92135
Part 2 of our glaucoma management series examines the expanding role of these technologies, how they're being used, and how to document for reimbursement.
BY ANDREW RABINOWITZ, M.D.

ILLUSTRATION: JOEL & SHARON HARRIS

Until about a decade ago, medical management of glaucoma patients was generally considered to be a time-consuming exercise that brought only limited benefits to patients, and few professional or financial rewards to the ophthalmologists who treated them. But beginning in 1995, revolutionary advances in treatment have gradually transformed glaucoma management into an exciting frontline area of everyday ophthalmology.

In the first installment of this five-part series, I described how the introduction of more efficacious drugs and the development of advanced diagnostic instruments has made long-term medical management of glaucoma an increasingly positive experience for patients, and more financially rewarding for practices.

This month, I'll discuss what I believe is the appropriate role of such instruments as the Heidelberg Retina Tomograph, the Laser Diagnostic Technologies GDxVCC, the Carl Zeiss Meditec Stratus OCT, and the Talia RTA in the early diagnosis and subsequent monitoring of glaucoma. I'll also explain when these tests should be administered, and how findings should be documented to qualify for reimbursement under CPT code 92135.

Detecting the Disease

Determining the appropriate time to initiate treatment of a glaucoma suspect is a highly complex task. Equally daunting is the challenge of assessing whether a glaucoma patient who's currently being treated is in fact stable, or undergoing progression.

As you know, achromatic automated perimetry is the current standard of care for detecting the presence of functional glaucomatous nerve damage. For patients with documented glaucoma, standard achromatic automated perimetry (SAP) is used to assess stability or progression at a given level of IOP.

The optic nerve is made up of a combination of p (parvocellular) and m (magnocellular) axons. The optic nerve consists of approximately 1 million p-cell axons and 200,000 m-cell axons. P-cell axons have profound neural redundancy. Large numbers of p-cell axons must be lost in order to appreciate the earliest visual field defects on SAP.

Approximately 50% of p-cell axons are theorized to have already been lost when visual field testing reveals the first glaucomatous defects. Anatomically, 40% of the neuroretinal rim area is lost by glaucomatous optic nerve damage before the first defects in visual field analysis appear.¹ The profound neural redundancy of p-cell axons precludes SAP from serving as a sensitive barometer of early glaucoma.

M-cell axons don't share the neural redundancy we see with p-cells. Loss of between 10 to 15% of the functional m-cell axons can lead to defects on short-wavelength automated perimetry (SWAP). M-cell axons are more vulnerable to early glaucomatous ischemia and consequent glaucomatous cell death compared with p-cell axons. Testing for m-cell loss offers greater sensitivity when looking for early glaucomatous damage.

Two modalities have been used to map m-cell loss. These modalities include contrast sensitivity testing in the form of frequency doubling testing (FDT) and SWAP. FDT is gaining momentum as a screening tool for early glaucoma. Historically, SWAP testing has been limited due to difficulty in seeing the test stimulus and cumbersome test algorithms. The introduction of SITA-SWAP should improve the utility of short-wavelength testing. SWAP testing will likely grow in popularity because it has shown promise in detecting functional loss earlier than SAP.²

Laser Diagnostic Technologies' GDxVCC	Heidelberg Engineering HRT II
Carl Zeiss Meditec Stratus OCT	Talia Technology, Ltd. RTA

The Goal: Earlier Detection

The current reliance on standard achromatic automated perimetry creates a delay in the ability to detect early glaucomatous cell death. This delay may be mitigated by using information garnered from biometric measurements of the optic nerve head (ONH) and retinal nerve fiber layer (RNFL). Changes in the ONH and RNFL may precede perimetric damage by a period ranging from months to years.³

Scanning computerized ophthalmic diagnostic imaging (SCODI) continues to evolve as a series of diagnostic modalities aimed at appreciating the earliest evidence of glaucoma.

