The classification of glaucoma (primary open-angle or angle-closure) relies heavily upon knowledge of the anterior segment anatomy, particularly that of the anterior chamber angle. Angle-closure glaucoma is characterized by obstruction of aqueous fluid drainage through the trabecular meshwork (the primary fluid egress site) from the eye's anterior chamber. The width of the angle is one factor affecting the drainage of aqueous humor. A wide unobstructed iridocorneal angle allows sufficient drainage of aqueous humor, whereas a narrow angle may impede the drainage system and leave the patient susceptible to angle-closure glaucoma. The treatment for this condition is a peripheral iridotomy (laser) or peripheral iridectomy (surgery). Slit lamp biomicroscopy is used to evaluate the anterior chamber, however, the chamber angle can only be examined with specialized lenses, the most common of these being the gonioscopic mirror. In this procedure a gonio lens is applied to the surface of the cornea under topical anesthesia and the image magnified with the slit lamp. Gonioscopy is the standard method for clinically assessing the anterior chamber angle. Other techniques for imaging the anterior eye segment include ultrasonography and optical coherence tomography. (1)
Ultrasonography uses high frequency mechanical pulses (10 to 20 MHz) to build up a picture of the front of the eye. An ultrasound scan along the optical axis assesses corneal thickness, anterior chamber depth, lens thickness and axial length. Ultrasound scanning across the eye creates a two-dimensional image of the ocular structures. It has a resolution of 100 microns, but only moderately high intra-observer and low inter-observer reproducibility. Ultrasound biomicroscopy (about 50 MHz) has a resolution of 30 to 50 microns. As with gonioscopy, this technique requires placement of a probe under topical anesthesia.
Optical coherence tomography (OCT) is a non-invasive method that creates an image of light reflected from the ocular structures. In this technique a reflected light beam interacts with a reference light beam. The coherent (positive) interference between the two beams (reflected and reference) is measured by an interferometer, allowing construction of an image of the ocular structures. This method allows cross-sectional imaging at a resolution of 6 to 25 microns. The Stratus OCT™ (Carl Zeiss Meditec), which utilizes a 0.8 micron wavelength light source, was designed for evaluating the optic nerve head, retinal nerve fiber layer and retinal thickness.* The Zeiss Visante OCT™ uses a 1.3 micron wavelength light source and is designed specifically for imaging the anterior eye segment. Light of this wavelength penetrates the sclera, allowing high-resolution cross-sectional imaging of the anterior chamber angle and ciliary body. The light is, however, typically blocked by pigment, preventing exploration behind the iris. Ultrahigh resolution OCT can achieve a spatial resolution of 1.3 microns, allowing imaging and measurement of corneal layers.
The Visante OCT received marketing clearance through the U.S Food and Drug Administration (FDA) 510(k) process in 2005, listing the Stratus OCT and Orbscan™ II as predicate devices. The 510(k) summary describes the Visante OCT as “a non-contact, high resolution tomographic and biomicroscopic device indicated for the in vivo imaging and measurement of ocular structures in the anterior segment, such as corneal and LASIK flap thickness.”
An early application of OCT technology was the evaluation of the cornea before and after refractive surgery. Since this is a non-invasive procedure that can be conducted by a technician, it has been proposed that this device may provide a rapid diagnostic and screening tool for the detection of angle closure in glaucoma. Also being investigated is the possibility that the 0.8 micron wavelength Stratus OCT, which is already available in a number of eye departments, may provide sufficient detail for routine clinical assessment of the anterior chamber angle in glaucoma patients.
*(see policy 9.03.06 regarding other ophthalmologic techniques of evaluating glaucoma)

Scanning computerized ophthalmic (e.g., OCT) imaging of the anterior eye segment is considered investigational.

Beginning January 1, 2008, there is a category III CPT code specific to computerized imaging of the anterior eye segment:
0187T: Scanning computerized ophthalmic diagnostic imaging, anterior segment, with interpretation and report, unilateral

BlueCard/National Account Issues

State or federal mandates (e.g., FEP) may dictate that all FDA-approved devices may not be considered investigational and thus these devices may be assessed only on the basis of their medical necessity.

