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Anterior Segment Scanning with SDOCT

HOW TO CORRECTLY MEASURE A CORNEA ANGLE IN DEGREES

Written by Bill Dilworth, Larry J Alexander OD FAAO Tuesday, 07 May 2013

With the advent of SDOCT a new world has opened for assessing the anterior chamber angle. It is now possible to measure the angle accurately with some of these instruments as well as watch the dynamics of the iris in vivo, such as watching a plateau iris actually close the angle when the lights are turned off and the pupil dilates.

However for accuracy purposes some specific guidelines have been created to assure proper initial and repeated measurement of the angle. This presentation outlines the proper method to obtain accurate and repeatable anterior chamber scans.

OBTAINING THE ACCURATE SCAN

The issue of measurement error of the anterior chamber angle occurs if the angle is measured with the patient looking straight ahead.  The error is due to the inaccuracy generated by the corneal refraction.  This refraction results in bending the measuring beam as illustrated below.  While minor, this is critical in making clinical decisions.

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View with the patient looking straight ahead.  While the angle is still occluded either way you can see that the cornea is not flattened to maximize the view.

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You can likewise see from the image below that the cornea is not in perfect position but still conveys the fact that with lights off the angle closes down.

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If you have the patient look to the side to flatten the cornea the “bending” does not occur resulting in a more accurate reading of the angle as illustrated below.

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The proper technique for accurate measurement of the angle follows. 

Step 1: Obtain the angle-scan with the cornea flattened, which will look like:

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Step 2: Lighten the scan to enhance the image and bring out the features of the angle.

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Step 3: Initiate the measuring protocol depending on the instrument used. Then identify the Scleral Spur by following the bottom of the cornea (yellow arrows) and the bottom of the sclera (red arrows) to the point where they meet.  The green arrow points to the scleral spur.

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Step 4.  Place the vertex of the angle tool at the scleral spur.

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Step 5.  Drag the bottom side of the angle into the anterior chamber.

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Step 6.  Pull the bottom line down until it touches the iris at some point and pull the end to that point.

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Step 7.  Pull the top line down until it touches the bottom of the cornea.  If there is a gap between the top line and the corneal bottom surface, shorten the line to eliminate that gap.

angle

Step 8.  Drag the angle measurement to the center of the angle.

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Step 9. Print

COMMENTARY:

It is important to realize that there are a number of factors involved in angle assessment beyond the measurement.  Among those is the fact that the only real study that classified the angles that are potentially occludable is Chan RY, Smith JA, Richardson KT.  Anterior Segment Configuration Correlated With Shaffer’s Grading of the Anterior Chamber Angle. Arch Ophthalmol  1981. 99:104-107.  In their article they refer to the Table of the Shaffer Grading System, which is shown below.  It should be clarified that this was done without the advantage of the accurate technology of SDOCT 

Angle Grade

Numerical Grade

Angle Width Degrees

Implied Clinical Interpretation

Wide-Open

IV

35-45

Closure Impossible

Wide-Open

III

20-35

Closure Impossible

Moderate Narrow

II

20

Closure Possible

Extreme Narrow

I

10

Closure Probable Eventually

Complete or Partial Closure

0

0

Closure Present or Imminent

The other issues that impact on angle assessment involve the architecture of the iris to the lens and cornea.  Issues of plateau iris, cataract-induced bowing of the iris, sub-acute angle closure, iridodyalysis, and the characteristic iris of pigmentary dispersion also confound the total analysis of the anterior chamber.  All of these architectural factors make accurate measurement of the angle difficult.  Gonioscopy must still be employed to uncover debris in the anterior chamber and trabecular meshwork such as pigmentary dispersion , neovascularization of the iris as well as a number of developmental anomalies.  Gonioscopy and SDOCT anterior chamber analysis are not mutually exclusive.

EFFECTS OF A CATARACT ON THE ANGLE

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THE PLATEAU IRIS

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While there are many potential uses for this technology one more must be mentioned….the ability to assess the patency of peripheral iridotomies.   While the technique of transillumination has long been used to assess the patency of peripheral iridotomies, an iridotomy with a membrane will transilluminate.  The use of the anterior chamber evaluation of SDOCT can be of tremendous benefit in the assessment of the patency.

THE YAG PI

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CONCLUSION

Anterior SDOCT offers the doctor multiple advantages including assessment of the architecture of the anterior chamber angle.  For proper interpretation, however, the test must be performed properly as outlined in this presentation.  Other applications of anterior segment SDOCT include but are not limited to the following:

ANTERIOR SEGMENT SDOCT APPLICATIONS

  • MONITORING DEPTH AND HEALING OF ULCERS
  • MONITORING AND MAPPING EPITHELIAL BASEMENT MEMBRANE DYSTROPHY
  • “DISCOVERING” SYNECHIAE AND INFLAMMATORY DEBRIS
  • MAPPING TERRIENS MARGINAL DYSTROPHY
  • MAPPING PELLUCID MARGINAL DEGENERATION
  • MAPPING LIMBAL EPITHELIAL DEFECTS
  • ASSESS FOR PENETRATING WOUNDS...HOW DEEP IS THAT ABRASION OR CORNEAL FOREIGN BODY
  • ASSESS THE DEGREE OF CORNEAL ECTASIA WITH DEGREE OF THINNING
  • MONITOR CORNEAL SWELLING REGRESSION
  • EVALUATE SCLERAL LENS FITTING AND ORTHOKERATOLOGY
  • PACHYMETRY
  • ASSESS DEGREE OF HYPHEMA, HYPOPYON AND MONITOR REGRESSION
  • INVESTIGATE FOR IRIDODYALYSIS

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About the Author(s)

Bill Dilworth

Bill Dilworth

Bill Dilworth began in ophthalmology as a technician at University of Pittsburgh Medical Center. During his 20 years there, he worked with all the sub-specialists and learned a great deal about eyes, practices, and patients. When the imaging center was stated under the direction of Drs. Schuman and Ishikawa, he was appointed to the lead position.

While running the Imaging Center at UPMC Bill worked with several companies to help develop and improve their devices and software. The move to Optovue was an extension of these efforts. In the last 7 years while working for Optovue, Bill has continued to work with doctors, engineers, technicians, and staff to promote visualizing and understanding of our eyes and the many things that can go wrong with them.

Larry J Alexander OD FAAO

Larry J Alexander OD FAAO

Dr. Alexander (1948-2016) was a 1971 graduate of Indiana University School of Optometry. He served in the US Navy then served as a Professor at the University of Alabama Birmingham School of Optometry. Larry contributed to a number of chapters in textbooks and has published three editions of Primary Care of the Posterior Segment, as well as contributed to the professional literature. He also lectured extensively in the area of ocular and systemic disease. His areas of special interest included dysfunctional tear syndrome, glaucoma and macular degeneration.  His lessons are the basis for this site and he will be dearly missed. 

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