Article Date: 3/1/2005

Scanning Laser Ophthalmoscopy vs. Visual Perimetry
Diagnostic laser imaging is here to stay -- no question about it.
BY IAN BEN GADDIE, O.D., F.A.A.O., Louisville, Ky.

One decade ago, I discovered the concept of scanning laser technology in the form of an optic nerve tomographer that sat in a crated box beside an A-scan in a room that not many people at the University of Alabama at Birmingham (UAB) School of Optometry's Ocular Disease Clinic saw. The instrument was basically a high-tech cup-to-disc ratio estimator. It rarely, if ever, worked properly and when it did, no one knew what to make of the data. Eventually, we got smarter and the technology got better, sprouting other commercial devices including Heidelberg Engineering's HRT (Heidelberg Retina Tomograph), Carl Zeiss Meditec's Stratus OCT (optical coherence tomographer) and GDxVCC as well as Talia Technology's RTA (retinal thickness analyzer). Imaging is now integral in every aspect of glaucoma and retinal care.

Colleagues ask me how I use the technologies. Their most common question, "Which is more useful: a visual field machine or a scanning laser?"


Which instrument is better?

The answer to this question lies in the premise that visual field instruments detect functional vision changes, whereas imaging detects structural changes in the retina, optic nerve and nerve fiber layer. In general, structural changes precede functional changes, although exceptions do exist.

Keep in mind that each test yields a piece of information, and each piece of information on its own isn't diagnostic for glaucoma. Therefore, you have to integrate all pieces of the information to solve the puzzle. Not all of the information will present itself at once and many times, the process takes several years to rule glaucoma in or out. There has never been and there likely never will be a silver bullet diagnostic test for glaucoma.

Visual field variability

Visual field testing can prove one of the most frustrating necessary evils in glaucoma care. Most importantly, perimetry is a subjective test. The machine isn't inexpensive, but it takes significant technician time (10 minutes of testing, five minutes of setting up, five minutes of saving and printing the test) for each patient -- and patients hate everything about the test. Patient performance usually reflects this sentiment, making the interpretation of that data more difficult. In the worst-case scenario, the findings open up a can of worms (possible progression or neurological field findings) that we must also deal with.

Most frequently, we encounter poor reliability on visual field testing, especially as the age group increases. Media opacities diminish the reliability and some people just can't take field tests reliably. In addition, a large percentage of visual fields that were initially suspect for glaucoma turn out to be nothing on retesting, thereby highlighting the subjective testing variability. For example, in the Ocular Hypertension Treatment Study (OHTS), investigators didn't enroll many patients because of the variability on the qualifying visual fields.

Visual fields represent the functional aspect of the visual system. We shouldn't underestimate the value of these data. The ability of the perimeter to detect nonglaucomatous neuropathy, both at the level of the optic nerve and the entire neuro-visual system, is truly irreplaceable by any imaging device. Carl Zeiss Meditec's new Glaucoma Progression Analysis software helps sort out variability and establishes trends in overall field sensitivity. Despite all of the practical shortcomings of perimetry, visual field testing is absolutely necessary and remains a standard of care in glaucoma management.

A need for something more

Scanning laser technologies have helped ignite a renaissance in glaucoma and retinal care. The capabilities of these devices collectively include retinal nerve fiber layer (RNFL) measurements, optic nerve tomography, retinal thickness and computed tomography of the macula. There's even a prototype-imaging device that documents death of individual retinal ganglion cells. Experts have estimated that up to 50% of the RNFL can be lost before a visual field defect develops. This means that we're theoretically waiting for 50% of the viable RNFL to die before instituting treatment if we depend on visual fields analysis alone. Clearly, this isn't desirable.

A bright future

The evolution of these devices will be exciting as different technologies merge into one streamlined testing unit. The laser scanning devices are synergistic in their contribution to the glaucoma evaluation. We commonly see patients who have early optic nerve signs of glaucoma without any evidence of visual field loss. The diagnostic laser imagers can confirm our clinical suspicions and allow early treatment. The ability of the scanning lasers to detect low levels of structural change is powerful in monitoring borderline or suspect cases.

