VISUAL FIELD REVIEW

Is this a case of early glaucoma?

In this issue I will discuss how to interpret visual field outcomes.

This 68 year old female has been noted with bilateral peripapillary atrophy (PPA, zone-beta) and cup-to-disc ratios of RE 0.5 and LE 0.4 with pallor and thinning of the left neural rim, especially in the macula bundle. Her acuities were R 6/9 (20/30) L 6/12 (20/40) with IOPs of 19 mmHg in both eyes. She has moderate bilateral cataracts and her visual fields are as shown. Her primary complaint is for blurred vision on the left side that looks like a shadow. Do you think she has glaucoma?


Well, let’s go over how to identify an abnormal field. A visual field can be called abnormal if ONE of the following is abnormal in the presence of a reliable test:

Glaucoma Hemifield Test (GHT: borderline or abnormal)
Pattern Standard Deviation (PSD, p<5%)
a cluster of abnormal (1 edge only) points flagged in the Pattern Deviation plot (PD lowest right panel)

In this case, the lady has abnormal GHT, MD, PSD and a cluster of abnormal points in the pattern deviation plots of both eyes. This means that both fields are abnormal. The RE is reliable (allowing 20% for fixation loss and 33% for false responses) but the left eye exceeds the expected fixation loss criterion. So let's review what these criteria mean.

Katz and Sommer(1) have shown that 30% of normal and 45% of glaucoma patients exceed these reliability limits -- sound familiar? The majority of failures being due to the presence of excessive fixation losses (41-67%). These arise from inaccurate mapping of the blind-spot so that remapping the blind-spot during testing reduces the number who fail to 14%. I use the eye monitor to gauge whether the patient is fixating and record that on the chart using a 0-4+ score. The authors propose that a conservative criterion of reliability for all indices should be 33%, as this would fail only 3% of all field exams. Applying this criterion would suggest reliable outcomes in both eyes.

Given reliable tests, the GHT is outside normal limits in both eyes, the PSD is likewise (P<5%) and points cluster on the PD plot. Clustering can be assessed against the Hoddap-Anderson-Parrish criterion (2) which requires abnormality at 3 neighboring points at p<5% (one edge point) with one of these being p<1%.

Both eyes show such clusters and fail at the other two criteria (GHT, PSD) indicating abnormal field outcomes. The question that now needs to be asked is: does this patient have glaucoma?

If you concluded that she has normal tension glaucoma, so did the consulting physician who started her on latanoprost, after which her IOPs were 10 and 11 mmHg some 3 months later.

Glaucoma is sometimes one of the hardest diseases to diagnose. In this case, peripheral RPE changes were found following dilated fundus examination some 2 years after the field tests. ERG evaluation exposed the true diagnosis of retinitis pigmentosa.

This case demonstrates how to evaluate a visual field and the importance of keeping an open mind on the diagnosis. It reinforces the need to perform a full battery of tests in considering a differential diagnosis; which should always include a peripheral retinal examination through a dilated pupil.

Algis Vingrys, BScOptom, PhD

1. Katz J, Sommer A. Reliability indexes of automated perimetric tests. Arch Ophthalmol 1988: 106: 1252-54.
2. Hodapp E, Parrish RK, Anderson DR. Clinical decisions in glaucoma. St Louis MO, Mosby-Year Book, 1993: 52-61.

QUESTIONS & ANSWERS

This section contains questions from readers of the journal with answers and comments from editorial board members.

Question: "In low tension glaucoma, it has sometimes been suggested to obtain an MRI to find a neuro or neuro-vascular etiology (in the anterior visual pathway) of compromised appearing optic nerve heads and to obtain a carotid duplex. Are those tests really indicated in cases of LTG?"

Answer: Douglas Anderson, MD, writes, "To say that all individuals in whom normal-tension glaucoma (NTG) is being considered should have neuro-imaging and evaluation of major vessels is to say that the diagnosis is one of exclusion. I believe one should first consider whether the clinical features support the diagnosis of NTG. Typically the discs have excavations that are associated with localized thinning of the rim, nearly always at the inferior or superior temporal meridian. Disc hemorrhages may be observed by ophthalmoscopy intermittently, so disc examination may be done over several visits 2 to 4 weeks apart before considering expensive diagnostic tests. The likelihood that NTG is the diagnosis increases if the visual field defect is pre-chiasmal with nerve-fiber bundle characteristics, especially if bilateral, and even more so if the patient has vasospastic symptoms or a family history of glaucoma. If a sufficient number of these characteristic traits of NTG are present, I would not feel the neuro-diagnostic tests are needed, at least at the time of initial presentation. The exceptions would be that I would undertake the tests at the time of presentation if the field defect is a unilateral prechiasmal defect with pallor of the rim without localized expansion of the cup and without ophthalmoscopically recognizable retinal disease, if the acuity is poor (central scotoma) and not explained by macular disease, or if the field defect was suspiciously hemianopic, and would undertake the test later if there is unexpected progression despite substantial lowering of the IOP."

