Diagnosis of open angle glaucoma (OAG)

The early stages of open-angle glaucoma are asymptomatic. Therefore an appropriate index of suspicion on the part of the clinician will improve early diagnosis. The diagnosis of OAG can be enhanced by having more primary-care clinicians:

• Recognize risk factors associated with the development of OAG,
• Develop a higher index of suspicion that OAG may actually be present,
• Actively question patients to uncover underlying risk factors.

Risk Factors underlying OAG

Risk factors can be broadly classified as demographic, familial, ocular, and systemic.

Demographic risk factors

As described in the section on epidemiology, age is one of the strongest risk factors for OAG. Across all populations studied, there is an exponential rise in the rate of OAG with increasing age. In the Beaver Dam Eye study, Americans 43 to 54 years of age had a prevalence rate of 0.9% compared with a prevalence rate of 4.7% for those Americans 75 years of age and older. Beaver Dam is a rural community in Wisconsin with a relatively uniform Caucasian constituency.

Race has been recognized as an important risk factor in OAG for the past 2 decades. By comparison to the population in the Beaver Dam Eye study, the population in the Baltimore Eye Survey was characterized by inner city ethnic diversity. The prevalence of glaucoma was higher in African Americans, and its increase with age was greater than in Caucasian Americans. The age-adjusted prevalence rates for OAG were 4 to 5 times higher in African Americans compared to Caucasian Americans. OAG affected approximately 1 in 10 African Americans aged 80 years or older [Tielsch et al., 1991].

Open-angle glaucoma may be a more severe disease among African Americans compared with Caucasian Americans because OAG in African Americans:

• has an earlier onset [Tielsch et al., 1996],
• is more refractory to medical therapy [Tielsch et al., 1996],
• visual field defects are generally more severe than those of Caucasians [AGIS, 1998].

While it is appropriate to begin screening for OAG in Caucasian Americans after age 50, screening African Americans should begin after age 30 [Quigley et al., 1997].

Family history

Some cases of POAG have a direct genetic origin and this is well supported by many family pedigrees that show dominant and recessive patterns of inheritance. Data derived from the Baltimore Eye Survey revealed that in POAG, family history is an important factor [Tielsch et al., 1994]. About 16% of cases had a positive family history of glaucoma among first-degree relatives vs. about 7% among controls. The strongest association was with the patients' siblings and the weakest was with the patients' children. A familial association of POAG among African Americans appeared stronger than among Caucasian Americans. In a separate study conducted in Australia, a positive association between IOP and a family history of glaucoma was significant only for the cohort with glaucoma [Weih et al., 2001].

Ocular risk factors

Elevated intraocular pressure (IOP)

In the Beaver Dam study, the median IOP was about 15 mm Hg [Klein et al., 1992]. In an Australian study [Weih, et al., 2001] the geometric mean was 14.2 mm Hg for those participants without glaucoma and 17.9 mm Hg for those with glaucoma. While IOP is an important factor in OAG, using IOP alone as a diagnostic test for OAG is clinically inadequate. Many recent studies rely solely on optic disc appearance, visual field defects, or both to diagnose OAG and relegate IOP to a parameter that should be collected but not strongly considered in the initial diagnosis. IOP remains a critical factor in the long-term management of patients with glaucoma. For additional information, see the section on pathogenesis of glaucoma in this disease module.

The fact remains, that in the American population, the prevalence rate of OAG rises monotonically as IOP increases [Sommer, et al., 1991]. The problem with setting an IOP cut-off to indicate the presence of OAG (such as greater than 21 mm Hg) is that no reasonable balance of sensitivity or specificity is achieved [Tielsch, 1996]. As a practical point, in the Baltimore Eye Survey most patients with glaucoma had a screening IOP less than 25 mm Hg. In about half the patients with glaucoma, their IOP was 20 mm Hg or lower [Sommer et al., 1991]. About 7% of the non-glaucomatous participants had an IOP greater than 21 mm Hg.

Issues surrounding IOP do change dramatically when considering the diagnosis of OAG in a Japanese patient. In contrast to Western populations, IOP decreases with age even though the prevalence of POAG increases with age [Shiose, 1990; Nomura et al., 1999].

