirable marginal necrosis (photoablation).
The superiority of the photoablative action of the excimer laser over all other kinds of laser for corneal surgery has been examined in detail. In the case of glaucoma, laser surgery allows the removal of the corneoscleral trabecular meshwork through contact, thus creating points of opening in the inner wall of Schlemm’s canal (see figure).
As between 70% and 90% of outflow resistance is situated in the inner wall of Schlemm’s canal, which means that the eye’s main outflow resistance is located in the corneoscleral trabecular meshwork and the inner wall of Schlemm’s canal, ELT has recently been the subject of trials both with the erbium:YAG laser and the excimer laser.
Materials and methods
All patients were treated with a pulsed XeCl excimer laser Aida ( = 308 nm). The laser pulses had a duration of 80 ns. The laser light was conducted to the trabecular meshwork by contact via a monofilament silicone light-conducting fiber with a core diameter of 200 µm. The fiber was mounted in a metal jacket with an external diameter of 500 µm. Mean pulse energy at the tip of the fiber was 1.2 mJ. The energy at the tip of the fiber was calibrated before each operation using an independent energy measuring device built into the laser.
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The tip of the fiber was cut to a 65° angle to facilitate contact with the corneoscleral trabecular meshwork. Two to eight laser spots were applied on the posterior side of the eye. Twenty laser pulses with a repetition rate of 20 Hz were applied per spot. This was sufficient to create holes in the trabecular meshwork and to open Schlemm’s canal.
A total of 14 patients with chronic open-angle glaucoma with preoperative IOP of at least 21 mm Hg were treated. None of the patients had iridocyclitis or trauma or had had a previous operation to the trabecular meshwork. All patients were informed of the experimental nature of the surgery, which was performed using peribulbar anesthesia.
First, the anterior chamber was opened with a 2-mm clear cornea incision and filled with viscoelastic to protect the lens, iris and corneal endothelium. The incisions were temporal at 3 o’clock (left eye) or 9 o’clock (right eye). The fiber was then brought into contact through the opening with the trabecular meshwork opposite the incision. The positioning of the fiber on the trabecular meshwork was observed using a gonioscope. Two to eight laser spots were applied on the temporal side between 8 and 10 o’clock (left eye) and 2 and 4 o’clock (right eye).
After the spots had been applied, the viscoelastic was completely removed. Follow-up examinations were made 1 day, 1, 3 and 5 weeks and 3 and 6 months after ELT. The administration of 1% pilocarpine drops was begun immediately after surgery to prevent synechiae of the iridocorneal angle. Pilocarpine drops were administered four times daily for 7 days and steroid was administered in the form of drops for 30 days to suppress fibroblast activity.
IOP was measured for each patient before and after the operation using three successive applanation measurements, and the highest value obtained was used in the study. Medication was determined on the basis of the number of topical antiglaucoma drops. Additional oral medication was not administered. The statistical analysis was carried out for connected pairs using the Wilcoxon test.
Results
Fourteen eyes of 14 patients were treated using the ELT method. The patients’ average age was 72.7±10 years. The patients — six men and eight women — had a mean preop IOP of 26.8±3.7 mm Hg (range 21 to 34 mm Hg). The mean number of laser spots applied was 5.7. Twenty laser pulses were applied for each incision.
All patients were examined 6 months after the operation. After 6 months, mean IOP was 15.5±2.7 mm Hg (see chart). This represents a 40% reduction in IOP and is statistically significant (P=3.8 × 10-7). Mean preop medication was 1.8 (minimum 0, maximum 3), and this sank 6 months after the operation by 47% to 1 (range 0 to 3).
After 6 months, IOP was below 20 mm Hg in all patients; in 12 cases it was below 17 mm Hg. For these 12 patients mean medication was 1.1 after 6 months compared with 1.8 before the operation (reduction by 38%).
No statistically significant correlation emerged between the number of laser spots applied and the reduction in pressure achieved. There were no serious intra- or postoperative complications (e.g., retinal detachment, corneal clouding or deterioration of vision). No progress in cataract could be observed within the 6-month period. Small, localized anterior synechiae were observed with the gonioscope in two patients.
Limited reflux hemorrhaging from Schlemm’s canal was observed in seven of the 14 eyes during the operation. No hyphema, iritis or other intraocular infections were observed. The laser-induced defects within the corneoscleral trabecular meshwork were clearly visible by gonioscope during and after the operation.
