Applying Anterior Segment Imaging in Practice

Find out how emerging technologies are transforming clinical care in glaucoma, cornea, cataract and refractive surgery.

BY SEAN MCKINNEY, CONTRIBUTING EDITOR

Great advances in digital imaging and scanning technology that depend on revolutionary uses of optics and light continue to improve how practitioners evaluate the anterior segment to diagnose and manage disease. New technologies, such as Fourier-domain optical coherence tomography (OCT) and Scheimpflug imaging are enabling clinicians to examine the anterior and posterior surfaces of the cornea, obtain more accurate corneal thickness measurements, improve IOL implantation in cataract surgery and enhance screening, management and follow-up in refractive surgery.

Driving this trend for enhanced digital imaging of the anterior segment is the need to improve the management of glaucoma, cataract and refractive surgery patients. Doctors also want to offer better care to those with keratoconus, corneal ulcers and other pathologies.

This article will review a wide range of the latest digital imaging technologies used to evaluate the anterior segment and the research supporting their use.

"This is a very exciting time," says Jack Holladay, M.D., of Holladay LASIK Institute in Houston. "In the last few years, our ability to image the anterior segment has improved much more significantly than it had during the previous 20 to 25 years. And more progress is on the way."

Burgeoning Trend

Ike K. Ahmed, M.D., from the University of Toronto, compares the increasing use of these technologies to the gradual acceptance of new instruments 6 years ago for the posterior segment.

"Back then, we saw an explosion of posterior imaging technology," he says. "At first, doctors didn't consider these instruments a necessity. Now they're becoming essential to managing glaucoma, retinal disease and other aspects of posterior segment care. Similarly, imaging the anterior segment eventually will become an integral part of our assessments in preoperative testing, postoperative care and pathology management."

Phil Buscemi, O.D., a consultant from Greensboro, N.C., who specializes in ophthalmic instruments, says there's been a significant increase in the use of OCT for posterior and anterior care. "It's rumored that one company sold more OCT instruments to O.D.s than to M.D.s last year," he says. "The bottom line is that the units that can perform both anterior and posterior procedures are very attractive to optometrists and ophthalmologists."

Measuring Corneal Thickness, Curvature

Anterior segment imaging has been shown to accurately measure corneal thickness and curvature. The findings of several studies show a reliable correlation among procedures.^1–4 Li and colleagues⁵ found that high-speed OCT "is equivalent to ultrasonography for central corneal thickness (CCT) measurements before and after LASIK."

"The precision of these measurements is very valuable to us," Dr. Ahmed says. "It's become a key component in the assessment and management of glaucoma. It also can help in other applications, such as screening LASIK candidates or improving the accuracy of limbal relaxing incisions."

However, the literature consistently cautions physicians that these instruments aren't the same. In particular, "important discrepancies among instruments exist," wrote Li and colleagues⁵ after comparing ultrasound pachymetry, Orbscan II (Bausch & Lomb) and Visante OCT (Carl Zeiss Meditec). "Clinicians should be aware that corneal thickness measurements are influenced by the method of measurement and that, although highly correlated, these instruments shouldn't be used inter-changeably for the assessment of corneal thickness."^6–8

The Visante OCT (Carl Zeiss Meditec) provides an unprecedented level of detail for the corneal refractive surgeon.

Supplementing Gonioscopy

Digital techniques also are being used to examine angle structures. Physicians are using ultrasound to image the ciliary body to get information on the nonpupil block mechanisms of primary angle closure and diagnose other abnormalities, such as cyclodialysis clefts.⁹

"Ultrasound may prove to be important with the advent of angioplastic procedures for Schlemm's Canal," Dr. Buscemi says. "Preoperative and postoperative imaging likely will establish appropriate criteria for treatment and retreatment."

Anterior OCT offers the advantage of a noncontact approach that provides a rapid image of the angle and anterior segment.

