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Central islands after LASIK can successfully be treated surgery
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March 1998
Central islands are the most common topographical abnormality observed in postoperative laser in situ keratomileusis (LASIK) patients. Central islands are defined as elevations that occur centrally in the postoperative corneal topography.
They usually have the following characteristics:
- 1 D to 5 D of focal steepening in the corneal topographical findings.
- 1 mm to 4 mm in diameter.
- Onset about 1 month postoperatively.
- Symptoms of monocular diplopia and ghost imaging.
Most commonly associated with broad-beam lasers, central islands may cause serious visual disturbances to the patient. Their etiology appears to be multi-factorial, and several theories have attempted to explain their origin. The most commonly accepted theories are the acoustic wave theory of Jeff Machat, MD, the differential hydration theory of David Lin, MD, and the vortex plume theory.
Cornea not homogeneous
According to the differential hydration theory, the cornea is composed of 70% water and is not homogeneous in nature. The central stroma is more hydrated than the peripheral stroma. During the ablation process, moisture diffuses through the cornea from the anterior chamber and accumulates centrally in the stromal bed.
Moisture absorbs excimer laser energy and results in less corneal ablation. Therefore, if the laser is applied evenly across the cornea, less ablation will occur centrally because of the moisture, resulting in a central island.
Attentiveness to fluid management during the ablation process and central flap compression are surgical techniques that help prevent or reduce central island formation. Also, some laser software programs incorporate a central pretreatment ablation to reduce the incidence of central islands. More fluid accumulates in cases of higher refractive error because of the time and depth of ablation.
Surgical intervention
---The Corneal Smoother is placed over a soft contact lens bandage surface in the area of the central island. Pressure is applied centrally as the instrument is rotated in a circular manner. The treatment should be applied for 60 seconds, noting striation on the retroilluminated cornea.
With photorefractive keratectomy, most central islands resolve spontaneously and do not require surgical intervention. This is because the loss of Bowman's layer stimulates a greater healing response. In contrast, LASIK takes place intrastromally and stimulates less healing response. This may be the reason why central islands tend not to resolve in the LASIK procedure. Surgically, central islands can be treated by lifting the flap and ablating over the area of steepness where the central island is located.
Noninvasive treatment
Central islands can now successfully be treated without surgery with a new instrument called a Corneal Smoother by Micra USA Inc. The inventor, Nick Caro, MD, from Chicago, has used the Corneal Smoother postoperatively to massage out irregularities of the flap with great success. The Corneal Smoother has a 5-mm button, angled 30° from the handle. The button is placed centrally, compressing the cornea, then a squeegee motion outward (toward the flap edges) is used to gently massage irregularities out of the flap immediately after surgery. I have found the instrument may also be effective in massaging away central islands.
Case report 1
A 27-year-old man had a preoperative cycloplegic refraction of -6 D sphere in his right eye and a -6.50 -0.50 X 168 in the left eye. Both eyes were correctable to 20/20.
After a complete evaluation, the patient was scheduled for bilateral LASIK surgery. The surgical procedure was unremarkable.
Postoperatively, the patient was given a topical antibiotic/steroid suspension four times a day for 1 week. After approximately 1 month, the patient complained of decreased vision in the right eye with ghosting images. The patient's manifest refraction in the right eye was +0.75 -0.50 X 105 with best-corrected visual acuity of 20/40. The left eye was -0.25 -0.50 X 35, yielding visual acuity of 20/20.
Topographical mapping indicated the presence of a central island in his right eye. It was about 5 D in steepness and about 2.5 mm in size. A soft bandage contact lens was placed on the eye, and one drop of topical anesthetic was instilled. The soft bandage contact was used to protect the epithelium during the procedure.
The Corneal Smoother was placed over the soft contact lens bandage surface in the area of the central island. Pressure was applied centrally as the instrument was rotated in a circular manner. The treatment was applied for 60 seconds, noting striation on the retroilluminated cornea. The flap should not dehisce or dislocate. The soft contact lens was removed.
The patient's uncorrected vision immediately improved from 20/40 to 20/25. Post-procedure topography showed significant reduction of the central island. Two weeks later, there was improvement in the size and steepness of the central island when compared to the pretreatment topography. Uncor-rected visual acuity in the right eye was 20/25 with a refractive error of +0.50 -0.75 X 95, yielding 20/20 vision. Because of the residual central island, a second treatment was applied with the Corneal Smoother.
Case report 2
A 43-year-old woman had a preoperative refraction of -0.50 -2.75 X 90 in her right eye and a -2.50 -0.75 X 75 in the left eye. Both eyes were correctable to 20/20. After a complete evaluation, the patient was scheduled for bilateral LASIK surgery. The surgical procedure was unremarkable.
The Visx Star laser and a Chiron LASIK microkeratome were used. Postoperatively, the patient was given a topical antibiotic/steroid suspension four times a day for 1 week. After approximately 1 month, the patient complained of blurry and double vision in the right eye. The patient's manifest refraction was -0.25 -1.00 X 45 with a best-corrected visual acuity of 20/25. The left eye was unremarkable at 20/20 uncorrected.
Topographical mapping indicated the presence of a central island in the right eye. It was approximately 3 D in steepness and approximately 1 mm in size. The corneal compression technique was applied. At 2 months follow-up, the patient had no diplopia, and vision was 20/30 uncorrected. The patient's manifest refraction was plano -0.50 x 60 corrected to 20/20.
Use serial topography
The corneal compression technique is a noninvasive procedure for enhancing central islands without additional ablation and surgical flap manipulation. Exceedingly vigorous force with the flap smoother may result in lacquer cracks in Bowman's layer and flap displacement or detachment, so a conservative approach is recommended. Endpoint flattening of the central island is much easier to ascertain with the aid of serial topography, an essential tool for this procedure. One should not forget the importance of monitoring the long-term effectiveness of corneal compression.
It is hoped that advancements in the excimer laser delivery systems, such as scanning capabilities, will decrease the incidence of central islands. Refinements in the LASIK surgical technique, such as attentiveness to the accumulation of fluid on the stromal bed during the procedure, should reduce the risk of central island formation. With the aid of the corneal flap smoother, the clinician has a new, noninvasive technique to manage central island formation.
For Your Information:
- The Corneal Smoother is available from Micra USA Inc., 2950 Niagara Falls Blvd., North Tonawanda, NY 14120; (800) 323-1177; fax: (716) 743-2702.