After 3 to 4 weeks the initially almost completely round holes began to change in shape and became oval. Circular pigmentation of the edges of the holes also became apparent after about 4 weeks and increased during the period of ob servation. No signs of cyclodialysis were apparent in any of the eyes when examined with the gonioscope. A postop check with the gonioscope showed that many of the spots, sometimes more than 50%, were clearly too posterior, regardless of the individual surgeon.
In other words, although the surgeon was aiming for the trabecular pigment strip, many of the spots were incorrectly positioned behind this at the scleral spur or even further back (see figures top left).
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Discussion
The morphological results of erbium:YAG laser trabecular ablation on donated human eyes have been described in detail. Vogel and Lauritzen showed in 1997 that IOP can be reduced by spot ablation of the trabecular meshwork ab interno with the excimer laser. The use of a classic photoablative UV laser (eg, XeCl excimer laser) was an obvious possibility in order to counteract the main problem in glaucoma surgery, ie, the healing process and scar suppression.
This was not generally available in the past because of the high cost and high maintenance required. The fact that classic photoablative lasers minimize scar formation has been extensively examined in refractive corneal surgery.
Our clinical observations show, however, that repair mechanisms also occur, at least to some extent, with ELT. These repair mechanisms may perhaps develop slowly and incompletely in glaucoma patients or the newly formed tissue guarantees a certain improvement in outflow in comparison with the preoperative status.
Limited reflux hemorrhaging from Schlemm’s canal was observed in seven of the 14 eyes during the operation. The question of whether this may be seen as a reliable indication for the complete perforation of the trabecular meshwork or indicated that the external wall of Schlemm’s canal was also perforated has not yet been fully clarified.
Unlike all ab externo laser operations, ELT does not cause a deterioration in the conjunctival situation thanks to its ab interno approach. This is of great importance to many patients whose glaucoma is difficult to treat because here a filtering glaucoma operation at a later date would present no problem. A further advantage of ELT is that neither intra- nor postoperative complications were observed.
And as for erbium:YAG laser trabecular ablation, no ocular hypotony in the early post-operative phase with the risk of a flat interior chamber, choroid detachment and subsequent cataract formation is to be expected. With ELT, due to outflow resistance in the episcleral vessels and the scleral tissue, both of which are not affected, a postop pressure level below 12 mm Hg need not be expected and is not desired.
Considerable improvements in surgical techniques and in fiber technology following the pilot study will further improve results in the near future. Now (following the completion of the pilot study) two different iridocorneal angle areas are being treated with four to five spots each, eg, localization between 5 o’clock and 7 o’clock and 8 o’clock and 10 o’clock for the right eye.
Improvements in the fiber and in surgical techniques should help avoid the frequently too posterior and thus incorrect positioning of ELT spots. An operation microscope with good magnification and the use of a good gonioscope (Osher Surgical Gonio Posterior Pole Lens) (mirror image) or working with an upright image (Swan-Jacob Gonioprisma) are helpful here.
ELT is a promising option in glaucoma surgery for patients with chronic open-angle glaucoma. It avoids the typical disadvantages of laser sclerostomy such as over-filtration and conjunctival scarring and has a very low risk of complications in comparison with traditional filtering glaucoma surgery. Further clinical studies on a larger number of patients and with longer observation periods are necessary before we can assess the future value of laser trabecular ablation in glaucoma surgery.
It will be particularly important to check whether permanent repair of trabecular outflow resistance and the accompanying reduction in pressure will be sufficient to prevent continued damage to the optical nerve caused by glaucoma.
For Your Information:References:
- Thomas Neuhann, MD, can be reached at Helene-Weber-Allee 19, D-80637 Munich, Germany; +(49) 189-159-31339; fax: +(49) 189-1578394; e-mail: ThomasNeuhann@compuserve.com.
- Armin Scharrer, MD, can be reached at Moststrasse 12, Fuerth 90762, Germany; +(49) 911-779-820; fax: +(49) 911-779-8251; e-mail: oberscharrer@t-online.de.
- Eduard Haefliger, MD, can be reached at Augenarzt FMH, Haupstrasse 55, Binningen 4102, Switzerland; +(41) 61-426-6000; fax: +(41) 61-426-6001.
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