Laser in siter keratomileusis (LASIK) is a well-known refractive procedure for correcting myopia, hyperopia, and astigmatism. The advantages of this method are fast and painless visual rehabilitation and lack of haze.1,3 Until recently, the ideal flap thickness was approximately 130 to 160 μm, to allow better surgical manipulations and less intra-operative complications.2
Nevertheless, the possibility of post-LASIK ectasia due to insufficient residual stromal bed and the need for higher refractive corrections, in combination with the trend for bigger ablation zones, wavefront and topography-guided treatments oriented our thinking towards thinner flaps.4 Ultra-thin flap LASIK seems to have several advantages over the traditional flap.10,11 This particular procedure preserves more untreated corneal tissue and therefore might be able to maintain the overall biomechanical integrity of the cornea.
We present a series of patients who underwent LASIK after ultra thin flap creation (less than 70 μm) and developed subepithelial haze during the first postoperative month.
Laser in situ Keratomileusis was performed using the Allegretto Wave Eye-Q, 400 Hz excimer laser (Wavelight, Germany) and the ablation algorithm adopted in all cases was the basic wavefront optimized algorithm for the correction of myopic astigmatism. In all patients, Schwind, Carriazo, Pendular 90 μm head microkeratome was used to create a superior hinged flap. After the microkeratome pass, the flap was lifted and central pachymetry (ultrasound pachymeter Corneo – Gage™ Sonogage Cleveland USA) was performed. Three measurements were taken, and the minimum value obtained was subtracted from the preoperative corneal thickness. The difference calculated equals the flap thickness (subtraction pachymetry).
A soft contact lens was applied in all eyes as a bandage and was removed on the first postoperative day.
All patients were examined 60 minutes postoperatively to check flap adherence. They were given flurbiprofen sodium 0.03% drops (Ocuflur; Allergan, Irvine, Calif) four times daily for 2 days, dexamethasone 0.1%, Tobramycin 0.3% drops (TobraDex, Alcon Laboratories Inc) four times daily for 2 weeks, and sodium hyaluronate 0.18% drops (Vismed; TRB Chemedica, Newcastle under Lyme, United Kingdom) four times daily for 1 month.
Anterior stromal haze was graded subjectively during slit-lamp biomicroscopy and reported as one of five standardized categories: clear, trace (haze seen only with broad-bean illumination), mild (haze visible by slit-beam illumination), moderate (haze somewhat obscuring iris detail), and marked (haze markedly obscuring iris detail).
A 25-year-old man was referred to our institute for myopia and astigmatism. The uncorrected visual acuity (UVA) was Counting Fingers and best spectacle corrected visual acuity (BSCVA) (Snellen scale) 20/20 for both eyes. Manifest refraction was −5.50 − 0.25 × 180 in the right eye and −5.25 − 0.50 × 180 in the left eye. Preoperative corneal thickness was 565 and 576 μm in the right and left eye, respectively, while flap thickness was assessed at 69 and 61 μm. The attempted correction was targeted to emmetropia.
A month after surgery the patient returned to our institute for the first complete postoperative examination. UVA was 20/32 and BSCVA was 20/20 in both eyes (manifest refraction −0.50 in the right eye and −0.75 in the left eye).
On slit-lamp examination, flaps were in place. Mild subepithelial haze was revealed in both eyes (Fig. 1). To investigate the patients’ cornea, confocal microscopy was performed with a modified confocal scanning laser ophthalmoscope (HRT II/Rostock Cornea Module, Heidelberg Engineering). The principle of operation of the modified scanning laser ophthalmoscope (SLO) has been described in previous studies.15 Qualitative evaluation of the images was performed. Corneal images revealed a normal epithelium with flat superficial cells, polygonal intermediate cells and cylindrical basal cells. Images of the flap interface exhibited highly increased scattering due to haze formation (Fig. 2A), whereas activated keratocytes’ bright nuclei with abnormal shape against a dark background were observed just posterior to the flap interface (Fig. 2B). Nonetheless, normal oval-shaped keratocytes were observed at the posterior stroma similar to normal LASIK procedures (Fig. 2C). Anterior segment ocular coherence tomography (optovue Inc Fremont, CA) was performed to confirm flap thickness (Fig. 3).