Question: "I heard in a lecture by a Glaucoma specialist that Xalatan works just as well when taken once a week as daily. Is there any truth in this?"

Answer: Thom Zimmerman, M.D., Ph.D., writes, "The basis for the statement that Xalatan works just as well when taken once a week as once daily is derived from the fact that there is a very, very long wash out period for Xalatan and the other prostaglandins. This is flawed though because the patients have been taking the prostaglandin analogues chronically and then when you stop it, there is a long wash out period. No one knows what will happen if you are using it once a week. My best guess is that it won’t be nearly as efficacious as Xalatan used once daily. My reasoning for this comes from a couple of different directions. Xalatan and the prostaglandin analogues are not as effective initially as they are later in therapy. I feel that you have to have a prostaglandin analogue aboard for at least four to six weeks before you really know what its maximal effect will be. Based on this I doubt if you’ll ever get to the maximal effect using the drug once weekly. Secondly, with all drugs there’s a "loading" and/or an "equilibrium" level that must be achieved before the drug reaches its maximal effect. Based on the previous statements, I doubt if this can be achieved with once weekly dosing.

In your practice, if you’d like to take a look at this topic, there are a number of studies that you could design. You could start new patients on Xalatan once weekly and follow them for awhile to see what the pressure response is. Alternatively, you could take patients who have been on chronic Xalatan and then, move those to once weekly and see what the difference from the chronic Xalatan to the once weekly dose is concerning the intraocular pressure. Should you choose to do either or both of these studies, I think it would be well worth reporting."

Thom J. Zimmerman, MD, P.D
Emeritus Professor and Chair, Department of Ophthalmology and Visual Sciences
Emeritus Professor, Department of Pharmacology and Toxicology
University of Louisville, School of Medicine

Question: "I have recently incorporated glaucoma management to the practice and I am asking if you have any clinically informative glaucoma flow sheets or glaucoma suspect examination sheets that you can share with me?"

Answer: Although we are unaware of any standardised examination forms that are widely available, there is a set of excellent flow sheets available to help you with clinical glaucoma decision-making. These cover everything from help with arriving at a diagnosis through to treatment stepladders and are available in "Terminology and Guidelines for Glaucoma, IInd edition" by the European Glaucoma Society. The entire book is published by Dogma Publications in Italy (ISBN: 88-87434-13-1). However, the introduction and all the flow charts are included in the preview version which can be downloaded for free as a *.pdf file from the Society’s website. Click on the following link to get directly to the webpage: www.eugs.org/ebook.asp

Question: "As to the glaucoma risk calculators: Is PSD a factor obtained from certain visual field units in particular?? Other than the Devers risk calculator, it was mentioned that there is a Mansberger glaucoma risk calculator; where can this be found?"

Answer: Firstly, PSD (pattern standard deviation) is one of the global visual field indices obtained from Statpac single field analysis by the Humphrey Field Analyzer, and provides a single figure quantification of irregularities of the field ‘surface’ and is conceptually similar (although not numerically equivalent) to ‘loss variance’ in Octopus instruments. It is sensitive to focal losses, being low in normal individuals with a smooth field surface, and rising when focal defects become apparent, and falling again in extinguished fields. PSD compliments the fellow global index Mean Deviation (MD) which quantifies the ‘average’ reduction between the patients test results and those of an age-matched normal, providing a good measure of generalised loss but is insensitive to early, focal defects typical of glaucoma. Risk calculators currently available are based upon the Ocular Hypertension Treatment Study and use of the Humphrey Field Analyzer (HFA). Unfortunately the Octopus, Oculus, FDT or any other perimeter creates different measurements that cannot be used with the risk calculator. With regard to risk calculators, the Devers Risk Calculator was developed by Dr SL Mansberger: they are the same thing. This risk calculator is available for use or download from the Devers Eye Institute website (www.discoveriesinsight.org/GlaucomaRisk.htm). For further information about the calculator, have a look at the review featured in the OGS E-Journal issue 2.