Early glaucomatous loss usually won't show a great deal of agreement between imaging and perimetry. You can do a decent job of following established to moderate cases with a combination of perimetry and imaging. The imaging determines small changes in structure, whereas perimetry qualifies the extent of progression. It's best to monitor advanced glaucomatous damage with perimetry, especially macular threshold algorithms. Imaging is usually noncontributory at this point. Interestingly, I have witnessed many cases where imaging has failed to reveal glaucomatous damage, yet optic nerve and perimetry data strongly suggest glaucoma.

A place for laser imaging

It's important to remember that we must integrate data from adjunctive testing into the complete management plans that we employ in our practices. Scanning laser devices will never supplant functional visual field testing; however, the information that we garner from these diagnostic instruments will continue to impact our understanding of glaucoma and retinal diseases as well as our management capabilities.

Dr. Alexander practices at John-Kenyon Eye Center, a medical/surgical practice. He was a former professor at the University of Alabama at Birmingham School of Optometry, has published many papers and is author of Primary Care of the Posterior Segment, now it its third edition.



Visual fields are critical in the evaluation and continued management of all patients.
BY LARRY J. ALEXANDER, O.D., Louisville, Ky.

With the advent of scanning laser technology and all of its attributes, primary eyecare professionals still need to perform visual fields on glaucoma patients. Rather than mandate that all glaucoma patients must have routine visual fields, perhaps we should base the decision on patient profiles and patient complaints. Do all glaucoma suspects or all who are managed for glaucoma require visual fields? There is no absolute answer, but the following true story is worth a thousand words.

What's wrong with this patient?

A patient enters your practice for a glaucoma evaluation. He has a positive family history of both glaucoma and cataracts and hasn't had been to an eyecare practitioner for an eye evaluation in years. The patient is a 55-year-old white male who's using reading glasses from the drugstore. He has no specific complaints, but feels that he just needs a comprehensive eye evaluation.

Upon further probing, the patient reports mild complaints that include brief bouts of headaches and some vague complaints of blurred distance vision in the left eye that seems to have started about six months ago. The patient is taking blood pressure medication and his physician recently placed him on fexofenadine HCl (Allegra) for sinus-related headaches.

Your physical findings include:

► Unaided distance acuity of OD 20/20 and OS 20/25

► Extraocular muscles (EOMs) full and unrestricted

► Pupils equal and reactive to light and accommodation

► Best-corrected visual acuity is OD +0.25-0.50 x 175 20/20 and OS +0.25-1.00 x 15 20/20. You determine an add at +2.00D

► Confrontation fields are full to finger count

► IOPs by Goldmann of 18 mmHg OD and 20 mmHg OS at 2:30 p.m.

► Pachymetry was 600 OD and 595 OS

► Gonioscopy reveals wide open angles with grade 1 pigment in the trabecular network

► Slit lamp examination reveals Grade 1 nuclear sclerosis

► Dilated fundus examination reveals:

► You ran automated disc tomography with the diagnosis of glaucoma suspect based on optic nerve head appearance and family history. The tomographic analysis was totally within age-matched guidelines. Purportedly, tomographic changes will present up to six years before visual field changes in glaucoma patients, so you don't perform a threshold visual field.

For patient disposition, one of the two following scenes could occur. You pick your ending.

Ending 1

You sit down with the patient to review the physical findings of your exam and you tell him, "You have a mild prescription that we could put in a pair of no-line bifocals. This will take care of your perceived blur in the left eye. I do recommend that.

"Your pressures are in the normal range, especially when corrected downward to 15 mmHg OD and 17 mmHg OS in consideration of your thick corneal readings.

"Your ocular health is totally normal with the exception of early cataracts that are of no consequence at this point.

"You have a congenital variation of the optic nerves called tilted discs,which give the appearance of glaucoma. We have evaluated the discs with the latest optic nerve head tomographic analysis and have found that you're totally within normal limits for your age. The technology I used actually discovers problems that are associated with glaucoma sooner than any other available test can.

