Managing femtosecond laser complications

Suction loss, gas bubble interference, unliftable flaps and other complications can be handled.

Issue: September 15, 2006

ByWilliam W. Culbertson, MD

Amar Agarwal

The near-infrared femtosecond laser is a unique instrument, which can produce incisions and lamellar interface planes in the cornea by photodisruption. Contiguous plasma gas bubbles are created in the cornea, which expand and cause microdelamination of the corneal collagen. At the same time, complications can occur. My special guest for this column is William W. Culbertson, MD, professor of ophthalmology at Bascom Palmer Eye Institute, to explain these complications and solutions.

Amar Agarwal, MS, FRCS, FRCOphth

In LASIK flap creation with the femtosecond laser, a female cone-shaped suction ring is placed on the eye to stabilize it, and a male cone-shaped applanation plate is “docked” to it. The cornea is flattened over the area of the intended treatment, and the eye is held stationary by the suction ring. A scanning pattern of laser pulses is placed at a predetermined depth and diameter by moving the beam in either a raster pattern for the lamellar interface or a circular spiral pattern for the side (or edge) cut of the flap.

Nothing moves across the cornea, and the resection is carried out only by scanning the laser beam. The flap interface still has some thin tissue attachments to the underlying bed that are broken when the flap is lifted up.

In comparison to a blade microkeratome, the IntraLase flap has a more predictable thickness and diameter and has a planar contour from side to side. The flap diameter is not influenced by corneal curvature, and the flap centration can be adjusted by the laser software before activation of the laser. As a result of its inherent precision and because nothing moves across the cornea while the flap is being created, minor complications and side effects are minimal, and transient and serious complications are rare.

Suction loss

During the creation of the flap, the IntraLase suction ring may lose vacuum, and the applanation plate may become separated from the cornea. If this occurs during the propagation of the lamellar interface, there is no serious consequence to the flap except that the interface is incomplete. In this case, the suction ring is reapplied, the interface cut is performed, and the side cut is made at the end. If suction is lost during the side cut, then the diameter of the side cut is decreased by 1 mm, the suction ring is reapplied, and the side cut is performed just inside the outside diameter of the lamellar cut.

Interference by gas bubbles

In the anterior chamber. Occasionally, the gas bubbles generated from the intrastromal photodisruption can dissect from the interface through the peripheral cornea and into the anterior chamber via the trabecular meshwork. With the patient supine and the anteroposterior axis of the eye oriented vertically in preparation for flap lifting and excimer laser treatment, the bubbles collect and coalesce in the apex of the anterior chamber, partially obscuring the pupil and the patient’s view of the fixation light (Figure 1).

If the bubble is large enough, it may prevent pupil margin tracking by the laser and inhibit the patient’s ability to fixate. The bubbles absorb into the aqueous humor in 2 to 3 hours and treatment may be completed. Often, the bubbles are small, the edge of the pupil is not obscured, and the patient is able to fixate around the bubble. Then, treatment may proceed without waiting for the bubbles to absorb.

In the cornea. Gas bubbles are routinely formed in the LASIK interface by femtosecond laser photodisruption (opaque bubble layer). These interface bubbles are released when the flap is lifted, and therefore, they do not interfere with treatment. However, sometimes the bubbles dissect into the superficial layers of the stromal bed during propagation of the laser interface (deep opaque bubble layer) (Figure 2). These deep bubbles are not released when the flap is lifted.

Depending on the location of the deep opaque bubble layer, the pupil or iris landmarks may be obscured, preventing either pupil localization for tracking and/or iris landmark-based iris registration. These superficial stromal bed bubbles usually resolve within 30 to 45 minutes. If the opaque bubble layer interferes in pupil tracking or iris registration, then the laser treatment should be delayed until the bubble layer resolves.

Gas bubbles in the anterior chamber obscuring the patient’s view of the laser fixation light.

Gas bubbles deep to the interface in the anterior stromal bed (deep opaque bubble layer).

Images: Culbertson WW

Unliftable flap

Occasionally, the interface is insufficiently dissected, and it is difficult to separate the flap from the underlying stromal bed. Attempts to forcefully open the interface with spatulas and blades may lead to torn flaps or rough or irregular surfaces.

