CLINICAL SCIENCE
Corneal Densitometry in Keratoconus Bernardo Lopes, MD,*†‡ Isaac Ramos, MD,*† and Renato Ambrósio, Jr, MD, PhD*†‡
Purpose: The aim of this study was to compare corneal densitometry measured by Scheimpflug tomography in normal and keratoconic eyes and to assess the differences in densitometry values among the stages of keratoconus.
Methods: Keratoconic and normal corneas were examined using the Pentacam. Corneal densitometry was measured over a 12-mm diameter area, divided by annular concentric zones and depths. Keratoconus was classified according to the topographic keratoconus classification. Results: We enrolled 1 eye randomly selected from each of 172 patients with normal corneas (N) and 98 patients with bilateral keratoconus (KC). There were significant differences between the groups for densitometry measurements in 2 annuli: central 2.0 mm in diameter (N = 16.85 6 2.42, KC = 18.93 6 2.78, P = 0.0001) and annulus 2.0 to 6.0 mm in diameter (N = 15.18 6 2.18, KC = 16.16 6 1.71, P = 0.005), and total diameter (N = 24.89 6 6.18, KC = 16.71 6 2.3, P = 0.033). Divided by layers, the inner parts of anterior (120 mm), central (from 120 mm to the last 60 mm), and posterior (last 60 mm) layers were also higher in the KC group (P , 0.001). There were differences according to the stages of KC for corneal densitometry of the central annuli at total thickness, anterior and central layers. More advanced cases presented a higher backscatter (P , 0.05). The anterior layer presented the smallest overlap between groups and KC stages.
Conclusions: The densitometry map reveals that light backscatter was higher in the central portion of the anterior keratoconic cornea than in the normal cornea. The densitometry level is higher in more advanced stages. Key Words: corneal densitometry, light backscatter, keratoconus, Scheimpflug imaging (Cornea 2014;33:1282–1286)
S
cheimpflug imaging has a large number of applications in corneal diagnosis. Corneal and anterior segment tomography based on a rotating Scheimpflug camera allows for imaging of both the anterior and posterior corneal surfaces, a full
Received for publication May 28, 2014; revision received August 1, 2014; accepted August 6, 2014. Published online ahead of print October 2, 2014. From the *Rio de Janeiro Corneal Tomography and Biomechanics Study Group, Rio de Janeiro, Brazil; †Instituto de Olhos Renato Ambrósio, Rio de Janeiro, Brazil; and ‡Department of Ophthalmology, Federal University of São Paulo, São Paulo, Brazil. R. Ambrósio is a consultant for Oculus Optikgeräte GmbH. The other authors have no funding or conflicts of interest to disclose. Reprints: Bernardo Lopes, MD, Rua Conde de Bonfim 211/712 Tijuca, Rio de Janeiro, RJ, Brazil 20520-050 (e-mail: [email protected]). Copyright © 2014 by Lippincott Williams & Wilkins
1282
| www.corneajrnl.com
pachymetric map, and thus additionally, with new software, it is possible to measure the amount of backscattered light in the different regions of the cornea. A normal cornea is expected to scatter light mostly in the interfaces air/cornea and cornea/water where there are higher differences in light refraction indices with only minimum scatter by the structures of corneal tissue. This optically transparent tissue results from regular spacing, small uniform diameter of orthogonally arranged collagen fibers, and balanced keratocyte components.1,2 Keratoconus is an ectatic disease that produces several changes in the cornea. These changes disturb the fragile balance of well-distributed corneal extracellular matrix and cells and lead to the thinning and anterior protrusion of the cornea.3,4 In advanced cases, this disarrangement may lead to corneal scars with evident opalescence. But mild cases are supposed to present increased backscatter detectable by noninvasive Scheimpflug analysis.5 In this study, we examined the difference in backscattered light between normal and keratoconic corneas and the distribution of corneal densitometry in different stages of the disease.
PATIENTS AND METHODS Patient Inclusion The retrospective observational study reviewed the chart of patients examined at the Instituto de Olhos Renato Ambrósio (Rio de Janeiro, Brazil). It followed the tenets of the Declaration of Helsinki and was approved by the ethics committee of the Federal University of São Paulo, Brazil. Patients were selected from a database of cases diagnosed as having bilateral keratoconus (KC) and also a database of preoperative patients with a stable cornea after at least 1 year of Lasik (normal corneas). All eyes were examined by a fellowship trained cornea and refractive surgeon (R.A.). Patients with KC were defined as those having the following ocular findings in both eyes3,4: typical Placido disc-based videokeratographic findings as paracentral inferior–superior asymmetry and/or asymmetric bowtie pattern, with or without skewed axes and at least 1 clinical sign including stromal thinning, conical protrusion of the cornea, Fleisher ring, Vogt striae, enlarged corneal nerves, increased intensity of the corneal endothelial reflex, subepithelial fibrillary lines, and Munson and Rizzuti signs. Contact lens wear was discontinued at least 3 weeks for rigid contact lens and 1 week for soft contact lens before the assessment. Patients with a history of eye surgery, eye disease, extensive corneal scarring, connective tissue disease, pregnancy at the time of the measurements, and with a history of pregnancy in the past 12 months were excluded. Cornea Volume 33, Number 12, December 2014
Cornea Volume 33, Number 12, December 2014
Corneal Densitometry in Keratoconus
FIGURE 1. Corneal densitometry display in the Pentacam.
Corneal Tomography All eyes were examined using rotating Scheimpflug corneal and anterior segment tomography (Pentacam HR, Oculus GmbH, Wetzlar, Germany). An expert examiner, who was masked to the clinical condition of the patient, acquired Pentacam images as follows: The patient’s chin was placed on the chin rest, and the forehead was placed against the forehead strap. After blinking a few times, the patient was asked to open both eyes and stare at the fixation target. Proper alignment was obtained using a joystick, and then the automatic release mode started the scan using 25 single Scheimpflug images captured within 2 seconds for each eye. Image quality was checked, so that only cases with acceptable quality images were included in the study. The Pentacam provides a topographic keratoconus classification for staging the disease in grades 1 to 4 with 3 subgrades (1–2, 2–3, and 3–4). Cases were divided into mild, less than grade 2; moderate, from grades 2 to 3; and severe, grades higher than 3 (3–4 and 4). An add-on to standard software of Pentacam provides corneal densitometry analysis (Fig. 1). It measures the backscattered light over a 12-mm-diameter area. Zonal densitometry can be measured by 4 annular concentric zones centered in the apex of the cornea. The first central zone covers the annulus with a 2-mm diameter, the second covers the annulus that extends from a 2- to 6-mm diameter, the third covers the one from a 6- to 10-mm diameter, and the last zone is composed of the annulus that extends from a 10- to 12-mm Ó 2014 Lippincott Williams & Wilkins
diameter. The analysis is also performed by depth in 3 layers: the anterior layer, which corresponds to the 120mm superficial corneal thickness; the posterior layer, which corresponds to the most posterior 60 mm; and the central, which has no fixed thickness value, and is the zone between the anterior and posterior layers. Densitometry is expressed in gray scale units, ranging from a minimum light scatter of 0 (maximum transparency) to a maximum light scatter of 100 (minimum transparency).
