This, the second of four articles on glaucoma, will briefly review the optometrist’s role in case finding and consider the relevant strategies that are being employed, including history taking, anterior eye and optic nerve head evaluation, tonometry, visual fields and allied techniques. A brief overview of referral pathways and NICE guidelines is also provided.
Background
As discussed in the previous article,1 glaucoma is a leading cause of blindness worldwide and, with its rising prevalence, it has become an increasingly important public health issue. Timely diagnosis and early treatment are vital to reduce the progression rate of primary open-angle glaucoma (POAG);2 however, the often asymptomatic nature and gradual progression of the condition, mean that large numbers of cases go undetected until advanced stages. Since glaucomatous vision loss is irreversible, disease prevention or minimisation of progression, is paramount to the maintenance of a sighted lifetime.3
Since glaucoma is an important health problem and there is effective treatment available, glaucoma does meet some of the criteria for a national screening programme. However, evidence suggests that a UK national screening programme based on age would not be cost-effective;4 currently available tests are not accurate enough for diagnosis and there are no randomised trials to demonstrate the efficacy of such programmes. Therefore it is likely that for the time being community optometrists will continue to play an important role in glaucoma case finding by carrying out evaluations for glaucoma on patients presenting for ‘routine’ eye examinations.
Evaluation of patients at risk of glaucoma
The College of Optometrists5 advises that patients who are at risk from POAG include patients with one or more of the following:
- High IOP
- Optic disc features suggestive of glaucoma (Figure 1)
- Symptoms of loss of peripheral vision.
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The guidance also states that even in the absence of signs or symptoms listed above, patients at greater than average risk include those:
- In certain ethnic groups, eg African/Caribbean people
- With first degree relatives with glaucoma
- Over the age of 40 years
- Taking topical/systemic steroids.
As highlighted in Part 1 other risk factors, such as myopia, need to be considered.
In addition to procedures carried out as part of a ‘routine’ eye examination, when evaluating patients at risk of glaucoma, an evaluation of the intraocular pressure and the optic nerve head (including disc size) should normally be carried out and may include central visual field assessment using perimetry with threshold control and assessment of the anterior eye and angle.5
Due to the low (around 2 per cent)6 prevalence of glaucoma in the general population in the UK, standard clinical tests do not have high enough sensitivity and specificity to make them ideal tests for glaucoma screening. Therefore, referrals for suspect glaucoma from optometrists can have a high proportion of false positives.6 Considering the low prevalence, it is of note that some audits of referrals from community optometrists have ‘commendable’ positive predictive values.7 The positive predictive value or ‘PPV’ is the proportion of patients with positive screening test results who are found to have the disease. Conversely, there are reports of significant proportions of previously undiagnosed cases of glaucoma having been recently seen by optometrists or ophthalmologists.8,9 Such reports have highlighted the problems of relying on standard tests such as tonometry. This is further emphasised by publications suggesting that around half of all glaucomatous eyes had IOP less than 21mmHg at screening.10
Optometrists are responsible for approximately 95 per cent of referrals for suspected glaucoma and ocular hypertension for ophthalmologist opinion. Although the diagnosis of early glaucoma is often challenging, it is important to ensure that available resources are used most appropriately. Repeat measures and referral refinement (where an initial suspicious finding is validated by subsequent enhanced assessment that adds value) both reduce false positive referrals.11,12 Repeat measures and/or referral refinement by accredited optometrists have become established as locally enhanced models for glaucoma referral.7,13,14,15
Over the past two decades there have been advances in the scope of optometric practice. In addition to the role in case finding, optometrists are increasingly becoming involved in community based co-management/shared care programmes for chronic eye disease. In parallel, greater numbers of optometrists are adopting state-of-the art equipment for ocular imaging or assessing visual function to enhance the detection and monitoring of eye disease.16 It is of note that glaucoma-related cases accounted for 30 per cent of 50 clinico-legal cases involving optometrists in 2006.17
Current strategies employed in glaucoma detection are outlined below.
History taking
The history is a vital part of assessing the risk of developing glaucoma. POAG is known to increase with advancing age18,19,20 which may reflect various factors that together make the optic nerve head more vulnerable, even at relatively normal IOP levels. Other risk factors are outlined in Table 1.
Questioning should include:
- When did you last have an eye examination?
- What is your age?
- What are your symptoms?