"Additionally, OCT and these other imaging technologies offer documentation beyond traditional gonioscopy, providing a sustainable digital record of the patient's anatomy," says Elliott M. Kirstein, O.D., F.A.A.O., director of Harper's Point Eye Associates in Cincinnati.

OCT vs. Ultrasound Biomicroscopy

Anterior segment OCT and ultrasound biomicroscopy "showed excellent performance" when used to evaluate the angles of 17 patients at the Cole Eye Institute in Cleveland, resulting in similar mean values, reproducibility, sensitivity and specificity.¹⁰

"The true advantage of anterior segment OCT is its noncontact scanning method that's performed as the patient is in the sitting position, which increases comfort and compliance," according to a 2007 editorial in the British Journal of Ophthalmology.¹¹ Ultrasound biomicroscopy requires patients to lie in the supine position, but it does offer "clinically useful information, such as angle opening distance, angle recess area and trabeculariris," the editorial reported.

Dr. Holladay believes all digital imaging technologies have an advantage over gonioscopy. "OCT, Scheimpflug and digital ultrasonic ultrasound biomicroscopy provide very good calculations and analysis to help us see the angle as it really exists, without the mashing or manipulation of the eye that can distort the true anatomy," Dr. Holladay says. "These instruments also help evaluate pupil constriction very well."

Researchers in Singapore determined that anterior segment OCT effectively detected primary angle closure when it was compared with gonioscopy in 203 Asian subjects who'd been diagnosed with primary angle closure, primary open-angle glaucoma, ocular hypertension or cataract. Data showed that anterior segment OCT identified angle closure in at least one quadrant in 71% of patients, compared to 49% of patients examined with gonioscopy. When performed in the dark, OCT confirmed angle closure in 98% of subjects who were found to have angle closure on gonioscopy. OCT found that 2% of these patients had open angles. OCT also found closed angles in 44.6% of patients who were mistakenly diagnosed with open angles with gonioscopy.¹² A total of 56.4% of these patients were diagnosed with open angles through the use of OCT.

Evolving Technologies

Practitioners have seen a proliferation of innovations, including: • Scheimpflug imaging, which produces a 3-D image of the anterior and posterior surfaces of the cornea (Pentacam Comprehensive Eye Scanner, Oculus) Pentacam offers 3-D images of the front/back corneal surfaces. • Double Scheimpflug-Placido imaging that uses Placido imaging to provide corneal curvature data, while imaging with two Scheimpflug cameras captures elevation data (Galilei, Ziemer Ophthalmology) • A device that provides information on corneal topography, wavefront, autorefraction, keratometry and pupillometry (OPD-Scan II, Nidek) The OPD-Scan II offers a wide variety of corneal data. • Fourier domain optical coherence tomography (OCT), a light-interference-based technology that uses high-speed digital signal processing to convert the interference spectrum into as many as 30,000 A-scans per second (RTVue-100, Optovue) • High-frequency time domain OCT and biometry, which employs high speed and an altered scan profile to optimize its use for anterior segment imaging (Visante, Carl Zeiss Meditec Inc.) • An instrument that combines a confocal microscope, a noncontact endothelial microscope and a precision pachymeter to conduct full thickness corneal measurements and to localize intracorneal structures, including haze (ConfoScan 4, Nidek) The ConfoScan 4 provides full thickness corneal measurements. • Digital slit lamps, such as the Haag-Streit EyeCap and Imaging Module IM 900, Topcon SI-D Series and CSO with Digital Vision System, enhance the basic anterior segment examination with high-quality photography and video capture • A confocal laser scanning tomograph, combined with a special microscope for the cornea, that allows for imaging of corneal cells with 1 micron resolution (HRT3 with the Rostock Cornea Module, Heidelberg) The HRT 3-Rostock Corneal Module can image corneal cells. • A corneal topographer that measures the shape of the cornea, including any irregularities that may exist on the anterior and posterior surfaces, and also measures corneal thickness (Orbscan II, Bausch & Lomb). Orbscan II measures thickness, shape and surfaces of the cornea.