Statistical Analysis Sample size and power calculations were performed. Patients were first divided into 2 groups: Normal and Keratoconus. Normality check was accomplished using the Kolmogorov–Smirnov goodness-of-fit test. As variables were nonnormally distributed, Mann–Whitney U test was used to assess the differences between the groups. The KC group was divided into 3 groups by disease staging. The difference between the groups was assessed with the Kruskal–Wallis test and post hoc Dunn test. Analysis was carried out using the software MedCalc 11.1 (MedCalc Software, Mariakerke, Belgium).
RESULTS Two hundred and seventy patients were included in the study: 172 patients with normal corneas and 98 patients with KC. There were 85 men and 87 women in the normal group, www.corneajrnl.com |
1283
Cornea Volume 33, Number 12, December 2014
Lopes et al
FIGURE 2. A, Corneal densitometry at total thickness and total diameter. B, Corneal densitometry at the central 2.0-mm annulus of the anterior cornea (120 mm).
and 53 men and 45 women in the KC group. The subjects’ age in the normal group was 36.1 6 14.6 years (range, 11– 82), and that in the KC group was 29.6 6 11.2 years (range, 10–74 years). In the KC group, there were 41 patients with
mild KC, 47 patients with moderate KC, and 10 patients with severe KC. Total corneal light backscatter was higher in the KC group (KC = 17.82 6 4.57, N = 16.71 6 2.3, P = 0.033). The densitometry map revealed a statistically significant difference between the groups in stratified analysis with less overlap between the groups. Considering the total thickness of the cornea, separating the densitometry by concentric annular zones, the backscatter at the first 2 annuli (center to 2 mm and 2–6 mm diameter) was statistically higher in the KC group (KC = 19.87 6 5.07, N = 16.58 6 1.8, P , 0.001; KC = 16.64 6 2.99, N = 14.89 6 1.61, P = ,0.001, respectively). Dividing the cornea into 3 layers, they showed the same behavior with a higher backscatter in the central cornea (P , 0.05) in the KC group. The differences were marked in the 2 central annuli in the 3 layers. The central annulus in the anterior layer showed the smallest overlap between the groups. These results are summarized in Figure 2 and Table 1. The analysis, considering different stages of KC, showed the same pattern when comparing the 3 groups with normal corneas. A statistically significant difference was present at the 2 central annuli in total thickness and in the 3 layers (P , 0.0004) with a trend to increase densitometry as the KC stage evolves. At the anterior layer, the post hoc test revealed that the higher values of KC moderate and severe were statistically significant in comparison with those of KC mild (2-mm central annulus: KC severe 37.26 6 11.77, KC moderate = 30.53 6 8.16, KC mild = 25.34 6 3.49, P , 0.05). Table 2 and Figure 3 summarize these results.
TABLE 1. Corneal Light Back Scatter in Gray Scale Units KC (n = 98) Total thickness
Anterior layer 120 mm
Central layer
Posterior layer 60 mm
Annulus 0.0–2.0 mm Annulus 2.0–6.0 mm Annulus 6.0–10.0 mm Annulus 10.0–12.0 mm Total diameter Annulus 0.0–2.0 mm Annulus 2.0–6.0 mm Annulus 6.0–10.0 mm Annulus 10.0–12.0 mm Total diameter Annulus 0.0–2.0 mm Annulus 2.0–6.0 mm Annulus 6.0–10.0 mm Annulus 10.0–12.0 mm Total diameter Annulus 0.0–2.0 mm Annulus 2.0–6.0 mm Annulus 6.0–10.0 mm Annulus 10.0–12.0 mm Total diameter
N (n = 172)
Mean 6 SD
Range
6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6
13.8–47.3 11–34.1 9.1–53.7 6.7–59.5 11.4–40.3 17.3–69.9 14.3–51.3 12.6–69.9 8.2–82.4 14.8–54.9 11.4–49.3 9.8–28.7 7.7–53.5 6.1–56.1 9.4–37.2 9.3–31.4 8.7–24 7–37.7 5.7–42.5 8.4–28.8
19.87 16.64 15.78 22.51 17.82 29.04 24.04 21.21 30 24.89 16.84 13.88 13.71 20.37 15.35 13.72 12.01 12.45 17.16 13.23
5.07 2.99 6.74 8.96 4.57 8.06 4.44 8.58 12.68 6.18 5.16 2.96 6.85 8.92 4.56 3.24 2.37 5.1 7.14 3.51
Mean 6 SD
Range
P
6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6
(12.8–22) (10.8–19.8) (10.2–26) (10.3–48) (11.7–25.1) (16.1–31.4) (15.4–29.6) (13.1–34.8) (13.6–59.4) (15.7–30.5) (9.9–22.4) (8.4–19.2) (8.3–25.5) (9.6–45) (9.5–24.8) (9.6–18.1) (8.2–15.2) (7.6–25) (7.7–44) (9.1–23.3)
,0.001 ,0.001 0.112 0.063 0.033 ,0.001 ,0.001 0.424 0.080 0.160 ,0.001 ,0.001 0.208 0.059 0.999 ,0.001 ,0.001 0.166 0.059 0.217
16.58 14.89 15.38 23.16 16.71 23.47 21.3 20.6 31.27 23.08 13.94 12.39 13.21 20.5 14.28 12.33 10.98 12.33 17.69 12.77
1.8 1.61 3.29 5.97 2.3 2.99 2.56 3.74 8.65 2.9 2.21 1.93 3.43 5.6 2.54 1.38 1.24 3.19 5.52 2.25
KC indicates keratoconus; N, normal corneas; bold, statistically significant results.
