Three Key Glaucoma Tests: When and How to Use Them
The final part of our glaucoma management series focuses on gonioscopy, fundus photography and serial tonometry -- their appropriate use and documenting for reimbursement.
BY ANDREW RABINOWITZ, M.D.

In previous installments of this series, I've discussed the appropriate use of some of the relatively new and highly sophisticated technologies that we can now utilize for early diagnosis and subsequent monitoring of glaucoma. In addition, we also have at our disposal several other more standard tests that can be performed on a relatively regular basis to obtain important information that can be of great benefit in diagnosing, treating and following glaucoma suspects and glaucoma patients alike.

In this article, I'll discuss three of these tests: gonioscopy, fundus photography and serial tonometry. I'll describe how these tests can be best used and how to document for reimbursement.

Gonioscopy

The anatomy of the anterior chamber angle holds vital information regarding the mechanism of IOP elevation. Though gonioscopy can be a challenging test to perform, once mastered it can become a powerful tool in helping to make accurate diagnoses.

Dynamic gonioscopy should be performed at the initial examination even if the patient doesn't appear to have narrowing. The test is performed with greatest facility using a four-mirrored lens coupled with the human tear film.

Once the lens is placed on the cornea, the next step is to properly identify Schwalbe's line, which is where the corneal light reflexes come together. Structures posterior to Schwalbe's line should then be identified. The trabecular meshwork is adjacent to Schwalbe's line and just posterior to it. Next, one can appreciate the scleral spur followed by the ciliary body band. In general, Schwalbe's line and the scleral spur are whitish in color. The trabecular meshwork can vary from blonde to deeply pigmented, as in cases of pseudoexfoliation and pigment-dispersion syndrome. In most instances, the primary goal of gonioscopy is to determine how much of the trabecular meshwork is available to facilitate aqueous egress from the eye.

If there's no available trabecular meshwork, as determined by gonioscopy, conventional outflow-enhancing drugs such as miotics will be of little use. The absence of meshwork also precludes the use of laser trabeculoplasty as a treatment modality. Thus, patients with no conventional trabecular outflow are typically surgical patients.

Gonioscopic grading of angle depth as well as pigmentation is important. Angle depth is determined based upon how deep (posterior) one can see. If only Schwalbe's line can be identified, the angle is grade I. If the trabecular meshwork can be appreciated, the angle is grade II. If visualization to the scleral spur is possible, the angle is grade III. Finally, if the ciliary body band can be appreciated, the angle is grade IV. The presence of an abnormally deep angle, either localized or diffuse, suggests previous trauma and consequent recession. Iris recession often doesn't lead to glaucoma until years after the initial trauma. The mechanism of angle-recession glaucoma is likely due to the proliferation of a fibrotic membrane over the trabecular tissue. Traumatic glaucoma cases usually have asymmetric gonioscopic findings. Traumatic glaucoma can rarely be diagnosed without a thorough gonioscopic examination.

Obtain More Clues

Angle pigmentation varies dramatically among individual patients. Meshwork pigmentation can be so mild as to be barely appreciated, or as dark as the iris and ciliary body band. The two conditions that manifest with heavy meshwork pigmentation are pigment-dispersion syndrome and pseudoexfoliation syndrome. Both conditions can be relatively asymmetric with regard to the degree of pigment in each eye. In bilateral cases, the eye with heaviest pigmentation usually reveals the higher IOP. Not all patients with pigment-dispersion syndrome develop glaucoma. Additionally, not all cases of pseudoexfoliation syndrome progress to glaucoma.

Once angle depth and pigment are graded, your attention should be turned to the presence or absence of synechiac adhesions and/or abnormal blood vessels. With gonioscopy, the lens is initially applied with minimal pressure on the globe. If no angle structures are appreciated at this point, then gentle pressure should be applied to the lens to attempt to open the angle. If the angle can be opened to the degree that the trabecular meshwork can be appreciated, the angle is said to have appositional closure.

The implication of appositional closure is that an iridotomy or lensectomy may serve to restore near physiologic anatomy. If the angle can't be opened to the degree that the trabecular meshwork can be appreciated, the angle is said to have synechial closure. The implication here is that even after an iridotomy or lensectomy, angle anatomy isn't likely to be restored to the degree that the trabecular meshwork will be visible to the examiner's eye, or to allow aqueous outflow.