It's long been known that pathologic changes to the optic nerve head and/or the RNFL are present months to years prior to obvious perimetric defects. Thus, when trying to determine which glaucoma suspects should be treated, SCODI offers great hope in that it may allow us to appreciate evidence of morphometric axonal loss prior to functional loss.

Technologies Vary

SCODI encompasses several distinct testing modalities.

The HRT II, developed by Heidelberg Engineering, evaluates the topography of the optic nerve head. Confocal scanning laser topography uses simultaneous stereoscopic videographic digitized images to make quantitative topographic measurements of the optic nerve head and surrounding retina.

Scanning laser polarimetry (SLP) is a modality used to study the nerve fiber layer. SLP is performed using the GDxVCC system made by Laser Diagnostic Technologies. Scanning laser polarimetry measures changes in the linear polarization of light (retardation). It uses a polarimeter, an optical device to measure linear polarization change, and a scanning laser ophthalmoscope together to measure the thickness of the nerve fiber layer of the retina. The device employs exclusive "variable corneal compensator" technology to mitigate the role of corneal birefringence.

Optical coherence tomography (OCT) uses low-coherence light to measure interference patterns. The Stratus OCT is an interferometer manufactured by Carl Zeiss Meditec. It provides real-time, non-contact, cross-sectional imaging and measurement of the optic nerve head, macula, and peripapillary region. OCT is used to identify structural abnormalities that precede or correspond to functional loss.

The Retinal Thickness Analyzer (RTA), produced by Talia Technology, Ltd., is an ophthalmic imaging device for the mapping and quantitative measurement of retinal thickness and disc topography. The RTA provides raw data as optical cross sections, 2D and 3D mapping, disc topography and fundus imaging of the optic nerve, peripapillary and macula regions. The RTA identifies subtle abnormalities and changes by mapping the retina and comparing the data to a normative database.

In the following section, I'll examine the four modalities in more detail and assess how they can appropriately be used in the detection and monitoring of glaucoma.

Scanning Laser Ophthalmoscopy (HRT II)

This instrument attempts to detect subtle morphometric changes to the optic disc. Kamal et al. identified change in a subset of ocular hypertensive patients, which predated the development of glaucomatous visual field loss. These studies followed ocular hypertensive patients for the potential development of primary open-angle glaucoma (POAG). During the course of the studies, no perimetric changes were noted among the glaucoma suspects. However, optic disc topographic changes were noted among ocular hypertensive patients who were subsequently deemed to have developed early POAG. The studies concluded that HRT could be used as a tool to help diagnose glaucoma prior to the development of visual field changes as measured by standard achromatic automated perimetry.^4,5

Work by Chauhan on patients with early glaucoma revealed that glaucomatous disc changes determined with scanning laser tomography occurred more frequently than field changes. Most patients with field changes also had disc changes; however, less than half of those with disc changes had field changes.⁶

HRT II has also been found to be of predictive value among patients with existing perimetric damage. Certain HRT II parameters have been shown to correlate with progression of visual field indices. A study by Lester et al. demonstrated the presence of significant correlations between progression of rim area and cup shape measure with visual field indices. These correlations suggest that these HRT parameters could be good indicators of the degree of glaucomatous ONH damage.⁷ It's likely that HRT II progression can be predictive of visual field and consequently glaucomatous progression.

A study by Hatch et al. attempted to assess which specific HRT II measurements may be of predictive value. In this study, the HRT II variables of rim volume, cup shape, and mean nerve fiber layer height distinguished:

► subjects with elevated IOP and normal nerve heads and subjects with glaucomatous optic nerve heads

► glaucomatous optic nerve heads with and without repeatable visual field abnormality⁸.

In addition to aiding in the diagnosis of early glaucoma and assessing progression in cases of documented glaucoma, HRT II can be useful in distinguishing between low-tension (LT) POAG and high-tension (HT) POAG.