1284
| www.corneajrnl.com
Ó 2014 Lippincott Williams & Wilkins
Cornea Volume 33, Number 12, December 2014
Corneal Densitometry in Keratoconus
TABLE 2. Values by Keratoconus Stage N (n = 172) Total thickness
Anterior layer 120 mm
Central layer
Posterior layer 60 mm
Annulus 0.0–2.0 mm Annulus 2.0–6.0 mm Annulus 6.0–10.0 mm Annulus 10.0–12.0 mm Total Diameter Annulus 0.0–2.0 mm Annulus 2.0–6.0 mm Annulus 6.0–10.0 mm Annulus 10.0–12.0 mm Total Diameter Annulus 0.0–2.0 mm Annulus 2.0–6.0 mm Annulus 6.0–10.0 mm Annulus 10.0–12.0 mm Total diameter Annulus 0.0–2.0 mm Annulus 2.0–6.0 mm Annulus 6.0–10.0 mm Annulus 10.0–12.0 mm Total diameter
KC Mild (n = 41)
Mean 6 SD
Range
6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6
16.1–31.4 15.4–29.6 13.1–34.8 13.6–59.4 15.7–30.5 9.9–22.4 8.4–19.2 8.3–25.5 9.6–45 9.5–24.8 9.6–18.1 8.2–15.2 7.6–25 7.7–44 9.1–23.3 12.8–22 10.8–19.8 10.2–26 10.3–48 11.7–25.1
23.47 21.3 20.6 31.27 23.08 13.94 12.39 13.21 20.5 14.28 12.33 10.98 12.33 17.69 12.77 16.58 14.89 15.38 23.16 16.71
2.99 2.56 3.74 8.65 2.9 2.21 1.93 3.43 5.6 2.54 1.38 1.24 3.19 5.52 2.25 1.8 1.61 3.29 5.97 2.3
KC Moderate (n = 47)
Mean 6 SD
Range
6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6
17.3–36 14.3–32.4 12.6–55 13.2–65.1 14.8–45.3 11.4–26.4 9.8–22.3 7.7–46.3 9.7–45.7 9.4–34.4 10–22 8.7–18.1 7–37.5 7.9–42.5 8.4–27.6 13.8–25.3 11–23.7 9.1–46.3 11–49.3 11.4–35.8
25.34 22.47 20.76 30.15 23.59 15.88 13.7 13.53 19.93 14.97 13.55 11.73 12.56 17.29 13.19 18.27 15.97 15.61 22.46 17.25
3.49 3.43 6.72 10.7 4.96 3.24 2.73 5.96 7.83 4.08 2.39 1.94 4.98 7.37 3.33 2.61 2.37 5.77 8.06 3.91
Mean 6 SD
Range
6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6
23.5–69.9 20.3–51.3 14.6–69.9 8.2–82.4 19.5–54.9 11.5–40.7 10.1–28.7 8.6–53.5 6.1–56.1 10.7–37.2 9.3–31.4 9.8–22.3 7.6–37.7 5.7–40.1 9.2–28.8 15.4–47.3 13.6–34.1 10.5–53.7 6.7–59.5 13.3–40.3
30.53 25.2 22.32 30.78 26.02 16.69 13.92 14.25 20.48 15.56 13.54 12.1 12.69 17 13.29 20.26 17.07 16.41 22.75 18.29
8.16 5.09 10.39 14.3 7.3 4.71 3.11 8 9.75 5.21 3.53 2.27 5.39 7.18 3.74 5.23 3.35 7.85 9.87 5.31
KC Severe (n = 10) Mean 6 SD
Range
6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6
25.4–64.7 20.7–30 15.3–26.5 12.4–48.5 19.8–28.7 16.3–49.3 11.6–22.9 10–20.2 10.2–35.2 12.5–22 11.2–26.8 10.2–24 8.4–21.6 9–28.5 10.8–22 19.2–46.9 14.7–25.6 11.1–22.8 10.6–34.9 14.5–24
37.26 25.03 17.82 27.7 24.89 21.44 14.4 11.93 21.66 15.88 15.28 12.71 10.86 17.4 13.2 24.65 17.38 13.49 21.58 18
11.77 2.46 3.19 11.69 3.13 9.42 3.01 2.96 8.83 2.7 4.26 3.82 3.77 5.85 3.15 7.85 2.92 3.3 7.85 2.66
Post Hoc Test
Total thickness
Anterior layer 120 mm
Central layer
Posterior layer 60 mm
P
N vs. KC Mild
N vs. KC Moderate
N vs. KC Severe
Mild vs. Moderate
Mild vs. Severe
Moderate vs. Severe
,0.0001 ,0.0001 0.0554 0.1265 0.0335 ,0.0001 ,0.0001 0.5332 0.3135 0.2819 0.0001 0.0003 0.1628 0.2680 0.9946 ,0.0001 ,0.0001 0.1086 0.3123 0.3799
P , 0.05 P , 0.05 NS NS NS P , 0.05 P , 0.05 NS NS NS P , 0.05 P , 0.05 NS NS NS P , 0.05 P , 0.05 NS NS NS
P , 0.05 P , 0.05
P , 0.05 P , 0.05
P , 0.05 P , 0.05
P , 0.05 P , 0.05
NS NS
P , 0.05 P , 0.05 P , 0.05
NS P , 0.05 P , 0.05
NS NS P , 0.05
NS P , 0.05 P , 0.05
NS P , 0.05 NS
P , 0.05 P , 0.05
P , 0.05 P , 0.05
NS NS
NS NS
NS NS
P , 0.05 P , 0.05
P , 0.05 P , 0.05
P , 0.05 NS
P , 0.05 NS
P , 0.05 NS
KC indicates keratoconus; N, normal corneas; bold, statistically significant results.
DISCUSSION The analysis of light backscatter has gained increasing relevance in corneal diagnosis. It has been described in postrefractive surgery,6–8 infectious keratitis,9 corneal dystrophies,10 corneal graft surgery,11,12 and for evaluating the crosslinking result in patients with KC.13,14 In the clinic it is also used complementarily to endothelial cell count Ó 2014 Lippincott Williams & Wilkins
in monitoring the disease progression in cornea guttata patients.15 Corneal transparency is caused by complex mechanisms; regular-arrangement and size regularity of collagen fibrils are part of it.1,16 High levels of corneal light backscatter can be seen even in corneas considered clinically clear.17 The disarrangement in corneal histology caused by KC can also alter densitometry levels.18 www.corneajrnl.com |
1285
Lopes et al
Cornea Volume 33, Number 12, December 2014
this region were the weakest in a normative study.5 Variation in white-to-white distance also occurs, leading to the mistaken examination of portions of the limbus and sclera, causing higher densitometry values. Scheimpflug tomography provides an automated, rapid, and reliable method for assessing corneal densitometry. Although corneal light backscatter is unclearly correlated with forward light scatter, corneal densitometry may play a valuable role in characterizing keratoconic corneas. REFERENCES
FIGURE 3. A, Corneal densitometry at the central 2.0-mm annulus of the anterior cornea (120 mm) for the different stages of keratoconus. B, Corneal densitometry at the 2.0- to 6.0-mm annulus of the anterior cornea (120 mm among the different stages of keratoconus).