Abnormal optic disc parameters

Whether using a conventional ophthalmoscope or a computer-assisted laser scanner to characterize the relative health of the optic nerve head, the data collected are markers of glaucomatous optic neuropathy (GON), i.e. the disease and not risk factors per se. Parameters that are often collected include:

• cup-to-disc ratio,
• neuroretinal rim width,
• cup volume,
• vessel 'hooking' or bridging,
• presence of a disc hemorrhage, and others.

As with IOP, there is no appropriate cut-off value for any of these parameters in the diagnosis of OAG. In the Beaver Dam Eye Study, the mean cup-to-disc ratio of participants without glaucoma was 0.36; these participants had a mean IOP of about 15 mm Hg [Klein et al., 1992].

Myopia

An association between POAG and myopia has been known for decades. It was systematically studied in the Blue Mountains Eye Study, a cross-sectional population-based study of older, predominantly white, Australians 49 to 97 years of age [Mitchell, et al. 1999]. OAG was diagnosed on the basis of optic disc findings, characteristic perimetric defects, and without reference to IOP. Of the 3,654 patients examined, probable or definite OAG was diagnosed in 108 patients (3%) and ocular hypertension was diagnosed in 135 patients (3.7%).

The association between OAG and myopia was strong. Glaucomatous optic nerve damage and visual field defects occurred in 4.2% of eyes with low myopia compared with 1.5% without myopia (more than a 2-fold difference). A stronger association was found between those patients with moderate to high myopia and OAG, which was 4.4% compared to eyes without myopia, 1.5%. The latter difference is almost 3 fold. The authors further note that the relationship between myopia and OAG is independent of IOP.

Systemic risk factors

Numerous studies suggest an association between GON, systemic cardiovascular disease, and hematological disease; a review of these studies has been compiled [Hayreh, 1999]. Factors to consider are:

• arterial hypertension in cases of POAG and normal tension glaucoma (NTG),
• low systemic blood pressure and postural hypotension in patients with NTG, and,
• nocturnal arterial hypotension (especially among hypertensive patients using oral hypotensives) in patients with GON.

In a population-based cross-sectional study, 4,297 participants older than 40 received an optic nerve head examination, applanation tonometry, computerized perimetry and a blood pressure reading [Bonomi et al., 2000]. Analyses of these data revealed that higher systemic blood pressure was correlated to a higher IOP but the end effect was too minor to count as a risk factor. In contrast, a lower diastolic perfusion pressure was associated with a marked increase in the occurrence of POAG. The latter is supported by data from the Baltimore Eye Survey in which a lower perfusion pressure was also strongly associated with POAG [Tielsch et al., 1995].

Vasospasm is another risk factor in the development of POAG. The circulatory system within the eye is unique. The retina has characteristics in common with the brain and the choroid has characteristics in common with coronary vessels and vessels in the fingers; the optic nerve shares features of both the brain and the choroid [Flammer et al., 2001]. While arteriosclerosis is of minor importance in glaucomatous optic neuropathy (GON), the vasospastic syndrome is associated with GON [Flammer et al., 1999]. The vasospastic syndrome is a constitutionally increased vascular response to emotional stress and cold temperatures. An association has been identified between a history of migraine or Raynaud's syndrome and low-tension glaucoma [Tielsch, 1996], which is further discussed in the section on pathophysiology.

Risk Factors for OAG progressing

During a 5-year retrospective study, 186 eyes with POAG and 138 eyes with normal-pressure glaucoma were evaluated [Tezel, et al., 2001]. In all cases, IOP was less than 21 mm Hg during the follow-up interval. Progressive optic disc damage was described by at least a 5% decline in the neural rim area / disc area ratio. During the 5-year follow-up interval, the progression rate was about 38% in eyes with POAG and about 51% in eyes with normal-pressure glaucoma. The authors observed that optic disc damage was more likely to progress in those patients who had advanced damage to the parapapillary tissue and neural rim. They commented that patients with more advanced optic nerve damage require maintenance at lower IOP values to delay or decrease progression. That glaucomatous damage to the disc may progress even though the patient's IOP has been reduced has been well established [Palmberg, 2001]. In a 5-year follow up of the Beaver Dam Eye Study cohort, IOP was shown to be associated with significant enlargement in the vertical cup / disc ratio [Klein, et al., 1997]. Enlargement of the vertical cup / disc ratio underlies progression of GON.

