C52261: Assessing the anterior chamber in optometric practice

In everyday practice we are required to examine the contents of the anterior chamber (AC) for clarity and signs of inflammation, often associated with uveitis or severe keratitis. It is also important to examine the structure of the AC as certain elements can point to an increased risk of acute angle closure and angle closure glaucoma. Recent research has found that a significant percentage of glaucoma patients classified as open angle actually have a narrow angle element to their disease 1 making examination and documentation essential. Intermittent angle closure can be difficult to detect due to the fact that IOP will be in the normal range between attacks, careful questioning and accurate assessment of AC structures are essential to prevent patients suffering sight loss through angle closure glaucoma.

Anterior Chamber Structure

The AC is bordered by the corneal endothelium anteriorly and the lens capsule and iris posteriorly. The anterior chamber depth (ACD) is deepest centrally, directly under the corneal apex and narrows as the cornea curves to the limbus. The anterior chamber angle (ACA) relates to the space between the iris and corneal endothelium at the limbus with the trabecular meshwork at the apex. The angle is generally narrowest superiorly (Figure 1).

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With increasing age, the lens will thicken, pushing the iris forward and narrowing the ACA and shortening the ACD. This greatly increases the risk of angle closure. In addition to age, risk factors include female sex, short stature, East Asian descent, hyperopia, increased lens thickness and decreased axial length. Many of these risk factors are linked, such as hyperopia and decreased axial length or age and increased lens thickness. As with many of our other clinical findings, having accurate grading or measures on record for every patient can allow us to build up a trend over time, ensuring that any further investigation or intervention is undertaken before pathological change occurs.

It is important to identify patients with narrow angles as there is a significant risk of progression to vision loss over time. Some 22% of patients with an occludable angle will have a primary angle closure within five years.2 Of those patients with primary angle closure 28% will progress to primary angle closure glaucoma (PACG) within a further five years.3 This means six in every 100 patients you examine with narrow, occludable angles will be diagnosed with PACG within 10 years. Add to this that PACG is significantly more blinding than primary open angle glaucoma (1:4 patients versus 1:10 with severe sight loss 4) and the fact that it can be easily avoided with prophylactic treatment and the argument for screening every patient is made. Recent studies have shown that PACG is far more prevalent than previously thought, and that a 19% increase in cases is expected in the UK over the next decade.5

Angle closure has traditionally been linked with very specific symptoms; red, painful eyes at night, haloes around lights and vomiting combined with extreme pain. A study which asked both angle closure suspects and open angle patients specifically about these symptoms found high rates of positive responses in both groups and concluded that these symptoms are simply not specific enough to be diagnostic.6 The only way to ensure your patient is not at risk is to examine these structures as part of your routine eye examination.

Both the ACD and ACA can be measured or graded by various means. Prior to cataract surgery, or refractive lens exchange, biometry accurately measures a number of components including the ACD. It has been shown that a greater percentage of refractive surprises following IOL placement come from inaccurate ACD measurements (42%), greater than both axial length (36%) and lens thickness (22%).7 It is, however, unlikely that optometric practice will have easy access to biometry.

In a hospital setting, and increasingly within community optometry, the gold standard of ACA assessment is being utilised. Gonioscopy allows us to view directly into the angle and assess which angle structures are visible beyond the iris. It allows us to assess 360 degrees of the ACA. It is a skilled technique, more in the assessment of the images than the practical aspects of placing the lens and setting up the slit lamp. This technique should be performed with the minimal amount of light possible as a constricted pupil will artificially deepen the angle by pulling the iris away from the angle. Indentation pressure with the gonioscopy lens can also widen the angle. It is invasive for the patient and more time consuming for the examiner than a non-contact process of assessing the angle depth. Because it requires time and considerable skill it is unlikely to be our first line screening of ACA.

Both methods described above are probably too time consuming to perform on every patient; there are, however, quick and easy ways to examine these structures during a routine slit lamp examination. Neither require additional equipment and both are simple techniques to learn. Both produce results that are reproducible between examinations and examiners.

Anterior chamber angles can be graded using Van Herick and anterior chamber depth can be measured using Smith’s technique. Both of these techniques are described below. Although Van Herick’s is a technique we are all familiar with, an updated decimal grading scale increases accuracy.

Anterior Chamber Angle Examination and Grading

Van Herick technique

In 1969 Van Herick published his process for assessing angle depth using a slit lamp (Table 1).8

Table 1 Step-by-step guide to Van Herick’s technique

• This technique should be carried out in a dark room
• Set the patient up at the slit-lamp and advise them to fixate straight ahead
• Lock the illumination at 60° temporally to the patient
• The viewing microscope should be set straight ahead
• Set the magnification to 10-16x
• Use a fine beam and high illumination
• Place the beam perpendicularly at the limbus
• Assess the depth of the dark space between the iris and corneal endothelium
• Compare that space with the thickness of the limbal corneal section
• Repeat the technique nasally and temporally for both eyes

His grading scale has been used ever since. (Table 2)

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Van Herick’s is a quick and easy technique to perform, it is comfortable and non-invasive for the patient. The grading is on a one to four 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. The grading does, however, correlate well to Shaffer’s grading system used in gonioscopy.

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% but between grade 1 and grade 2 less than 25%. 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 (see Figure 2 a to c).

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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 (Figure 3).

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Expressing the angle depth decimally as we do cup to disc ratio 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% narrowing of the angle. This greater accuracy allows us to identify which patients should have further investigation. 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.9 It should also be noted that you can grade angles deeper than 1.0, which are regularly seen following lens implantation where the angle deepens beyond 100% of the limbal corneal section. In these cases angles can be graded beyond 1.0, for example if the limbal angle depth was 120% of the corneal section a grading of 1.2 would be recorded (see Table 3 for a summary of decimal and percentage grades).