- Are you of African or Asian descent?
- Do you have any first degree relatives with eye disorders?
- Do you have existing eye conditions?
- Do you have any other medical conditions?
- Are you taking or using any prescription or over the counter medicines, if so what?
Inevitably, a growing elderly population will result in an increase in age-related diseases such as diabetes and hypertension. Some studies have demonstrated an association between glaucoma and diabetes 22 and patients who experience dips in blood pressure have an increased risk of developing normal tension glaucoma. The same is true for patients who suffer from vasospastic disorders such as Raynaud’s phenomenon and migraine.
It is important to enquire about ocular trauma, since patients who have had an ocular injury may develop glaucoma either in the short term or many years later. Patients may be using corticosteroid medications for a variety of reasons hence careful questioning is required to ascertain this possibility, since an increase in IOP will occur in around 30 per cent of patients on steroids. For example, younger patients may not appreciate that the steroid-containing ointments they are using intermittently for skin conditions, count as ‘medications’ when questioned.
Although POAG is typically asymptomatic, symptoms suggestive of field loss should naturally be explored. Patients with primary angle closure glaucoma (PACG) may complain of frontal headaches, blurring and/or haloes around lights in mesopic conditions. In acute ACG, symptoms will include reduced vision, and a red and painful eye. Pigment dispersion syndrome can be symptomatic (discomfort, misty vision, haloes) if IOP is elevated substantially. This may occur after rapid release of pigment due to vigorous exercise.
In terms of refractive associations, moderate to high myopia is a well-established risk factor for POAG, whereas small hyperopic eyes are more susceptible to PACG (particularly in older females with diabetes, shallow central anterior chambers, as are those of South East Asian descent). African heritage which can be associated with large optic discs with relatively fewer nerve fibres, thinner corneas and in some areas relatively poor access to eye care, increases the risk and severity of POAG.23
Evaluation of the optic nerve head
Slit-lamp biomicroscopy
Slit-lamp binocular indirect ophthalmoscopy is considered an accurate way of identifying optic nerve damage and may be associated with lower costs compared with approaches such as retinal tomography, optical coherence tomographers and laser polarimetry.9 If a combination of IOP measurement, optic disc examination and visual field measurement provides possible evidence of glaucoma, NICE recommend that patients be referred to a specialist for further assessment (which includes gonioscopy and pachymetry) to confirm the diagnosis. Damage to the optic nerve head may be seen as changes in optic disc cupping, disc notching, haemorrhage, disc pallor and peripapillary atrophy, abnormalities of the peripapillary retinal nerve fibre layer, changes in the appearance of the lamina cribrosa, and vessel calibre changes. Fingeret et al24 recommend a systematic approach to evaluation:
- Observation of the scleral ring to identify limits of optic disc and evaluate size
- Identification of the size of the rim (glaucomatous damage suspected if ISNT rule is not followed)
- Examination of the rim and nerve fibre layer
- Examination of the region outside the optic disc for peripapillary atrophy
- Looking for retinal and optic disc haemorrhages.
Eyes with large optic disc diameters tend to have larger C/D ratios, which can lead to the erroneous diagnosis of glaucoma and small cups can be glaucomatous in small discs.
A red-free filter and fundus photography can be used to look for retinal nerve fibre defects and haemorrhages and a graticule with a continuously variable slit can be useful to measure disc size. An appropriate correction factor for the lens being used is applied. A vertical slit is adjusted until it approximates the vertical disc diameter; the measurement is then read off the slit lamp. A 60D lens has a correction factor of x1.0, a 78D lens x1.1 and a 90D lens x1.3. Several studies have suggested that a vertical C/D ratio of greater than 0.6 and or asymmetry in C/D ratios gives an increased risk for developing glaucomatous field loss.
Optic nerve and retinal nerve fibre layer imaging
Over the past few years there has been a myriad of optic nerve and retinal nerve fibre layer imaging devices; confocal scanning laser tomography (Heidelberg Retinal Tomograph ‘HRT’) (Figure 2), optical coherence tomographers (OCTs) and a device that measures the retinal nerve fibre layer using scanning laser polarimetry (GDx).
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The uptake of OCTs by UK optometrists has doubled in recent times.16 The principle is that high resolution imaging of the optic disc and/or retinal nerve fibre layer will identify glaucomatous damage and progression earlier and more accurately. Digital stereo disc photography has been regarded as the reference standard for clinical trials in glaucoma, although subjective interpretation of stereophotographs can be highly variable.