Finally, the presence or absence of abnormal blood vessels must be determined. If abnormal vessels are found in the angle, the patient likely has iris rubeosis. The presence of abnormal angle vessels suggests retinal or panocular ischemia. The ischemia leads to the production of angiogenic compounds, which circulate throughout the eye. The presence of neovascularization in the angle mandates a thorough evaluation for possible causes of posterior segment or panocular ischemia.

The most common cause of retinal ischemia is diabetic vasculopathy. The most common cause of panocular ischemia is carotid artery disease. If the etiology of the ischemia isn't identified, the neovascularization won't abate. If retinal photocoagulation is performed to curb production of angiogenic compounds, repeat gonioscopy should be performed to monitor for evidence of regression of the neovascularization. Prompt regression of the abnormal vessels should occur following adequate panretinal photocoagulation.

Repeating the Test

The need to repeat gonioscopy depends on the type and mechanism of glaucoma. In the case of an anatomic narrow angle, repeat gonioscopy can be helpful in determining progression toward angle closure. With a potential angle-closure patient, gonioscopy can reasonably performed at 6-month intervals. Your Medicare carrier may have a local coverage determination on this and you should always check the carrier's Web site for policies and coding guidelines. Billing in excess of guidelines may prompt medical review and require the submission of more documentation.

In the case of pigment-dispersion syndrome, gonioscopy should be performed at 6-month intervals. This allows for appropriate staging of the condition. Additionally, gonioscopy allows for appreciation of the anatomic position of the peripheral iris. Repeat gonioscopy is also of value in following iris cysts and pigmented lesions with malignant potential, and is mandated in the evaluation and monitoring of suspicious iris lesions. Pigmented iris neoplasms have been known to spread locally to the filtration angle.

Careful gonioscopy is also of therapeutic importance. Grading meshwork pigmentation is helpful in selecting optimal candidates for laser trabeculoplasty. Through experience, we've learned that the tissue response to argon laser trabeculoplasty (ALT), and to a lesser degree selective laser trabeculoplasty (SLT), is greatly influenced by the degree of angle pigmentation. In general, the heavier the pigmentation, the greater the tissue response and consequently the degree of IOP reduction achieved. This is the reason that patients with pigment dispersion and pseudoexfoliation show the greatest response to laser trabeculoplasty. Thus, if the patient is being considered as a candidate for laser trabeculoplasty, the examination upon which this decision is being made can and should include a thorough gonioscopic examination. The rationale in this case is to assess the chances of success, as well as to determine what laser settings would be optimal in a given case.

Documenting Gonioscopy

The role of gonioscopy in glaucoma evaluation can't be overestimated. The CPT code for gonioscopy is 92020 and isn't currently bundled with the office visit and consultation codes. The national average reimbursement for 2004 is $26.50, bilateral. To bill for the procedure, a good diagram must be included in the chart. It must include angle and pigment grading, as well as the presence or absence of peripheral anterior synechiae (PAS) as well as abnormal angle vessels. In the case of traumatic glaucoma, the areas of recession must be delineated. Again, check with your Medicare carrier for utilization guidelines, as they may vary from carrier to carrier.

Gonioscopic examination is often neglected if the patient doesn't appear to have narrow-angle anatomy on slit-lamp examination. But gonioscopy is very valuable from a clinical perspective for determining the mechanism of glaucoma, monitoring various types and stages of glaucoma, and in understanding the response to various therapeutic treatment options, including the response to certain classes of medications, as well as to laser trabeculoplasty.

Fundus Photography

Fundus photography using stereo photography remains the gold standard for evaluating the optic nerve head. Stereo photos should be obtained at the time of initial evaluation, and then used for prospective analysis. The use of stereo photography allows for careful viewing of the optic nerve head vasculature as well as disc morphology.

If the patient's clinical condition suggests progression, photographs should be obtained at that time and be compared to those obtained at the time of initial examination. If the patient's condition is relatively stable to firmly stable, photos can be repeated at 3 to 5 year intervals. Disc photos should be obtained at any time during the course of treatment or observation if a disc hemorrhage is noted. Photos should be repeated when the hemorrhage resolves.

Unlike scanning laser ophthalmoscopy (92135), fundus photography is reimbursed by every Medicare carrier. It's billed using CPT code 92250, with the national average reimbursement for 2004 at $81.76, bilateral. The frequency for which Medicare reimburses for the test depends on the local coverage determination and coding guidelines, and is diagnosis-driven.