A search of the MEDLINE database to December 2007 focused on the use of anterior imaging techniques to diagnose or manage closed angle glaucoma. Although the search identified a number of technical reviews, clinical research appears to be at an early stage of development.
One study assessed the ability of a prototype of the Visante OCT to detect primary angle closure in 203 Asian patients. (2) The patients, recruited from glaucoma clinics, had been diagnosed with primary angle closure, primary open-angle glaucoma, ocular hypertension and cataracts; some had previously been treated with iridotomy. Images were assessed by 2 glaucoma experts, and the results compared to an independently obtained reference standard (gonioscopy). Data were reported from 342 eyes of 200 individuals. A closed angle was identified in 152 eyes with gonioscopy and 228 eyes with OCT, agreement was obtained between the two methods in 143 eyes. Although these results suggest low specificity for OCT, it is noted that gonioscopy is not considered to be a “gold” standard. The authors suggest 3 possible reasons for the increase in identification of closed angles with OCT: lighting is known to affect angle closure, and the lighting conditions were different for the 2 methods (gonioscopy requires some light); placement of the gonioscopy lens on the globe may have caused distortion of the anterior segment, and; landmarks are not the same with the two methods. The authors noted that longitudinal studies will be required to determine whether eyes classified as closed by OCT but not by gonioscopy are at risk of developing primary angle closure glaucoma.
Another prospective observational study (n=26) evaluated imaging of the anterior angle chamber with the Stratus OCT, which had been developed for retinal imaging. (3) Ten eyes with normal open angles and 16 eyes with narrow or closed angles or plateau iris configuration as determined by gonioscopy were assessed. The OCT image was rated for quality, for ability to demonstrate the anterior chamber angle, and for ability to visualize the iris configuration; patients were classified as having open angles, narrow angles, closed angles or plateau iris configuration. Ultrasound biomicroscopy was performed for comparison if plateau iris configuration was diagnosed. The investigators reported that the Stratus OCT provided high-resolution images of iris configuration and narrow or closed angles, and imaging of the angle was found to be adequate in cases of acute angle-closure glaucoma where the cornea was to cloudy to enable a clear gonioscopic view. Open angles and plateau iris configurations could not be visualized with the 0.8 micron wavelength Stratus OCT.
Ideally, a diagnostic test would be evaluated based on its technical performance, diagnostic performance (sensitivity and specificity) and clinical validity. Current literature consists primarily of assessments of qualitative and quantitative imaging and detection capabilities. Technically, the Visante OCT has the ability to create high resolution images of the anterior eye segment. Studies indicate that the Visante OCT detects more eyes with narrow or closed angles than gonioscopy, showing high sensitivity and low specificity in comparison with the reference standard. However, if the reference standard is flawed (e.g., does not detect all cases), the information provided by sensitivity and specificity is limited. Evaluation of the diagnostic performance of the Visante OCT depends, therefore, on demonstration of an improvement in clinical outcomes. Although the resolution of the images and the ease of use might be considered advantageous, evidence is insufficient to determine whether use of OCT can improve detection and management of patients at risk of developing primary angle-closure glaucoma. Given the number of questions regarding the impact of this new technology on health outcomes, this procedure is considered investigational.

References:

1. Wolffsohn JS, Peterson RC. Anterior ophthalmic imaging. Clin Exp Optom 2006; 89(4):205
2. Nolan WP, See JL, Chew PT et al. Detection of primary angle closure using anterior segment optical coherence tomography in Asian eyes. Ophthalmology 2007; 114(1):33-9.
3.  Kalev-Landoy M, Day AC, Cordeiro MF et al. Optical coherence tomography in anterior segment imaging. Acta Ophthalmol Scand 2007; 85(4):427-30.

Closed angle glaucoma
Optical coherence tomography
Visante OCT
Stratus OCT