"You should come back so I can re-evaluate your cataract progression and your IOPs in one year and take a look at the laser scan of your optic nerves also to monitor for progression.

"Do you have any questions so far?"

And the patient answers, "No."

So you end the conversation by saying, "I'll send you out to our optician to take care of choosing your no-line bifocals. I also want you to set up a follow-up appointment in one year."

Ending 2

You sit down with the patient to review the physical findings of your exam and you ask him, "You have a mild prescription that we could put in a pair of no-line bifocal glasses. Doing so will take care of the perceived blur in your left eye. But tell me more about when this blur started and about the blur itself. It seems strange to me that you notice this when there's so little difference in your prescription. Do you actually cover one eye and then the other to compare vision?"

Patient: "Well doc, I don't cover one eye and then the other. About six months ago, I noticed that things off to the left of my field of vision were actually a little blurry, especially in the upper part of my vision. And it actually seems to be getting a little worse."

You: "When did you start getting these sinus headaches and are they getting worse?"

Patient: "Well doc, that's also strange. I've never really had sinus problems or headaches all my life, and these started almost a year ago. My family physician says they're sinus-related and put me on Allegra (fexofenadine) on a trial basis. However, the headaches are also getting worse. I go back to that physician in a month after we try the Allegra for a while."

You: "I know you have another appointment today, but I want to get a threshold field on you before you leave. This will test your side vision to make sure nothing else is going on besides your concerns about glaucoma. With our new system, performing this test will only take about four minutes for each eye."

You performed the test on each of the patient's eyes and the visual fields demonstrated a congruous upper left homonymous hemianopic defect approaching within five degrees of fixation. All patient reliability factors were good. After reviewing the fields, you continue your discussion with this patient:

You: "Your pressures are in the normal range, especially when corrected downward to 15 mmHg OD and 17 mmHg OS in consideration of your thick corneal readings.

"Your ocular health is totally normal with the exception of early cataracts that are of no consequence at this point.

"You have a congenital variation of the optic nerves called tilted discs that give the appearance of glaucoma. We have evaluated the discs with the latest optic nerve head tomographic analysis and have found that you're totally within normal limits for your age.

"As you can see, your visual field shows a loss of vision in the upper left of both eyes. This test implies that you have something interfering with the fibers that conduct the image from your eyes to the seeing part of your brain (the occipital cortex). I have concern that this is causing you to have "blur" in the left side of your vision and may be associated with your headaches.

"I'm going to set up an appointment for you to see a neurological specialist. Unfortunately, these appointments are often four to six months out, so I'm also going to order a magnetic resonance imaging (MRI) of your brain and eyes over the next few days. With your permission, I'm also going to inform your family physician of what I'm doing."

Don't put all your eggs in one basket

While evolving technology is wonderful, significant pitfalls do exist in placing all of your faith in tests. The true practice of health care involves listening to signs, symptoms and the temporal presentation of both. The clinical decision-making algorithm begins and ends with the doctor-patient discussion, which differential diagnosis tests influence.

When evaluating and managing my glaucoma patients, I rely heavily on optic nerve head tomographic analysis because it's a true objective test. The subjectivity and questionable repeatability of visual fields often confound the value of the information. The technology of the scanning laser is so remarkable that I have to remind myself daily of the importance of the fact that I must listen to the patient and remember that he may have other issues besides glaucoma.

Visual fields add dimension and depth to managing a patient and neither confirm nor deny the scanning laser findings. In the diagnostic-management algorithm, tests are important only when viewed in the gestalt.

As an additional note, within the framework of visual field testing, I have concerns about frequency doubling technology that's seemingly supplanting threshold testing. Remember that frequency-doubling technology is exquisite in detecting and managing glaucoma patients, but that it may fall short in the detection of "neurological" anomalies. We often lose sight of the fact that the eyeball is connected to the occipital cortex. 

Dr. Gaddie is an adjunct assistant professor of optometry at Northeastern State University College of Optometry. He's a member of the American Optometric Association and is a Fellow of the American Academy of Optometry.



Optometric Management, Issue: March 2005