AIGS CONSENSUS MEETING ON ANGLE CLOSURE GLAUCOMA

The third consensus meeting of the Association of International Glaucoma Societies was held on May 3, 2006 in Hollywood, Florida. This particular meeting was on Angle Closure Glaucoma (ACG). The faculty was made up of experts from around the world with the goal of developing consensus statements related to ACG. Primary angle closure glaucoma is one of the leading causes of blindness throughout Asia and may be more common than previously thought in the western countries. Overall, open angle glaucoma is more common than ACG but many more cases of blindness are related to ACG because it is more severe when it occurs. With the aging population, more individuals will be afflicted with ACG. New technologies like ultrasound biomicroscopy and anterior segment optical coherence tomography allow pathologic mechanisms to be better understood. Still, therapies remain similar to what they were years ago, starting with iridotomy and moving on from there.

In 2002, a classification system was put forward that is used in epidemiologic research. The International Society of Geographic and Epidemiological Ophthalmology (ISGEO) describes three stages for ACG;

a. Primary Angle Closure Suspect: occludable angle, but normal IOP, disc and field without evidence of peripheral anterior synechiae (PAS);

b. Primary Angle Closure: occludable angle with either raised IOP and/or primary PAS: disc and field normal;

c. Primary Angle Closure Glaucoma: occludable angle with either raised IOP and/or primary PAS: disc and field abnormal.

A large part of the discussion for the meeting was what is considered an "occludable" or narrow angle, which conveys that there is an anatomical predisposition to angle-closure. Prior studies have used the definition of a narrow angle to be that the posterior (usually pigmented) trabecular meshwork is obstructed by the peripheral iris for three or more quadrants of its circumference. This stringent definition is not embraced by all, and there was discussion to take a more liberal stance. Another issue was to differentiate appositional versus synechial closure, because the management will differ between these situations.

The preliminary consensus statements included:

1. Primary angle closure is characterized by iridotrabecular contact that may lead to peripheral anterior synechiae, ocular hypertension, glaucomatous optic neuropathy, damage to ocular tissues including corneal endothelium and lens, loss of visual function and in a large population of untreated cases, blindness.

2. Gonioscopy is indispensable to the diagnosis and management of all forms of glaucoma and is an integral part of the eye examination. Dynamic gonioscopy is the only way to determine if iridotrabecular contact is appositional or synechial and is an essential component of gonioscopy. Access to a magnifying Goldmann-style lens enhances the ability to identify important anatomical landmarks and signs of pathology. The ideal standard is to use both types of lenses.

3. Anterior segment imaging devices may augment the evaluation of the anterior chamber angle.

4. Identification of the underlying cause of angle-closure is essential to accurate diagnosis and treatment.

5. Angle-closure can be caused by one or a combination of abnormalities in the relative or absolute sizes or positions of anterior segment structures or abnormal forces in the posterior segment that may alter the anatomy of the anterior segment. Angle closure may be understood by regarding it as resulting from blockage of the trabecular meshwork caused by forces acting at four successive anatomic levels: the iris (pupillary block), the ciliary body (plateau iris), the lens (phacomorphic glaucoma), and vectors posterior to the lens (malignant glaucoma).

6. Although the amount of pupillary block may vary among eyes with angle-closure, all eyes with angle-closure require treatment with iridotomy.

7. The ISGEO framework should be used as the standard framework for classification of the natural history of angle closure, to determine prognosis and describe an individual’s need for treatment at different stages of natural history of the disease.

8. Additional clinical sophistication can be gained by describing sequelae of angle-closure affecting the cornea, trabecular meshwork, iris, lens, optic disc and retina. Specifically, the extent of PAS, level of presenting IOP (in asymptomatic cases) and presence of glaucomatous optic neuropathy.

9. In order to effectively target management at the cause of angle-closure, the ISGEO scheme must be used in parallel with the 4-point classification of mechanisms.

10. Further refinement of these systems should be made on the basis of peer-reviewed evidence.

11. The term "anatomically narrow" should be substituted for "occludable" angle, as a more intellectually honest description.

12. Gonioscopy should be promoted as a standard part of the comprehensive eye examination. The choice of lens is a matter of personal preference.

13. It is desirable to record gonioscopic findings in clear text, avoid "derivative" schemes. The ideal standard is to assess and record all features identified in the Spaeth grading system.