In a study of ambulatory blood pressure in patients with glaucoma, patients were divided into "dippers" or "non-dippers" depending on whether their mean systolic and diastolic blood pressure fell by more than 10% at night [Graham et al., 1999]. The mean follow up interval was about 5 years; 70 patients were studied. During that interval, those patients with larger nocturnal blood pressure dips were those who where more likely to have visual field progression even with good IOP control. Patients with visual field progression were also more likely to have a history of disc hemorrhage.

Intraocular pressure (IOP) determination

Applanation tonometry, frequently referred to as Goldmann tonometry, is considered the most accurate method and it is the most frequently used.

To perform applanation tonometry, the cornea is lightly anesthetized using a topical agent. With the patient seated comfortably and head resting gently against the headband of the slit lamp, the tip of the tonometer is positioned to touch the central cornea lightly. The examiner turns a small dial that quantitatively applies a force to the tonometer tip. The IOP within the eye determines the force within the eye that presses outwards and resists the tonometer tip. The greater the IOP, the greater the force that must be quantitatively applied with the tonometer tip. When a defined visual pattern appears, the force of the tonometer equals the IOP and the examiner reads the IOP from the small dial. Because of the variability inherent with the procedure, the reading is accurate to within 1.5 mm Hg. The utility of the applanation tonometer is limited, however, if the patient has corneal scarring, high corneal astigmatism, corneal edema or corneal thinning following laser refractive surgery. If a patient has had laser refractive surgery, pneumotonometry may be a preferred method of IOP determination since Goldmann tonometry underestimates the post-LASIK IOP [Duch et al., 2001].

Since IOP can be measured using several techniques, and since the IOP data can vary with the technique, the method used in the clinic is also recorded. In a medical chart, the recording of IOP will look something like:

19 mm Hg

23 mm Hg                10:00 a.m.

In the above designation, the 'T' is for Tension and is synonymous with IOP. IOP in the right eye (OD) is 19 mm Hg (the top number). The IOP in the left eye (OS) is 23 mm Hg (bottom number). This IOP was recorded using APPlanation tonometry at 10:00 a.m.

In the above example, this patient has at least 2 risk factors. One is that the IOP in the left eye is 23 (i.e. greater than 21). The second is an asymmetry in IOP between the two eyes of more than 3 mm Hg. In this example the asymmetry is 4 mm Hg, i.e. 23 -19. To uncover another risk factor related to IOP, this patient would be requested to return late in the afternoon for a repeat IOP check. While most patients experience their highest IOP in the morning, there are some patients whose IOP is highest later in the day.

While an elevated IOP is a distinct risk factor in the development of glaucomatous optic neuropathy (GON), a significant proportion of people may never be affected even if their IOP levels are considerably elevated. These patients are categorized as having ocular hypertension. Even though their IOP is elevated compared with normal values, the physical structure and chemistry within their eye offers an as yet unknown protection from the elevated IOP. Patients with ocular hypertension convert to POAG at the rate of 2% per year [Rasker et al., 2000].

Anterior chamber angle depth determination

The anterior chamber angle can be routinely determined in several ways. The easiest method, useful in screening, is simply to project a slit beam of light through the cornea near the limbus using a slit lamp. By comparing the thickness of the reflected beam within the cornea with the thickness of its posterior shadow, the depth of the angle can be estimated.

Penlight evaluation of the anterior chamber angle also has clinical value.

Note that in the above illustration of the shallow anterior chamber angle (right), the bowing forward of the iris results in a shadow being cast on to the iris when light from a penlight is projected in from the side.

A more accurate method to determine the depth of the anterior chamber angle is to use the gonioscope. The gonioscope is similar to a large contact lens and is approximately the size of a small spool of thread. A gonioscope has small mirrors built into it. Gonioscopy permits visualization of the anterior angle and estimation of the angle width. Typically the angle is described as being wide, intermediate, narrow, excessively narrow, or closed. For general purposes, if the trabecular meshwork within the anterior chamber angle is visible in the gonioscope, the angle is open.

The width of the angle normally varies as one looks around the circumference of the angle. Anatomically, the angle is narrowest at the 12:00 position in most people.

Optic nerve head (ONH) examination

To get a good view of the ONH using conventional instruments such as the direct ophthalmoscope, the pupil should be dilated; dilation is routine in ophthalmological practices. By comparison, internists and other physicians who are not ophthalmologists rarely dilate the pupil prior to ophthalmoscopy [Patel et al., 1995]. A primary concern may be provoking acute angle-closure glaucoma. In the Baltimore Eye Survey [Patel et al., 1995], 4,870 subjects were dilated; no subject developed acute angle-closure glaucoma although 38 were later judged to have occludable angles. These authors note that the risk of occluding a potentially occludable angle was less than 1:333. Prior to dilating, the physician should:

• perform a penlight evaluation of the anterior chamber angle, and,
• inquire about a prior history of glaucoma.

