Ординатура / Офтальмология / Английские материалы / Elevation Based Corneal Tomography 2nd_Belin, Khachikian, Ambrósio_2011
.pdf
240 |
ELEVATION BASED CORNEAL TOMOGRAPHY |
CAPSULAR BAG DISTENSION SYNDROME
Capsular bag distention syndrome (CBDS) is an uncommon but well recognized complication after cataract surgery. It is associated with enlargement of the space between intraocular lens (IOL) implant and posterior capsule. It often produces anterior vaulting of IOL optic, anterior bowing of the iris, shallowing of the anterior chamber, and a myopic shift. The fluid within the capsular space can turn turbid or cloudy, resulting in decreased vision. Pentacam Scheimpflug images documents CBDS24 including the exact measurements of the distended posterior capsule, density or turbidity of the fluid, IOL position, and its relation to the capsular bag and iris to rule out pupillary block (FIGURE 13). The decrease in distention of the posterior capsule post-capsulotomy could be measured with Pentacam. Scheimpflug imaging in such cases is quicker and more precise than ultrasound biomicroscopy and highfrequency ultrasonography. Screening pseudophakic eyes using Scheimpflug imaging could help to detect and quantify CBDS at an early stage and to document changes over a period of time and the post-capsulotomy status. In a series of 11 eyes with CBDS we found that the average amount of myopia induced due to CBDS was 0.75 D and the average hyperopic shift after capsulotomy was 0.64 D.
Figure 13. Slit lamp photograph of capsular bag distension syndrome showing posterior bowing of the opacified posterior capsule with Elschnig pearls (Left), Pentacam Tomogram (Top center) and Scheimpflug Image (Top right) showing capsular bag distension syndrome allowing for calculation of exact measurements of the distended posterior capsule, assessment of density or turbidity of the fluid, IOL position, and its relation to the capsular bag and iris to rule out pupillary block with pre-capsulotomy measurements . The post-capsulotomy tomogram (Bottom Center) and Scheimpflug image (Bottom right) illustrate the change in measurements. Size of the distended capsular bag may help predict the refractive outcome after capsulotomy.
CHAPTER 17. THE USE OF PENTACAM IN A CATARACT PRACTICE |
241 |
POSTERIOR CAPSULE OPACIFICATION
Posterior capsule opacification (PCO) remains the most common cause of impaired postoperative visual acuity following cataract extraction. There is continued focus on several experimental and clinical trials, including studies of surgical techniques, IOL design, and drugs to reduce the incidence of PCO. An objective quantitative measurement of PCO is of paramount importance to assess the efficacy of such trials. Although several imaging systems have been reported, at present there is no consensus on an optimal quantification method for PCO analysis. The use of Scheimpflug imaging to quantify PCO was first reported in 1995 by Lasa et al.25 The earlier Scheimpflug systems could only capture images in one meridian at a time,25 and Hayashi et al.26,27 had analyzed data using single-slit Scheimpflug images in up to four meridians. Subsequently, several studies have been published using Scheimpflug images, and the results have been correlated with histologic findings.28 Pentacam is the first Scheimpflug camera–based instrument that can capture images in multiple meridians in a single automated scan. We recently described29 a new system of measurement to objectively quantify PCO using ImageJ software (National Institutes of Health, Bethesda, MD) based analysis of Pentacam Scheimpflug Tomograms which produced highly reproducible results that correlated with the results obtained from analysis of slit lamp based retro-illumination photographs using the POCOman system.30
Pentacam Scheimpflug tomogram images have a distinct advantage over the previous Scheimpflug camera (Anterior Eye Segment Analysis System EAS 1000; Nidek, Tokyo, Japan) on which PCO density was analyzed in the central 3-mm region. Because the tomogram is reconstructed from 50 Scheimpflug images, it covers almost the entire area of the posterior capsule instead of a single slit beam meridian or the average density calculated from four meridians. The rotating Scheimpflug camera allows 50 images to be reconstructed into a single image. Before the availability of the Pentacam tomograms, there was no way to correlate the value of PCO obtained on Scheimpflug images with the slit lamp images because the principles of the two photographic techniques are different. Tomograms allow the creation of Scheimpflug-based PCO tomogram in the same plane as a slit lamp retro-illumination image, and the two may be compared. Given that the tomograms are easier and quicker to obtain, provide PCO pixel intensity in up to 100 meridians, have no observer bias, and allow for a more objective analysis than slit lamp images, Pentacam tomograms have the potential to become an efficient grading system for PCO (FIGURE 14).