Improving Outcomes Post Glaucoma Surgery

Beyond diagnostics, the role of OCT is increasing in glaucoma surgery. In one study¹³ of seven patients, anterior segment OCT showed features of bleb morphology that changed after laser suture lysis. On OCT, six of seven eyes showed increases in total bleb height, greater bleb wall thickness and separation of the scleral flap from the sclera.

In another study,¹⁴ Müller and colleagues evaluated postoperative images of filtering blebs and deep sclerectomies, using slit-lamp-adapted 1310-nm OCT. Functioning and dysfunctioning filtering blebs delivered different OCT patterns and correlated with clinical outcomes. "This could be a new way to assess the postoperative healing process with the possibility of earlier intervention in cases of impending scarring," the study reported.

Corneal Applications

Doctors also rely on anterior segment imaging for effective management of corneal procedures.

Drs. Fine, Hoffman and Packer¹⁵ used OCT to establish a profile of a successful clear corneal cataract incision in 9,000 cases performed without one incident of endophthalmitis over 10 years.

"I use OCT to guide phototherapeutic keratectomy treatment for corneal scars," says David Huang, M.D., Ph.D., director of the Doheny Laser Vision Center in Los Angeles. "You can see the epithelial hyperplasia very clearly. I apply transepithelial ablation down to the maximum epithelial thickness in many cases to take advantage of epithelial smoothing, followed by further ablation outward, combined with refractive treatment or with masking to further smooth."

Imaging also has played a role in the following situations:

• When measuring graft central thickness with a Scheimpflug camera after penetrating keratoplasty.¹⁶

• When using OCT to compare visual function after deep anterior lamellar keratoplasty with visual function after penetrating keratoplasty for keratoconus, and also when correlating this comparison with corneal thickness.¹⁷

Managing Refractive Procedures With OCT

Surgeons and other clinicians continue to expand the use of OCT to improve screening, management and follow-up in refractive surgery. Some of the early work in this area gained momentum about 5 years ago when doctors began using OCT to monitor corneal flap and residual stromal bed thickness after LASIK.^18,19

"When you image the cornea, you can see the LASIK flap much easier and determine its anatomy," Dr. Holladay says. "This helped us determine that the IntraLase laser was much more precise than the microkeratome. We wouldn't have been able to determine this otherwise."

Dr. Ahmed says that digital imaging has increased the precision of preoperative assessments of potential refractive patients. "We can identify corneal ectasia or keratoconus," he says. "Imaging helps us evaluate the effects of previous LASIK when deciding whether to do an enhancement. These are all very critical points in refractive surgery."

Dr. Kirstein says that imaging provides needed information on corneal thickness and shape. "The ongoing challenge to screen for corneal disease, such as forme fruste keratoconus before refractive surgery, seems to be better managed when surgeons have sufficient knowledge of corneal thickness, as well as epithelial and endothelial curvature from limbus to limbus," he says.

Striving for Perfection

Dr. Ahmed says imaging increases the desire to produce refractive perfection. The new technology helps monitor healing and confirm reasons for less-than-ideal outcomes.

"Confocal microscopy allows you to examine the microscopic structure of the stromal bed," Dr. Buscemi says. "Often, there are changes in cellular morphology associated with aberrant corneal wound healing."

When using OCT in LASIK cases, "the main advantage you have is that you can measure the front and back surfaces of the cornea," Dr. Huang says. "The RTVue-100 (Optovue) provides higher resolution, which enables you to visualize the flap with higher precision."

Consider these recent findings related to anterior segment OCT found in the literature:

• The precision of anterior OCT was used to prevent LASIK retreatment in a 43-year-old man whose stromal bed was too thin. As a result, he was spared what could have been a harrowing outcome. Instead, he underwent successful PRK 4 months later.²⁰

The RTVue-100 (Optovue) improves screening, management and follow-up in refractive surgery.