In our study, we found an increase in light backscatter in the central cornea (6 mm in diameter) and total diameter in the KC group (2-mm central annulus: KC = 19.87 6 5.07, N = 16.58 6 1.8, P , 0.001; 2- to 6-mm annulus: KC = 16.64 6 2.99, N = 14.89 6 1.61, P = ,0.001; total diameter: KC = 17.82 6 5.57, N = 16.71 6 2.3, P = 0.033). A higher densitometry in the central cornea was also seen in all 3 layers (P , 0.001). The difference was marked in the central annulus of the anterior layer, where the difference was found in the different stages of KC (2-mm central annulus: KC severe 37.26 6 11.77, KC moderate = 30.53 6 8.16, KC mild = 25.34 6 3.49, P , 0.05). This is consistent with KC pathology, where the anterior cornea is the first and most affected. The primary lesion in the keratoconic eye is supposed to be located in the basal epithelial cell layer, leading to its disappearance and thinning of epithelial layer.19 Breaks in the Bowman layer and thickened subbasal nerve plexus have also been described.4,20 In the central layer, the higher densitometry in the KC group (2-mm central annulus: KC = 16.84 6 5.16, N = 13.94 6 2.21, P = 0.0001; 2- to 6-mm annulus: KC = 13.88 6 2.96, N = 12.39 6 1.93, P = 0.003) is consistent with the findings of a previous study using the principle of Scheimpflug photography for the assessment of corneal light scatter, where most corneal light scatter is produced by the stroma,21 and with the haze seen in confocal microscopy studies.22 In the periphery, no statistically significant difference was found between the groups. The values of peripheral zones, however, must be interpreted with caution, especially the 10 to 12 mm, as the repeatability and reproducibility in
1286
| www.corneajrnl.com
1. Maurice DM. The structure and transparency of the cornea. J Physiol. 1957;136:263–286. 2. Jester JV, Moller-Pedersen T, Huang J, et al. The cellular basis of corneal transparency: evidence for “corneal crystallins”. J Cell Sci. 1999;112:613–622. 3. Krachmer JH, Feder RS, Belin MW. Keratoconus and related noninflammatory corneal thinning disorders. Surv Ophthalmol. 1984;28:293–322. 4. Rabinowitz YS. Keratoconus. Surv Ophthalmol. 1998;42:297–319. 5. Ni Dhubhghaill S, Rozema JJ, Jongenelen S, et al. Normative values for corneal densitometry analysis by Scheimpflug optical assessment. Invest Ophthalmol Vis Sci. 2014;55:162–168. 6. Fares U, Otri AM, Al-Aqaba MA, et al. Wavefront-optimized excimer laser in situ keratomileusis for myopia and myopic astigmatism: refractive outcomes and corneal densitometry. J Cataract Refract Surg. 2012;38:2131–2138. 7. Rozema JJ, Trau R, Verbruggen KH, et al. Backscattered light from the cornea before and after laser-assisted subepithelial keratectomy for myopia. J Cataract Refract Surg. 2011;37:1648–1654. 8. Cennamo G, Forte R, Aufiero B, et al. Computerized Scheimpflug densitometry as a measure of corneal optical density after excimer laser refractive surgery in myopic eyes. J Cataract Refract Surg. 2011;37: 1502–1506. 9. Otri AM, Fares U, Al-Aqaba MA, et al. Corneal densitometry as an indicator of corneal health. Ophthalmology. 2012;119:501–508. 10. Elflein HM, Hofherr T, Berisha-Ramadani F, et al. Measuring corneal clouding in patients suffering from mucopolysaccharidosis with the Pentacam densitometry programme. Br J Ophthalmol. 2013;97:829–833. 11. Bhatt UK, Fares U, Rahman I, et al. Outcomes of deep anterior lamellar keratoplasty following successful and failed “big bubble”. Br J Ophthalmol. 2012;96:564–569. 12. Koh S, Maeda N, Nakagawa T, et al. Quality of vision in eyes after selective lamellar keratoplasty. Cornea. 2012;31:45–49. 13. Greenstein SA, Fry KL, Bhatt J, et al. Natural history of corneal haze after collagen crosslinking for keratoconus and corneal ectasia: Scheimpflug and biomicroscopic analysis. J Cataract Refract Surg. 2010;36: 2105–2114. 14. Gutierrez R, Lopez I, Villa-Collar C, et al. Corneal transparency after cross-linking for keratoconus: 1-year follow-up. J Refract Surg. 2012;28: 781–786. 15. Weyns M, Rozema J, Koppen C, et al. Clinical applications of corneal Scheimpflug densitometry in DSAEK and cornea guttata patients. Acta Ophthalmol. 2010;88:0. 16. Freegard TJ. The physical basis of transparency of the normal cornea. Eye (Lond). 1997;11:465–471. 17. Patel SV, McLaren JW, Hodge DO, et al. The effect of corneal light scatter on vision after penetrating keratoplasty. Am J Ophthalmol. 2008; 146:913–919. 18. Rozema JJ, Koppen C, Bral N, et al. Changes in forward and backward light scatter in keratoconus resulting from corneal cross-linking. AsiaPacific J Ophthalmol. 2013;2:15–19. 19. Mathew JH, Goosey JD, Bergmanson JP. Quantified histopathology of the keratoconic cornea. Optom Vis Sci. 2011;88:988–997. 20. Ucakhan OO, Kanpolat A, Ylmaz N, et al. In vivo confocal microscopy findings in keratoconus. Eye Contact Lens. 2006;32:183–191. 21. McCally RL, Farrell RA. The depth dependence of light scattering from the normal rabbit cornea. Exp Eye Res. 1976;23:69–81. 22. Efron N, Hollingsworth JG. New perspectives on keratoconus as revealed by corneal confocal microscopy. Clin Exp Optom. 2008;91: 34–55.