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Anterior OCT

Another way to view the ACA is with OCT. There are some significant differences to angle viewing with OCT versus the gold standard of gonioscopy. In OCT the image is taken in the dark with a more dilated pupil, this could be viewed as a benefit as this is when the patient is most at risk of angle closure (Figure 4).

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The most important difference is that with OCT, and Van Herick, we are simply getting a ‘snapshot’ of the ACA, at the nasal and temporal points. Although the angle tends to be narrower temporally than nasally, it is usually narrowest superiorly so OCT images should be viewed with this in mind. In combination with an accurate, decimal Van Herick grading an OCT image can alert us to a patient that requires a more detailed angle examination by gonioscopy.

Anterior Chamber Depth

The anterior chamber depth (ACD) is the depth as measured from the corneal endothelium (at the corneal apex) to the anterior lens surface. The measurement is expressed in millimetres. Measuring ACD in addition to ACA also allows us to assess a patient’s risk for angle closure glaucoma. As with ACA the shallower the ACD the more at risk of angle closure the patient is. Normative values vary across populations: one study with centres in nine countries over six continents found the shallowest ACD in New Zealand and the deepest in the USA.10 Table 4 shows a guide to the technique.

Table 4 Smith’s technique – step by step

This technique should be carried out in a dark room

• Set the patient up at the slit-lamp and advise them to fixate straight ahead
• Lock the illumination at 60° temporally to the patient (as temporal Van Herick's)
• The viewing microscope should be set straight ahead
• Set the magnification to 10-16x
• Use a thin 1-2mm beam and turn it horizontally
• Place the beam over the temporal pupil margin, focused on the cornea
• A second beam on the iris/anterior lens will be visible on the nasal pupil margin
• Increase the height of the beam until the edges of the two beams just touch
• The illumination must be rotated 60° to the other side to measure the other eye

An average adult ACD is 3.15mm, with a shallowing of 0.01mm per year. Eyes with ACD lower than 2.5mm should be considered at risk of angle closure glaucoma.

Smith’s technique

Biometry instrumentation to take accurate ACD measurements is rarely available in optometric practice, but like ACA grading with Van Herick’s there is a way to utilise your slit lamp skills to take this important measurement. Dr Redmond Smith published details of a technique in 1979 that with a little practice can allow us to identify those at risk of occlusion.11

Smith’s technique is non-invasive and quick to perform. Unlike Van Herick’s, Smith’s does not use a grading scale – it is a quantitative method and we are measuring the ACD in millimetres. This allows for greater accuracy and ongoing monitoring of structure change as we age. Figure 5 shows various stages of Smith’s technique and Figure 6 shows the optical principles behind the technique.

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An original correction factor of 1.4 was documented following work on 56 eyes and comparing Smith’s technique to pachymetry data. Lower correction factors have since been demonstrated in research when comparing Smith’s to A Scan Ultrasound (1.34), Pachymetry (1.31) and Orbscan (1.22).12 Using the 1.4 correction factor we are overestimating the depth of the ACD (and underestimating the risk) and it is therefore advisable to use the correction factor of 1.31.

Summary

Simply by utilising the equipment we currently have we can grade and measure the anterior chamber. Given the increase in narrow angle disease routine assessment of these structures should become an integral part of our examination.

Morven Campbell is clinical services manager for Black and Lizars

References

1 Moutsou M, Arora A, Goyal S. ARVO 2014, EGS 2014

2 Thomas R, George R, Parikh R et al. Five year risk of progression of primary angle closure suspects to primary angle closure: a population based study. Br J Ophthalmol 200387450–454.454

3 Thomas R, Parikh R, Muliyil J et al. Five-year risk of progression of primary angle closure to primary angle closure glaucoma: a population-based study. Acta Ophthalmol Scand 200381480–485.485

4 H A Quigley, A T Broman. The number of people with glaucoma worldwide in 2010 and 2020 Br J Ophthalmol 2006;90:262-267

5 Day AC1, 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.

6 Ong EL, Baasanhu J, Nolan W, Uranchimeg D, Lee PS, Alsbirk PH, Johnson GJ, Foster PJ. The utility of symptoms in identification of primary angle-closure in a high-risk population. Ophthalmology. 2008 Nov;115(11):2024-9

7 Young Rae Roh, Sang Mok Lee, Young Keun Han, Mee Kum Kim, Won Ryang Wee, and Jin Hak Lee Intraocular Lens Power Calculation Using IOLMaster and Various Formulas in Short Eyes Korean J Ophthalmol. 2011 Jun; 25(3): 151–155.

8 Van Herick W, Shaffer RN, Schwartz A. Estimation of width of angle of anterior chamber. Incidence and significance of the narrow angle. Am J Ophthalmol 1969;68:62–9.

9 Cockburn DM. Slit lamp estimate of anterior chamber depth as a predictor of the gonioscopic visibility of angle structures. (1982)Am J Optom Physiol Opt 59, 904–8

10 Matthew T, Feng et al. Anterior chamber depth in normal subjects by rotating scheimpflug imaging Saudi Journal Ophthalmology July–September, 2011Volume 25, Issue 3, Pages 255–259

11 Smith RJ. A new method of estimating the depth of the anterior chamber. Br J Ophthalmol. 1979;63:215-220.

12 Eperjesi F1, Holden C. Comparison of techniques for measuring anterior chamber depth: Orbscan imaging, Smith’s technique, and van Herick’s method. Graefes Arch Clin Exp Ophthalmol. 2011 Mar;249(3):449-54.