Maolong Tang

Keratoconus Diagnosis with Optical Coherence Tomography Pachymetry Mapping

OBJECTIVE To detect abnormal corneal thinning in keratoconus using pachymetry maps measured by high-speed anterior segment optical coherence tomography (OCT). DESIGN Cross-sectional observational study. PARTICIPANTS Thirty-seven keratoconic eyes from 21 subjects and 36 eyes from 18 normal subjects. METHODS The OCT system operated at a 1.3 microm wavelength with a scan rate of 2000 axial scans per second. A pachymetry scan pattern (8 radials, 128 axial scans each; 10 mm diameter) centered at the corneal vertex was used to map the corneal thickness. The pachymetry map was divided into zones by octants and annular rings. Five pachymetric parameters were calculated from the region inside the 5 mm diameter: minimum, minimum-median, inferior-superior (I-S), inferotemporal-superonasal (IT-SN), and the vertical location of the thinnest cornea. The 1-percentile value of the normal group was used to define the diagnostic cutoff. Placido-ring-based corneal topography was obtained for comparison. MAIN OUTCOME MEASURES The OCT pachymetric parameters and a quantitative topographic keratoconus index (keratometry, I-S, astigmatism, and skew percentage [KISA%]) were used for keratoconus diagnosis. Diagnostic performance was assessed by the area under the receiver operating characteristic (AROC) curve. RESULTS Keratoconic corneas were thinner. The pachymetric minimum averaged 452.6+/-60.9 microm in keratoconic eyes versus 546+/-23.7 microm in normal eyes. The 1-percentile cutoff was 491.6 microm. The thinnest location was inferiorly displaced in keratoconus (-805+/-749 microm vs -118+/-260 microm; cutoff, -716 microm). The thinning was focal (minimum-median: -95.2+/-41.1 microm vs -45+/-7.7 microm; cutoff, -62.6 microm). Keratoconic maps were more asymmetric (I-S, -44.8+/-28.7 microm vs -9.9+/-9.3 microm; cutoff, -31.3 microm; and IT-SN, -63+/-35.7 microm vs -22+/-11.4 microm; cutoff, -48.2 microm). Keratoconic eyes had a higher KISA% index (2641+/-5024 vs 21+/-19). All differences were statistically significant (t test, P<0.0001). Applying the diagnostic criteria of any 1 OCT pachymetric parameter below the keratoconus cutoff yielded an AROC of 0.99, which was marginally better (P = .09) than the KISA% topographic index (AROC, 0.91). CONCLUSIONS Optical coherence tomography pachymetry maps accurately detected the characteristic abnormal corneal thinning in keratoconic eyes. This method was at least as sensitive and specific as the topographic KISA. FINANCIAL DISCLOSURE(S) Proprietary or commercial disclosure may be found after the references.

Measuring total corneal power before and after laser in situ keratomileusis with high-speed optical coherence tomography.

PURPOSE: To measure total corneal power using optical coherence tomography (OCT). SETTING: Refractive surgery practices at 2 academic eye centers in Cleveland, Ohio, and Los Angeles, California, USA. METHODS: Thirty‐two eyes of 17 patients having myopic laser in situ keratomileusis (LASIK) were enrolled in a prospective observational study. Manifest refraction, OCT, and Placido ring corneal topography with the Atlas 995 (Carl Zeiss Meditec, Inc.) were performed preoperatively and 3 months after laser in situ keratomileusis (LASIK). A high‐speed (2000 axial scans/second) corneal and anterior segment OCT prototype was used. The total corneal power was calculated by summation of the anterior and posterior surface powers, and the value was compared with that determined by simulated keratometry. Two methods of measuring total corneal power were tested: the direct method, which used OCT to measure both corneal surfaces directly, and the hybrid method, which combined OCT with anterior corneal topography. RESULTS: The repeatability (pooled standard deviation) of measuring total corneal power using the hybrid method was 3 times better than that using the direct method. It was 0.23 diopter (D) before LASIK and 0.26 D after LASIK. Preoperative total power was 1.13 D (2.6%) lower than the simulated keratometry. Compared to the LASIK‐induced change in spherical equivalent refraction, the change in total corneal power was equivalent, while the change in simulated keratometry power was significantly smaller (−18.8%) (P<.001). CONCLUSIONS: Keratometry using the traditional index of 1.3375 overestimated the total power in preoperative corneas and underestimated LASIK‐induced refractive change. Measuring both corneal surfaces using a combination of OCT and Placido ring topography provided a better measure of total corneal power that closely tracked the refractive change in post‐LASIK eyes.

