Corneal ectasia after photorefractive keratectomy

Graefe’s Arch Clin Exp Ophthalmol DOI 10.1007/s00417-006-0507-z

SHORT COMMUNICATION

Corneal ectasia after photorefractive keratectomy Antonio Leccisotti

Received: 5 August 2006 / Revised: 26 October 2006 / Accepted: 16 November 2006 # Springer-Verlag 2006

Abstract Background Corneal ectasia after photorefractive keratectomy (PRK) has only been occasionally reported, and its incidence has not been assessed. Methods In a retrospective, non-comparative case series, incidence of corneal ectasia was evaluated in a personal consecutive 6453-case series of myopic PRK with a minimum follow-up of 18 months. Features and prognosis were evaluated in all patients with ectasia after PRK (in both personal and referred cases). Results Ectasia was detected in three eyes of two patients (patients 1 and 2) 3 years and 1 year after PRK performed elsewhere. In addition, in the personal PRK series, two eyes of two patients (patients 3 and 4) developed corneal ectasia 5 months after surgery (incidence: 0.03%). Pre-operatively, forme fruste keratoconus was present in patients 1, 3 and 4; keratoconus in the fellow eye in patient 2; pachymetry 1.5 diopters (D) on tangential map in the central 6 mm (in any area within the 6-mm zone, considering the colour coded curvatures displayed on topography) which could not be explained by an irregular excimer ablation (e.g. decentration) or by an underlying corneal disease (e.g. previous scars). A myopic and/or myopic astigmatic refractive change >1 D in the postoperative period was considered essential to confirm the diagnosis, as well as a reduction in best spectaclecorrected visual acuity (BSCVA). Exclusion criteria were: (1) incomplete data or follow-up, and (2) any kind of preoperative corneal scar, including severe post-PRK haze, previous corneal surgery other than PRK (LASIK, incisions, lamellar or penetrating keratoplasty). The pre- and post-operative refractive data, uncorrected visual acuity (UCVA), BSCVA, videokeratography and US pachymetry were reviewed, as well as (when available) the attempted correction, the instrument used, and the postoperative course. Keratoconus stage was assessed by the expert system included in the computed videokeratographer Keratograph (Oculus, Wetzlar, Germany). Evaluated indexes are the following (Oculus Keratograph Instruction Manual, Version 1.53, 2002): – – – – – – –

ISV (index of surface variation): deviation of single corneal radium from average radium (abnormal >37) IVA (index of vertical asymmetry): degree of symmetry of corneal radium with the horizontal meridian as symmetry axis (abnormal >0.28) KI (keratoconus index): elevated in keratoconus (abnormal >1.07) CKI (central keratoconus index): elevated in central keratoconus (abnormal >1.03) Rmin (minimum corneal radium in mm): reduced in keratoconus (abnormal 19). ABR (aberration index): degree of surface aberration calculated by Zernike analysis (abnormal >1). The keratoconus stages are the following:

Stage I: Rmin 7.5 to 6.5, ISV 30 to 55, KI 1.07 to 1.15 Stage II: Rmin 6.9 to 5.3, ISV 55 to 90, KI 1.10 to 1.25 Stage III: Rmin: 6.6 to 4.8, ISV 90 to 150, KI 1.15 to 1.45 Stage IV: Rmin 150, KI>1.5.

Statistical calculations were done by CIA program (BMJ, Bristol, England).