Technologies such as in vivo high resolution imaging using spectral domain OCT can help detect glaucoma, assist with monitoring and help in our understanding of the disease. While the features of OCT instruments for imaging the optic nerve head vary, they all tend to provide measurements of the optic nerve head, peripapillary retinal nerve fibre layer thickness, macular ganglion cell/inner plexiform layer thickness and these measures can be compared to age and/or disc size corrected normative values (Figure 3).25,26
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Measures of the optic nerve head parameters include the vertical cup-to-disc ratio and cup volume which can be considered in conjunction with the assessment obtained with slit-lamp binocular indirect ophthalmoscopy. The neuroretinal rim thickness display provides an indication of the likelihood of the value for each location falling within the normal range. Qualitative information by way of a B-scan display provides an indication of cup depth and slope. Average RNFL thickness values can be evaluated in the context of disc size, and colour-coded en-face thickness maps highlight thicker and thinner areas with warm and cool colours.
A plot of the peripapillary retinal nerve fibre thickness profile provides values referenced to the normative range for age. While OCT technology offers additional capability for diagnosis and monitoring, it is essential the information is interpreted appropriately and considered along with other clinical signs and information.
Tonometry
Raised intraocular pressure is a modifiable risk factor for glaucoma, the higher the IOP at presentation, the greater the risk of developing POAG. It has been stated that there is no level of IOP above which POAG can be said to always develop and no level below which POAG never develops. 21mmHg is quoted as the upper limit of normal and Goldmann Applanation Tonometry (GAT) has served as the standard for IOP measurement for more than five decades (Figure 4). Alternative tonometers such as dynamic contour or noncontact air tonometers, Tono-Pen and icare rebound tonometers have yet to be evaluated in large scale studies for cost effectiveness and durability and their advantages have not been consistently demonstrated.27 However, recent studies have shown improvements in the measurements obtained, and home monitoring may become feasible in future with devices designed for home use and contact lens based sensor devices for continuously measuring pressure related profiles available.
The College of Optometrists recommends where IOP is borderline the test should be repeated noting the time of day of each test. NICE recommends that patients whose IOP by applanation tonometry is consistently or recurrently greater than 21 mmHg:
- Have a formal diagnosis of ocular hypertension by a healthcare practitioner who has appropriate training or qualifications
- Are monitored in view of the greater risk of developing glaucoma.
The College of Optometrists guidelines5 state that it is good practice to follow up equivocal results from non-contact tonometry with contact applanation tonometry. If non-contact tonometry is being used, before referral, four readings per eye should be taken and the mean used as the result. In the absence of other signs of glaucoma, referral should be considered when the mean is >21mmHg. If the patient has not had non-contact tonometry before, and the mean of four readings is >21mmHg, a new set of readings should be taken. Guidance on the referral of glaucoma suspects issued by the College of Optometrists and the Royal College of Ophthalmologists states asymptomatic patients should be referred if the optometrist identifies one or more of the following:
- Optic disc signs consistent with glaucoma in either eye
- IOP exceeds 21mmHg
- A visual field defect consistent with glaucoma is detected in either eye
- A narrow anterior drainage angle on van Herick testing, consistent with a significant risk of acute angle closure within the foreseeable future
- Conditions often associated with glaucoma, eg pigment dispersion syndrome or pseudoexfoliation.
Practitioners may consider not referring patients at low risk of significant visual field loss in their lifetime:
? Patients aged 80 years or over with measured IOPs <26mmHg with normal ocular examinations
? Patients age 65 years and over with IOPs <25mmHg and normal ocular examinations
Such patients may be advised they should be reviewed by a community optometrist every 12 months.
Visual fields
The most sensitive measure of the visual field is standard automated perimetry (SAP) or ‘threshold perimetry’. Alternative approaches to perimetry, designed to detect functional deficits at an earlier stage have been developed; however, techniques such as frequency doubling technology perimetry, short wavelength automated perimetry and critical flicker fusion perimetry have not consistently shown significant advantages over standard automated perimetry in this regard.