242 |
ELEVATION BASED CORNEAL TOMOGRAPHY |
|
|
|
|
Figure 14. Assessment of posterior capsule opacification (PCO) with Pentacam tomograms. Pentacam tomogram (which is a 3D reconstruction from Scheimpflug images) (Top Left) demonstrating an opacified posterior capsule and the corresponding slit lamp retroillumination image (Top Right). ImageJ software (NIH, Bethesda, MD) was used to detect the density of PCO on the Pentacam tomogram (Bottom Left) which correlated with the PCO grades obtained using an established method of assessing PCO on slit lamp images: POCOman software (Bottom Right).
TRAUMATIC CATARACT
The utility of Scheimpflug images in assessment of posterior capsule rupture following closed globe injury (FIGURE 15)31 traumatic intra-lenticular foreign bodies (FIGURE 16)32, electric cataract following exposure to high voltage electric shock (FIGURE 17)34 has been documented.
CHAPTER 17. THE USE OF PENTACAM IN A CATARACT PRACTICE |
243 |
|
|
|
|
|
|
|
Figure 15. Pentacam Scheimpflug image of posterior capsule tear following blunt ocular trauma showing the least (above) and greatest (below) dimensions of the tear. While slit-lamp examination will illustrate the defect, the primary advantage of the rotating Scheimpflug camera is that it allows accurate and objective quantification of the PCT. Additionally changes in the dimensions of the tear may be followed in cases where the surgeon decides to delay the surgery. The centration and tilt of the IOL can also be objectively documented following surgery.
Figure 16. Pentacam Scheimpflug image showing Intralenticular Foreign Body (ILFB) and the corneal wound of entry with an intact posterior capsule. Pentacam helps to accurately localize and map the trajectory of foreign bodies lodged in anterior segment allowing better decisions for management.
244 |
ELEVATION BASED CORNEAL TOMOGRAPHY |
|
|
|
|
Figure 17. Electric Cataract: Pentacam Scheimpflug images showing increased lens density with multiple anterior subcapsular opacities raised in the center of the lens following high voltage electrical shock.
ASSESSMENT OF IOL TILT
Decentration and tilt are important for the optical functioning of the IOL. A decentered IOL will increase the risk for halos in scotopic conditions, and tilt may introduce aberrations. The ultimate limitation of customized IOL’s like multifocal and pseudo-accommodating IOL’s is precision in their positioning. Scheimpflug images suffer from geometric distortion (resulting from tilt of the object, lens, and image planes) and optical distortion (because the different surfaces are viewed through anterior refracting surfaces). Ray-tracing techniques are therefore required to obtain reliable crystalline surface geometry from Scheimpflug images.34,35
De Castro and associates36 recently described software algorithms to correct the effect of optical distortion on tilt and decentration measurements by correcting the geometrical distortion of the images, detecting the edges of the cornea, IOL, pupil and lens to find the pupil center, IOL center, IOL tilt, and eye rotation to each of the 25 sections obtained with the Pentacam. As IOL optics become more complex in design to overcome presbyopia and provide better visual quality, it is important to be able to assess IOL centration and the interaction between the capsule and the IOL haptics, and the position of the IOL optics relative to the posterior capsule and iris post-surgery.
The phakic IOL simulation software simulates the phakic IOL fit in the anterior chamber and calculates the minimum distance from each point of the phakic IOL to adjacent eye structures. The phakic IOL fit can also be modified manually.
CHAPTER 17. THE USE OF PENTACAM IN A CATARACT PRACTICE |
245 |
CONCLUSION
Pentacam imaging is simple to perform, rapid, and has an easier learning curve in contrast with slit-lamp–based photographic lens grading systems. It provides lens densitometry measurements as an easy, quick, objective, repeatable assessment of cataract and is a step forward for precise grading and tracking lens changes over time. It has great potential in documenting progression of cataract in longitudinal studies, as well as for epidemiologic studies and clinical trials.
The versatility of Pentacam in assessing posterior capsule opacification, different types of cataract and its utility in challenging cataract cases such as those with preexisting posterior capsule rupture and traumatic cataracts makes it an exciting tool for the modern cataract surgeon.
