DIAGNOSTICS

Correctly Obtain and Analyze Serial Scans
Measuring changes in the optic nerve head via serial scans is the key to successful glaucoma management.

J. James Thimons, O.D., F.A.A.O., Fairfield, Conn. & Mary Jameson, B.H.S., C.O.A., C.P.O.T., Philadelphia.

Glaucoma is like an identity thief. Instead of stealing your identity a little at a time without warning until you suddenly receive bills for exorbitant purchases you never made, however, it gradually steals your sight without warning, until you suddenly find yourself blind.
To prevent identity theft, many employ credit-monitoring agencies. To prevent further vision theft from glaucoma, we use state-of-the-art diagnostic and monitoring instruments.
But simply employing these devices isn’t enough to stop further vision theft. You must both acquaint yourself with these instruments’ operating features and determine your specific role in this process to ensure you can get them to meet your expectations.
Here, I will discuss the two steps you should take to ensure success with your Heidelberg Retinal Tomograph (HRT). Although several of these recommendations are specific to HRT, you can use the majority of them (ocular surface management, dilation, etc.) to obtain better results regardless of the type of instrument you employ. Remember: All instruments yield improved outcomes when you maximize the quality of the image.

1. Enhance image acquisition
Without a high-quality scan, you can’t accurately assess the patient’s optic nerve to determine glaucoma progression and whether this progression necessitates treatment. To make sure this is never an issue when employing your HRT III, follow these steps to enhance image acquisition:
• Instill a drop or two of artificial tears prior to obtaining the image. One of the key factors in obtaining a good image is the quality of the patient’s tear film. An “amoeboid” movement on the HRT’s screen is most likely the result of an irregular or “oily” tear film, and this can preclude you from obtaining a highquality image. But, by instilling a drop or two of artificial tears in the patient prior to taking the image, you will see a remarkable improvement in the photo you obtain. (The ocular surface is one of three refractive elements that can affect image quality. The others are cataracts, vitreous opacities and refractive error.) Instill the drops, and have the patient blink a few times right before you take the scan.
• Dilate your senior patients and patients who have unstable accommodation. Typically, you’ll get a better image if you dilate your senior patients who have any noticeable cataract because cataract can affect image quality.
Patients younger than age 25 can have unstable accommodation. This will influence the acquisition of data while the instrument is capturing the standard image set (you can observe this on screen as the instrument evaluates the patient). This can result in a final image that doesn’t represent the actual optic nerve head (ONH) anatomy and will cause problems with progression analysis when compared with future images. To avoid this, use a mild cycloplegic, which produces a consistent accommodative status in those patients who show variability. (Make a note in the patient’s chart that you did this, so you can complete his next visit under the same circumstances.) The mydriatics take a little while to work, so I instill the artificial tears just before the scan.
If, however, you choose not to dilate the patient, perform the HRT prior to the installation of any anesthetics to avoid the corneal surface degradation seen in most patients with the use of these agents. (Many patients have pupils large enough to image without dilation. This is typically greater than 4- to 5mm.)

• Correct or adjust for refractive status. While refractive error isn’t usually a concern, it’s occasionally necessary to correct or adjust for refractive status to improve the image quality. Just like quantified visual fields, you will most commonly notice a poor image quality (out of focus) in patients who have greater than 1.00D to 1.25D of cylinder. Remember that the instrument has an astigmatic correction feature. Insert the correction and align it to the appropriate axis. You will be pleasantly surprised by the improvement of image quality when you do this.
Another way to create better images from patients who have higher refractive errors: See whether the patient’s image quality is better with his own prescription or with the dioptric correction the HRT III provides. Remember: In patients with extremely high refractive errors who’ve had refractive surgery, it may be difficult to obtain a superior image due to the axial length vs. corneal steepness issues that the procedure induces.
• Assess the level of image quality. The level of image quality is indicated by the Standard Deviation number in the upper left of the printout and is an important indicator of the validity of the image. Typically, a number below 50 is considered reasonable, but the lower the achieved number, the better.
Another way to obtain better image quality: Image the same eye several times until you get an image that meets your criteria for acceptability. (This is the number developed in the initial normative data analysis.) An important caveat: Remember to delete those images you don’t use to prevent them from being utilized in the future analysis of progression.
• Make sure you’ve appropriately placed the contour line. The contour line is the basis for the topographic interpretation of the optic nerve and, as such, is an essential component of your interface with the device. While you can plot most patients on either the topographic or reflectance image, occasionally, individuals require a better view to assure that the device has identified a true optic disc margin. In situations like this, utilize the 3- D image feature to assure you’ve appropriately placed the contour line. This will ensure a good initial value and a better potential for accurate analysis of progression.