With a direct ophthalmoscope, the examiner can visualize the cup within the disk to:

• compute the cup-to-disk ratio (especially the vertical cup-to-disc ratio);
• analyze the neuroretinal rim width and color; and,
• estimate the cup depth.

The disadvantage of the direct ophthalmoscope is that the illumination is relatively dim and that can impair visualization of the ONH in eyes with a cloudy media, such as a cataractous lens. In addition, there is no stereoscopic perception of depth (since it is a monocular technique). The examiner's findings are hand drawn onto the patient's medical chart.

The following figure shows the normal appearance of the posterior pole, including the optic nerve head.

The next figure shows 'bean pot' cupping in advanced glaucoma.

The following figure provides a link between the contours within the structure of the optic nerve head and the appearance of the optic nerve head as seen upon ophthalmoscopy.

Note how and where the Nerve Fiber Layer (NFL) corresponds to the rim area; the rim area is made up of retinal ganglion cells from the NFL. The rim area is an important parameter that changes in concert with the cup-to-disc ratio; the greater the cup-to-disc ratio, the less the rim area. Correspondingly, as the rim area declines, so does the number of viable retinal ganglion cells; loss of retinal ganglion cells is linked to visual field loss.

Another method uses the slit-lamp, which has a bright and well-controlled illumination source, in conjunction with one of several hand-held lenses, i.e. a +78 or a +90 lens. Since it is binocular, it also provides for stereoscopic viewing. The examiner's findings are hand-drawn on to the patient's medical chart.

A photographic method utilizes the fundus camera. The magnification as well as the illumination can be well controlled. The photographic data obviate the need for hand drawings. While truly a valuable clinical tool, the expense of a fundus camera is high.

Using the slit lamp or fundus camera, the examiner also evaluates the cup-to-disk ratio, the neuroretinal rim width and color and the cup depth and color. The advantage is that the thickness of the nerve fiber layer (NFL) can also be evaluated in those patients in whom it can not be viewed.

Confirming signs of glaucomatous optic neuropathy are:

• Thinning of the NFL on the disc rim,
• Notching (i.e. appearance of dents) of the NFL on the disc rim,
• Displacement of vessels on the disc rim (indicating a loss of underlying support secondary to neuronal loss),
• Vertical elongation of the optic cup.

In the last few years, devices have become commercially available that utilize confocal imaging, a sophisticated and expensive technique. Using scanning lasers and digitization technology, optical slices (tomographic sections) can be acquired and then reconstructed to provide 3-dimensional views of the in vivo ONH. Measurements obtained from the tomographic sections agree well with stereoscopic photographs taken with the fundus camera. Reproducibility of these images is good. . The HRT II and the GDx VCC are sophisticated laser scanners that collect lots of quantitative data for statistical analyses and longitudinal follow-up. The archival abilities of these computerized scanners are substantial and growing. These scanners are expensive.

Perimetry

The general technique of mapping the visual field is referred to as perimetry. There are two fundamental methods: kinetic and static. The kinetic method uses a test object (usually a white spot of light) that is moved from 'non-seeing' to 'seeing'. This means that the white disc is either moved from the far periphery (non-seeing) towards the macula/fovea (seeing) or from inside of a scotoma outwards, i.e. non-seeing to seeing.

Static methods use a non-moving test object, hence the term static. A static light source is illuminated for a brief interval and patient is queried as to whether or not it was seen. Alternatively, the intensity of the light source is increased from dim to bright until the spot becomes visible. The latter method is referred to as static threshold perimetry.

Kinetic (manual) perimetry

Using this technique, the patient inserts their face into a large curved bowl. The patient sees a background of white light uniformly reflected from the surface of the bowl. Superimposed on this white background, is a spot of white light that is referred to as the target. The target is physically moved on the white background and this movement is the source of the term 'kinetic' perimetry. The target is sequentially moved to test the areas of seeing and non-seeing. The patient signals that he has seen the target by pressing a button that triggers an audible tone so that the perimetrist marks a dot on a sheet of paper with a pencil. After many manual presentations, the perimetrist connects the dots and plots the visual field by hand. One of the most important factors in the process is having a well-trained perimetrist who knows how to search for scotomas. Today, well-trained perimetrists are in short supply. Until the late 1980's, the visual fields obtained using kinetic perimeter were the international gold standard, now they are becoming antiquated.