Proper documentation mandates that the photos be accompanied by an interpretation and report that includes vertical and horizontal cup/disc ratio based upon vessel pattern and/or coloration, the presence or absence of diffuse or focal pallor, the presence or absence of asymmetry, and the presence or absence of progression regarding any of the above parameters. Photos should be properly labeled as to which eye they represent, the date they were taken, and the date they were reviewed. Finally, the implications or consequences of the photographic evidence should be discussed. The implications should address whether any changes in treatment plan will be instituted as a result of the photographs. If the photos include red-free images, commentary on the status of the retinal nerve fiber layer should accompany the images.

Fundus photos are equally important to document normal anatomy as well as pathology. Care should be taken at the earliest stages to address the presence or absence of asymmetry. If the discs are atypical but not abnormal in appearance, it's important to provide an explanation. Anomalous disc conditions include physiologic cupping, tilted discs, small crowded discs, disc drusen, and peripapillary atrophy. Each of these conditions can contribute to atypical perimetric results. If anomalous disc anatomy is appreciated, the visual fields should be re-evaluated to look for resultant scotomas. The value of fundus photos is greatly enhanced if the results are correlated with functional deficits.

To date, no single technology has shown clinical superiority to the time-tested value of fundus photography.

Serial Tonometry

Perhaps equally important to IOP reduction in the treatment of glaucoma is reducing the range of diurnal fluctuation a given patient experiences over the course of a day. Asrani et al. have shown that flattening the diurnal curve is a paramount goal of glaucoma treatment.¹ Patients whose IOP varied more than 11 mmHg over a given day had a less than 15% stability rate. This rate was nearly tripled by lowering the diurnal range to around 7.0 mmHg. The therapeutic value of flattening the diurnal curve to physiologic levels (3.8 mmHg) is of incalculable value.

The best way to determine the range of IOPs a patient experiences over the course of a day is to obtain a diurnal curve by performing serial tonometry. Although we no longer hospitalize patients to obtain IOP readings around the clock, we can at least obtain a fair assessment of diurnal range over the hours our practices are open. When serial tonometry is performed during the course of a single day, CPT code 92100 is used.

CPT code 92100 is a bilateral code and is no longer bundled with office visits/consultations. The national average payment for 2004 is $82.14. The code has two uses: measurement of diurnal IOP fluctuations and monitoring IOP during the acute treatment phase of an attack of acute narrow-angle glaucoma. Check with your local Medicare carrier for utilization guidelines, including frequency.

Serial tonometry is helpful prior to initiating therapy in a newly diagnosed case. This enables the doctor to gain an understanding of a given individual's circadian readings. It's been my experience that repeating serial tonometry several months into a treatment plan can help determine if the medical regimen being used is effective throughout the day.

Many working patients can't devote all or part of a day to having their IOP checked. It's possible, however, to have patients vary the time of their visit to our offices so as to obtain a more varied set of measurements. Thus, each visit can be at a unique time point. This allows for the acquisition of diurnal measurements over the course of a year as opposed to a single day.

Serial tonometry is a valuable tool if a patient appears to be progressing in spite of IOP readings that are repeatedly within the specified target range. Additionally, as our patients become increasingly enamored with once-daily therapy, we must ensure that their IOPs are remaining in an acceptable range in the waning hours of the day during the trough effect of their medications. Regardless of whether diurnal readings are obtained over a single day, or over a series of visits each at a different time of day, understanding the circadian patterns of IOP levels is of vital importance in the diagnosis and treatment of glaucoma.

Use These Important Tools

Overall current utilization of the three tests discussed in this article may be less than that of scanning laser ophthalmoscopy and standard achromatic perimetry. The information obtained, however, is of equally vital importance. Each test adds to the depth of knowledge we can attain with each individual. Adding these modalities to your treatment program will yield tremendous benefits to both you and your patients. The minimally invasive nature of each of these tests makes them excellent adjuncts to our core knowledge. They also add depth to the two-dimensional database defined by perimetry and scanning laser ophthalmoscopy.

In addition to improving the standard of care you can provide to your patients, using these tests allows for improved reimbursement without having to increase spending on new technologies.

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.

REFERENCES

1. Asrani et al. Diurnal intraocular pressure fluctuation and glaucomatous progression. Journal of Glaucoma. 2000;9:134-142.