Perimetry is not a statutory requirement for optometrists providing General Ophthalmic Services (GOS) in England, it is carried out at the discretion of the optometrist. In contrast, the Scottish GOS contract specifies that suprathreshold automated perimetry is required during a primary NHS eye examination if the individual has a family history of glaucoma (and is 40 years or over), raised IOP, suspect glaucoma or is aged over 60 years and is presenting for eye examination for the first time.
When interpreting the results of any visual field test, a check is indicated on what test was carried out, under what settings/circumstances (including the refractive correction worn) and whether the results obtained appear to be reliable (by observing the patient and observing whether values are acceptable for false-positive, false negative, and fixation loss catch trials) (see Wild 2015 for recent review). Plots from the Humphrey Field Analyser show the absolute values of sensitivity, a grey scale presentation of the light sensitivity value at each location tested, total deviation plot showing the difference between the measured value at each location and the age-corrected normal value, a pattern deviation plot shows the difference between the measured value of sensitivity and the age-corrected normal after correcting for overall differences from that of a normal (eg due to media opacities or refractive blur) (Figure 5).
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Figure 5 shows a single field printout of the visual field recorded with Program 24-2 and SITA Fast algorithm for the left eye of a patient with glaucoma. The report indicates a reliable respondent with severe loss in the superior hemifield. The glaucoma hemifield test confirms the impact of this altitudinal defect as do the visual field index, mean deviation and pattern standard deviation.
Visual field defects in glaucoma include nerve fibre bundle defects (paracentral, arcuate, nasal step) and overall depressions, baring of the blind spot and enlargement of the blind spot. Defects tend to follow the distribution of nerve fibres, be asymmetric between eyes and hemispheres and can be variable and intermittent. The two probability analysis plots give the statistical probability of the given deviation lying within the range encountered in the age-corrected normal population. This is a useful feature of the report, indicating the overall and localised abnormalities. The glaucoma hemifield test compares the number and severity of the pattern deviation probability symbols between the top and bottom halves of the field and has been shown to be sensitive to early glaucoma visual field loss. The mean deviation, pattern standard deviation and visual field index provide summary measures that may be useful metrics in the assessment of progression. Promising tools are now available that enable a combined evaluation of structural (using OCT) and functional (using visual field analysers) glaucoma deficits at the same time. Advances in software have also enabled automated progression analysis capability for structural and functional deficits.
Anterior segment evaluation
Glaucoma may be associated with pseudoexfoliation, rubeosis (often secondary to vascular occlusion or diabetes), uveitis, iridocorneal endothelial/corneal conditions (eg Fuchs’ endothelial dystrophy), or complications of ocular surgery or trauma and signs may be detected on examination with a slit-lamp biomicroscope.
Iris transillumination and pigment on the corneal endothelium (sometimes forming a Krukenberg spindle) and anterior segment may be indicative of pigment dispersion syndrome, which is a relatively common cause of secondary open-angle glaucoma. This should prompt evaluation of the IOP, optic nerve head and visual field regardless of the patient’s age.
In pseudoexfoliation syndrome greyish material produced by epithelial cells is deposited on structures within the eye and these signs can often be seen on the corneal endothelium, iris pupillary margin or on the anterior lens surface when the pupil is dilated.
Uveitis can lead to IOP elevation due to obstruction of aqueous outflow and pupil block, synechiae, inflammatory cells or due to the effect of steroid therapy. The possibility of uveitic glaucoma should be considered in patients affected by juvenile rheumatoid arthritis, sarcoidosis etc.
Angle imaging
In addition to optic nerve and retinal applications, OCT instruments can provide a view of the anterior chamber angle (Figure 6). If glaucoma is suspected the College of Optometrists5 recommends that the anterior eye and angle be assessed. Van Herick or Redman-Smith method can be used for estimating the anterior chamber angle and central anterior chamber depth.
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The slit lamp permits gonioscopy to distinguish open-angle from angle-closure disease as well as identification of secondary causes of the glaucomas. Community optometrists may seek alternatives to gonioscopy which is not a core competency for optometrists, although since publication of the NICE guideline, gonioscopy has had more prominence at continuing professional development events.16 New angle imaging technologies such as anterior segment OCT are becoming more widely available in specialist practices.