Ó 2014 Lippincott Williams & Wilkins
Corneal Densitometry in Keratoconus Bernardo Lopes, MD,*†‡ Isaac Ramos, MD,*† and Renato Ambrósio, Jr, MD, PhD*†‡
Purpose: The aim of this study was to compare corneal densitometry measured by Scheimpflug tomography in normal and keratoconic eyes and to assess the differences in densitometry values among the stages of keratoconus.
Methods: Keratoconic and normal corneas were examined using the Pentacam. Corneal densitometry was measured over a 12-mm diameter area, divided by annular concentric zones and depths. Keratoconus was classified according to the topographic keratoconus classification. Results: We enrolled 1 eye randomly selected from each of 172 patients with normal corneas (N) and 98 patients with bilateral keratoconus (KC). There were significant differences between the groups for densitometry measurements in 2 annuli: central 2.0 mm in diameter (N = 16.85 6 2.42, KC = 18.93 6 2.78, P = 0.0001) and annulus 2.0 to 6.0 mm in diameter (N = 15.18 6 2.18, KC = 16.16 6 1.71, P = 0.005), and total diameter (N = 24.89 6 6.18, KC = 16.71 6 2.3, P = 0.033). Divided by layers, the inner parts of anterior (120 mm), central (from 120 mm to the last 60 mm), and posterior (last 60 mm) layers were also higher in the KC group (P , 0.001). There were differences according to the stages of KC for corneal densitometry of the central annuli at total thickness, anterior and central layers. More advanced cases presented a higher backscatter (P , 0.05). The anterior layer presented the smallest overlap between groups and KC stages.
Conclusions: The densitometry map reveals that light backscatter was higher in the central portion of the anterior keratoconic cornea than in the normal cornea. The densitometry level is higher in more advanced stages. Key Words: corneal densitometry, light backscatter, keratoconus, Scheimpflug imaging (Cornea 2014;33:1282–1286)
S
cheimpflug imaging has a large number of applications in corneal diagnosis. Corneal and anterior segment tomography based on a rotating Scheimpflug camera allows for imaging of both the anterior and posterior corneal surfaces, a full
Received for publication May 28, 2014; revision received August 1, 2014; accepted August 6, 2014. Published online ahead of print October 2, 2014. From the *Rio de Janeiro Corneal Tomography and Biomechanics Study Group, Rio de Janeiro, Brazil; †Instituto de Olhos Renato Ambrósio, Rio de Janeiro, Brazil; and ‡Department of Ophthalmology, Federal University of São Paulo, São Paulo, Brazil. R. Ambrósio is a consultant for Oculus Optikgeräte GmbH. The other authors have no funding or conflicts of interest to disclose. Reprints: Bernardo Lopes, MD, Rua Conde de Bonfim 211/712 Tijuca, Rio de Janeiro, RJ, Brazil 20520-050 (e-mail: [email protected]). Copyright © 2014 by Lippincott Williams & Wilkins
1282
| www.corneajrnl.com
pachymetric map, and thus additionally, with new software, it is possible to measure the amount of backscattered light in the different regions of the cornea. A normal cornea is expected to scatter light mostly in the interfaces air/cornea and cornea/water where there are higher differences in light refraction indices with only minimum scatter by the structures of corneal tissue. This optically transparent tissue results from regular spacing, small uniform diameter of orthogonally arranged collagen fibers, and balanced keratocyte components.1,2 Keratoconus is an ectatic disease that produces several changes in the cornea. These changes disturb the fragile balance of well-distributed corneal extracellular matrix and cells and lead to the thinning and anterior protrusion of the cornea.3,4 In advanced cases, this disarrangement may lead to corneal scars with evident opalescence. But mild cases are supposed to present increased backscatter detectable by noninvasive Scheimpflug analysis.5 In this study, we examined the difference in backscattered light between normal and keratoconic corneas and the distribution of corneal densitometry in different stages of the disease.
PATIENTS AND METHODS Patient Inclusion The retrospective observational study reviewed the chart of patients examined at the Instituto de Olhos Renato Ambrósio (Rio de Janeiro, Brazil). It followed the tenets of the Declaration of Helsinki and was approved by the ethics committee of the Federal University of São Paulo, Brazil. Patients were selected from a database of cases diagnosed as having bilateral keratoconus (KC) and also a database of preoperative patients with a stable cornea after at least 1 year of Lasik (normal corneas). All eyes were examined by a fellowship trained cornea and refractive surgeon (R.A.). Patients with KC were defined as those having the following ocular findings in both eyes3,4: typical Placido disc-based videokeratographic findings as paracentral inferior–superior asymmetry and/or asymmetric bowtie pattern, with or without skewed axes and at least 1 clinical sign including stromal thinning, conical protrusion of the cornea, Fleisher ring, Vogt striae, enlarged corneal nerves, increased intensity of the corneal endothelial reflex, subepithelial fibrillary lines, and Munson and Rizzuti signs. Contact lens wear was discontinued at least 3 weeks for rigid contact lens and 1 week for soft contact lens before the assessment. Patients with a history of eye surgery, eye disease, extensive corneal scarring, connective tissue disease, pregnancy at the time of the measurements, and with a history of pregnancy in the past 12 months were excluded. Cornea Volume 33, Number 12, December 2014
Cornea Volume 33, Number 12, December 2014
Corneal Densitometry in Keratoconus
FIGURE 1. Corneal densitometry display in the Pentacam.
Corneal Tomography All eyes were examined using rotating Scheimpflug corneal and anterior segment tomography (Pentacam HR, Oculus GmbH, Wetzlar, Germany). An expert examiner, who was masked to the clinical condition of the patient, acquired Pentacam images as follows: The patient’s chin was placed on the chin rest, and the forehead was placed against the forehead strap. After blinking a few times, the patient was asked to open both eyes and stare at the fixation target. Proper alignment was obtained using a joystick, and then the automatic release mode started the scan using 25 single Scheimpflug images captured within 2 seconds for each eye. Image quality was checked, so that only cases with acceptable quality images were included in the study. The Pentacam provides a topographic keratoconus classification for staging the disease in grades 1 to 4 with 3 subgrades (1–2, 2–3, and 3–4). Cases were divided into mild, less than grade 2; moderate, from grades 2 to 3; and severe, grades higher than 3 (3–4 and 4). An add-on to standard software of Pentacam provides corneal densitometry analysis (Fig. 1). It measures the backscattered light over a 12-mm-diameter area. Zonal densitometry can be measured by 4 annular concentric zones centered in the apex of the cornea. The first central zone covers the annulus with a 2-mm diameter, the second covers the annulus that extends from a 2- to 6-mm diameter, the third covers the one from a 6- to 10-mm diameter, and the last zone is composed of the annulus that extends from a 10- to 12-mm Ó 2014 Lippincott Williams & Wilkins
diameter. The analysis is also performed by depth in 3 layers: the anterior layer, which corresponds to the 120mm superficial corneal thickness; the posterior layer, which corresponds to the most posterior 60 mm; and the central, which has no fixed thickness value, and is the zone between the anterior and posterior layers. Densitometry is expressed in gray scale units, ranging from a minimum light scatter of 0 (maximum transparency) to a maximum light scatter of 100 (minimum transparency).
