OD News Articles

4th April 2017

The Occludable Angle

by Myung Jung, OD Chehalis, WA

As optometric physicians, we routinely make quick angle closure risk assessments before dilating patients for fundus exams. Although uncommon, acute angle closure can cause vision loss, eye pain, and irreversible optic nerve damage. We know patients with hyperopia and shorter axial lengths are more likely to have narrow angles. But of those, which are occludable? And how can we determine if they are good candidates for surgical treatment to reduce the risk?

DEFINITION

Let’s start by defining the occludable angle. Here are some quantifiable measures:

  1. On gonioscopy without indentation, posterior trabecular meshwork is seen in less than one-fourth of the entire angle1.
  2. The width of the iridocorneal angle is less than 20 degrees (Shafer grade I-II)2.
  3. Anterior chamber depth is less than 2.5 mm3.
DIAGNOSTIC METHODS

Now let’s review the methods available to diagnose an occludable angle:

  • Anterior Chamber Depth
     Anterior chamber depth (ACD) is measured from the corneal epithelium to the anterior capsular bag of the lens. Normal depth is between 2.5 mm and 3.5 mm. Patients with an ACD of less than 2.5 mm are at greater risk of angle closure1. ACD can be measured with A-scan, anterior segment optical coherence tomography (AS-OCT), or optical biometry. A shallow measurement can serve as a screening tool, leading us to examine the angle further with gonioscopy. 
     
  • Gonioscopy
    Gonioscopy is the gold standard for diagnosing the occludable angle. To get the most information from the angle, it should be performed dynamically. The four-mirror goniolens allows for this. As previously mentioned, the angle is occludable when the full posterior trabecular meshwork is not visible in three quadrants or more.
    • If there is contact between the peripheral iris and trabecular meshwork, you can gently indent the cornea with a four-mirror goniolens to see if deeper structures are visible.
    • If the iris and trabecular meshwork separate on indentation, you are looking at an appositional angle.
    • If there is no change in the structure, what you may be seeing is peripheral anterior synechiae (PAS).

Gonioscopy should be done in dim lighting. Shining a bright light during the exam will constrict the pupil and open areas of apposition. This will make the angle appear more open than it actually is. It is also important to distinguish iris processes which appear as fine, lacy projections of the iris into the scleral spur or trabecular meshwork. While iris processes are normal, PAS or apposition between the iris and trabecular meshwork are abnormal. Visit www.gonioscopy.org for excellent tips, videos and examples of this technique.

  • Imaging Technology
    Advanced technologies such as the ultrasound biomicroscopy (UBM) and anterior segment optical coherence tomography (AS-OCT) have made detailed, cross-sectional views of the angle possible. Like gonioscopy, this imaging should be performed under dim lighting conditions.

    There are similarities in UBM and AS-OCT imaging technologies. But there are also key differences, as outlined in the table above.

    The scleral spur is often used as a reference point for quantitative measurements of the angle on UBM and AS-OCT. Although both imaging instruments are excellent tools, it can be difficult to locate the scleral spur and other angle structures on every image. Because of this limitation, imaging technology should be used in conjunction with gonioscopy to diagnose occludable angles.
     
  • Provocative Dilation Test
    This is done with either pharmacologic pupillary dilation or darkroom adaptation, and is considered positive when IOP rises 8 mmHg or more. Dilation attempts to incite an angle closure under the provider’s watch, but this is not a favored diagnostic technique. There is significant risk that angle closure could happen hours later as the pupil comes back down.

    When patients with narrow angles have been dilated for a fundus exam and you are concerned about possible angle closure, it may be helpful to recheck their IOP. Eyes are most vulnerable to angle closure about 3 to 5 hours after dilation—when there is greatest contact between the iris and lens. 
TREATMENT OPTIONS

Several treatment options are available for patients with occludable angles:

  • Cataract Surgery
    For patients with visually significant lens opacification, cataract surgery can decrease the risk of primary angle closure from pupillary block. Patients can be examined without dilation until the day of cataract surgery. At PCLI, on the day of surgery, a dilated fundus exam is performed.
     
  • Laser Peripheral Iridotomy
    Patients without visually significant cataracts are candidates for laser peripheral iridotomy (LPI). 
     
  • Goniosynechialysis
    For patients with significant PAS, goniosynechialysis may be done in conjunction with cataract surgery. This surgical technique strips PAS from the trabecular surface, giving aqueous renewed access to the trabecular meshwork.
PLATEAU IRIS

Plateau iris is a condition in which the ciliary processes are positioned anteriorly, displacing the peripheral iris forward. Up to 20 percent of angle closure glaucoma may be due to plateau iris4. On gonioscopy, prominent last rolls of the iris are visible with a sharp drop-off of the peripheral iris. This is also known as the double-hump sign. What really differentiates plateau iris configuration from the appositional angle is the anteriorly positioned ciliary processes. We cannot view structures posterior to the iris on gonioscopy or AS-OCT, which makes the UBM incredibly useful in the diagnosis of plateau iris.

Because peripheral iridotomy cannot change the displaced ciliary processes, a patent iridotomy can still lead to angle closure in plateau iris syndrome. However, appositional angle component can still be present in conjunction with plateau iris. The appositional angle can be treated with LPI or cataract surgery to prevent pupillary block. When LPI is unsuccessful in opening the angle due to plateau iris, and cataract surgery is not indicated, peripheral iridoplasty can widen the angle. Laser burns contract and pull the iris root away from the angle structures.

CONCLUSION

Although acute angle closure is relatively rare, we want to be on the lookout for patients at risk. Newer imaging technologies have become popular assessment methods. When used in conjunction with gonioscopy, they provide quantitative and qualitative data that guide our diagnoses and help us select appropriate treatment.

Questions

If you ever have questions about occludable angles, feel free to contact any of our optometric physicians. We are always happy to help.

REFERENCES
  1. Foster PJ, Devereux JG, Alsbirk PH, et al. Detection of gonioscopically occludable angles and primary angle closure glaucoma by estimation of limbal chamber depth in Asians: modified grading scheme. British Journal of Ophthalmology 2000;84:186-192.
  2. Kim, Y.Y. & Jung, H.R. (1997). Clarifying the nomenclature for primary angle-closure glaucoma. Surv Ophthalmol, Vol. 42, No. 2, (Sep-Oct 1997) pp. (125-136).
  3. George R, Paul PG, Baskaran M, et al. Ocular biometry in occludable angles and angle closure glaucoma: a population based survey. The British Journal of Ophthalmology. 2003;87(4):399-402
  4. He M, Friedman DS, Ge J, Huang W, Jin C, Lee PS, Khaw PT, Foster PJ. Laser peripheral iridotomy in primary angle-closure suspects: Biometric and gonioscopic outcomes: the Liwan Eye Study. Ophthalmology 2007;114:3:494-500. Epub 2006 Nov 21.