A new glaucoma guideline was published on March 28, 2015 by the Scottish Intercollegiate Guideline Network (SIGN). SIGN was formed in 1993 to improve the quality of health care in Scotland, by reducing the variation in clinical practice and patient outcomes.1 SIGN is part of NHS Quality Improvement Scotland. In Scotland, the SIGN glaucoma guideline supersedes the National Institute for Health and Clinical Excellence (NICE) glaucoma guideline still applicable for primary care in England and Wales (viewable at nice.org.uk/guidance/cg85 and covered in the next article in this series).
The SIGN glaucoma guideline provides recommendations for community optometrists on referral and safe discharge of glaucoma, ocular hypertensive, and glaucoma suspect patients in Scotland. The NICE glaucoma guideline gives evidence-based recommendations for the diagnosis and management of glaucoma and ocular hypertension, which is most likely to take place in secondary care, and so continues to be most relevant to hospital based glaucoma care. The SIGN guideline is evidence-based and was developed by a committee of optometrists, ophthalmologists, researchers and patient organisations and is most relevant to primary eye care. This article will outline the key recommendations of the SIGN guideline, and, where appropriate, the differences between recommendations of the NICE and SIGN guidelines will be discussed.
Disc Assessment
Disc assessment, intraocular pressure measurement and visual fields testing are the mainstay of both glaucoma detection and the monitoring of glaucoma progression. When considering referral for glaucoma, optometrists should, firstly, carefully examine the optic disc for structural changes which could be evidence of glaucoma. Prior to the publication of the SIGN guidelines, practitioners would normally note the cup to disc ratio, along with any pertinent disc features. While a cup to disc ratio greater than 0.5 is an independent risk factor for glaucoma,2 the sensitivity of using cup-to-disc ratio alone to diagnose glaucoma is poor.3 Use of the cup-to-disc ratio as a predictor of glaucoma is of limited use due to the wide variation in normal disc appearance. Normally large discs tend to larger cups and smaller discs tend to have smaller cups, hence, in order to differentiate between normal and glaucomatous discs, disc size must be assessed.3,4
The SIGN guideline included new recommendations related to disc assessment to improve differentiation between normal and glaucomatous discs. The disc assessment recommendations in the SIGN glaucoma guideline are based on the Disc Damage Likelihood Scale (DDLS) which was developed by Professor George Spaeth.5 Unlike the cup-to-disc ratio, the DDLS takes into account both focal changes in neural retinal rim and disc size. The DDLS has been shown to be repeatable and correlated with visual field loss.6,7 The DDLS also has a higher sensitivity and specificity than cup-to-disc ratio 8 when differentiating between normal and glaucomatous discs. In order to classify the disc using the DDLS, the disc size should be measured and the narrowest rim to disc ratio assessed. The recommendation in the SIGN guideline is that the narrowest rim to disc ratio should be assessed, and patients should be referred when the value is below a threshold value which is based on the disc size.
Disc size can be assessed using a condensing lens. To use a condensing lens to measure vertical disc height, simply use a narrow width slit beam (1-2mm in width) and adjust the height of the beam to match the vertical height of the disc (see Figure 1).
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Read the value from the slit lamp scale and multiply the reading by the correction factor appropriate for the condensing lens used. Table 1 shows magnification correction factors for common condensing lenses.
Furthermore, OCT disc scans give an accurate reading of vertical disc height. Discs less than 1.5mm in vertical diameter are classified as small, discs between 1.5mm and 2.0mm are medium sized, and discs greater than 2.00mm in diameter are classified as large.
The narrowest rim ratio is assessed by determining the width of the neuroretinal rim at its narrowest point compared to the overall size of the disc. Figure 2 shows an image of a suspicious disc (on the left) and a schematic drawing representing the disc (on the right). The narrowest rim to disc ratio for the disc shown in Figure 2 is 0.1.
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The recommendation for referral based on disc signs, irrespective of any other glaucomatous signs, is included in Figure 3.
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These referral criteria based on the narrowest rim ratio and disc size correspond to DDLS grade 4 or above. The Disc Damage Likelihood Scale can be seen in Table 2.
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The SIGN guideline also recommends that disc photography should be carried out and enclosed with all electronic referrals for glaucoma, glaucoma suspect and ocular hypertensive patients, and also at baseline for ocular hypertensive patients being monitored in the community. Direct electronic referrals from community optometry to ophthalmology are now available in most Health Boards throughout Scotland. The guideline recommends that the narrowest rim to disc ratio should be considered alongside other signs of glaucomatous changes, such as cup-to-disc asymmetry and retinal nerve fibre layer haemorrhage, when considering referral. The disc assessment required to comply with the SIGN glaucoma guideline is more comprehensive and represents a more standardised approach to referral of glaucoma suspects based on disc signs.