Work by Eid and Spaeth has suggested that optic cups were larger in patients with LT-POAG than in those with HT-POAG. Measurements of sectors of the optic disc correlated better with visual field changes in LT-POAG than did global measurements of the whole nerve head, indicating more vulnerability of the optic nerve to focal damage with low IOP. The inferior neuroretinal rim area was significantly smaller (P> .05) in the LT-POAG group. The mean deviation of the superior arcuate area was significantly greater than the opposite sector in patients with LT-POAG but not in those with HT-POAG.⁹

Like all these new technologies, the role of HRT II in diagnosing and treating glaucoma is in its infancy. But with 12 years of clinical studies demonstrating its capabilities, its role in pre-perimetric glaucomas can't be overstated. Further studies will continue to define the value of HRT II in the evaluation and treatment of pre-perimetric and perimetric glaucoma. As HRT II continues to gain momentum, the size of the statistical database will increase. Accordingly, the data that we can use to evaluate the diagnostic and predictive power of HRT II will likely grow in an exponential fashion over the next several years.

Scanning Laser Polarimetry (GDxVCC)

The GDxVCC is a scanning laser polarimeter. Scanning laser polarimetry (SLP) relies on the birefringent property of the RNFL. The goal is to provide an indirect measurement of RNFL thickness. This measurement is based on the linear relationship between the retardation of monochromatic reflected light and histologically measured RNFL tissue thickness. There are other structures along the visual axis that are birefringent. These structures include the cornea, the Henle fiber layer of the macula, and the lens. Of these three layers, the cornea is of greatest importance with regard to birefringence.

To minimize the impact of corneal birefringence on RNFL measurement, Laser Diagnostic Technologies developed the variable corneal compensator (VCC.) The incorporation of VCC into the GDx has been shown to dramatically improve the reliability of the device. Numerous studies published in peer-reviewed journals have supported the conclusion that the device can accurately discriminate healthy from glaucomatous eyes.^10-13

A study by Burrow revealed that information provided by GDx SLP aids in the clinical decision-making process for patients with various types of glaucoma. The study also suggests that GDx analysis may be helpful in determining patients at risk for glaucoma, even in eyes in which the cup-to-disc ratio and visual fields hasn't demonstrated progression.¹⁴

Finally, GDx SLP has shown correlation with both HRT II and standard achromatic perimetry. These correlations were quantitated by Eid, Spaeth, and Katz. In this study, retinal nerve fiber layer height parameters (mean RNFLH and RNFL cross-sectional area) were decreased significantly in patients with glaucoma compared with healthy individuals. Retinal nerve fiber layer height parameters were correlated strongly with rim volume, rim area, and cup/disc area ratio. Retinal nerve fiber layer (RNFL) parameters and cup/disc area ratio showed the strongest correlation with visual field mean deviation in patients with glaucoma. Retinal nerve fiber layer height measures were reduced substantially in patients with glaucoma compared to age-matched healthy subjects. Retinal nerve fiber layer height was correlated strongly with topographic optic disc parameters and visual field changes in patients with glaucoma.

Results demonstrated a more marked thinning of the neuroretinal nerve fiber layer in the eye with the higher IOP in normal-tension glaucoma patients.¹⁵

The preceding paragraphs provide a glimpse into the growing body of scientific evidence supporting the use of GDxVCC in the work-up of glaucoma suspects and beyond. In sum, numerous clinical studies exist that support the use of scanning laser polarimetry.

Optical Coherence Tomography (OCT)

Optical coherence tomography is based on the principal of Michelson interferometry. Interference patterns produced by low-coherence light reflected from retinal tissues and a reference mirror are processed into an "A-scan" signal. Multiple A-scan signals are aligned to produce a two-dimensional image that can be thought of as a form of in-vivo histology.

OCT can be used to measure retinal nerve fiber layer thickness to differentiate glaucomatous eyes from normal eyes. RNFL and macula thickness normative data provide age-matched comparison of the patient to a population of normal patients.

