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Post-Laser Elevation of Intraocular Pressure

Marshall N. Cyrlin, MD

Many of the complications that can occur with traditional incisional surgery have been eliminated by anterior segment laser surgery for glaucoma and for post-cataract surgery capsulotomy. Laser surgery has its own substantial risks. These include inflammation, hemorrhage, synechiae formation, and transient or prolonged intraocular pressure (IOP) elevation. Post-laser IOP elevations may be significant, particularly in the patient with preexisting advanced glaucoma. IOP spikes, if undetected or untreated, can result in further visual loss or blindness.

MECHANISMS OF INTRAOCULAR PRESSURE ELEVATION

Argon laser for iridotomy (ALI) or trabeculoplasty (ALT) interacts with pigmented tissue through its coagulative effect. The laser can contract collagen (lower energies) or vaporize tissue with explosive force (higher energies). Laser treatment can result in the formation of vapor bubbles, pigment debris, and the release of prostaglandins, plasma, or fibrin that can reduce facility of outflow through the trabecular meshwork (TM) by mechanical obstruction or by inciting inflammation.

Selective laser trabeculoplasty (SLT) was developed as a more tissue-specific and possibly safer alternative to ALT. It is performed with a Q-switched, frequency-doubled, 532-nm neodymium:yttrium-aluminum-garnet (Nd:YAG) laser. The potential advantage of SLT over ALT to reduce the complications of laser trabeculoplasty (LTP) is based on its mechanism of selective photothermolysis. This mechanism allows for the selective absorption of short laser pulses to pigmented cells within the TM. This targeted treatment reduces collateral damage to the surrounding tissue of the TM and has been shown in histopathologic specimens to eliminate coagulative damage.^1,2 The lesser amount of tissue interaction and destruction from SLT would be expected to result in a lower incidence and magnitude of IOP elevations. In a study comparing treatments of 180 degrees of the angle with ALT vs SLT, the energy used for the treatment, discomfort during the treatment, and the immediate post-laser inflammation in the anterior chamber were significantly lower for SLT.³

Nd:YAG laser employed for iridotomy or capsulotomy exerts its physical effect on pigmented or nonpigmented tissue by photodisruption with the creation of an explosive, expanding plasma shock wave. IOP elevation may result from the previously noted mechanisms, as well as from hemorrhage or hyphema in the case of iridectomy, or from capsular or cortical debris in the case of capsulotomy.

INCIDENCE OF INTRAOCULAR PRESSURE ELEVATION

Postoperative elevation of IOP may commonly follow either ALI or Nd:YAG laser.^4,5 IOP elevation, which occurs within 2 hours post-laser in 96% of the cases, has been reported to have an incidence from 60% to 65% and a range from 1 to 38 mm Hg over pretreatment levels, with no significant difference found between the type of laser used, total energy employed, or the amount of resulting inflammation.^6,7

IOP elevation after ALT may occur in approximately one-third8 to one-half⁹ of patients, is dependent on the treatment variables, and can be visually threatening if the optic nerve is already compromised by advanced glaucomatous optic atrophy.^10,11 The Glaucoma Laser Trial¹² evaluated the immediate post-laser IOP elevation in 271 eyes of patients with primary open-angle glaucoma who were assigned to ALT as an initial treatment. The patients received 2 treatments, 1 month apart, to 180 degrees of the TM. The treatments were standardized as to power intensity (threshold of bubble formation) and to location (straddling the pigmented and nonpigmented anterior TM). IOPs were measured at 1 hour and 4 hours after each treatment session. In creases of IOP greater than 5 mm Hg occurred in 34% of eyes after one or both treatment sessions, and increases of greater than 10 mm Hg occurred in 12%. Only a small percentage of eyes had IOP rises greater than 5 mm Hg 4 hours after laser treatment without an elevation 1 hour after treatment. Eyes with increases of IOP after the first treatment were more likely to have increases after the second treatment. Of all of the parameters evaluated, only pigmentation of the TM was associated with IOP increases.¹² Other studies have found a greater incidence and severity of post-laser elevations of IOP from treatments of 100 burns delivered to 360 degrees than from 50 burns delivered to 180 degrees.^8,13