Intraocular Pressure
Intraocular pressure (IOP) is another key element of glaucoma referral, diagnosis and monitoring. Prior to publication of the SIGN glaucoma guideline, all patients with IOP over 21mmHg were recommended to be referred for further investigation under NICE glaucoma guidance. The type of tonometer used for referral purposes is not specified in the NICE glaucoma guideline. The SIGN guideline recommends that when referring ocular hypertension, glaucoma suspect, and glaucoma patients, intraocular pressure should be measured using only contact tonometry (either Perkins or Goldmann tonometry) in order to promote consistency of measurement between primary and secondary care.
Furthermore, SIGN guidance recommends at least two measurements should be taken per eye on a single occasion. The reason that at least two measurements are required on a single occasion is to reduce ‘measurement noise’. By doing this, ‘noise’ in the measurement can be reduced by around 3mmHg, and hence false positives, due to this error being minimised. The SIGN glaucoma guideline recommends that protocols must also be in place to ensure regular calibration of tonometers so that intraocular pressure measurements taken by community optometrists are accurate.
The SIGN glaucoma guideline recommends that intraocular pressure measurements should be considered in combination with central corneal thickness (CCT) measurements, typically measured using ultrasound pachymetry (see Figure 4).
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All routine glaucoma referrals should include a CCT thickness. Central corneal thickness is an independent risk factor for glaucoma and people with a CCT under 555µ are more at risk of developing glaucoma than those with CCT over 555µ.9 Age is also an independent risk factor for glaucoma and the SIGN referral recommendation based on intraocular pressure differs for patients below age 65 and for those aged over 65. While the NICE glaucoma guideline recognises the importance of central corneal thickness when diagnosing and treating glaucoma, at the point of referral the recommendation is for referral of all patients with intraocular pressure over 21mmHg irrespective of central corneal thickness. Table 3 summarises the SIGN guideline recommendations for referral based on intraocular pressure.
Under the SIGN guidance the type of pachymeter should be recorded along with the mean CCT and standard deviation (if available) as part of the referral. The Scottish government recently announced that each practice in Scotland will be issued with one pachymeter so that all referrals can be made in compliance with the SIGN guidelines.
Visual Fields
The SIGN guideline states that, when referring glaucoma suspect, ocular hypertensive or glaucoma patients based on findings from the visual field test, irrespective of other findings, a minimum of two visual fields assessments showing a reproducible visual field defect should be carried out. If the result is unequivocal then one visual field test will suffice. The guideline does not state specifically the testing strategy which should be used. However, the General Ophthalmic Services (GOS) Contract in Scotland states that a full threshold strategy should be used when conducting repeated visual fields as part of the referral refinement process for glaucoma suspect patients. Figure 5 shows a full threshold visual field plot with evidence of a glaucomatous defect.
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The SIGN guideline also suggests that a Humphrey visual field analyser should be used for glaucoma referrals to promote consistency with secondary care. However, note that this is a ‘good practice point’ and not a firm recommendation in the guideline.
Other recommendations included in the SIGN guideline
The guideline encourages practitioners to take a thorough history and identify any relevant risk factors for glaucoma and record these on any referral related to glaucoma or ocular hypertension. The guideline is not only concerned with primary open angle glaucoma, normal tension glaucoma and ocular hypertension. It also contains recommendations related to secondary glaucoma and closed angle glaucoma.
Before referring patients, an anterior chamber assessment must be conducted. Specific criteria exist as to when a patient should be referred based on anterior chamber depth. Patients with an occludable peripheral angle should be referred for further investigation. If the peripheral anterior chamber depth is one quarter or less of the width of the corneal thickness (as assessed using the van Herrick technique), then referral should be considered under SIGN guidance. Alternatively, if gonioscopy is used, then, where the posterior pigmented trabecular meshwork is not visible around more than 270° of peripheral angle in either eye, referral should be considered. Evidence has shown that when differentiating between open and closed angles, van Herrick’s technique and gonioscopy are well correlated, hence, the SIGN guideline allows the use of either Van Herrick’s technique or gonioscopy to determine if the angle is occludable.10
The guideline has specific guidance on monitoring of patients with ocular hypertension in the community. The guideline recommends repeated intraocular pressure (measured using applanation tonometry) and visual fields (using standard automated perimetry) as well as baseline optic nerve status. Evidence suggests there is little benefit to intensive monitoring of ocular hypertensive patients and so the guideline suggests two-yearly monitoring of ocular hypertensive patients with normal optic nerve appearance and normal visual fields.11
Previously, all patients with a first degree relative who had been diagnosed with glaucoma were monitored at yearly intervals under GOS regulations in Scotland. The guideline recommends that, where a first degree relative diagnosed with glaucoma is the only risk factor for glaucoma, the patient may be monitored at two-yearly intervals, whereas, if additional risk factors are also present, then monitoring should take place yearly.