Schuman and associates reported good correlation of OCT parameters with visual field findings.¹⁶ Optical coherence tomography has been shown to have relatively good reproducibility on repeated measurements.^17,18

OCT has been shown to discriminate early glaucoma from normal eyes in cases of glaucoma in which the diagnosis was based on abnormal visual field results.^19,20

OCT was also helpful in discriminating early glaucoma from normal eyes in cases of glaucoma in which the diagnosis was based on suspicious optic nerve head cupping.²¹

Both scanning laser polarimetry (GDxVCC) and optical coherence tomography (OCT) attempt to measure nerve fiber thickness. It's imperative that when two varied methods are applied to measure the same structures that they demonstrate strong correlation. Sek et al. compared OCT and GDxVCC in normal, ocular hypertensive, and glaucomatous eyes. OCT and GDxVCC were capable of differentiating glaucomatous from non-glaucomatous populations. Retinal nerve fiber layer thickness by OCT correlated with retardation measurements by GDxVCC.²²

Mistlilberger, Liebmann, Greenfield and Ritch evaluated the correlation between OCT and HRT II. They evaluated the optic disc and retinal nerve fiber layer appearance in normal, ocular hypertensive, and glaucomatous eyes. Both OCT and HRT II differentiated glaucomatous from non-glaucomatous eyes.

RNFL thickness measurements using OCT showed good correspondence to disc topographic parameters using HRT II. Specifically, OCT RNFL thickness showed no difference between normal and ocular-hypertensive eyes, but was significantly less in glaucomatous eyes. HRT II measurements of rim area, cup-disc ratio, cup shape measure, RNFL thickness, and RNFL cross-sectional area were significantly less in glaucomatous eyes and were correlated with mean OCT RNFL thickness. RNFL thickness obtained using OCT or HRT II were highly correlated with visual field mean defect when tested using standard achromatic perimetry.²³

Retinal Thickness Analyzer (RTA)

RTA is the most recently introduced scanning laser ophthalmoscope. The RTA is capable of creating 2-D and 3-D thickness and topography maps, as well as deviation probability maps from a normative database and quantitative numerical values.

A series of 5 to 13 scans are acquired in 3 to 5 minutes. Up to 208 optical cross sections are analyzed by a thickness algorithm at the posterior pole and peripapillary area. A topography algorithm is applied to map the optic disc.

A vertical green slit is generated by a helium-neon (543.3nm) laser. The slit beam is projected at an angle on the retina while a CCD camera records the backscattered light. Due to the oblique projection of the beam and the transparency of the retina, the backscattered light returns two peaks corresponding to the vitreoretinal and the chorioretinal interfaces. A 3 x 3-mm scan comprised of 16 optical cross sections is acquired within 0.3 seconds.

Five scans are obtained at the macula, three scans at the disc, and an additional five scans cover the peripapillary area.

As the CCD camera records the reflected image of the retinal cross sections, a thickness algorithm identifies the location of the anterior and posterior retinal borders. The calculated distance between the two light peaks determines the retinal thickness at a given point.

When used to evaluate disc topography, RTA requires that the operator draw a contour line along the disc edge. The same contour line is used in follow-up testing to ensure accurate monitoring of subtle changes. The disc topography report displays a rim/cup area map as well as a pseudo 3-D representation of the disc topography. Additional values that are analyzed and/or calculated include optic nerve head size and neuroretinal rim area. The parameters are crucial in detecting and monitoring for glaucomatous optic nerve head changes.