Static perimetry

Since the early 1990s, many studies that utilize visual fields use static perimetric methods. These have become automated using computer technology. Similar to the kinetic perimeter, the automated perimeter also has a large white bowl. The patient puts his head inside the bowl. From here on, the computer takes over. In most testing paradigms, a small dim white spot will light up in different regions of the bowl. This testing paradigm is referred to as, white-on-white perimetry, i.e. a white target is projected onto a white background.

The brightness of the white target is varied until the threshold for seeing is determined for an individual spot. The computer digitally records the brightness level for each spot tested. The computer repeats this process for all of the spots tested within the visual field. At the conclusion of the test, the computer maps the sensitivity in different regions of the visual field. Using special statistics to analyze clusters of points, it looks for patterns of visual field loss that identify glaucomatous optic neuropathy. The principal advantage of automated perimetry is that a well-trained perimetrist is not required - although a caring technician is. An additional advantage is the randomized nature of how the targets are presented. To map the visual fields in patients with glaucoma, it is now typical to test only the central 24 degrees of the visual field since that is the region of the visual field where defects are identified in the early and mid stages of the disease. The use of automated perimeters has created a new vocabulary for describing the resulting data. These are described below.

A Decibel Unit

Imagine a small light bulb about 30 centimeters in front of your eye. Arbitrarily assign a brightness level to the bulb of 100 units. Visual scientists count by factors of 10 to quantify brightness. If a certain gray filter is placed between your eye and the bulb, the filter may decrease the apparent brightness of the bulb from 100 units to 10 units. By definition then, there has been 1 log (base 10) unit of attenuation. Thus 1 log unit of attenuation is the same as decreasing the brightness by 10-fold.

Go back to the small light bulb analogy above. If you can still see the bulb after it has been attenuated by 3 log units (i.e., 1,000-fold dimmer), your vision would be far more sensitive than if you could only see the bulb if no filters were interposed. Note that each '0' in 1,000 is equivalent to a 10-fold reduction, thus 1,000-fold = 3 log units.

Each log unit is made up of 10 discrete steps, i.e., 0.1 log units per step. Each 0.1 log unit step is called a decibel (abbreviated dB). dBs are the basis for measuring sensitivity as described using automated perimetry.

Mean Deviation (MD)

The Mean Deviation (MD) describes the general sensitivity of the overall visual field. For example, if a dense cataract greatly reduces the amount of light that gets from the cornea to the retina, there will be an overall loss in sensitivity. The presence of a cataract can reduce the MD by -10 dB (or even more). -10 dB corresponds to 1.0 log units.

Corrected Pattern Standard Deviation (CPSD)

The Corrected Pattern Standard Deviation (CPSD) is also quantified using dB. The CPSD can reveal patterns of decreased sensitivity, scotomas. Scotomas are the same as small or large areas of visual field loss, i.e. 'non-seeing'. Probability scores are assigned that convey the likelihood that the scotoma is linked to underlying disease. The PSD is the Pattern Standard Deviation and it is similar to the CPSD. A study concluded that there was almost perfect agreement between the pattern standard deviation and the corrected pattern standard deviation [Thomas et al., 2000].

When using white-on-white perimetry, a general statement is that a 5 dB change in threshold sensitivity is clinically significant and reflects an underlying physiological change. For meaningful diagnosis, the number of dB loss must be compared to the actual pattern of loss.

The advantages of automated perimetry include:

• The results are not dependent on the skill level of the person administering the test,
• The targets are randomly presented compared with kinetic perimetry where the targets were often predictable,
• Threshold methods are more quantitative than kinetic methods,
• Patient fixation is automatically monitored,
• Most of the parameters measured are accompanied by statistics that convey the relative reliability of the test result.

A disadvantage of most perimetric tests, including automated perimetry, is the time required to test each eye, about 15 minutes. Fifteen minutes of test taking causes fatigue, which reduces reliability. An advanced algorithm that speeds testing time is known as SITA, which stands for Swedish Interactive Testing Algorithm. Recent techniques, like frequency doubling methods described below, reduce testing time.