Statistical Analysis Sample size and power calculations were performed. Patients were first divided into 2 groups: Normal and Keratoconus. Normality check was accomplished using the Kolmogorov–Smirnov goodness-of-fit test. As variables were nonnormally distributed, Mann–Whitney U test was used to assess the differences between the groups. The KC group was divided into 3 groups by disease staging. The difference between the groups was assessed with the Kruskal–Wallis test and post hoc Dunn test. Analysis was carried out using the software MedCalc 11.1 (MedCalc Software, Mariakerke, Belgium).
RESULTS Two hundred and seventy patients were included in the study: 172 patients with normal corneas and 98 patients with KC. There were 85 men and 87 women in the normal group, www.corneajrnl.com |
1283
Cornea Volume 33, Number 12, December 2014
Lopes et al
FIGURE 2. A, Corneal densitometry at total thickness and total diameter. B, Corneal densitometry at the central 2.0-mm annulus of the anterior cornea (120 mm).
and 53 men and 45 women in the KC group. The subjects’ age in the normal group was 36.1 6 14.6 years (range, 11– 82), and that in the KC group was 29.6 6 11.2 years (range, 10–74 years). In the KC group, there were 41 patients with
mild KC, 47 patients with moderate KC, and 10 patients with severe KC. Total corneal light backscatter was higher in the KC group (KC = 17.82 6 4.57, N = 16.71 6 2.3, P = 0.033). The densitometry map revealed a statistically significant difference between the groups in stratified analysis with less overlap between the groups. Considering the total thickness of the cornea, separating the densitometry by concentric annular zones, the backscatter at the first 2 annuli (center to 2 mm and 2–6 mm diameter) was statistically higher in the KC group (KC = 19.87 6 5.07, N = 16.58 6 1.8, P , 0.001; KC = 16.64 6 2.99, N = 14.89 6 1.61, P = ,0.001, respectively). Dividing the cornea into 3 layers, they showed the same behavior with a higher backscatter in the central cornea (P , 0.05) in the KC group. The differences were marked in the 2 central annuli in the 3 layers. The central annulus in the anterior layer showed the smallest overlap between the groups. These results are summarized in Figure 2 and Table 1. The analysis, considering different stages of KC, showed the same pattern when comparing the 3 groups with normal corneas. A statistically significant difference was present at the 2 central annuli in total thickness and in the 3 layers (P , 0.0004) with a trend to increase densitometry as the KC stage evolves. At the anterior layer, the post hoc test revealed that the higher values of KC moderate and severe were statistically significant in comparison with those of KC mild (2-mm central annulus: KC severe 37.26 6 11.77, KC moderate = 30.53 6 8.16, KC mild = 25.34 6 3.49, P , 0.05). Table 2 and Figure 3 summarize these results.
TABLE 1. Corneal Light Back Scatter in Gray Scale Units KC (n = 98) Total thickness
Anterior layer 120 mm
Central layer
Posterior layer 60 mm
Annulus 0.0–2.0 mm Annulus 2.0–6.0 mm Annulus 6.0–10.0 mm Annulus 10.0–12.0 mm Total diameter Annulus 0.0–2.0 mm Annulus 2.0–6.0 mm Annulus 6.0–10.0 mm Annulus 10.0–12.0 mm Total diameter Annulus 0.0–2.0 mm Annulus 2.0–6.0 mm Annulus 6.0–10.0 mm Annulus 10.0–12.0 mm Total diameter Annulus 0.0–2.0 mm Annulus 2.0–6.0 mm Annulus 6.0–10.0 mm Annulus 10.0–12.0 mm Total diameter
N (n = 172)
Mean 6 SD
Range
6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6
13.8–47.3 11–34.1 9.1–53.7 6.7–59.5 11.4–40.3 17.3–69.9 14.3–51.3 12.6–69.9 8.2–82.4 14.8–54.9 11.4–49.3 9.8–28.7 7.7–53.5 6.1–56.1 9.4–37.2 9.3–31.4 8.7–24 7–37.7 5.7–42.5 8.4–28.8
19.87 16.64 15.78 22.51 17.82 29.04 24.04 21.21 30 24.89 16.84 13.88 13.71 20.37 15.35 13.72 12.01 12.45 17.16 13.23
5.07 2.99 6.74 8.96 4.57 8.06 4.44 8.58 12.68 6.18 5.16 2.96 6.85 8.92 4.56 3.24 2.37 5.1 7.14 3.51
Mean 6 SD
Range
P
6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6
(12.8–22) (10.8–19.8) (10.2–26) (10.3–48) (11.7–25.1) (16.1–31.4) (15.4–29.6) (13.1–34.8) (13.6–59.4) (15.7–30.5) (9.9–22.4) (8.4–19.2) (8.3–25.5) (9.6–45) (9.5–24.8) (9.6–18.1) (8.2–15.2) (7.6–25) (7.7–44) (9.1–23.3)
,0.001 ,0.001 0.112 0.063 0.033 ,0.001 ,0.001 0.424 0.080 0.160 ,0.001 ,0.001 0.208 0.059 0.999 ,0.001 ,0.001 0.166 0.059 0.217
16.58 14.89 15.38 23.16 16.71 23.47 21.3 20.6 31.27 23.08 13.94 12.39 13.21 20.5 14.28 12.33 10.98 12.33 17.69 12.77
1.8 1.61 3.29 5.97 2.3 2.99 2.56 3.74 8.65 2.9 2.21 1.93 3.43 5.6 2.54 1.38 1.24 3.19 5.52 2.25
KC indicates keratoconus; N, normal corneas; bold, statistically significant results.