Although there is evidence suggesting that anterior segment imaging provides useful information in the evaluation of angle closure, it is suggested that none of these imaging methods appears to provide sufficient information about the anterior chamber angle anatomy to be considered a substitute for gonioscopy. Long-term studies are needed to validate the diagnostic significance of the parameters measured by such instruments for prospectively identifying individuals at risk for PAC.28
Pachymetry
Central corneal thickness (CCT) affects the estimate of IOP using applanation tonometry and pachymetry has an important role to play in glaucoma case finding and management. The IOP tends to be overestimated where the central cornea is thicker than average and underestimated in thin corneas.CCT is an integral part of the NICE guideline on the diagnosis and management of chronic open-angle glaucoma and ocular hypertension. Although Goldmann applanation tonometry remains the current gold standard, the newer forms of tonometry, eg dynamic contour tonometry and Ocular Response Analyser, have been designed to provide IOP measurements which are less influenced by the biomechanical properties of the cornea including CCT. However, these are yet to be widely used in practice.
It should be remembered that measured values of IOP have been shown in a number of studies to reduce after laser vision correction. One large scale study appeared to demonstrate that the decrease in measured values of IOP is considerably more marked in higher myopic corrections. This change is thought to be attributable to corneal biomechanical factors resulting from surgical changes to corneal shape and thickness.29
Referral pathways
Different patient referral pathways from community optometrists are in place throughout the UK. In Scotland suspect glaucoma patients are referred directly to the hospital eye service (HES), whereas in England referral is often via the patient’s GP, unless there is an alternative pathway in place, perhaps as part of a local glaucoma referral refinement scheme. It has been apparent for some time that ophthalmologist-delivered NHS care, within the NHS, has encountered ever-increasing demand for these services. This has led to alternative models of service provision for management of stable glaucoma and ocular hypertension involving health care professionals including optometrists.7 There is accumulating evidence supporting the monitoring and management of patients with, or at risk of, glaucoma by appropriately trained optometrists practicing within integrated pathways. Guidelines specifying the qualifications, competencies and experience required for these extended roles are in place. There is evidence that supports the reliability of measures, clinical decisions and outcomes for various models where care is delivered by accredited optometrists in the community or in the HES.30-34 The approaches have varied in terms of their design35 and are naturally influenced by local requirements. The practitioner’s role may range from data collection to clinical decision making according to clearly defined protocols, to independent prescribing within the realms of that individual’s competence and experience.7
NICE
The 2009 NICE guidelines on the diagnosis and management of chronic open-angle glaucoma and ocular hypertension offer recommendations as to how glaucoma services can be organised including the involvement of other healthcare professionals within care pathways by defining the permitted roles and the training, qualifications and experience required for them.36 The 2011 NICE Quality Standard for Glaucoma extends its scope into referral for people with possible glaucoma, defining the requirements of services across the whole glaucoma pathway. An integrated approach to service provision is essential to deliver sustained high quality care to those with glaucoma.37
Summary
Until there is evidence to support the introduction of a national screening programme for glaucoma, it is likely that optometrists will continue to play a vital role in detection by means of opportunistic case finding. Patients, particularly those at increased risk, need to be made aware of the importance of attending for eye examinations and any barriers to accessing eye care services need to be better understood so they can be addressed. Many areas have already made progress in reorganising glaucoma services with improved access and alternative referral pathways. Further research is needed to fully evaluate such schemes and the potential advantages to be gained from implementing the latest assessment technologies being used to enhance the decision making progress.
Model answers
Which of the following is not a recognised risk factor for primary open angle glaucoma?
B van Herrick grade 1
Which of the following is likely to most improve the positive predictive value of optometrist referral of open angle glaucoma suspects?
C Assessment of the full range of possible risk factors
Low systemic blood pressure during the resting stage of the cardiac cycle has what impact on the relative risk of developing primary open angle glaucoma?
D Increase risk by 3 times
Which of the following fundus viewing lenses allows a measurement of the disc height straight from the slit lamp height without the need for conversion?
A 60D
Which of the following field defects is unlikely to result from primary open angle glaucoma?
A Altitudinal defect of rapid onset
Which of the following statements about angle assessment is true?
B Gonioscopy is useful in assessing secondary open angle glaucoma
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Dr Clare O’Donnell is head of Optegra Eye Sciences and reader at Aston University. Mr Dan Nguyen is a consultant ophthalmologist and the lead clinician for glaucoma at Mid-Cheshire Hospitals NHS Foundation Trust. He is a consultant ophthalmologist at Optegra Manchester Eye Hospital where he also collaborates with Optegra’s Eye Sciences associates