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• Researchers used high-speed OCT in a study²¹ of 51 eyes to create a noncontact, rapid and repeatable measurement of LASIK flap thickness after surgery. Using a flap profile pattern (horizontal line, 512 axial scans) and a flap map pattern (4 radials, 256 axial scans each), they developed a new method for using high-speed OCT to measure LASIK flap thickness after surgery. This new method, they concluded, provided more information than previous OCT measurements had provided.

• OCT was found to be comparable to ultrasound pachymetry and Orbscan II (Bausch & Lomb) when measuring corneal thickness in a study²² of 59 eyes of 30 patients. Because of OCT, researchers were able to confirm that the femtosecond laser "created highly reproducible flaps that corresponded with the preoperative intended thickness."

• Besides imaging and measuring LASIK anatomy and monitoring postsurgical results, OCT helped surgeons at the Doheny Eye Institute at UCLA evaluate complications.²³

• Researchers who evaluated 32 eyes of 17 patients concluded that combining OCT (measuring posterior corneal surface) with Placido ring topography (measuring the anterior corneal surface) provided a better measurement of total corneal power than keratometry in the eyes of LASIK patients.²⁴ Using keratometry with the traditional index of 1.3375 was found to overestimate the total power in preoperative corneas and underestimate LASIK-induced change.

Scheimpflug and LASIK

The use of Scheimpflug Pentacam (Oculus) topography in LASIK cases also appears to be increasing, as demonstrated in recent studies.

Researchers at Albany Medical College used the technology to disprove reports that ectatic changes routinely occurred in the posterior corneal surface after LASIK surgery. Ciolino and Belin^25,26 reached this conclusion after evaluating 121 consecutive myopic eyes (103 LASIK and 18 PRK) and by examining 102 myopic eyes of 52 consecutive patients 1 year after surgery. "Contrary to results in previous studies, progressive changes to the posterior corneal surface didn't routinely occur after LASIK performed within established parameters," the study reports.

Other researchers documented the ability of Scheimpflug topography to evaluate the posterior surface of the cornea in myopic eyes after LASIK surgery.²⁷ "Pentacam has a high degree of repeatability for the posterior corneal curvature, which has a potential for early detection of keratectasia in these eyes," the study says. "Post-LASIK pachymetry with Pentacam also showed excellent repeatability."

Meanwhile, the Galilei system combines the advantages of two technologies: Placido imaging provides curvature data while imaging with two Scheimpflug cameras captures elevation data.

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Galilei (Ziemer) is a dual Scheimpflug imaging system that provides corneal curvature and elevation data.

Dr. Kirstein, who serves as research coordinator for Ziemer Ophthalmology, the company that produces Galilei, says this dual Scheimpflug system offers additional benefits, including pachymetry across the entire cornea, even when the camera is decentered by eye movement. Besides the dual Scheimpflug images, the Galilei uses Placido imaging to determine central curvature.

"Combining these three technologies into a single device not only saves office space and investment cost but creates the basis for obtaining consistent, combined diagnostic information from a single set of merged measurement data from three separate sources," he says.

Improving Cataract Surgery

Dr. Holladay says new imaging systems that measure the posterior surface of the cornea improve cataract procedures, especially in postrefractive surgery patients.

"As soon as you do PRK or LASIK, the normal relationship between the front and back surfaces of the cornea changes because we've created a new curvature on the front," Dr. Holladay says.

Anterior segment OCT and Scheimpflug technology can address this issue. "The predicted refractions after cataract surgery still aren't as good as they would be with a virgin cornea, but we're getting better," Dr. Holladay says. "It's just a matter of coming up with algorithms to measure the front and back surfaces. We have the data we need. Now we need the software that can help us calculate the appropriate powers."