Pressure elevations are similarly noted following SLT. An immediate post-SLT pressure spike accompanied by persistent anterior chamber reaction has been described and successfully treated medically.¹⁴ In a large multicenter clinical trial, all cases of post-laser increased IOP responded to medical therapy.¹⁵ Patients with previous ALT may be safely treated with SLT. However, patients with a history of previous ALT and eyes with heavy pigmentation have been reported to be at increased risk for post-laser IOP elevation.¹⁶ Lowering the energy settings to result in only slight cavitation bubbling at the laser spot or reducing the number of laser spots may be helpful in preventing IOP elevations when treating heavily pigmented angles.

After Nd:YAG laser capsulotomy, IOP can rise within the first few hours and rarely may last for weeks or months.^17–20 The amount of laser energy delivered, preexisting glaucoma, pre-laser elevated IOP, absence of an intraocular lens, or the sulcus fixation of a posterior cham ber lens are additional risk factors.

MANAGEMENT

Preoperative pressure measurement and treatment with prophylactic medical therapy are tantamount in the prevention of IOP elevations after anterior segment laser surgery. Management continues with appropriate post- laser IOP monitoring and further medical treatment. Patients currently taking glaucoma medication should be instructed to take their medication before arriving for laser, or it should be administered on arrival. Baseline IOP measurements are taken before the laser treatment is performed. Routinely measure the IOP of the fellow eye to use as a control. Historically, pilocarpine, beta-blockers, oral carbonic anhydrase inhibitors, oral osmotic agents, topical steroids, and nonsteroidal anti-inflammatory agents have been used with little or variable successes for either preoperative prophylaxis or post-laser treatment of IOP elevations.^10,21–27

Apraclonidine hydrochloride, an alpha-2 adrenergic agonist that reduces the rate of aqueous formation, was the first medication to be significantly and reliably effective in reducing postoperative elevations of IOP following laser iridotomy, ALT, or Nd:YAG laser capsulotomy.^10,28–33 In a study of 261 eyes assigned to receive either apraclonidine 1%, pilocarpine 4%, timolol maleate 0.5%, dipivefrin 0.1%, or acetazolamide 250 mg both 1 hour before and immediately after 360-degree ALT, apraclonidine was the only medication to reduce the mean IOP significantly from baseline.³⁴ Only 3% of the apraclonidine eyes had elevations of IOP greater than 5 mm Hg, compared with 39% for acetazolamide, 38% for dipivefrin, 33% for pilocarpine, and 32% for timolol. Bri monidine 0.5%, another alpha-2 adrenergic, was also found to reduce the incidence of post-laser IOP elevation when administered either before or after ALT.^35,36 Increases in IOP greater than 10 mm Hg have been reported in 17% to 27% of laser iridotomy patients successfully managed with apraclonidine.^37,38

Current prophylactic therapy dictates that the laser eye is pretreated with a drop of medication, preferably 0.5% apraclonidine or brimonidine 0.1% to 0.2%, either 30 to 60 minutes preoperatively and immediately postoperatively. The IOP should be measured again in both eyes at 1 and up to 2 hours after the laser. In cases of advanced glaucoma, a 3- or 4-hour post-laser pressure evaluation may be beneficial. Should the IOP become elevated 5 mm Hg or more from the baseline or should it reach a clinically unacceptable level, an additional drop of apraclonidine should be administered as well as any other topical miotic, beta-blocker, or carbonic anhydrase inhibitor that the patient is able to take.³⁹ For eyes unresponsive to the previous regimen or for more marked elevations of IOP in patients with advanced glaucoma, consider administering oral osmotic agents, if available, at a dose of at least 1 g/kg. Patients with significant elevations of post-laser IOP should be monitored until the IOP is reduced to a clinically acceptable level. They should be examined again the following day. Usually, the IOP will return to pretreatment levels or lower within 24 hours. A sustained increase in IOP may result in a very small percentage of patients following LTP and may require increased medical therapy or possibly surgery.

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