Advanced visual field techniques

Greater sensitivity to early glaucomatous changes and decreased variability are the goals of the advanced techniques along with improved sensitivity and specificity. Two techniques will be discussed here, short-wavelength automated perimetry and frequency doubling perimetry.

Short-wavelength automated perimetry (SWAP)

'Short-wavelength' is another way of saying 'blue' since blue is a visual sensation derived from short wavelength visible light. For comparison, red is a sensation derived from long wavelength visible light. Blue-sensitive cones are cones that are stimulated by short-wavelength light.

Defects in blue-sensitive cones, in ganglion cells connected to the blue cones, or in both may be present early in the progress of glaucoma. These cones and the ganglion cells to which they are connected are part of the 'short-wavelength-sensitive pathways'. SWAP, as the acronym implies, is a perimetric device that selectively tests the blue-sensitive pathways.

SWAP technology is comparable to that of conventional automated perimetry except that the background is bright yellow and the target is blue. In conventional static perimetry the background is white and the target is white (W/W). Data suggest that the blue-on-yellow (B/Y) stimulus configuration may be more sensitive to early glaucomatous changes than (W/W) because the blue-sensitive pathway is compromised earlier in the course of GON.

The advantages of SWAP are:

• Visual field defects (scotomas) can be detected 2 to 5 years earlier,
• Progression of visual field defects can be detected 1 to 3 years earlier,
• The extent of the visual field defect can be mapped better,
• Areas of visual field loss mapped using SWAP correlate with specific damage to the ONH rim and NFL defects,
• During the progression of glaucoma, defects in the B/Y visual field and the NFL defects follow each other more closely than do NFL defects and the W/W field changes [Teesalu et al., 1998],
• In those patients with ocular hypertension, functional damage can be detected with B/Y perimetry before being detected with NFL imaging.

The disadvantages of SWAP are:

• Limitations are imposed by lens yellowing, which increases naturally with age,
• Diffuse NFL loss in advanced glaucoma or advanced cataracts skew the results,
• SWAP is even more tedious for elderly patients to perform than W/W automated perimetry. Fortunately, a SITA program has been worked out for SWAP, thus reducing the testing time.

In a clinical study, a Humphrey Field Analyzer was used to evaluate progressive glaucomatous visual field changes; both W/W and SWAP parameters were used. Serial stereophotographs of the optic disc served as a reference [Girkin et al., 2000]. In this study 47 glaucoma patients with a minimum of 2 stereophotographs taken a minimum of 2 years apart were included. The follow-up time was about 4 years. Baseline data was compared to follow-up data. Progressive visual field loss was compared using SWAP and W/W perimetry in the 22 of 47 patients with progressive changes as viewed on the stereophotographs. There was a statistically significant mean change for both SWAP (p<0.001) and W/W (p<0.004) data between those eyes that did show progression and those that did not. Although the mean change score was greater for SWAP, this was not statistically significant. The authors conclude that serial SWAP corresponded better than W/W to glaucomatous changes of the optic disc. The sensitivity of SWAP was higher and the specificity was similar. The above evidence indicates that serial SWAP may improve detection of glaucomatous optic neuropathy progression.

Frequency doubling perimetry (known as FDT, i.e. Frequency Doubling Test)

Frequency doubling testing is a rapid, accurate method for detecting visual field defects associated with glaucoma. FDT uses a flickering target to stimulate motion-sensitive ganglion cells in the visual pathways (referred to as M-cells). FDT has a screening mode and a full-threshold testing mode. FDT results correlate well with static threshold visual field data.

The advantages of FDT are:

• Much faster than other forms of perimetry; i.e., screening fields require 45 seconds per eye and full-threshold testing requires 4 minutes per eye,
• Good test-retest reliability,
• Unaffected by eyeglasses, contact lenses or refractive error (up to 6.0 D),
• Unaffected by pupil size (if larger than 2 mm),
• Portable.

The disadvantages of FDT are:

• Thresholds are diffusely depressed by cataracts
• It may miss early nasal steps (a specific type of scotoma found in glaucomatous visual fields)

Comparisons between computerized optic disc scanning equipment and visual fields.

In recent years, computer-assisted scanning laser technology has evolved. As it has, its resolution and reproducibility have improved. Even now, however, there are two schools of thought about the early detection of glaucoma and best methods for monitoring its progression. One says that visual fields are the key and the second says that the appearance of the optic nerve head is the key. There is no clear answer and no gold standard at the present time. The current thinking is that there is considerable redundancy and overlap of the unique receptive field of each ganglion cell. About 50% of ganglion cells that serve a localized area must die before there is any loss in sensitivity that can be detected in a corresponding region of the visual field.