Precise IOL Placement

Some surgeons are using imaging to improve IOL implantation. Baikoff²⁸ reported that 3 years of using OCT helped him develop safety criteria for the placement of phakic IOLs.

"For preoperative cataract patients, we use axial lens power, intraocular lens power, refraction and keratometry," Dr. Ahmed says. "But optimal outcomes soon will require more than calculating IOL powers. With these imaging technologies, you can better evaluate astigmatism, corneal aberration and patients who've undergone previous LASIK.

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"You can look at the anterior chamber dimensions preoperatively to determine full surgical depth, size of the capsule, and other surgical issues, such as cataract density and position of the lens. This could affect your approach and choice of IOLs, such as those that accommodate more. Imaging in the future may influence how you set your phacoemulsification platform to determine surgical parameters."

Evaluating IOLs Postoperatively

Using Scheimpflug imaging and a custom-built Purkinje instrument to measure tilt and decentration of IOLs, researchers in Spain found that both systems demonstrated high reproducibility.²⁹

"Using imaging postoperatively to evaluate for tilt or decentration of IOLs is a lot more important now," Dr. Ahmed says. "There's less tolerance of decentration when a patient has a multifocal IOL, for example."

Surgeons at Johannes Gutenberg-University in Mainz, Germany, used Scheimpflug imaging to increase postoperative safety after implanting irisfixated phakic IOLs.³⁰ The system was used to simulate the effect of aging on the position of the IOL to optimize preoperative evaluation and patient selection.

"Another extremely important point here is that wavefront aberrometry can be used to detect when the IOL has been distorted due to capsular fibrosis or surgical technique," Dr. Buscemi says. "This has become very important as IOLs become thinner and more subject to torsional distortion. Wavefront may provide the only method of understanding why a patient doesn't see well following IOL implantation."

Managing Pathology

A variety of other disease-management applications are emerging for anterior segment imaging.

"The resolution of RTVue-100 is now high enough to see pathogens, such as Acanthamoeba," Dr. Huang says. "I haven't been able to catch a clinical case yet, but in our laboratory, we've been able to see them as a few pixel spots."

The high resolution of the RTVue-100 (Optovue) allows physicians to see pathogens, such as Acanthamoeba, as indicated by the three pixel spots.

Dr. Buscemi says physicians can employ confocal microscopy to assess herpetic and fungal keratitis.

Recently, researchers used Scheimpflug imaging to document evidence that led to a diagnosis of a novel CYP4V2 gene mutation in a patient with Bietti's crystalline corneoretinal dystrophy.³¹ The diagnosis was made possible by visualizing signs of corneal deposits that weren't apparent with the use of other tests, including slit lamp examination, applanation tonometry, Goldmann kinetic perimetry, fundus photography, fluorescein angiography and OCT.

Using Digital Slit Lamps

Digital slit lamp imaging, however, has become particularly helpful. Allan Rutzen, M.D., of Rutzen Eye Specialists in Severna Park, MD, says he's incorporated the use of the Haag-Streit Imaging Module IM 900 with his EyeCap system. "The freeze frame technology enables me to capture high-resolution images, even in unsteady patients," he says. "The freeze technology is particularly useful for high-magnification images because I can choose from multiple images to select one that's not blurred by microsaccades. I use anterior segment photography mainly to manage pathology.

"Although my notes are thorough and descriptive, and drawings are helpful, I find that high-quality photographs are the best method for following pathologies over time," Dr. Rutzen continues. "Some examples are corneal infiltrates and scarring from herpetic keratitis, epithelial ingrowth after LASIK surgery and pterygium progression."

The Imaging Module IM 900 (Haag-Streit) offers freeze frame technology to capture high-resolution images — even in unsteady patients.

Digital slit lamps also are useful for patient education. "The photos enable me to better explain the diagnosis, treatment and visual symptoms," he says. "For example, when I show a patient his photos of a corneal ulcer close to the visual axis, he's better able to understand the seriousness of the condition as well as the need for prompt, aggressive treatment to prevent the infection from enlarging and spreading to the visual axis."