1284
| www.corneajrnl.com
Ó 2014 Lippincott Williams & Wilkins
Cornea Volume 33, Number 12, December 2014
Corneal Densitometry in Keratoconus
TABLE 2. Values by Keratoconus Stage N (n = 172) Total thickness
Anterior layer 120 mm
Central layer
Posterior layer 60 mm
Annulus 0.0–2.0 mm Annulus 2.0–6.0 mm Annulus 6.0–10.0 mm Annulus 10.0–12.0 mm Total Diameter Annulus 0.0–2.0 mm Annulus 2.0–6.0 mm Annulus 6.0–10.0 mm Annulus 10.0–12.0 mm Total Diameter Annulus 0.0–2.0 mm Annulus 2.0–6.0 mm Annulus 6.0–10.0 mm Annulus 10.0–12.0 mm Total diameter Annulus 0.0–2.0 mm Annulus 2.0–6.0 mm Annulus 6.0–10.0 mm Annulus 10.0–12.0 mm Total diameter
KC Mild (n = 41)
Mean 6 SD
Range
6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6
16.1–31.4 15.4–29.6 13.1–34.8 13.6–59.4 15.7–30.5 9.9–22.4 8.4–19.2 8.3–25.5 9.6–45 9.5–24.8 9.6–18.1 8.2–15.2 7.6–25 7.7–44 9.1–23.3 12.8–22 10.8–19.8 10.2–26 10.3–48 11.7–25.1
23.47 21.3 20.6 31.27 23.08 13.94 12.39 13.21 20.5 14.28 12.33 10.98 12.33 17.69 12.77 16.58 14.89 15.38 23.16 16.71
2.99 2.56 3.74 8.65 2.9 2.21 1.93 3.43 5.6 2.54 1.38 1.24 3.19 5.52 2.25 1.8 1.61 3.29 5.97 2.3
KC Moderate (n = 47)
Mean 6 SD
Range
6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6
17.3–36 14.3–32.4 12.6–55 13.2–65.1 14.8–45.3 11.4–26.4 9.8–22.3 7.7–46.3 9.7–45.7 9.4–34.4 10–22 8.7–18.1 7–37.5 7.9–42.5 8.4–27.6 13.8–25.3 11–23.7 9.1–46.3 11–49.3 11.4–35.8
25.34 22.47 20.76 30.15 23.59 15.88 13.7 13.53 19.93 14.97 13.55 11.73 12.56 17.29 13.19 18.27 15.97 15.61 22.46 17.25
3.49 3.43 6.72 10.7 4.96 3.24 2.73 5.96 7.83 4.08 2.39 1.94 4.98 7.37 3.33 2.61 2.37 5.77 8.06 3.91
Mean 6 SD
Range
6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6
23.5–69.9 20.3–51.3 14.6–69.9 8.2–82.4 19.5–54.9 11.5–40.7 10.1–28.7 8.6–53.5 6.1–56.1 10.7–37.2 9.3–31.4 9.8–22.3 7.6–37.7 5.7–40.1 9.2–28.8 15.4–47.3 13.6–34.1 10.5–53.7 6.7–59.5 13.3–40.3
30.53 25.2 22.32 30.78 26.02 16.69 13.92 14.25 20.48 15.56 13.54 12.1 12.69 17 13.29 20.26 17.07 16.41 22.75 18.29
8.16 5.09 10.39 14.3 7.3 4.71 3.11 8 9.75 5.21 3.53 2.27 5.39 7.18 3.74 5.23 3.35 7.85 9.87 5.31
KC Severe (n = 10) Mean 6 SD
Range
6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6
25.4–64.7 20.7–30 15.3–26.5 12.4–48.5 19.8–28.7 16.3–49.3 11.6–22.9 10–20.2 10.2–35.2 12.5–22 11.2–26.8 10.2–24 8.4–21.6 9–28.5 10.8–22 19.2–46.9 14.7–25.6 11.1–22.8 10.6–34.9 14.5–24
37.26 25.03 17.82 27.7 24.89 21.44 14.4 11.93 21.66 15.88 15.28 12.71 10.86 17.4 13.2 24.65 17.38 13.49 21.58 18
11.77 2.46 3.19 11.69 3.13 9.42 3.01 2.96 8.83 2.7 4.26 3.82 3.77 5.85 3.15 7.85 2.92 3.3 7.85 2.66
Post Hoc Test
Total thickness
Anterior layer 120 mm
Central layer
Posterior layer 60 mm
P
N vs. KC Mild
N vs. KC Moderate
N vs. KC Severe
Mild vs. Moderate
Mild vs. Severe
Moderate vs. Severe
,0.0001 ,0.0001 0.0554 0.1265 0.0335 ,0.0001 ,0.0001 0.5332 0.3135 0.2819 0.0001 0.0003 0.1628 0.2680 0.9946 ,0.0001 ,0.0001 0.1086 0.3123 0.3799
P , 0.05 P , 0.05 NS NS NS P , 0.05 P , 0.05 NS NS NS P , 0.05 P , 0.05 NS NS NS P , 0.05 P , 0.05 NS NS NS
P , 0.05 P , 0.05
P , 0.05 P , 0.05
P , 0.05 P , 0.05
P , 0.05 P , 0.05
NS NS
P , 0.05 P , 0.05 P , 0.05
NS P , 0.05 P , 0.05
NS NS P , 0.05
NS P , 0.05 P , 0.05
NS P , 0.05 NS
P , 0.05 P , 0.05
P , 0.05 P , 0.05
NS NS
NS NS
NS NS
P , 0.05 P , 0.05
P , 0.05 P , 0.05
P , 0.05 NS
P , 0.05 NS
P , 0.05 NS
KC indicates keratoconus; N, normal corneas; bold, statistically significant results.
DISCUSSION The analysis of light backscatter has gained increasing relevance in corneal diagnosis. It has been described in postrefractive surgery,6–8 infectious keratitis,9 corneal dystrophies,10 corneal graft surgery,11,12 and for evaluating the crosslinking result in patients with KC.13,14 In the clinic it is also used complementarily to endothelial cell count Ó 2014 Lippincott Williams & Wilkins
in monitoring the disease progression in cornea guttata patients.15 Corneal transparency is caused by complex mechanisms; regular-arrangement and size regularity of collagen fibrils are part of it.1,16 High levels of corneal light backscatter can be seen even in corneas considered clinically clear.17 The disarrangement in corneal histology caused by KC can also alter densitometry levels.18 www.corneajrnl.com |
1285
Lopes et al
Cornea Volume 33, Number 12, December 2014
this region were the weakest in a normative study.5 Variation in white-to-white distance also occurs, leading to the mistaken examination of portions of the limbus and sclera, causing higher densitometry values. Scheimpflug tomography provides an automated, rapid, and reliable method for assessing corneal densitometry. Although corneal light backscatter is unclearly correlated with forward light scatter, corneal densitometry may play a valuable role in characterizing keratoconic corneas. REFERENCES
FIGURE 3. A, Corneal densitometry at the central 2.0-mm annulus of the anterior cornea (120 mm) for the different stages of keratoconus. B, Corneal densitometry at the 2.0- to 6.0-mm annulus of the anterior cornea (120 mm among the different stages of keratoconus).