Figure 1. Slit-Lamp Image One Month Postoperatively Showing Mild Central Subepithelial Haze (case 1).
Figure 2. Confocal Images of the Cornea Revealed Increased Scattering at the Flap Interface at 69 μm Depth (A), Activated Keratocytes Just Posterior to the Flap Interface (B) and Normal Keratocytes at the Posterior Stroma (C).
Figure 3. Anterior Segment Ocular Coherence Tomography (OCT) Confirming Flap Thickness Assessed at 69 μm.
The patient received flouorometholone 0.1% (FML, Alvia Greece) four times a day, which was gradually tapered (1 drop every 10 days).
A significant improvement in corneal haze formation was found during the following three post-LASIK months. On the fourth month’s postoperative examination, both UVA and BSCVA were 20/20 with manifest refraction of −0.25 in the right eye and plano −0.25 × 100 in the left eye.
A 23-year-old woman underwent LASIK for the correction of moderate myopia and astigmatism. Manifest refraction was −7.50 − 1.00 × 5 for the right eye and −8.00 − 1.00 × 175 for the left eye. Preoperative corneal thickness was assessed at 592 and 588 μm for right and left eye and flap thickness was estimated at 62 and 60 μm, respectively.
At first month’s complete postoperative examination, UVA was 20/25 for both eyes and BSCVA was 20/20 with a manifest refraction of −0.75 in both eyes. On slit-lamp examination, flaps were in place and the biomicroscopy revealed the presence of mild subepithelial corneal haze without triggering any discomfort to the patient (Fig. 4). The patient received flouorometholone 0.1% (FML, Alvia Greece) four times per day for 10 days, which was gradually tapered (1 drop every 10 days). Corneal confocal microscopy revealed increased scattering and activated keratocyte’s nuclei at the flap’s interface (Fig. 5). After 4 months, the slit-lamp examination revealed a clear cornea, while UVA was 20/20 in both eyes.
Figure 4. Slit-Lamp Image One Month Postoperatively Showing Subepithelial Mild Corneal Haze (case 2).
Figure 5. Confocal Images of the Second Patient’s Cornea. (A) Increased Scattering at the Flap Interface in an Extension Corresponding to the Presence of Haze, (B) Activated Keratocytes Just Posterior to the Flap Interface, and (C) Normal Keratocytes at the Posterior Stroma.
A 37-year-old man referred to our institute for the correction of moderate myopia. Laser in situ Keratomileusis was performed and the attempted correction was −7.75 for the right eye and −7.50 for the left eye. Corneal pachymetry preoperatively was 592 and 595 μm, respectively, and flap thickness was estimated at 69 μm in the right eye and 65 μm in the left eye.
On first month’s postoperative examination, UVA was 20/32 in both eyes with a manifest refraction of −0.75 in the right eye and −0.50 in the left eye. Slit-lamp examination revealed mild subepithelial corneal haze in both eyes. The patient received flouorometholone 0.1% (FML, Alvia Greece), which was gradually tapered over the following 2 months.
At the fourth month’s postoperative examination, UVA was 20/20 in both eyes. The slit-lamp biomicroscopy revealed a clear cornea without any evidence of haze.
A 24-year-old underwent LASIK for the correction of moderate myopia and astigmatism. Preoperatively, UVA was Counting Fingers for both eyes and BSCVA was assessed at 20/20 on both eyes with manifest refraction of −5.75 − 1.25 × 35 and −5.75 − 0.50 10 on right and left eye, respectively. Corneal pachymetry was 618 μm for the right eye and 620 μm for the left eye. Flap thicknesses were 63 and 62 μm for right and left eye respectively.
During first month’s complete postoperative examination, both UVA and BSCVA were assessed at 20/20 with −0, 50 manifest refraction on right and left eye. During slit-lamp biomicroscopy, flaps were in place, and a mild subepithelial haze was revealed on both eyes of the patient. The patient was satisfied from the outcome of the surgery without citing any complaints. To facilitate the disappearance of haze, the patient received a course of Fluorometholone 0.1% that was gradually tapered during the following month.