The clinical applications of RTA are varied and highly promising. Use of the RTA at the baseline evaluation of a glaucoma suspect or early diabetic may prove valuable. Once baseline RTA images are obtained, patients can be followed with serial examinations to evaluate for subsequent changes in retinal thickness. Gradual or rapid increase in retinal thickness associated with decreased vision in a diabetic may signify macular edema. Gradual or rapid decrease in retinal thickness, preceding or concurrent with perimetric field loss, may signify early glaucoma. In a patient whose retinal thickness appears to be thinning, additional evaluation of disc topography using RTA can be performed. Several recent studies support the validity of RTA testing.^24,25,26

New Tests Add to Knowledge

The objective of the biomorphometric tests is to allow for earlier detection of glaucoma prior to the development of perimetric damage, and aid in the determination of progression or stability in existing glaucomas. It's the goal of these new glaucoma diagnostic tests to discriminate among patients with normal IOP who have glaucoma, patients with elevated IOP who have glaucoma, and patients with elevated IOP who don't have glaucoma.

The role that these technologies play in the initial diagnosis and subsequent follow-up varies with the degree of pathology present at the time of initial evaluation.

The role of SCODI is greatest in glaucoma suspects and pre-perimetric glaucoma. ONH and RNFL changes often precede perimetric evidence of glaucoma. Abnormal findings or progression appreciated by SCODI testing may provide the first evidence of pathologic glaucomatous damage.

How I Use SCODI

For glaucoma suspects, and pre-perimetric glaucomas, I order baseline SCODI at the time of my initial evaluation. If the initial field is full, I repeat SCODI at 6-month intervals. I'll continue to repeat the test at 6-month intervals until such time as progression is noted or the visual field test uncovers an initial abnormality. At any time during follow-up, if either perimetric damage or morphometric progression is noted, I'll adjust the frequency of testing. Standard achromatic perimetry will be repeated at 6-month intervals instead of annually. This continues until target pressure is defined and reached. Once target pressure is achieved, I'll reduce SCODI frequency to annually, and increase perimetry to twice annually.

In glaucoma suspects and early glaucomas, an abnormal SCODI result mandates repeat testing usually at 6 months. A repeatable SCODI defect at this stage of the disease usually results in the initiation of medical treatment, even in the absence of perimetric damage.

In my view, the role of SCODI in mild-to-moderate glaucomas should be slightly less prominent. If SCODI is being performed on a patient with documented visual field defects, it should be ordered once annually. If the results of SCODI suggest progression, this finding must be used in conjunction with the other variables used to follow the disease -- IOP level and visual field (VF) status.

If IOP and VF appear to be stable, then the suggestion of progression on SCODI is usually not weighted to the degree that the therapeutic course is augmented. If the clinical variables of IOP and VF suggest progression, a similar suggestion from SCODI serves to strengthen the case for more aggressive treatment. Thus, at the mild-to-moderate stages of glaucoma, SCODI results are helpful if they're consistent with the standard clinical variables. If however, they contradict standard variables, they can usually be discounted.

SCODI plays its least prominent role in advanced glaucomas. By this stage, patients have definitive evidence of perimetric damage.

It's still reasonable to utilize SCODI at baseline and again annually. The results of SCODI at this juncture are used not to alter the course of therapy, but rather to further bolster the case for stability or progression.

Use All Your Tools

Thus, the application of SCODI varies with degree of disease. The test provides useful information at all stages of the disease, but in general, the utility tends to decrease as the degree of perimetric damage increases. This isn't necessarily a linear relationship, and thus I feel that annual SCODI is reasonable in all but "end-stage" glaucomas. A majority of end-stage cases require IOP reduction to 12mm HG or less, and very little objective data can sway this aggressive target zone.

New technologies offer growing opportunity to treat glaucoma with greater degrees of certainty. It's important, however, not to abandon the gold standard of biomorphometric analysis, stereophotographs of the optic nerve head.

A note of caution for all who treat glaucoma with enthusiasm as we apply new technology to the treatment of an age-old disease: Greaney, Hoffman, et al. compared SLO, SLP, and OCT with stereophotographic analysis of the optic nerve head. The study concluded that all three were no better than quantitative assessment of optic nerve head stereophotographs by experienced observers at distinguishing normal eyes from those with early-to-moderate glaucoma. A combination of imaging methods significantly improves their capability.²⁷

If we can learn to harness the information these newer technologies provide, we can begin to initiate treatment earlier in patients who truly are at risk for developing pathologic damage, while at the same time protect patients who represent physiologic extremes yet are not likely to develop pathologic glaucomatous cell death.