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Other Digital Slit Lamp Options

Topcon Medical Systems offers several digital slit lamps and accessories. Options range from the Digital Camera DC-1, which facilitates digital documentation, to the IMAGEnet2000 System, which allows for versatile diagnostic processing and interpretation. The SL-D2, SL-D4, SL-D7 and SL-D8Z are distinct slit lamps with varying capabilities, such as zoom, documentation and anterior-posterior range.

The Digital Vision System used by CSO slit lamps is designed to acquire and save high-quality photographs and videos by using a color correction algorithm. Software upgrades are made available through internet downloads.

The Digital Solutions Slit Lamp Imaging System (Escalon Digital Solutions) captures high-resolution digital images from any photo slit lamp for documentation, presentation and patient education. It connects to any computer and includes software that allows you to analyze, archive and integrate slit images with fundus and other diagnostic photos.

TTI Imaging Digital Image Capture (TTI Medical) is a software solution for surgical microscopes and ophthalmic slit lamps. It's adaptable to most slit lamps.

The Digital Slit Lamp Imager (OIS Inc.) integrates with your existing slit lamp, providing live-motion documentation of anterior and posterior segments. It features a foot switch and direct storage to your computer hard drive. Ophthalmic Imaging's Photo Slit Lamp Imager includes a high-resolution digital still frame camera that connects to your photo slit lamp for images of the anterior and posterior segments. The journal-quality images are ideal for documentation, presentation and patient education.

Looking Ahead

In the future, you can expect to see hybrid instruments, including a new Humphrey machine from Carl Zeiss Meditec that will combine placido rings and OCT, Dr. Holladay says. In this same unit, placido rings will sensitively measure corneal surface irregularities while OCT evaluates the interference of two light paths to determine corneal thickness and enhance imaging. "We'll see more alternatives as time passes. The future of anterior imaging has just begun."

That future, predicts Dr. Buscemi, will be characterized by increasingly interdependent technologies for the benefit of patients and your practice.

"The variety of modern modalities in anterior segment imaging offers the practitioner the ability to more accurately diagnose disease, prevent surgical complications and provide better outcomes for patients," he says.

"Telemedicine now offers the ability to comanage diabetes, glaucoma, cataract surgery and refractive surgery more efficiently. New and exciting applications continue to increase the bottom line of the practice, while at the same time reducing overall costs and providing better patient care." OM