In our study, we found an increase in light backscatter in the central cornea (6 mm in diameter) and total diameter in the KC group (2-mm central annulus: KC = 19.87 6 5.07, N = 16.58 6 1.8, P , 0.001; 2- to 6-mm annulus: KC = 16.64 6 2.99, N = 14.89 6 1.61, P = ,0.001; total diameter: KC = 17.82 6 5.57, N = 16.71 6 2.3, P = 0.033). A higher densitometry in the central cornea was also seen in all 3 layers (P , 0.001). The difference was marked in the central annulus of the anterior layer, where the difference was found in the different stages of KC (2-mm central annulus: KC severe 37.26 6 11.77, KC moderate = 30.53 6 8.16, KC mild = 25.34 6 3.49, P , 0.05). This is consistent with KC pathology, where the anterior cornea is the first and most affected. The primary lesion in the keratoconic eye is supposed to be located in the basal epithelial cell layer, leading to its disappearance and thinning of epithelial layer.19 Breaks in the Bowman layer and thickened subbasal nerve plexus have also been described.4,20 In the central layer, the higher densitometry in the KC group (2-mm central annulus: KC = 16.84 6 5.16, N = 13.94 6 2.21, P = 0.0001; 2- to 6-mm annulus: KC = 13.88 6 2.96, N = 12.39 6 1.93, P = 0.003) is consistent with the findings of a previous study using the principle of Scheimpflug photography for the assessment of corneal light scatter, where most corneal light scatter is produced by the stroma,21 and with the haze seen in confocal microscopy studies.22 In the periphery, no statistically significant difference was found between the groups. The values of peripheral zones, however, must be interpreted with caution, especially the 10 to 12 mm, as the repeatability and reproducibility in
1286
| www.corneajrnl.com
1. Maurice DM. The structure and transparency of the cornea. J Physiol. 1957;136:263–286. 2. Jester JV, Moller-Pedersen T, Huang J, et al. The cellular basis of corneal transparency: evidence for “corneal crystallins”. J Cell Sci. 1999;112:613–622. 3. Krachmer JH, Feder RS, Belin MW. Keratoconus and related noninflammatory corneal thinning disorders. Surv Ophthalmol. 1984;28:293–322. 4. Rabinowitz YS. Keratoconus. Surv Ophthalmol. 1998;42:297–319. 5. Ni Dhubhghaill S, Rozema JJ, Jongenelen S, et al. Normative values for corneal densitometry analysis by Scheimpflug optical assessment. Invest Ophthalmol Vis Sci. 2014;55:162–168. 6. Fares U, Otri AM, Al-Aqaba MA, et al. Wavefront-optimized excimer laser in situ keratomileusis for myopia and myopic astigmatism: refractive outcomes and corneal densitometry. J Cataract Refract Surg. 2012;38:2131–2138. 7. Rozema JJ, Trau R, Verbruggen KH, et al. Backscattered light from the cornea before and after laser-assisted subepithelial keratectomy for myopia. J Cataract Refract Surg. 2011;37:1648–1654. 8. Cennamo G, Forte R, Aufiero B, et al. Computerized Scheimpflug densitometry as a measure of corneal optical density after excimer laser refractive surgery in myopic eyes. J Cataract Refract Surg. 2011;37: 1502–1506. 9. Otri AM, Fares U, Al-Aqaba MA, et al. Corneal densitometry as an indicator of corneal health. Ophthalmology. 2012;119:501–508. 10. Elflein HM, Hofherr T, Berisha-Ramadani F, et al. Measuring corneal clouding in patients suffering from mucopolysaccharidosis with the Pentacam densitometry programme. Br J Ophthalmol. 2013;97:829–833. 11. Bhatt UK, Fares U, Rahman I, et al. Outcomes of deep anterior lamellar keratoplasty following successful and failed “big bubble”. Br J Ophthalmol. 2012;96:564–569. 12. Koh S, Maeda N, Nakagawa T, et al. Quality of vision in eyes after selective lamellar keratoplasty. Cornea. 2012;31:45–49. 13. Greenstein SA, Fry KL, Bhatt J, et al. Natural history of corneal haze after collagen crosslinking for keratoconus and corneal ectasia: Scheimpflug and biomicroscopic analysis. J Cataract Refract Surg. 2010;36: 2105–2114. 14. Gutierrez R, Lopez I, Villa-Collar C, et al. Corneal transparency after cross-linking for keratoconus: 1-year follow-up. J Refract Surg. 2012;28: 781–786. 15. Weyns M, Rozema J, Koppen C, et al. Clinical applications of corneal Scheimpflug densitometry in DSAEK and cornea guttata patients. Acta Ophthalmol. 2010;88:0. 16. Freegard TJ. The physical basis of transparency of the normal cornea. Eye (Lond). 1997;11:465–471. 17. Patel SV, McLaren JW, Hodge DO, et al. The effect of corneal light scatter on vision after penetrating keratoplasty. Am J Ophthalmol. 2008; 146:913–919. 18. Rozema JJ, Koppen C, Bral N, et al. Changes in forward and backward light scatter in keratoconus resulting from corneal cross-linking. AsiaPacific J Ophthalmol. 2013;2:15–19. 19. Mathew JH, Goosey JD, Bergmanson JP. Quantified histopathology of the keratoconic cornea. Optom Vis Sci. 2011;88:988–997. 20. Ucakhan OO, Kanpolat A, Ylmaz N, et al. In vivo confocal microscopy findings in keratoconus. Eye Contact Lens. 2006;32:183–191. 21. McCally RL, Farrell RA. The depth dependence of light scattering from the normal rabbit cornea. Exp Eye Res. 1976;23:69–81. 22. Efron N, Hollingsworth JG. New perspectives on keratoconus as revealed by corneal confocal microscopy. Clin Exp Optom. 2008;91: 34–55.
Ó 2014 Lippincott Williams & Wilkins