Results Five eyes of four patients met the criteria of post-PRK corneal ectasia, divided as follows: (a) three eyes of two patients (patients 1 and 2) had been referred after PRK performed elsewhere, and (b) two eyes of two patients (patients 3 and 4) were retrieved from our myopic and astigmatic-myopic PRK personal series, which ultimately included 6543 eligible eyes (incidence of ectasia: 0.03%; 95% confidence interval with Wilson method: 0.03 to 0.03%). Patient 1 (female, 38 years old, right and left eye) In 1999, the patient had undergone bilateral PRK elsewhere with an attempted correction of −7 −3 @ 0° in OD, and −6 −4 @ 0° in OS, with a preoperative US pachymetry of 520 μm in the OD and 510 μm in the OS. Pre-operative axial videokeratography showed in both eyes an inferior steepening (1.3 D in RE, and 1.6 D in LE); maximum curvature was 44.3 D in OD and 45.5 in OS. The instrument used, the presumed optical zone and ablation depth are unknown. In March 2002, the patient was referred to our clinic for worsened vision in both eyes. Refraction was −6 −5 @ 55° in OD, and −8 −6 @ 130° in OS, with a BSCVA of 0.6 in OD and 0.4 in OS. Tangential videokeratography showed bilaterally an inferior-temporal ectasia (49 D in OD, 51.5 D in OS); central US pachymetry was 405 μm in OD and 393 μm in OS. Rigid gas-permeable contact lenses were therefore prescribed, achieving a contact lens-corrected visual acuity of 0.9 in both eyes. Patient 2 (male, 31 years old, right eye) The patient had undergone penetrating keratoplasty in the OS for keratoconus elsewhere in 2000, with a resulting refraction of −0.5 @ 90°. The OD had a refraction of −4.5 −1.75 @ 175°, a 1.0 BSCVA, an US central pachymetry of 487 μm, and an axial videokeratography showing a regular bow-tie pattern, with a maximum curvature of 45.75 D. The patient, unable to tolerate a contact lens, underwent PRK in the RE elsewhere in November 2001. The attempted correction, the instrument used, the presumed optical zone and ablation depth are unknown. In December 2002, the patient was referred to our clinic for worsened vision in OD. In the OD, corneal ectasia was so prominent that videokeratography was severely disturbed. BSCVA was 0.2 with −6 −5 @ 90°; central US pachymetry was 382 μm. Deep anterior lamellar keratoplasty was

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performed in February 2003, achieving a regular surface and a BCVA of 0.8 with a +0.75 −1.75 @ 10° refraction. Patient 3 (male, 31 years old) Pre-operatively, the patient had a RE refraction of −3.75 −0.75 @ 10° and a OS refraction of −3.75 −0.5 @ 150°, with a bilateral 1.0 BSCVA. US pachymetry in the OD was 506 μm centrally, 532 μm superiorly (at 3 mm from geometrical centre)

and 545 μm inferiorly (at 3 mm from geometrical centre); in the OS it was 492 μm centrally, 520 superiorly and 515 μm inferiorly. Corneal topography was regular in the OD, and showed a moderate inferior steepening in the OS, with a maximum curvature of 45.7 D. Topographic indexes were normal except for the aberration index in OS; a keratoconus was however ruled out in both eyes by the Keratograph expert system (Fig. 1).

Fig. 1 Patient 3. Pre-operative corneal topography (Oculus Keratograph, Wetzlar, Germany), showing a moderate inferior steepening. Only the aberration index is abnormal; no keratoconus is indicated by the videokeratographer expert system. Top right, axial; bottom right, tangential

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Fig. 2 Patient 3. Five months after photorefractive keratectomy, on the tangential keratography the post-myopic ablation yellow/red ring is irregularly more curved in its inferior part

PRK was carried out bilaterally in December 2003, by Bausch & Lomb Technolas 217c in Planoscan mode. Because of a scotopic pupil (Colvard pupillometry) of 6.5 mm, a 6.5 mm optical zone and an ablation depth of 91 μm were planned in the OS. In April 2004, UCVA was 1.0 in both eyes; the corneal topography was normal in the OD but showed a moderate inferior steepening in the OS (Fig. 2). In May 2006, while the OD had maintained a 1.0 UCVA, the OS had a 0.5 UCVA and a 0.9 BSCVA with a −0.5 −1.5 @ 120° correction. Tangential corneal topography showed a normal myopic ablation pattern in the OD, whereas in the OS the ‘red ring’ was concealed by a 47.5 D steep central-inferior area, similar to a keratoconus pattern. Most keratometric indexes were pathological, and a ‘keratoconus level 1’ was indicated by the Keratograph expert system (Fig. 3). Central US pachymetry was 420 in the OS. Driving spectacles were prescribed. Patient 4 (female, 38 years old, left eye) Pre-operatively, the patient had a OD refraction of −2.5 −0.75 @ 130° and a OS refraction of −1.5 −1.25 @ 130°, with a bilateral 1.0 BSCVA. US pachymetry in the OD was 510 μm