Angle closure glaucoma is a common and serious condition. Gonioscopy becomes a crucial procedure when confronted with the patient at risk for or having angle closure glaucoma.

Murray Fingeret, OD

POLL RESULTS FROM OGS E-JOURNAL VOLUME 1, ISSUE 2

Gonioscopy, while challenging to master, is a rewarding examination technique that is integral to glaucoma diagnosis and management. A majority of our respondents always perform gonioscopy on newly diagnosed glaucoma patients, glaucoma suspects and ocular hypertensives. Gonioscopy is indicated in these circumstances. Assessment of the anterior chamber angle using the slit lamp and von Herrick technique is incomplete. The distinction of open or closed angle glaucoma is made with the help of gonioscopy. Some forms of secondary glaucoma are identified based on gonioscopic findings. One cannot make the diagnosis of primary open angle glaucoma without performing gonioscopy.

Gonioscopy may need to be repeated during the course of follow-up. Eighty-four percent of our respondents will repeat gonioscopy less often than once a year or only if new signs or symptoms develop.

Some potential confusion may be associated with question 3. Zeiss and Posner lenses are examples of 4 mirror type lenses. A majority of our respondents (55%) prefer a 4 mirror type goniolens which have a smaller diameter and a flatter curve than the cornea. While initially more difficult to master, these offer some advantages over 3 mirror type lenses for example the ability to easily perform indentation gonioscopy.

The technique and interpretation of gonioscopy is an important part of patient assessment in glaucoma care. Look for an expanded review of this in a future issue.

A PERSPECTIVE ON GONIOSCOPY

The question is often asked: "How often should gonioscopic assessment of the iridocorneal angle be performed?" We have read many divergent answers, and so are happy to offer ours.

For perspective, in most cases of glaucoma, gonioscopy is the least important tool in establishing the diagnosis or quantifying risk. However, there are indeed many cases where knowledge of angle patency can be clinically important. One of us (RKT) relates a classic encounter by one of our good M.D. friends. He had a patient whose IOP was recalcitrant to medical therapy, and was keen to perform argon laser trabeculoplasty. Upon placement of the laser-goniolens, however, it became apparent that the patient’s IOP was elevated because the angle was in near total closure. So instead of trabeculoplasty, a peripheral photoiridotomy was performed, which resulted in successful reduction in IOP. In retrospect, perhaps proactive gonioscopic assessment would have been wise.

There appears to be uniform consensus that gonioscopic assessment should be accomplished at the first or second evaluative visit. If the angle is wide open and there is no nuclear sclerotic cataract, then we repeat the procedure about every five years - every three years if there is moderate nuclear sclerotic cataract. If the angle is moderately narrow without nuclear sclerotic cataract, then we repeat the gonioscopic assessment perhaps every two to three years. In the presence of concurrent cataract, the gonioscopic evaluation is done annually. If the angle is narrow, the examination is done annually regardless of cataract status. The enlargement of nuclear sclerotic cataract can slowly press the iris diaphragm forward, potentially causing progressive compromise to the patency of the iridocorneal angle. This is why the presence or absence of nuclear sclerosis can influence the frequency of repeat gonioscopy.

In Volume 7-3, 2006 of the International Glaucoma Review, Paul Foster reports on page 360, "Limbal chamber depth assessment (LCD -- the Van Herick test) was used as method of identifying subjects with anatomically narrow angles. The authors cite the high sensitivity of LCD in identifying people with very narrow angles. The omission of gonioscopy for all subjects was an understandable, pragmatic decision." Yes, there is always the "plateau iris" to confound what would otherwise be a rather straightforward assessment. So, perhaps not "standard-of-care," it may well be that if the IOP is CCT-adjusted normal, and the Van Herick grade is 3 or 4, gonioscopic assessment will bring precious little to the diagnostic table. Certainly for grade II or less, gonioscopy should be performed because there appears to be a substantial prevalence of undiagnosed intermittent and/or asymptomatic angle closure-related ocular hypertension/glaucoma.

We most always use either Zeiss or Posner 4-mirror goniolens when performing gonioscopy. Certainly a Goldmann 3-mirror goniolens enables an excellent view, but requires a high viscosity interface liquid which is more cumbersome and unpleasant for the patient. If the 3-mirrow is used, we always take an extra minute to irrigate away the gonio-gel.

R. Melton and R. Thomas