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Evidence referenced in the SIGN guideline states that around 30-50% of patients with pseudoexfoliation syndrome will go on to develop secondary glaucoma,12 and around 15 to 50% of patients with pigment dispersion syndrome will develop glaucoma within a 15-year period.13 Key signs of pigment dispersion syndrome include mid-peripheral iris transillumination, Kruckenberg’s spindle (Figure 6), and a pigmented trabecular meshwork (which might be seen on gonioscopy). Key signs of pseudoexfoliation syndrome include deposition of pseudoexfoliative material at the pupillary border (which is best seen on dilation – see Figure 7) and heavily pigmented trabecular meshwork, particularly inferiorly (seen on gonioscopy). As patients with pigment dispersion syndrome and pseudoexfoliation are at increased risk of glaucoma, the guideline also states that patients with pigment dispersion and pseudoexfoliation should be monitored regularly in the community. The guideline further states they do not need to be referred to secondary care unless there are other signs of ocular hypertension or glaucoma.
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Discharge Criteria
The guidelines set out specific categories of patients who can be reviewed in the community following diagnosis at secondary care. These include patients with untreated ocular hypertension patients with pressures below 26mmHg where the central corneal thickness is >555µm and where all other ocular findings are normal. Untreated ocular hypertensives may also be discharged when pressure is above 25mmHg and there is a low lifetime risk of visual disability as a result of glaucoma. Finally, treated ocular hypertensives may be monitored in the community when specific re-referral criteria are documented.
Conclusions
The SIGN guidelines represent a significant shift in responsibility from secondary care to primary care as practitioners working in Scotland under SIGN guidance are now expected to conduct more comprehensive examination prior to any glaucoma referral and to review patients with significant risk factors.
The guidelines also set out to improve diagnostic accuracy and reduce inappropriate referrals to secondary care. The guidelines also mark a shift in the role of the community optometrist because treated and untreated ocular hypertensive patients may be discharged to community optometrists for monitoring in the community environment.
These guidelines provide a framework where community optometrists are more involved in the care of patients with glaucoma and ocular hypertension and will allow more patients to be monitored safely in the community.
Dr Laura Sweeney is an independent prescribing optometrist and lecturer in Vision Sciences at Glasgow Caledonian University and Postgraduate Optometry Research and Development Tutor at the University of Dundee
Key points
- The SIGN glaucoma guideline sets out new recommendations on the referral and safe discharge of glaucoma and ocular hypertensive patients in Scotland
- Pachymetry and detailed disc assessment using a new framework based on the Disc Damage Likelihood Scale are now important part of community glaucoma care under the guideline
References
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2 Gordon M, Beiser J, Brandt J, Heuer D, Higginbotham E, Johnson C, Keltner J, Miller J, Parrish R, 2nd, Wilson M, Kass M, 2002. The Ocular Hypertension Treatment Study: baseline factors that predict the onset of primary open-angle glaucoma. Arch Ophthalmol, 120, 714-20; discussion 829-30.
3 Garway-Heath D, Ruben S, Viswanathan A, Hitchings R, 1998, Vertical cup/disc ratio in relation to optic disc size: its value in the assessment of the glaucoma suspect. British Journal of Ophthalmology, 82, 1118-1124.
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8 Shetty R, Bayer A, Harasymowycz P, Chung J, Henderer J, Spaeth G, 2002, Validity of a New Disc Grading Scale for Estimating Glaucomatous Damage: Correlation with visual field damage, Investigative Ophthalmology & Visual Science, 43, 1064-1064.
9 Brandt JD, Beiser J, Kass M, Gordon M & Group, 2001, Central corneal thickness in the ocular hypertension treatment study (OHTS), Ophthalmology, 108, 1779-1788.
10 Park S, Sung K, Kang S, Jo J, Lee K, Kook M, 2011, Assessment of Narrow Angles by Gonioscopy, Van Herick method and anterior segment optical coherence tomography, Japanese Journal of Ophthalmology, 55, 343-350.
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12 Karger R, Jeng S, Johnson D, Hodge D, Good M, 2003, Estimated incidence of pseudoexfoliation syndrome and pseudoexfoliation glaucoma in Olmsted County, Minnesota. Journal of Glaucoma, 12, 193-197.
13 Siddiqui Y, Ten Hulzen R, Cameron J, Hodge D, Johnson D, 2003, What is the risk of developing pigmentary glaucoma from pigment dispersion syndrome? American Journal of Ophthalmology, 135, 794-799.