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REFERENCES

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Accutome (800)979-2020 (610)889-3233 (fax) www.accutome.com AMA Optics Inc. (877) 744-3937 tsm@amaoptics.com www.amaoptics.com Canon Medical Systems A division of Canon USA Inc. (800) 970-7227 eyecare@cusa.canon.com www.usa.canon.com/eye-care Carl Zeiss Meditec (800) 342-9821 (925) 557-4100 (925) 557-4101 (fax) info@meditec.zeiss.com www.meditec.zeiss.com Clarity Medical Systems Inc. (800) 215-6005 (925) 463-7984 (925) 251-0078 (fax) ussales@retcam.com intlsales@retcam.com www.claritymsi.com Clement Clark Inc. (800) 787-5426 (513) 336-7260 (fax) DGH Technology Inc. (800) 772-3883 (610) 594-0390 (fax) info@dghkoi.com www.pachymeter.com Ellex Inc. sales.us@ellex.com www.ellex.com Escalon Digital Solutions (A Subsidiary of Escalon Medical Corp.) (800) 676-0043 www.escalondigitalsolutions.com EyeQuip (800) 393-8676 info@eyequip.com www.eyequip.com EyeSysVision Inc. (281) 885-3800 (281) 749-8139 (fax) info@eyesys.com www.eyesys.com Gulden Ophthalmics (800) 659-2250 info@guldenophthalmics.com www.guldenophthalmics.com Haag-Streit USA (800) 787-5426 sjuenger@haag-streit-usa.com www.haag-streit-usa.com Heidelberg Engineering Inc. (800) 931-2230 (760) 598-3770 (760) 598-3060 (fax) contactUSA@heidelbergengineering.com www.heidelbergengineering.com Heine (800) 367-4872 info@heine.com www.heine.com Keeler Instruments (800) 523-5620 keeler@keelerusa.com www.keelerusa.com Kowa Optimed Inc. (800) 966-5692 kowa-usa-info@kowa.com www.kowa-usa.com Marco Ophthalmic Inc. (800) 874-5274 (904) 642-9338 (fax) info@marco.com www.marco.com Micro Medical Devices (818) 222-3310 Direct (866) 730-0663 Toll free sales@micromedinc.com www.micromedinc.com M&S Technologies Inc. (877) 225-6101 (877) 473-9806 (fax) sales@mstech-eyes.com www.mstech-eyes.com MSS (800) 728-9615 info@ms-services.com www.ms-services.com www.foreseephp.com Nidek Inc. (800) 223-9044 info@nidek.com www.usa.nidek.com Ocular Instruments Inc. (800) 888-6616 (425) 455-5200 (international) contact@ocular-instruments.com www.ocular-instruments.com Oculus Inc. (888) 284-8004 sales@OculusUSA.com www.OculusUSA.com OIS (800) 338-8436 info@oisi.com www.oisi.com Ophthonix Inc. (858) 899-2100 sales@ophthonix.com www.ophthonix.com OPKO Instrumentation/OTI (416) 631-9123 (Canada) (305) 575-4179 (Miami) www.opko.com Optos North America (800) 854-3039 www.optos.com Paradigm Medical Industries Inc. (801) 977-8970 info@paradigm-medical.com www.paradigm-medical.com Propper Manufacturing Company Inc. (800) 832-4300 (718) 392-6650 (fax) marketing@proppermfg.com www.proppermfg.com Precision Vision (800) 772-9211 precisionvision@mindspring.com www.precision-vision.com Reichert Ophthalmic Instruments (716) 686-4500 (716) 686-4545 (fax) info@reichert.com www.reichert.com RH Burton Company (800) 848-0410 sales@rhburton.com www.rhburton.com Right Medical (888) 224-6012 info@rightmedical.com www.rightmedical.com Shin-Nippon Ophthalmic Instruments (800) 630-5048 (440) 543-5742 fax www.shin-nippon.jp Sonomed Inc. (A Subsidiary of Escalon Medical Corp) (800) 227-1285 www.sonomedinc.com Stereo Optical Co. Inc. (800) 344-9500 (773) 777-2869 (International) sales@stereooptical.com www.stereooptical.com www.functionalvisionanalyzer.com Tomey USA (888) 449-4045 www.tomeyusa.com Topcon Medical Systems Inc. (800) 223-1130 tmsinfo@topcon.com www.topconmedical.com Tracey Technologies Corporation (877) 872-2393 info@traceytech.com www.traceytechnologies.com Veatch Ophthalmic Instruments (800) 447-7511 ask@veatchinstruments.com www.veatchinstruments.com VectorVision Inc. (800) 526-7703 info@vectorvision.com www.vectorvision.com Vision Sciences Research Corporation (925) 837-2083 inquiry@contrastsensitivity.net www.contrastsensitivity.net Visual Pathways Inc. (928) 778-5002 admin@vispath.com www.visualpathways.com Volk Optical Inc. (800) 345-8655 volk@volk.com www.volk.com Welch Allyn (800) 535-6663 www.welchallyn.com Woodlyn Inc. (800) 331-7389 (847) 952-0045 (fax) woody@woodlynintl.com www.woodlynintl.com Ziemer Ophthalmic Systems (866) 708-4490 (618) 251-9537 (fax) Pam.kaizer@ziemergroup.com www.ziemergroup.com