The patient turned to our institute the fourth postoperative month. UVA and BSCVA continued to be 20/20 with manifest refraction plano in both eyes. Patient’s cornea was clear with perfectly alligned flaps. There was no evidence of the haze revealed during first month’s biomicroscopy
A-33 year-old man underwent LASIK in our institute for the correction of mild myopia and astigmatism. The UVA was Counting Fingers in both eyes, while BSCVA was 20/20 in both eyes with manifest refraction of −6.50 − 0.75 × 170 on the right eye and −6.50 − 0.25 × 60 on the left eye. The preoperative pachymetric measurements revealed a cornea of 526 μm in the right and 529 μm in the left eye. Flaps were estimated at 53 and 62 μm in right and left eye, respectively. The correction was targeted to emmetropia.
On first month’s postoperative examination, UVA was 20/20 for the right eye and 20/25 for the left eye. The BSCVA was assessed at 20/20 for both eyes with manifest refraction of −0.25 and −0.75 for right and left eye, respectively. Mild subepithelial haze was discovered during the examination and the patient received Fluorometholone 0.1% four times daily that was gradually tapered (1 drop every week) over the next month.
The patient was re-examined after 3 months and both UVA and BSCVA were assessed at 20/20 with manifest refraction plano for the right eye and −0.25 for the left eye. At slit-lamp biomicroscopy, the cornea was clear without any evidence of haze.
Summary of patients’ flap thickness, attempted correction, first month manifest refraction and manifest refraction at last follow-up are presented in Table 1.
Table 1: Patients’ Flap Thickness, Attempted Correction and Manifest Refraction at First and Last Follow-Up Examination
LASIK during the last decades showed increased popularity as a refractive procedure of choice because, in dissociation with PRK, provided faster and painless visual rehabilitation and lacked haze formation. The increasing need for higher refractive corrections with less risk of post-LASIK ectasia has led surgeons to prefer thin and ultra thin flaps. According to studies, an ultra thin flap allows an immediate visual rehabilitation due to less postoperative edema and stability of the refractive result.9
In this case series, patients underwent LASIK with the Schwind Carriazo Pendular microkeratome 90 μm head. Flap thickness was calculated less than 70 μm in all eyes. To the best of our knowledge, this is one of the thinnest flap creations ever published for a mechanical microkeratome or a femtosecond laser.12–13 On the first month’s postoperative examination, slit-lamp evaluation revealed a mild subepithelial haze in all eyes. The objective manifest refraction revealed the presence of a myopic shift that disappeared by the fourth month’s postoperative evaluation. Presumably, the presence of haze can explain the myopic shift incidence and its disappearance after corneal haze reduction.
The incidence of haze formation after ultra thin flap LASIK (less than 70 μm) cannot be determined because this is the first case series ever reported in the literature. Kymionis et al.16 in a prospective study of the Schwind Carriazo Pendular microkeratome 90 μm head did not report any similar finding. Furthermore in the prospective study mentioned above, mean flap thickness was 79.88 ± 6.94 μm (range from 70 to 93 μm) whereas in all cases presented in this case series flap thickness is less than 70 μm.
Haze formation is usually caused by increased keratocyte activity in the anterior stroma and Bowman’s layer. Histologic studies in animals with corneal haze demonstrate abnormal glycosaminoglycans or nonlamellar collagen deposited in the anterior stroma as a consequence of epithelial-stromal wound healing. Subepithelial corneal haze typically peaks in intensity at 1 to 2 months and gradually disappears during the following months.8
Sub-bowman anterior corneal stroma photo-ablation could activate corneal keratocytes resulting in corneal haze. The expression of different inflammation factors, such as transforming growth factor (TGF-1), increases the number of keratocytes that proliferate in the sub-epithelial fibrous layer of the laser-ablated area.14 We can only presume that in these patients, ultra thin flap LASIK may result in haze formation due to increased wound healing response of the anterior stroma and Bowman’s layer.
A temporary subepithelial haze may be caused in the presence of thin flaps as in this particular case report (less than 70 μm). In similar studies with thicker flaps (more than 70 μm), no haze formation was observed postoperatively.9 Other additional predisponding factors such as individual response to healing process could be expressed as factors leading to haze formation.
In conclusion, post-LASIK subepithelial corneal haze after ultra thin flap (less than 70 μm) is a temporary potential complication that could affect patient’s refractive error during the first postoperative month.