Who and When to Test

Often the first question I hear when these newer tests are discussed among ophthalmologists is: "Who should be tested?"

Candidates for these tests include any patient who's deemed to be a glaucoma suspect by virtue of elevated or asymmetric IOP, suspicious optic nerve heads, or the appearance of characteristic glaucomatous defects on perimetric testing. Family history alone doesn't substantiate a diagnosis of glaucoma suspect. Similarly, diabetes mellitus, myopia, and race in absence of objective or subjective findings don't justify a diagnosis of glaucoma suspect.

Some private insurers -- as well as Medicare -- now reimburse for these tests using CPT code 92135. The code is a unilateral code, which means that you can bill for each eye separately. In addition, it falls under the category of "general supervision" for Medicare, which means that a physician's presence on the premises isn't required.

For Medicare, how often CPT code 92135 can be billed varies with each carrier. No National Coverage Decision (national policy) currently exists. The best recommendation is to contact your local Medicare carrier and study its policies. These policies are available on your carrier's Web site and are currently listed under "Local Medical Review Policies" or "Local Coverage Decisions." Policies can vary significantly from carrier to carrier.

Reimbursement Varies

The following guidelines may serve as a reasonable example of the reimbursement you can expect. They reflect the policies of my local carrier, Noridian Medicare for Arizona.

For patients characterized as glaucoma suspects and those with "mild damage," the test can be performed once per year. But as I stated above, I tend to use SCODI on glaucoma suspects and pre-perimetric glaucomas at 6-month intervals, which means that not all of these tests qualify for reimbursement.

Patients with "moderate damage" may be followed with SCODI or visual fields. One or two tests per year may be appropriate. If both SCODI and visual fields are used, only one of each test each year is necessary. When used together, their sensitivity approaches 90%.

In "advanced damage," visual field testing is preferred over SCODI. It's rarely necessary to perform more than four visual fields in a year in "advanced damage," and SCODI would probably not be necessary or beneficial. SCODI may be appropriate as one test per year.

Fully Document Your Claims

As with any diagnostic test, when submitting a claim for CPT code 92135, it's the responsibility of the treating physician to document the rationale for performing the test. The CPT description mandates "with interpretation and report." Third-party payers reserve the right to withhold or deny reimbursement if the appropriate documentation isn't provided when requested. Additionally, it's imperative that the results of the interpretation be shared with the patient. Patients are less likely to react adversely to the charges on their statement if the physician takes the time to explain the rationale for ordering the test.

When submitting a claim for a test under CPT code 92135, you're responsible for ordering, interpreting, and signing a written report. (See sample form on page 32.)

The written report should include the name of the ordering -- as well as the interpreting -- physician. The reason the test was ordered should be clearly displayed at the top of the report. The name and signature of the technician, and the date of the test should be included. The results of the test, as well as their implications for clinical management, are mandatory. Comparative data should be noted when applicable.

It's important to comment on whether the results will result in continuation of present management or a change of treatment plan. Finally, commentary on whether the test should be repeated and, if so, when, should be noted.

In the next installment of this series, I'll discuss the current standard of care for automated perimetry. This article will thoroughly discuss standard achromatic perimetry, short-wavelength automated perimetry, and frequency doubling technology.

Dr. Andrew Rabinowitz is a board-certified ophthalmologist specializing in glaucoma management. He's currently in private practice at the Barnet Dulaney Perkins Eye Center in Phoenix, Ariz., and can be reached at Barnet Dulaney Perkins Eye Center, 4800 North 22nd St., Phoenix, AZ 85016.

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