The etiology of the inadequate dissection is uncertain but appears to occur bilaterally in individual patients. When the ophthalmologist is able to forcefully elevate the flap, there often is some keratocyte activation and associated interface haze. The haze is corticosteroid sensitive and resolves with treatment within 3 to 4 months. There is no effect on vision. If the flap appears difficult to lift, then it is reasonable to abort the procedure and replace the already lifted edges of the flap to allow for healing over approximately a 1-month period. The procedure may be reattempted with a blade microkeratome set to cut the flap 50 µm deeper than the original femtosecond flap interface.

Non-dissected islands

If gas bubbles dissect through the stroma anteriorly, the bubbles will come to lie between the applanation plate and the corneal surface. The bubbles will spread ahead of the advancing propagation of the laser raster pattern and block the focused femtosecond laser light. This blocking leaves an undissected zone wherever it occurs. The interface then is not separable in this area. Forceful attempts to delaminate the corneal collagen fibers in this area can result in a tear through to the surface, leaving an isolated “island” of undissected tissue similar to the central islands that may occur with blade microkeratome-created flaps.

This phenomenon of dissection of gas bubbles through the anterior stroma can occur with thin flaps, through incisions such as after RK or penetrating keratoplasty and through scars such as after previous microbial keratoplasty or conductive keratoplasty.

A similar process may occur when there has been a previous surgical lamellar plane created in the cornea, such as from previous LASIK or keratomileusis. In this event, the gas may dissect along this existing intralamellar plane anterior to the intended plane and block the laser. The new plane is not dissected under this area, resulting in what amounts to a partial flap cut. Again, the management in these cases is to not initially attempt to lift the flap, allow it to heal for 6 weeks and then recut the flap with a blade microkeratome at a level at least 50 µm deeper or more superficial to the original femtosecond laser plane.

Transient light sensitivity

There are two complications that are encountered after LASIK with the IntraLase laser. The first is transient light sensitivity or good acuity photophobia syndrome, in which patients with good vision develop photophobia in the absence of any apparent finding on examination. Corticosteroid drops are prescribed, and symptoms improve within 1 week of treatment. Invariably, symptoms resolve with or without treatment, leaving no residual abnormality or symptoms. Its etiology is unknown, and speculation is varied among keratocyte activation to laser-induced iritis, scleritis or neuritis. The majority of patients feel more comfortable wearing sunglasses during the 2 to 6 weeks that it takes to resolve. The incidence of this symptom is approximately 1%.

Keratitis

Intrastromal inflammation may be localized around the edge of the flap, which can occur 2 to 7 days after flap creation. The corneal stromal tissue becomes hazy or white along the side cut, and there is associated cellular infiltration in the interface and in the superficial cornea in a narrow band along the edge of the flap. There may be some associated photophobia. Presumably this inflammation results from microscopic corneal tissue damage caused by the laser photo disruption, perhaps exaggerated by exogenous inflammatory factors in the tear film.

Although this process may share some features with diffuse lamellar keratitis, it is differentiated by its later onset and stromal inflammation outside the edge of the flap. Treatment consists of frequent topical corticosteroid drops and adjunctive measures. In mild to moderate cases, the process resolves without sequelae. In rare cases, the inflammation is severe, and scarring may develop in the area of the side cut and haze in the interface.

For more information:

Amar Agarwal, MS, FRCS, FRCOphth, is director of Dr. Agarwal’s Group of Eye Hospitals. Dr. Agarwal is the author of several books published by SLACK Incorporated, publisher of Ocular Surgery News, including Phaco Nightmares: Conquering Cataract Catastrophes, Bimanual Phaco: Mastering the Phakonit/MICS Technique, Dry Eye: A Practical Guide to Ocular Surface Disorders and Stem Cell Surgery and Presbyopia: A Surgical Textbook. He can be reached at 19 Cathedral Road, Chennai 600 086, India; fax: 91-44-28115871; e-mail: dragarwal@vsnl.com; Web site: www.dragarwal.com.

William W. Culbertson, MD, can be reached at P.O. Box 016880, Miami, FL 33101; 305-326-6364; e-mail: w.culbertson@miami.edu. Dr. Culbertson is on the Medical Advisory Board of IntraLase and has received speaking honoraria and travel expenses from IntraLase. He is also a clinical investigator for AMO-Visx and has received research support, speaking honoraria and travel expenses.

References:

Agarwal A. Handbook of Ophthalmology. Thorofare, NJ: SLACK Incorporated; 2005.

Agarwal A. Refractive Surgery Nightmares. Thorofare, NJ: SLACK Incorporated; 2007 (in press).

Agarwal S, Agarwal A, Agarwal A. Textbook on Corneal Topography. Jaypee, India; 2005.