In a clinical setting, the question was whether scanning laser technology or automated perimetry could better discriminate between normal and glaucomatous eyes. One study used a case-control design with 91 normal subjects and 94 patients with glaucoma [Lauande-Pimentel et al., 2001]. All patients underwent automated optic nerve head laser scanning and visual field assessments. One eye per individual was randomly assigned. The authors concluded that combining the scanning laser data of the ONH with the visual field data provided a high sensitivity/specificity ratio suggesting that using both measures may improve the diagnosis of glaucoma. Furthermore, the scanning laser data may be more sensitive in the diagnosis of early and moderate glaucoma.

In a prospective longitudinal study, 77 patients with early OAG glaucomatous visual field loss were evaluated for a median of 5.5 years [Chauhan et al., 2001]. Patients received visual field testing with a Humphrey perimeter (30-2) and scanning laser tomography with the Heidelberg Retina Tomograph. Of the 77 patients evaluated, 27% showed no progressive visual field loss or optic nerve head changes. Of interest, while only 4% progressed using visual field criteria only, 40% progressed using scanning laser tomography criteria only. The authors concluded that disc changes occur more frequently than visual field changes and that most patients with visual field progression also had disc changes. Less than half of those patients with disc changes had visual field progression. The authors suggest that scanning laser tomography is a good alternative to disc photography in routine patient management. The findings of progressive disc damage prior to field loss are corroborated by others [Zeyen et al., 1993].

Diagnosis of primary angle closure glaucoma

Angle closure glaucoma is diagnosed when aqueous fluid cannot reach the anterior chamber angle. Aqueous continues to be produced and accumulates behind the iris forcing the base of the iris to occlude the trabecular meshwork. Anything that causes the pupil to dilate can provoke an attack of primary angle closure glaucoma. Typically an attack occurs in a patient who already has a narrow angle.

Acute primary angle closure can be dramatic. Suddenly the eye becomes red. The pain can be severe. Patients frequently experience blurred vision, multi-colored halos around lights, headache, and sometimes nausea and vomiting. Examination of the eye reveals:

• conjunctival injection,
• corneal edema,
• cells and flare floating in the aqueous,
• a mid-dilated pupil,
• a shallow anterior chamber, and,
• an elevated intraocular pressure (which is sometimes dramatic).

The diagnosis is clinched using gonioscopy. Depending upon how many structures (striations) within the anterior chamber angle are visible, the angle is classified as:

• closed,
• occludable, or,
• open.

If the angle is closed or occludable, the physician is well advised to check the opposite eye since the configuration of the iris and the anterior chamber angle are likely to be similar in both eyes.

Acute angle closure glaucoma is a true emergency. It requires prompt diagnosis and treatment to prevent irreversible ONH damage. A variety of topical agents can be used to constrict the pupil and pull it away from the anterior chamber angle, reduce aqueous production, and slightly dehydrate the vitreous. Once the IOP has been effectively lowered, a special laser can be used to make a small hole in the peripheral iris, creating a passageway from the posterior chamber to the anterior chamber. Thus treatment of acute angle closure glaucoma is surgical.

While acute angle closure glaucoma presents dramatic signs and requires immediate treatment, many patients present with sub-acute angle closure glaucoma, which is comparatively more common and more subtle. In this condition, the angle intermittently closes and then spontaneously reopens. When it does close, the IOP rises and the ONH is damaged. When the angle re-opens, the IOP goes down. If recurrent closure is frequent, the appearance of the ONH can look like open-angle glaucoma.

If the anterior chamber angle looks like it can occlude, provocative tests can be used in the office. As their name implies, these provoke angle closure under clinical scrutiny. One test utilizes a mydriatic drop like phenylephrine [Lee et al., 1999] or tropicamide. As the pupil dilates, sometimes the angle occludes and the IOP rises. Another test involves placing the patient in a dark room with the head in face down position for 45 minutes. Unfortunately, false positives and false negatives are common with both tests. In a study, the risk involved in dilating a potentially occludable angle was less than 1:333 [Patel et al., 1995].

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Flammer J, Pache M, Resink T. Vasospasm, its role in the pathogenesis of diseases with particular reference to the eye. Prog Retin Eye Res 2001;20:319-349.

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