centrally, 550 μm superiorly (at 3 mm from geometrical centre) and 533 μm inferiorly (at 3 mm from geometrical centre); in the OS it was 509 μm centrally, 540 μm superiorly and 525 μm inferiorly. Corneal topography was regular in the OD, and showed a moderate inferior steepening in the OS, with a maximum curvature of 48.3 D (Fig. 4, top). Topographic indexes were normal (Oculus Keratograph), except for the vertical asymmetry index in OS; the keratoconus was considered ‘possible’ by the Keratograph expert system in OS, ruled out in OD. Considering the low correction needed, PRK was carried out bilaterally in January 2005, by Bausch & Lomb Technolas 217z in Planoscan mode. In the OS, a 6-mm optical zone and an ablation depth of 50 μm was planned. In May 2005, while the OD had a 1.0 UCVA, the OS had a 0.5 UCVA and a 0.8 BCVA with a −1 @ 5° correction. Tangential corneal topography showed a normal myopic ablation pattern in the OD, whereas in the OS the ‘red ring’ indicating a myopic ablation was accompanied by a 49.7 D steep inferior area (Fig. 4, bottom), resembling to a keratoconus pattern. Most keratometric indexes were pathological, and a ‘keratoconus level 1’ was indicated by the Keratograph expert system. Central US pachymetry was 443 in the OS.

Graefe’s Arch Clin Exp Ophthalmol

Fig. 3 Patient 3. Two and a half years after photorefractive keratectomy, the post-myopic ablation yellow/red ring has disappeared, replaced by a frankly ectatic pattern. Most indexes are

abnormal, keratoconus stage 1 is indicated by the Keratograph expert system. Top right, tangential; bottom right, axial

Discussion

forme fruste keratoconus (patients 1, 3, and 4); keratoconus in the fellow eye (patient 2); low pre-operative pachymetry (patients 2 and 3); deep ablation (patient 1). Even low myopic attempted PRK corrections (in our patient 4 and in the literature [10]) can induce ectasia in presence of an inferior steepening. The third finding is that ectasia can be a progressive phenomenon, and can induce visual symptoms years after

The first data resulting from our study is that corneal ectasia after PRK is rare, its incidence (approximately 1/3000) being similar to ectasia after LASIK (approximately 1/2500 [2]). Secondly, each observed case had at least one of the predisposing factors for ectasia identified for LASIK [2]:

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ƒFig. 4

Patient 4. Superior half: Pre-operative axial corneal topography, showing a moderate inferior steepening. Inferior half: 5 months after photorefractive keratectomy, on tangential corneal topography the post-myopic ablation yellow/red ring is accompanied by inferior ectasia

PRK (as in our patient 3). This latency was observed also in LASIK-induced ectasia [4]. It has been suggested that in some cases of late ectasia the thinning induced by surgery is followed by chronic inflammation with proteolysis caused by various factors, including eye rubbing [13]. The weakness of the present study is due to its retrospective design and to a minimum follow-up of 18 months, which might have underestimated the real incidence of ectasia after PRK. In addition, the inclusion of referred cases does not help in the determination of the time interval between PRK and ectasia. In conclusion, PRK can induce corneal ectasia in predisposed eyes. Although previous reports have shown a relative safety of PRK in forme fruste keratoconus [5, 6, 14], our series indicates that ectasia can follow even low myopic ablations.

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