glaucoma

Imprison The Silent Thief

To ensure accurate diagnosis and the most appropriate disease management plan, use these nine methods of glaucoma surveillance.

BY J. JAMES THIMONS, O.D., F.A.A.O., Fairfield, Conn.

When law enforcement wants to prevent an elusive criminal from causing harm, they often perform surveillance on the individual with the hope of eventually confining him or her to jail. We, as eyecare practitioners, are similar to law enforcement with regard to glaucoma, or "the silent thief." We recognize that an elusive eye disease is stealing the sight and quality of life of our patients and we perform surveillance on it, in hopes of preventing or stopping the disease process in its tracks.

While the essential components of surveillance for law enforcement are informants, undercover officers, long-lens cameras and wiretaps, among other methods, the following are the essential methods of glaucoma surveillance.

1 ONH documentation

Documenting the appearance of the optic nerve head (ONH) at both the initial assessment and through long-term care is crucial, as it enables you to track disease progression, which guides your treatment plan. For this reason, every practitioner who manages glaucoma patients should employ ONH photography via a standard fundus camera — and, in particular, stereo images in the global examination and documentation of the ONH. This is because stereo images provide a three-dimensional view, enabling you to see subtle changes, such as atrophied nerve fibers, in the depth of the optic nerve — a location from which a good portion of glaucoma changes occur.

Additional benefits of ONH photography: It allows us to assess the ONH between dilation visits and obtain a view of the three key indicators of glaucoma progression at every visit, regardless of whether you perform dilation. These indicators are:

1. Drance hemorrhages. These disc hemorrhages, named for Stephen M. Drance, M.D., are one of the "tell tale" signs of progression of the disease.

2. Progressive peripapillary atrophy. This sometimes-subtle sign of disease progression is an indication that the microvascular supply to the ONH has been compromised.

3. Acquired optic pits. Although the appearance of this condition may be subtle, it's typically not subtle in its effect on vision and visual field function. One study on this anomaly revealed that up to 20% of glaucoma patients experience this phenomenon during the course of treatment.¹ This condition causes progression in the Z-axis of the ONH without significant enlargement of the existing cup/disc ratio.

The recent development of ONH photography software systems has been an enormous benefit, as they enable us to rapidly and efficiently process, analyze and record quality images.

Several ONH imaging devices exist that provide detailed information regarding specific aspects of the tissue. These devices: optical coherence tomography (OCT), Heidelberg retinal tomographer (HRT) and GDx (Carl Zeiss Meditec).

2 Cup/disc ratio and ONH measurement

You must evaluate the cup/disc ratio of the ONH in relation to the overall ONH size. The reason: Given that a relatively fixed number of nerve fibers exist in the optic nerve, if the patient's scleral foramen varies in size, this variation will markedly influence the size of the cup/disc, independent of disease.

Therefore, identify and record the vertical height of the ONH in all patients using small (1.5mm), medium (2.0mm) or large (2.5mm) via a 60D or 70D lens.

3 Visual field testing

Despite the evolution of ONH assessment technologies, visual field testing remains a significant method of diagnosing and monitoring glaucoma. Consider this: Visual field testing made approximately one third of initial glaucoma diagnoses in the Ocular Hypertension Treatment Study (OHTS). And as opposed to ONH assessment, which reveals structural damage, visual field testing reveals the visual "function" of the patient — critical information for determining their safety in the real world.

For these reasons, the results from visual field testing are just as important in guiding our diagnosis and development of a treatment plan as any other technology in the glaucoma arena.

Something else to consider: New developments in technology have markedly improved the outcomes of visual field testing. They include Selected Image Target Area (SITA)-based algorithms and their counterparts as well as glaucoma progression analysis (GPA) software.

4 Retinal nerve fiber layer (RNFL) view

Many ways exist for you to view the RNFL, so you can establish a baseline for future assessment. They are: The use of a red-free filter on the binocular direct or indirect (BIO) ophthalmoscope, digital fundus photography or optic nerve head (ONH)/RNFL imaging systems.

In viewing the RNFL, it's crucial to remember that seeing the detail of the RNFL in blue-eyed, light-skinned patients is difficult, as they have less eye pigment than dark haired and/or dark-skinned patients. For this reason, you may want to consider using one of the available imaging devices to make an accurate assessment.

Additionally, make sure you look for "slit" defects in the early aspects of the disease and"wedge" defects and "diffuse" loss in the later phases of the disease.