REFERENCES
1.Chylack LT, Jr, Wolfe JK, Singer DM, et al. The Lens Opacities Classification System III. Arch Ophthalmol. June 1, 1993 1993;111(6):831-836.
2.Chylack LT, Wolfe JK, Singer DM, et al. The Lens Opacities Classification System III. The Longitudinal Study of Cataract Study Group. Arch Ophthalmol. 1993;111(6):831-836.
3.The age-related eye disease study (AREDS) system for classifying cataracts from photographs: AREDS report no.
4.Am J Ophthalmol. 2001;131(2):167-175.
4.Hall NF, Lempert P, Shier RP, Zakir R, Phillips D. Grading nuclear cataract: reproducibility and validity of a new method. BRIT J OPHTH. 1999;83(10):1159-1163.
5.West SK, Rosenthal F, Newland HS, Taylor HR. Use of photographic techniques to grade nuclear cataracts. Invest Ophthalmol Vis Sci. 1988;29(3335435):73-77.
6.Brown N. An advanced slit-image camera. Br J Ophthalmol. 1972;56(5079412):624-631.
7.Brown N. Quantitative slit-image photography of the lens. Trans Ophthalmol Soc U K. 1972;92(4515515):303-
8.Hockwin O, Dragomirescu V, Laser H. Measurements of lens transparency or its disturbances by densitometric image analysis of Scheimpflug photographs. Graefes Arch Clin Exp Ophthalmol. 1982;219(7160634):255-262.
9.Brown NAP, Bron AJ, Sparrow JM. Methods for evaluation of lens changes. International Ophthalmology. 1988;12(4):227-235.
10.Kashima K, Trus BL, Unser M, Edwards PA, Datiles MB. Aging studies on normal lens using the Scheimpflug slit-lamp camera. Invest Ophthalmol Vis Sci. 1993;34(8425834):263-269.
11.Magno BV, Freidlin V, Datiles MB. Reproducibility of the NEI Scheimpflug Cataract Imaging System. Invest Ophthalmol Vis Sci. 1994;35(8206726):3078-3084.
12.Foo KP, Maclean H. Measured changes in cataract over six months: sensitivity of the Nidek EAS-1000. Ophthalmic Res. 1996;28 Suppl 2(8883087):32-36.
13.Pei X, Bao Y, Chen Y, Li X. Correlation of lens density measured using the Pentacam Scheimpflug system with the Lens Opacities Classification System III grading score and visual acuity in age-related nuclear cataract. Br J Ophthalmol. 2008;92(18586899):1471-1475.
14.Kim J-S, Chung S-H, Joo C-K. Clinical application of a Scheimpflug system for lens density measurements in phacoemulsification. J Cataract Refract Surg. 2009;35(19545809):1204-1209.
15.Nixon D. Preoperative cataract grading by Scheimpflug imaging and effect on operative fluidics and phacoemulsification energy. J Cataract Refract Surg. 2010;36(2):242-246.
246 |
ELEVATION BASED CORNEAL TOMOGRAPHY |
16.Kirkwood BJ, Hendicott PL, Read SA, Pesudovs K. Repeatability and validity of lens densitometry measured with Scheimpflug imaging. J Cataract Refract Surg. 2009;35(19545810):1210-1215.
17.Grewal DS, Brar GS, Grewal SP. Correlation of nuclear cataract lens density using Scheimpflug images with Lens Opacities Classification System III and visual function. Ophthalmology. 2009;116(8):1436-1443.
18.Abramoff MD, Magalhaes, P.J., Ram, S.J. Image Processing with ImageJ. Biophotonics International. 2004;1(7):36-42.
19.Duncan DD, Shukla OB, West SK, Schein OD. New objective classification system for nuclear opacification. J Opt Soc Am A Opt Image Sci Vis. 1997;14(6):1197-1204.
20.Qian W, Söderberg PG, Chen E, Magnius K, Philipson B. A common lens nuclear area in Scheimpflug photographs. Eye. 1993;7 ( Pt 6):799-804.
21.Drews-Bankiewicz MA, Caruso RC, Datiles MB, Kaiser-Kupfer MI. Contrast sensitivity in patients with nuclear cataracts. Arch Ophthalmol. 1992;110(7):953-959.
22.Chylack LT, Wolfe JK, Friend J, et al. Validation of methods for the assessment of cataract progression in the Roche European-American Anticataract Trial (REACT). Ophthalmic Epidemiol. 1995;2(2):59-75.