Corneal power measurement with Fourier-domain optical coherence tomography

PURPOSE: To study the accuracy and repeatability of anterior, posterior, and net corneal power measured by Fourier‐domain optical coherence tomography (OCT). SETTING: Doheny Eye Institute, Los Angeles, California, USA. DESIGN: Cross‐sectional study. METHODS: A Fourier‐domain OCT system (RTVue) was used to scan normal eyes, eyes after myopic laser in situ keratomileusis (LASIK), and keratoconic eyes. After the corneal surfaces were delineated, the system calculated anterior and posterior corneal powers by curve fitting over the central 3.0 mm diameter area. Net corneal power was calculated using a thick‐lens formula. The repeatability of the calculations was evaluated by the pooled standard deviation of 3 measurements from the same visit. The net corneal power values were compared with standard automated keratometry measurements (IOLMaster). RESULTS: The repeatability of Fourier‐domain OCT net corneal power was 0.19 diopters (D), 0.26 D, and 0.30 D in the normal, post‐LASIK, and keratoconus groups, respectively. The Fourier‐domain OCT net corneal power was significantly lower than keratometry by a mean of −1.21 D, −2.89 D, and −3.07 D, respectively (P<.001). The anterior–posterior curvature ratio was lower in post‐LASIK and keratoconic eyes than in normal eyes (P<.001). CONCLUSIONS: Corneal power measured by Fourier‐domain OCT achieved good repeatability in all 3 groups. The repeatability was better than slower time‐domain OCT systems. Because Fourier‐domain OCT directly measures both anterior and posterior corneal surfaces, it may produce more consistent results than standard keratometry in post‐LASIK and keratoconic eyes in which the anterior–posterior corneal curvature ratios are altered by surgery or disease. Financial Disclosure: No author has a financial or proprietary interest in any material or method mentioned. Additional disclosures are found in the footnotes.

Optical coherence tomography to assess intrastromal corneal ring segment depth in keratoconic eyes.

PURPOSE: To investigate intrastromal corneal ring segment depth with a high‐speed corneal optical coherence tomography (OCT) system. SETTING: Doheny Eye Institute, University of Southern California, Los Angeles, California, USA. METHODS: A prospective observational case series comprised 4 eyes of 4 patients receiving Intacs intrastromal corneal ring segments (Addition Technology, Inc.) for keratoconus. Optical coherence tomography (OCT) was performed between 7 days and 43 days after implantation. RESULTS: The slitlamp impression of intrastromal corneal ring segment implantation depth did not correlate well with OCT measurements (r2 = 0.68). The fractional implantation depth was correlated with several surgical variables using a stepwise multivariate regression model, and 2 statistically significant correlations were found. The position of the distal portions of the ring segments was shallower than that of the portion closer to the insertion site (P = .003). Segments placed in the inferior cornea (P = .008) experienced more distal shallowing. Shallower depth was associated with greater fractional anterior stromal compression (P = .04). CONCLUSIONS: Shallower placement of intrastromal corneal ring segments may result in more complications, such as epithelial–stromal breakdown and extrusion, because of the greater anterior stromal tensile strain. The distal and inferior portions of intrastromal corneal ring segments tended to be placed at a shallower depth. Optical coherence tomography provided precise measurement of ring segment depth and may help identify implants that pose a greater risk for depth‐related complications.

Pachymetric mapping with Fourier-domain optical coherence tomography

PURPOSE: To evaluate the repeatability of Fourier‐domain optical coherence tomography (OCT) pachymetric mapping and compare central corneal thickness (CCT) measurements by OCT, ultrasound pachymetry, and scanning‐slit tomography. SETTING: Doheny Eye Institute, University of Southern California, Los Angeles, California, USA. METHODS: A Fourier‐domain OCT system was used to map the corneal thickness in normal eyes with scans centered on the corneal vertex or the pupil. Repeatability of central and pericentral map sectors was assessed by pooled standard deviation. The CCT measurements were compared between the OCT, ultrasound, and scanning‐slit devices. RESULTS: Pupil centration (SD: 1.3 μm central, 1.8 to 3.8 μm pericentral) provided better repeatability than vertex centration (SD: 1.7 μm central, 2.4 to 5.7 μm pericentral) in all sectors (P<.035). The mean CCT was 536.9 μm ± 27.0 (SD) by OCT, 556.6 ± 30.5 μm by ultrasound, and 537.2 ± 32.6 μm by scanning‐slit tomography (acoustic factor 0.92). The CCT measured by OCT was significantly thinner than by ultrasound pachymetry (P = .007; mean difference −19.7 μm; 95% limits of agreement [LoA], −40.4 to 0.9 μm) but not than by scanning‐slit tomography (P = .2637; mean difference −0.3 μm; 95% LoA, −24.0 to 23.5 μm). The CCT by OCT correlated well with ultrasound and scanning‐slit CCTs (r = 0.940 and r = 0.934, respectively). CONCLUSION: Pachymetric mapping with Fourier‐domain OCT was highly repeatable. Repeatability was better with pupil‐centered scans than with corneal vertex–centered scans. Ultrasound pachymetry, Fourier‐domain OCT, and scanning‐slit tomography should not be used interchangeably for CCT assessment. Financial Disclosure: No author has a financial or proprietary interest in any material or method mentioned. Additional disclosures are found in the footnotes.

