Glaucoma Imaging: Getting Closer

Optometric Management, Jul 2006 by Dul, Mitchell W

Optometric Management is proud to present this installment in our series of articles on glaucoma that have been planned in partnership with the Optometric Glaucoma Society (OGS). The Society has provided OM with expert authors who will discuss therapies, epidemiology, diagnostic equipment and other current issues in glaucoma management. For additional information on the Society, contact the OGS through the Web site optometricglaucomasociety.org. The views expressed in this article are the author's and do not necessarily represent the views of the OGS or Optometric Management.

New technologies such as scanning laser tomography (HRT3, Heidelberg Engineering), laser polarimetry (GDx, Carl Zeiss Meditec) and ocular coherence tomography (OCT Stratus) will likely play a growing role in optometry, affecting the way we assess and manage glaucoma. These tools will enable us to acquire objective and highly reproducible images that will assist in the work-up and management of our patients.

In addition, the images provide an excellent tool for patient education - obviously, a very important factor that influences a patient's adherence to future follow-up and treatment. These devices are beginning to provide a technological platform from which we can expect future software enhancements. These advancements will enable us to better assess clinically significant change in a patient's condition over time.

Use sound judgement

By now we're all familiar with scanning laser tomography, laser polarimetry and ocular coherence tomography, devices that assess the structure of the optic nerve, retinal nerve fiber layer and retina. These present data in various forms, including the use of probability scales that provide a sense how a patient compares with each device's internal database. This information supplements our clinical assessment, which entails comparing each patient's findings with our own clinical experience. I wouldn't advise you use any of these devices in the absence of sound clinical assessments and judgment. Further research is required.

Fortunately, several National Eye Institute studies are currently underway that will shed light on the issue of whether these technologies are more sensitive for detecting glaucoma progression than a thorough clinical examination that includes both structural optic nerve head (ONH) and functional visual field (VF) assessments.

Anterior segment

Recent technological advances for glaucoma management include ocular coherence tomography (OCT) available for the anterior segment of the eye. Examples of this include Visante (Carl Zeiss Meditec) and SL OCT (Heidelberg Engineering). In addition, Portable Ophthalmic Devices is seeking PDA-approval for an ultrasonic device that is presently used for small-animal research and veterinary ophthalmic diagnosis. This device provides dynamic images of anterior segment. For instance, we can visualize changes in the configuration of the anterior chamber angle, iris, lens and ciliary body in the light and dark. This can add important information in the work-up of a glaucoma patient with a suspected narrow angle component.

Studies conducted across the United States show that nonglaucoma specialists often do not perform critical elements of the assessment of glaucoma patients. Most notably, these include gonioscopy, optic nerve assessment and optic nerve head documentation on a regular basis. Moreover, researchers found this to be the case among both comprehensive ophthalmologists/optometrists and glaucoma specialists. These studies appear to suggest that practitioners are under-utilizing gonioscopy in their assessment of glaucoma patients.

The new anterior segment devices could provide high quality, dynamic, reproducible images of the anterior chamber angle without touching the eye.

Assessing the anterior chamber angle

The utility of gonioscopy in the management of glaucoma is critical for an accurate diagnostic assessment. Although primary open-angle glaucoma is by far the most common form of glaucoma in the United States, it is a diagnosis we should reserve for patients who have had a thorough gonioscopic assessment to rule out secondary or contributing etiologies.

For instance, a practitioner could misdiagnose intermittent or chronic angle closure glaucoma (ACG) as chronic open angle glaucoma, which could lead to inappropriate treatment and exposure to medications that may not be necessary. Although there are several mechanisms responsible for ACG, most close (acutely, intermittently, or chronically) the anterior chamber filtration angle by the peripheral iris. This makes gonioscopy an essential element in the differential diagnosis of these and other forms of glaucoma.

Imaging the anterior segment angle

The Stratus OCT (Carl Zeiss Meditec), used clinically for the evaluation of the retina, employs a 0.8 nm wavelength that cannot penetrate the sciera. As such, it is not ideal for assessing the anterior segment. However, the anterior segment version, the SL OCT, uses a longer wavelength that produces clinically useful results. Preliminary clinical studies suggest that anterior segment OCT compares favorably with conventional gonioscopy in its ability to identify potentially occludable angles.