Typically, if I don't have access to ONH photography or imaging, I use the BIO with a red-free filter because it provides an excellent field of view, and its intense illumination enhances the contrast of the RNFL's fine striations. I utilize the 90D lens in patients who've undergone BIO and reveal pattern changes because this lens gives me a large field of view, so I can see the individual detail of the RNFL.

5 Gonioscopy

Because gonioscopy provides a view of the anterior chamber between the cornea and iris, enabling you to determine whether the drainage angle is open or closed, it's an important diagnostic and monitoring tool.

Unfortunately, many practitioners don't use a goniolens because it deviates from the normal glaucoma exam pattern and causes patient discomfort.

One study highlighted this fact, revealing an almost 50% rate of non-use in the initial assessment of glaucoma.² This, despite the fact that gonioscopy is a necessity to classify the disease into its basic component parts (i.e., narrow angle and open angle glaucoma).

Given that treatment varies dramatically between these two forms of the disease and the prognosis is exceedingly different, this is a test that every clinician committed to excellence in patient care should utilize.

6 Tonometer use

Although many clinicians employ a pneumatic tonometer to obtain both individual and diurnal IOP measurements, it's well known that the medical literature doesn't support this testing for final glaucoma diagnosis and treatment decisions.

The Goldman tonometer is currently the gold standard in IOP measurement for final diagnosis and treatment decisions. Tonometer technology, as is the case with other ophthalmic technology, however, is constantly evolving. In fact, devices now provide IOP measurements independent of inter-individual variations in corneal properties, such as thickness, scleral rigidity and corneal hysteresis.

7 Measurement responsibility limitation

Unfortunately, clinician-induced IOP measurement variation is common, compromising the treatment decision-making process and, therefore, the effectiveness of glaucoma management. For instance, IOP measurements obtained on one patient by multiple individuals is likely to create differing measurements secondary to different interpretations of mire endpoints. The difference between a thick and thin mire pattern can be as much as 5mmHg.

Because consistent technique is essential for the acquisition of accurate results, limit the number of clinicians responsible for obtaining the patient's IOP measurement, whether doctor or technician.

Also, while operating the tonometer, strive to curb aspects of the measurement process itself that could obfuscate therapeutic decisions. An example: body position at the slit lamp. If the patient has to squeeze himself behind the slit lamp to get his chin into a position in which you can obtain an IOP measurement, this can induce a valsalva-like compression of the chest wall, causing an increase in the patient's IOP. To avoid this, have the patient stand, or employ a hand-held tonometer.

8 Pachymetry

Although being dogmatic about specific tests in a disease of this diversity is dangerous, I believe that saying that you cannot accurately diagnosis nor efficiently treat glaucoma without employing a pachymeter is a reasonable statement.

Consider this: The OHTS revealed that eyes that had a corneal thickness of 555 microns or less were at threefold greater risk for developing glaucoma vs. eyes that had a corneal thickness greater than 588 microns. This finding prompted the study's authors to recommend corneal thickness measurement in the clinical evaluation of ocular hypertension patients.

Additional studies have expanded the impact of this information to patients who have glaucoma.^3,4

The bottom line: Because corneal thickness is a risk factor for visual field loss and a predictor of glaucoma, regardless of type, it's essential you use pachymetry not only in the decision to initiate therapy, but in the assessment of therapeutic regiments in existing patients, as modification may be necessary.

9 Diurnal curve assessment

Because research has revealed that diurnal fluctuations may be an independent risk factor for the progression of visual field loss, and that glaucoma patients who have a normal IOP reading during an office visit often have fluctuations in diurnal IOP independent of office measurements, you must assess diurnal IOP fluctuations prior to making disease management decisions, such as establishing a target IOP.^5,6

Have the newly diagnosed glaucoma patient present to your practice three times during one day (i.e. morning, mid-day and evening) for IOP measurements. Or, have him return for multiple visits during the diagnostic baseline workup. Several appointments are ideal, but both the patient's schedule and insurance coverage often prevent more than one such appointment.

I've found, however, that several health insurance companies do cover more than one diurnal assessment appointment in cases in which a patient who's currently taking glaucoma drugs presents with one or more of the glaucoma indicators.

Educate patients that the data you obtain from these appointments enables you to design the best treatment plan. This often dissuades their reluctance to present for these exams.

Glaucoma, like crime, currently has no cure. But, through the nine essential methods of surveillance, you can accurately diagnose and create the best treatment plan, enabling you to prevent and often stop the disease process. (In addition to these methods, consider using the Glaucoma Five-Year Risk Estimator or the Electronic Risk Calculator [Pfizer].) OM

References furnished upon request.

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