Reproducibility of Tear Meniscus Measurement by Fourier-Domain Optical Coherence Tomography: A Pilot Study

BACKGROUND AND OBJECTIVE To study the reproducibility of tear meniscus measurement with high-speed high-resolution Fourier-domain optical coherence tomography (FD-OCT). PATIENTS AND METHODS Twenty normal participants were enrolled in this prospective study. The lower tear meniscus in the right eye of each subject was imaged by vertical scans centered on the inferior cornea and the lower eyelid using an FD-OCT system (RTVue; Optovue, Inc., Fremont, CA) with a corneal adaptor. The system performs 26,000 axial scans per second and has a 5-micron axial resolution. Each subject was examined at two visits 30 to 60 days apart. Each eye was scanned twice on each visit. The scans were taken 2 seconds after a blink. The lower meniscus height, depth, and cornea-meniscus angle were measured with a computer caliper. The cross-sectional area was calculated using a two-triangle approximation. RESULTS The between-visits coefficient of variation was 17.5%, 18.0%, 35.5%, and 12.2% for meniscus height, depth, area, and angle, respectively. The intraclass correlations for these parameters were 0.605, 0.558, 0.567, and 0.367, respectively. CONCLUSION FD-OCT measures lower tear meniscus dimensions and area with higher between-visits reproducibility than previous OCT instruments. FD-OCT may be a useful way to measure dry eye severity and treatment effectiveness.

Optical coherence tomography of clear corneal incisions for cataract surgery

PURPOSE: To study the architecture of clear corneal incisions for phacoemulsification cataract surgery using optical coherence tomography (OCT). SETTING: Doheny Eye Institute and Department of Ophthalmology, Keck School of Medicine of the University of Southern California, Los Angeles, California, USA. METHODS: This prospective study comprised 20 eyes of 20 patients 1 month after cataract surgery performed by 1 of 2 experienced surgeons. Temporal clear corneal single‐plane incisions were made with 3.0 mm metal keratomes. Each eye was scanned before and 1 month after surgery with a prototype high‐speed anterior segment OCT system (Carl Zeiss Meditec, Inc.). The OCT scans were repeated 3 times during the same visit. The length of the corneal incision, thickness of the cornea, and position of the incision (distance from external wound edge to scleral spur) were measured using a computer caliper. The angle of the incision relative to the corneal surface was then calculated. RESULTS: The mean corneal incision length was 1.81 mm ± 0.27 (SD), the mean corneal thickness at the incision was 747 ± 67μm, and the mean distance between the incision and the scleral spur was 1.46 ± 0.24 mm. The mean angle of the incision was 26.8 ± 5.5 degrees. The measurements were repeatable to within ±0.072 mm (pooled standard deviation) for the incision length, ±11 μm for the corneal thickness, and ±0.042 mm for the position of the incision. CONCLUSIONS: Optical coherence tomography allowed excellent evaluation of corneal incisions in cataract surgery postoperatively. Measurements of wound dimensions using OCT were highly repeatable.

Intraocular lens power calculation after previous myopic laser vision correction based on corneal power measured by Fourier-domain optical coherence tomography.

PURPOSE: To use Fourier‐domain optical coherence tomography (OCT) to measure corneal power and calculate intraocular lens (IOL) power in cataract surgeries after laser vision correction. SETTING: Doheny Eye Institute, Los Angeles, California, and Cullen Eye Institute, Houston, Texas, USA. DESIGN: Prospective comparative case series. METHODS: Patients with previous myopic laser vision correction who had monofocal IOL implantation were enrolled. A Fourier‐domain OCT system was used to measure corneal power and pachymetry. Axial length and anterior chamber depth were measured with partial coherence biometry. An OCT‐based IOL formula was developed, and the mean absolution error (MAE) of postoperative refraction was compared with that for the Haigis‐L formula. At Doheny, corneal power was also measured using the clinical history method, the contact lens overrefraction method, and slit‐scanning tomography total optical power. RESULTS: Sixteen eyes of 16 patients were enrolled at the 2 sites. Previous laser vision correction ranged from −9.81 to −0.88 diopter (D). The MAE was 0.50 D for OCT‐based IOL calculation and 0.76 D for the Haigis‐L formula (P=.14). In the 6 eyes enrolled at Doheny, the MAE of OCT‐based IOL calculation was 0.60 D. In comparison, the contact lens overrefraction (MAE = 1.46 D, P<.05) and clinical history (MAE = 1.78 D, P<.05) methods were worse. Slit‐scanning tomography gave an MAE of 1.28 D (P>.05). CONCLUSION: The predictive accuracy of OCT‐based IOL power calculation was equal to or better than current standards in post‐laser vision correction eyes. Financial Disclosures: Drs. Tang, Li, and Huang receive grant support from Optovue Inc., Fremont, California, USA. Dr. Huang received patent royalty, stock options, travel support, and speaker honorarium from Optovue, Inc., and receives patent royalty from the Massachusetts Institute of Technology related to optical coherence tomography technology licensed to Carl Zeiss Meditec, Inc. Dr. Wang received research support from Ziemer USA, Inc., Alton, Illinois, USA. Dr. Koch is a consultant to Alcon Surgical, Inc., Fort Worth, Texas, USA.