23.Wong AL, Leung CK, Weinreb RN, et al. Quantitative assessment of lens opacities with anterior segment optical coherence tomography. BRIT J OPHTH. 2009;93(1):61-65.
24.Jain R, Grewal D, Gupta R, Grewal SP. Scheimpflug imaging in late Capsular Bag Distention syndrome after phacoemulsification. Am J Ophthalmol. 2006;142(6):1083-1085.
25.Lasa MS, Datiles MB, Magno BV, Mahurkar A. Scheimpflug photography and postcataract surgery posterior capsule opacification. Ophthalmic Surg. 1995;26(2):110-113.
26.Hayashi H, Hayashi K, Nakao F, Hayashi F. Quantitative comparison of posterior capsule opacification after polymethylmethacrylate, silicone, and soft acrylic intraocular lens implantation. Arch Ophthalmol. 1998;116(12):1579-1582.
27.Hayashi K, Hayashi H. Posterior capsule opacification after implantation of a hydrogel intraocular lens. BRIT J OPHTH. 2004;88(2):182-185.
28.Saika S, Miyamoto T, Ishida I, et al. Comparison of Scheimpflug images of posterior capsule opacification and histological findings in rabbits and humans. J Cat Ref Surg. 2001;27(7):1088-1092.
29.Grewal D, Jain R, Brar GS, Grewal SP. Pentacam tomograms: a novel method for quantification of posterior capsule opacification. Invest Ophthalmol Vis Sci. 2008;49(5):2004-2008.
30.Bender L, Spalton DJ, Uyanonvara B, et al. POCOman: new system for quantifying posterior capsule opacification. J Cataract Refract Surg. 2004;30(10):2058-2063.
31.Grewal DS, Jain R, Brar GS, Grewal SP. Posterior capsule rupture following closed globe injury: Scheimpflug imaging, pathogenesis, and management. EJO. 2008;18(3):453-455.
32.Grewal SP, Jain R, Gupta R, Grewal D. Role of scheimpflug imaging in traumatic intralenticular foreign body. Am J Ophthalmol. 2006;142(4):675-676.
33.Grewal DS, Jain R, Brar GS, Grewal SP. Unilateral electric cataract: Scheimpflug imaging and review of the literature. J Cat Ref Surg. 2007;33(6):1116-1119.
34.Dubbelman M, Van der Heijde GL. The shape of the aging human lens: curvature, equivalent refractive index and the lens paradox. vis res. 2001;41(14):1867-1877.
35.Coppens JE, van den Berg TJ, Budo CJ. Biometry of phakic intraocular lens using Scheimpflug photography. J Cat Ref Surg. 2005;31(10):1904-1914.
36.de Castro A, Rosales P, Marcos S. Tilt and decentration of intraocular lenses in vivo from Purkinje and Scheimpflug imaging. Validation study. J Cataract Refract Surg. 2007;33(3):418-429.
Chapter
18
Glossary
Stephen S. Khachikian, MD
Michael W. Belin, MD, FACS
Renato Ambrósio Jr., MD, PhD
Aberrometry: A technique used to measure refractive errors in much greater detail than is possible with traditional clinical refraction. Aberrometry measures the shape of a wavefront of light that has passed through the optical elements of the eye. Analyzing the wavefront’s shape determines the amount and type of refractive error present.
Apex: Geometric center of the Pentacam’s (or other devices) exam. This is the first Purkinje’s reflex on the cornea while having the patient fixating on the fixation target.
ART: Ambrósio Relational Thickness, which considers the Thinnest Point (TP) in relation to the PPI. ART is calculated as the ratios between the TP and the maximal PPI meridian (ART Max = TP/PPI Max) and the average (ART Ave = TP/PPI Ave).
Astigmatic Surface: A surface that has two meridians of different curvature. The Pentacam reports the astigmatism of the corneal surface in the central 3 mm of the cornea.
Axis: Axis of corneal astigmatism (red for steep, blue for flat and user selectable).
BAD: An abbreviation for the Belin-Ambrósio Display, which is a refractive screening display incorporating both anterior and posterior elevation data and pachymetric data into one unified screening screen.
Best-fit Shape: A reference shape or surface (sphere, ellipse or toric ellipse) mathematically generated by the elevation topographer of which corneal elevation is measured.
