Ординатура / Офтальмология / Английские материалы / Step by Step Minimally Invasive Glaucoma Surgery_Garg, Melamed, Bovet, Pajic, Carassa, Dada_2006

262 Step by Step Minimally Invasive Glaucoma Surgery

INTRODUCTION

Currently, glaucoma is defined as a disturbance of the structural or functional integrity of the optic nerve that causes characteristic atrophic changes in the optic nerve, which may also lead to specific visual field defects over time. This disturbance usually can be arrested or diminished by adequate lowering of intraocular pressure (IOP). The generic term glaucoma should only be used in reference to the entire group of glaucomatous disorders as a whole, because multiple subsets of glaucomatous disease exist.

People who maintain elevated pressures in the absence of nerve damage or visual field loss exist. They are considered at risk for glaucoma and have been termed glaucoma suspects or ocular hypertensives. POAG is a major worldwide health concern, because of its usually silent, progressive nature, and because it is one of the leading preventable causes of blindness in the world. With appropriate screening and treatment, glaucoma usually can be identified and its progress arrested before significant effects on vision occur.

Following keratomileusis in situ and lamellar keratectomy in 1990, the author detected a decrease in the IOP of some patients. At first it was thought that this IOP dropping was due to modification of radial curvature or thickness of the cornea. Later it was supposed that suction was the lowering pressure mechanism, so in 1995 the application of the suction ring was started in a group of ocular hypertensive patients with no eye surgery antecedents. Intraocular pressure was measurably lower in a significant number of cases. This has also been seen in laser-assisted in situ keratomileusis (LASIK). It has been proposed that the decrease in IOP may be a real event.

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The mechanism may well involve stretching of the zonule which stretching produces some form of change in the trabecular meshwork either physiologically through chemical mediators, or through a mechanical opening of the trabecular pores.

Pneumatic trabeculoplasty (PNT) is a noninvasive treatment, performed in an ophthalmologist office, which has been demonstrated to reduce the intraocular pressure (IOP) in patients with primary open-angle glaucoma (POAG), pigmentary glaucoma and ocular hypertension (OH).

The ophthalmic international PNT device consists of a suction ring, a vacuum pump, and connecting tubing. The suction ring is made of disposable plastic. This ring is connected to a vacuum pump via three-way silicone tubing, which in turn connects to a single tube attached to the pump inlet (Figs 15.1 and 15.2).

The pump is preset to deliver a maximum vacuum pressure corresponding to 65 mm Hg within the eye. The pump also provides a digital timer which counts down the treatment time selected by the user. The ring is supplied sterile, while the other components, which do not contact the patient, are supplied no sterile.

To use the device to perform the PNT procedure, the physician first administers topical anesthesia to the patient. The patient is then placed in a supine position, and the eyelids are spread either manually (a speculum can be used but is not recommended). The ring must be positioned to clear the upper eyelid. The ring is then centered on the clear cornea and pressed downward slightly. In addition, to ensure proper suction, the surface of the eye should be wet when the ring is applied.

Once the ring is properly positioned, the physician activates the pump which has been previously set to the

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desired treatment time in seconds. Suction is applied for 60 seconds, followed by a 5 minute rest period, then repeated for an additional 60 seconds. Immediately prior to the end of each treatment period, the ring should be depressed slightly to minimize any discomfort to the patient when suction is released. Patches need not be applied following treatment, but one drop of antibiotic should be administered on completion of the PNT procedure. Tobradex or other steroid containing drops are to be avoided during this study. If steroid-containing drops and/or medications must be administered during the follow-up period, report(s) to that effect, identifying the patient, the eye(s) involved, dosage and duration of treatment must be remitted to the medical monitor.

Patients should be examined at regular intervals (approximately every 3 to 4 months) to determine whether additional treatments are needed to maintain IOP control. Preliminary trials have shown that a repeat of the procedure at 1 week provides a more profound and lasting decrease in IOP

The mechanism of action of PNT is unclear, but there is supporting evidence to show that it acts on the trabecular meshwork. This evidence comes in the form of measured increases in accommodative amplitude in early presbyopic patients undergoing PNT, albeit of a temporary nature. There is corroborating evidence from the studies of Schachar and Thornton in which expansion of the sclera over the cilliary body either by means of implanted plastic ring segments (Schachar) or radial incisions (Thornton) was accompanied by a measured decrease in post-surgical IOP.

As we stated before the mechanism may well involve stretching of the zonule which stretching produces some form of change in the trabecular meshwork either physiologically through chemical mediators, or through a

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mechanical opening of the trabecular pores. Additional evidence that the mechanism of action involves improvement in outflow is that patients who respond well to latanoprost also seem to do well with PNT.

There is no evidence that PNT causes any form of cyclodialysis and no cases of PNT have shown either flare or cells post-treatment.

Clinical Trials

Clinical trials have demonstrated that approximately 75 percent of POAG patients will demonstrate a PNT response. Of these patients, approximately 50 percent will eliminate their need for medication and the remainder will demonstrate a reduction in medication requirements.

Pneumatic trabeculoplasty, when used in combination with antiglaucoma medication, was evaluated in two studies: a feasibility study involving 177 patients, and a separate efficacy study involving 317 eyes.

Both studies were nonblinded, single-armed, and nonrandomized; the primary efficacy end point in each study was a decrease in intraocular pressure (IOP) compared with baseline. The first study reported a mean drop in IOP of 6.3 mmHg across the entire group. The second study showed a mean IOP after PNT treatment level at least 1 mmHg less than the pretreatment mean; except at 3, 6, 9, and 12 months, when it was at least 2 mmHg less than the initial mean IOP. The lesser reduction observed in the second study can be explained by the fact that a number of the patients were at least partially controlled by antiglaucoma medications at enrollment, and, as a result, the group had a lower starting IOP than those enrolled in the first study. In both studies, a clear trend to less medication was observed when PNT was added to a

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patient’s treatment regime. The ability of PNT to reduce IOP and medication requirements, along with its relatively benign safety profile, supports the use of PNT as part of a glaucoma patient’s treatment regimen.

Adverse Events

Adverse events reported following PNT are generally mild in nature and resolve within a few days. Patients receiving PNT typically experience transient ‘gray-out’ of vision sometimes associated with multicolored light patterns during the application of the vacuum ring. These phenomena typically vanish with 30-40 seconds upon release of the vacuum. Patients may experience some mild ocular discomfort (conjunctival hyperemia and conjunctival hemorrhage) following the PNT procedure. This discomfort will typically resolve, without treatment, within a few hours but may last as long as a day or two. Long-term side effects are absent following PNT.

Considerations for Reduction in Anti-glaucoma Medication

Generally speaking, reduction of anti-glaucoma medications can begin three weeks following the PNT repeat application. Given the numerous variations in antiglaucoma medication regimes, it is not possible to recommend a single specific medication reduction strategy.

The Substantive Equivalence of PNT and ALT

Argon laser trabeculoplasty is the procedure probably closest to being equivalent to pneumatic trabeculoplasty in its affect and possible mode of action. Clinical data shows that, overall, PNT produces the same or similar reduction

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in IOP with fewer serious side effects and has the advantage of being both totally noninvasive and repeatable with high success rates in repeated treatment.

PNT lowers IOP in glaucoma patients at least as well, if not better than ALT and with much greater safety since no complications such as those reported to occur following ALT.

CONCLUSION

PNT can produce a significant reduction in IOP; reduction in IOP can be permanent; it is repeatable with similar or greater effect; can reduce or eliminate medication dependency; there is no damage to optic nerve fibers; does not accelerate/produce VF changes; may improve VA in some patients.

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