Keratoconus diagnosis with optical coherence tomography–based pachymetric scoring system

Purpose To develop an optical coherence tomography (OCT) pachymetry map–based keratoconus risk scoring system. Settings Doheny Eye Institute, University of Southern California, Los Angeles, California, and Brass Eye Center, New York, New York, USA; Department of Ophthalmology, Affiliated Eye Hospital of Wenzhou Medical College, Wenzhou, China. Design Cross‐sectional study. Methods Fourier‐domain OCT was used to acquire corneal pachymetry maps in normal and keratoconus subjects. Pachymetric variables were minimum, minimum−median, superior–inferior (S–I), superonasal–inferotemporal (SN–IT), and the vertical location of the thinnest cornea (Ymin). A logistic regression formula and a scoring system were developed based on these variables. Keratoconus diagnostic accuracy was measured by the area under the receiver operating characteristic (ROC) curve. Results One hundred thirty‐three eyes of 67 normal subjects and 82 eyes from 52 keratoconus subjects were recruited. The keratoconus logistic regression formula = 0.543 × minimum + 0.541 × (S–I) − 0.886 × (SN–IT) + 0.886 × (minimum–median) + 0.0198 × Ymin. The formula gave better diagnostic power with the area under the ROC than the best single variable (formula = 0.975, minimum = 0.942; P<.01). The diagnostic power with the area under the ROC of the keratoconus risk score (0.949) was similar to that of the formula (P=.08). Conclusion The OCT corneal pachymetry map–based logistic regression formula and the keratoconus risk scoring system provided high accuracy in keratoconus detection. These methods may be useful in keratoconus screening. Financial Disclosure Oregon Health and Science University (OHSU) and Drs. Huang, Li, and Tang have a significant financial interest in Optovue, Inc., a company that may have a commercial interest in the results of this research and technology. These potential conflicts of interest has been reviewed and managed by OHSU. Dr. Brass receives speaker honoraria from Optovue, Inc. No other author has a financial or proprietary interest in any material or method mentioned.

Beveled femtosecond laser astigmatic keratotomy for the treatment of high astigmatism post-penetrating keratoplasty.

Purpose: To use beveled femtosecond laser astigmatic keratotomy (FLAK) incisions to treat high astigmatism after penetrating keratoplasty. Methods: Paired FLAK incisions at a bevel angle of 135 degrees, 65% to 75% depth, and arc lengths of 60 to 90 degrees were performed using a femtosecond laser. One case of perpendicular FLAK was presented for comparison. Vector analysis was used to calculate the changes in astigmatism. Fourier domain optical coherence tomography was used to examine incision morphology. Results: Wound gaping requiring suturing was observed in the case of perpendicular FLAK. Six consecutive cases of beveled FLAK were analyzed. Fourier domain optical coherence tomography showed that beveled FLAK caused a mean forward shift of Bowman layer anterior to the incisions of 126 ± 38 &mgr;m, with no wound gaping. The mean magnitude of preoperative keratometric astigmatism was 9.8 ± 2.9 diopters (D), and postoperatively it was 4.5 ± 3.2 D (P < 0.05). Uncorrected visual acuity improved from 1.24 ± 0.13 logarithm of the minimum angle of resolution preoperatively to 0.76 ± 0.38 postoperatively (P < 0.05). Best spectacle–corrected visual acuity improved from 0.43 ± 0.33 logarithm of the minimum angle of resolution preoperatively to 0.27 ± 0.24 postoperatively (P = 0.22). Visual results were reduced in 2 patients by cataract progression. Between 1 and 3 months after beveled FLAK, the keratometric cylinder was stable (<1 D change) in 5 of 6 patients, and regressed in 1 patient. No complications occurred. Conclusions: Beveled FLAK incisions at varied depth are effective in the management of postkeratoplasty astigmatism. Early postoperative changes stabilized within 1 month in most patients. Further studies are needed to assess long-term outcomes.