2. Accurately measure progression
To accurately measure glaucoma progression in a patient with the HRT III:
• Assess images three times over a one-year period. For most patients, glaucoma is a slowly progressive disease. For this reason, you should obtain an initial image in the first year you identify the disease, six months later and six months after that to elucidate a baseline over approximately a one-year period. This gives you the three necessary images over a period that is reasonable to measure progression for the average patient.
Still, patients who have normal tension glaucoma with migraine headaches or other vascular abnormalities, such as Raynaud’s Syndrome, may require more intensive monitoring. In addition, patients with existent early- to moderate- visual field loss and intraocular pressures (IOP) below goal levels may also need an accelerated series of HRTs, so you can carefully monitor them for the type of minimal change that is indicative of poor control. (A stable patient is usually imaged one to two times per year. In some instances, it may be necessary to image the patient at a rate that is significantly greater than that.)
• Carefully assess ONH area size. The idea of measuring ONH size has gained a real foothold in glaucoma assessment and can now be measured and recorded for every patient, who is a glaucoma suspect, ocular hypertensive or has active glaucoma, as part of the basic data collection in the office (pachymetry, IOP, Cup/Disc {C/D} ratio, mean deviation & ONH size). Still, it is not yet standard-of-care.
Do this in the mm-squared area of the optic nerve. The normative data range is from 1.69- to 2.82mm squared. This gives you a critical weapon in the analysis of the patient with large C/D ratios. You can find the numeric for this in the parameters dropdown menu on the HRT’s image screen or on the printout in the stereometric parameter section.
• Measure the total area of both nerves, and compare that finding with the C/D asymmetry. While in the past C/D ratios with more than a 0.2mm difference in size were considered indicative of glaucoma, the HRT III has shown that it’s unreasonable to make a decision regarding this issue until you’ve measured the total area of both nerves and compared that finding with the C/D asymmetry.

For example, a patient was recently referred for imaging with a 0.3mm difference between O.D. and O.S. with normal Sita 24 to 2 visual fields. Upon measuring the ONH area, I noted that the O.D. (smaller C/D) was a 2.0mm sq. area. The O.S. ONH was 3.0mm sq. area. This gave a relative comparison of O.D. vs. O.S. that showed the percentage of C/D ratio to ONH size was equal between the two eyes and thusly, the patient wasn’t at risk for glaucoma. Instead, this patient had a unilateral megalo-papilla and simply required regular follow-up at routine intervals. (The definition of megalo-papilla is ONH area > than 2.9mm square whereas a hypoplastic disc is <1.68mm.)
Remember: State-ofthe- art diagnostic and monitoring instruments for glaucoma are only as good as, you, their operator. In the case of HRT, this means, you must enhance image acquisition and accurately measure disease progression to prevent further vision loss and successfully manage your glaucoma patients.

Dr. Thimons practices in Fairfield, Conn., and is adjunct clinical professor at the New England College of Optometry & Pennsylvania College of Optometry and chairman of the National Glaucoma Society. E-mail him at jthimons@sbcglobal.net.

Ms. Jameson is an instructor in the Specialty Services Department of the Eye Institute, Pennsylvania College of Optometry, in Philadelphia. E-mail her at mary@pco.edu.