Viewing the fundus with the aid of mydriatic agents is becoming more and more standard in optometric practice. The 2006 updated GOS contract in Scotland ensured fundal exam with mydriasis became standard practice for all patients over 60. A dilated fundal view brings benefits to both the patient and examiner but this increased use of mydriatic agents raises some challenges. The risk of angle closure is ever present but how great a risk is it, and how effective are we in mitigating that risk?
Angle variation
Various studies have looked at the prevalence of narrow angles and the risk of an angle occlusion following pharmacologic mydriasis. Primary angle closure glaucoma (PACG) affects 0.4 per cent of the European population over 40. The prevalence increases with each decade peaking at 0.94 per cent for patients aged over 70. In addition to this a 19 per cent increase is expected in the UK over the next decade.1 When we consider that one in 100 of over 70-year-old patients potentially has PACG we see how important it is to evaluate the angle width as part of a thorough eye examination, whether we are dilating the pupils or not.
[CaptionComponent="510"]One study found that twice as many women than men had narrow angles.2 While another that occludable angles were three times more prevalent in the over 70s than the 40-69 age group with use of mydriatic agents.3
We accept that there is a risk of angle closure involved in dilation, but what can we do to mitigate that risk and when do we decide that the risk is too great? Are there clear criteria within your practice stating which patients are not dilated and what, if any, additional investigations those patients should receive?
It has been shown that the best indicators of risk of angle closure following mydriasis are angle depth and history of glaucoma. Utilising both of these factors we can reduce the risk of an angle closure attack to one in 333.3
Clinical assessment
Gonioscopy is the standard measure for angle examination but for various reasons it is not the first line of examination for most optometrists. As a technique it is invasive for the patient and more time-consuming for the examiner than a non-contact process of assessing the angle depth. As it requires considerable skill to interpret the angle structures it is therefore unlikely that its place within optometric practice is as a first-line screening. More recently, OCT technology has allowed a non-invasive cross-sectional view of the anterior chamber angle (described as AS-OCT).
Knowing the population we examine is largely made up of normals, we are looking to identify those outliers with an accurate, comfortable and quick screening test that is reproducible between examinations and examiners.
In 1969 when Van Herick published his process for assessing angle depth using a slit lamp, these criteria were fulfilled. His grading scale has been used ever since. (Table 1).
It is an easy technique to master. With a separation of 60 degrees between the illumination and viewing systems, a fine slit beam is placed perpendicularly at the limbus. The ratio of the thickness of the corneal section to the ‘dark space’ between the endothelium and the anterior iris is measured (Figure 1).
The thinner the dark space in proportion to the limbal corneal section, the narrower the angle. This is repeated nasally and temporally on each eye. This is a quick and easy technique to perform. It needs no additional equipment and is comfortable and non-invasive for the patient. The grading is on a 1-4 scale but counter-intuitively the highest grade of 4 is preferable. This is in direct contrast to the grading of all other slit-lamp findings (CCLRU gradings) where the highest grade of 4 notates the less desirable finding (for example hyperaemia or neovascularisation).
From the grading it can be seen that it is not a linear scale with the range between grade 3 and grade 4 covering 50 per cent but between grade 1 and grade 2 less than 25 per cent. The grouping of grading at the narrower end of the range makes sense, as these are the angles we are most concerned with. However, in addition to recording angles on the day of examination, grading scales are used for monitoring change over time. With this range a patient’s angle would need to halve in depth before they dropped from a grade 4 to a grade 3. In reality many angles are between these grades and the recording of grade 2/3 and grade 3/4 has led to many clinicians looking for a better system to record this finding.
As optometrists, we are used to examining a large number of optic discs. We review and record many aspects of disc appearance to decide whether any findings now, or any changes in the future, are of a pathological basis. One of the main measurements we use is the cup to disc ratio which we notate on a decimal linear scale of 0.0 to 1.0 in 0.1 steps. Making this visual judgement is something we are used to and shows that we are comfortable with a finer scale than the van Herick’s scale used for angle measurement.
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Decimal grading
Expressing the angle depth decimally as we do cup to disc ratio, or as a percentage of the limbal corneal section allows greater accuracy in comparison to the traditional van Herick’s grade. If you record a change from 1.0 to 0.7, you know that there has been a significant change, whereas a change from grade 4 to grade 3/4 would have been unlikely to alert you to a 30 per cent narrowing of the angle. This greater accuracy allows us to identify which patients should have further investigation before dilation. Viewing the angle with gonioscopy or imaging with anterior segment OCT for patients graded 0.3 or less ensures more confidence when performing a dilation. When this grading scale was first published it was found that the threshold of 0.3 should be the limit to which an angle should be considered occludable.4
In conclusion, a clear protocol will ensure confidence for patients and practitioners. Within our practices (Black & Lizars, Scotland), that protocol states that any patients with an angle grading of 0.3 or less should be examined further by gonioscopy and/or AS-OCT.
Having OCT images (Figure 2) allows us to share the angle scans with ophthalmology for further clinical advice should it be required. It also ensures any onward referrals for prophylactic iridotomies are well illustrated.
References
1 Day AC, Baio G, Gazzard G, Bunce C, Azuara-Blanco A, Munoz B, Friedman DS, Foster PJ. The prevalence of primary angle closure glaucoma in European derived populations: a systematic review. Br J Ophthalmol, 2012; Sep;96(9):1162-7. doi: 10.1136/bjophthalmol-2011-301189. Epub 2012 May 31.
2 Wolfs RC1, Grobbee DE, Hofman A, de Jong PT. Risk of acute angle-closure glaucoma after diagnostic mydriasis in non-selected subjects: the Rotterdam Study. Invest Ophthalmol Vis Sci, 1997; Nov;38(12):2683-7.
3 Patel KH1, Javitt JC, Tielsch JM, Street DA, Katz J, Quigley HA, Sommer A. Incidence of acute angle-closure glaucoma after pharmacologic mydriasis. Am J Ophthalmol, 1995, Dec; 120(6):709-17.
4 Cockburn DM. Slit lamp estimate of anterior chamber depth as a predictor of the gonioscopic visibility of angle structures. Am J Optom Physiol Opt, 1982; 59, 904–8.
? Morven Campbell is clinical services manager for Black & Lizars Optometrists, Scotland