Center Keratoconus Index (CKI): Pachymetry based numerical index used to assess the likelihood of a cornea having keratoconus. Elevated especially in central keratoconus.
Corneal Thickness Spatial Profile (CTSP): A graphic display of the average of the thickness values along 22 imaginary circles centered on the thinnest point, with the diameter of the circles increasing in 0.4-mm steps.
248 |
ELEVATION BASED CORNEAL TOMOGRAPHY |
Ectatic Change: Progressive change in the cornea associated with increasing curvature, increased elevation and often associated thinning. Ectatic change is seen in keratoconus, pellucid marginal degeneration and post refractive ectasia.
Elevation Topography: Method of imaging the corneal surface(s) that generates an X, Y, and Z coordinate systems (locates points in space) and creates maps of corneal surface compared to a reference surface (sphere, ellipse, toric ellipse). Curvature and pachymetry maps can be computed form this elevation data.
Enhanced Elevation Maps: These are corneal elevation maps where the reference surface has been calculated after excluding an area with potential corneal abnormalities. The resulting corneal elevation map will better highlight areas of abnormal corneal elevation.
Equivalent Keratometer Readings (EKR): Used for post refractive IOL calculations, this reading utilizes both the anterior and posterior corneal surfaces to produce a graphical and tabular representation of the “adjusted” post surgical “K” readings at varying pupil sizes.
Forme Fruste Keratoconus: This refers to a mild or abated form of keratoconus with few clinical signs and subtle topographical changes. It has often been used incorrectly to describe “suspect” cases without any signs or symptoms of ectatic change.
Index of Height Asymmetry (IHA): This index gives the degree of symmetry of height data with respect to the horizontal meridian. This index is analogous to the Index of Surface Variance (IVA), though it is sometimes more sensitive.
Index of Height Decentration (IHD): This index is calculated from Fourier analysis of corneal height and gives the degree of vertical decentration of the apex. This value is elevated in keratoconus.
Index of Surface Variance (ISV): Gives the deviation of individual corneal radii from the mean value. This index is elevated in all types of irregularity of the corneal surface (scars, astigmatism, deformities caused by contact lenses, keratoconus, etc.).
Index of Vertical Asymmetry (IVA): Gives the degree of symmetry of the corneal radii with respect to the horizontal meridian. This index is elevated in cases of oblique astigmatism, in keratoconus or in limbal ectasias.
Irregular Astigmatism: Type of astigmatism where the principal meridians are nonorthogonal. This type of astigmatism is not correctable fully by spectacles.
CHAPTER 18. GLOSSARY |
249 |
Keratoconus-Index (KI): Compares measurements of the central and peripheral corneal thickness allowing quantification of corneal thinning. This index tends to be elevated in keratoconus.
Keratometer: Also known as an ophthalmometer, is a diagnostic device used for measuring corneal curvature at a defined and set optical zone.
KMax: Point of highest curvature on the axial or sagittal curvature map.
Lens Densiometry Graph: A graphical display depicting the level of opacity of the ocular media. Higher values equate to reduced light transmission. It is most useful in evaluating lens clarity.
Orbscan (Bausch & Lomb): An early device employing slit scanning elevation topography combined with a Placido topographer which provides topographic maps of the anterior and posterior corneal surfaces and images of the anterior chamber.
Par CTS (Par Vision): An early corneal imaging system which used close-range photogrammetry (rasterphotogrammetry) to measure and produce a topographic elevation map of the anterior corneal surface. No longer commercially available.
Pellucid Marginal Degeneration (PMD): A bilateral, non-inflammatory, peripheral corneal thinning disorder characterized by a band of thinning of the peripheral inferior cornea.
Pentacam (Oculus): A rotating Scheimpflug elevation based corneal topographer which provides elevation and curvature maps of the anterior and posterior corneal surfaces, corneal thickness maps, anterior chamber dimensions and objective readings of lens densitometry.
Percentage Thickness Increase (PTI): A graphical display showing the percentage increase in the average thickness along 22 imaginary circles centered on the thinnest point, with the diameter of the circles increasing in 0.4-mm steps.
Placido Disk: A planar keratoscope made of concentric rings that when reflected off the cornea, permit evaluation of the smoothness and an estimation of the curvature of the cornea.
Placido Topography: Curvature based corneal topography which uses a modified Placido disk, reflected off the corneal surface. The rings of the disk are digitally measured to create “topographic” maps of corneal curvature.