Laser technology has revolutionized many medical fields. In ophthalmology, lasers are used to photocoagulate, cut, remove, shrink, and stretch ocular tissues. New types of lasers and novel applications continue to be developed. This article will focus on the Nd:YAG laserand its role in the treatment of ophthalmic disorders.
Laser is an acronym for light amplification by stimulated emission of radiation. A laser contains material that releases photons. This process is amplified so the emitted photons are in phase and produce monochromatic coherent high intensity polarized light. The power is modulated by altering the energy or time (P = E/t). Q-switching and mode-locking refer to methods of increasing laser power by using shutters that synchronize the light phase, compressing output in time.
The Nd:YAG laseris a solid state laser that uses a neodymium-doped yttrium-aluminum-garnet crystal as the lasing medium. It is optically pumped with a lamp or diode and most commonly emits infrared light at 1064nm. It can be used in either a pulsed or continuous mode. Pulsed YAG lasers are typically Q-switched to achieve high-intensity pulses, which can be frequency doubled to emit light at 532nm.
There are numerous ophthalmic applications for Nd:YAG lasers. They are most commonly used to treat posterior capsular opacification after cataract surgeryand to create a peripheral iridotomy in patients with narrow angles or angle-closure glaucoma. YAG lasers can also be used to cut the anterior capsule for capsular block syndrome and capsular phimosis, as well as to cut vitreous strands in the anterior chamber. In malignant glaucoma, disruption of the anterior hyaloid face is performed with the YAG laser, and in refractory glaucomas, these lasers can be used for cyclophotoablation of the ciliary body. They have also been helpful for draining premacular subhyaloid hemorrhages in patients with Valsalva retinopathy. Panretinal photocoagulation can be performed with frequency-doubled Nd:YAG lasers. Other applications include the treatment of recurrent corneal erosions and vitreous floaters.
A more detailed description of some of these procedures follows:
Posterior capsulotomy: When a patient has a visually significant posterior capsular opacity or "secondary cataract", a YAG laseris used to open the posterior capsule centrally. Patients are pretreated with iopidine or Alphagan-P to prevent an IOP spike, and then under topical anesthesia, the laser treatment is performed with a slit-lamp delivery system using an appropriate contact lens (i.e., Abraham capsulotomy YAG lens) to stabilize the eye and focus the laser beam. The energy setting depends upon the density of the capsular opacification, but the typical starting point is 1-2mJ and the energy is then titrated according to the tissue response. The YAG laser causes photodisruption with the shock wave travelling anteriorly. Therefore, most lasers have a focus offset control to allow the surgeon to place the laser beam posterior (up to 250 microns) to the HeNe beam focus point on the capsule. This helps prevent intraocular lens (IOL) pitting. Most surgeons will also place the initial laser spots off-center to avoid inadvertently damaging the IOL near the visual axis.
Anterior capsulotomy:The YAG laser is also utilized to cut the capsule in other conditions. Capsular block syndrome occurs when there is retained viscoelastic in the capsular bag behind the IOL. This causes a myopic shift and is evident on slit-lamp examination as an obvious space between the posterior IOL surface and the posterior capsule. A YAG laser is used to puncture the anterior capsule peripheral to the IOL optic to allow the trapped material to drain. Alternatively, a posterior capsulotomy can be created to achieve the same result. Anterior capsular contraction syndrome or capsular phimosis may occur with a small capsulorhexis. Creating radial anterior capsulotomies with a YAG laser effectively treats this condition.
Peripheral iridotomy:Lasers have long replaced surgical iridectomies for the treatment of angle-closure glaucoma. This noninvasive laser procedure is performed prophylactically in eyes with narrow or occludable angles. The laser energy required ranges from 4-10mJ depending on the iris thickness and pigmentation. Although the YAG laser may produce some bleeding, this laser creates an iridotomy much more easily and efficiently than an Argon laser. A peripheral iridotomy may also be beneficial in pigmentary glaucoma to change the iris configuration.
Vitreolysis:YAG lasers are also used to treat aphakic and pseudophakic malignant glaucoma by disrupting the anterior hyaloid face either peripherally through an iridectomy or centrally through the pupil with 3-11mJ. Laser vitreolysis can also be performed on strands of incarcerated vitreous in the anterior chamber that cause cystoid macular edema. Clear, thin vitreous wicks may be difficult to lyse, so it is best to pretreat with pilocarpine to induce miosis and stretch the incarcerated vitreous, then use bursts of 5-10mJ aimed at a pigmented area of the strand or near the wound. A change in the pupil shape back to round indicates successful vitreolysis.
Corneal stromal reinforcement:For the treatment of recurrent corneal erosions, the Nd:YAG laser may be used as an alternative to anterior stromal puncture with a needle, epithelial debridement with a spatula or alcohol, superficial keratectomy with a diamond burr, or phototherapeutic keratectomy with an excimer laser. The advantages of the YAG laser for this procedure are that it doe not require removal of the overlying loose epithelium, it is less painful than other options, and there is minimal risk of scarring. Laser pulses of 0.3-0.6mJ are placed in the subepithelium or superficial stroma.