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Automated structural technologies can detect changes in the optic nerve head and the retinal nerve fiber layer associated with glaucoma before perimetric testing.
Dr. Rhee, associate chief, Department of Ophthalmology, Massachusetts Eye and Ear Infirmary, Harvard Medical School, Boston, described the advantages and disadvantages of various imaging devices for use in diagnosing and following patients with glaucoma.
Structural testing is used to diagnose glaucoma early and to observe changes in eyes with glaucoma. In both frameworks, the patient benefits.
The "fly in the ointment," however, is that about 35% to 40% of ganglion cells are lost before the appearance of the first visual field defect on automatic achromatic (i.e., white-on-white) visual field perimetry, he explained.
The key, then, is to detect signs that indicate that the ganglion cells are dying before the insult is clinically apparent, the concept of pre-perimetric glaucoma. To do this, investigators will have to be able to recognize structural signs in the optic nerve or the RNFL that point to future damage, "that is, damage in a so-called 'normal' visual field test [Humphrey, Carl Zeiss Meditec]," Dr. Rhee said.
"It is important to note that in both the Ocular Hypertension Treatment Study and the European Glaucoma Prevention Study, [in] at least 50% of cases, ocular hypertension will progress by visual field changes first," he said.
Optic nerve head imaging
A topographic map of the optic nerve can be obtained using a confocal scanning laser ophthalmoscope (Heidelberg Retina Tomograph [HRT], Heidelberg Engineering) and optical coherence tomography (OCT).
"It is important to know that the two technologies are not interchangeable," he said. "The two machines used to measure the same optic nerve head provide two different readings."
He added that using a topographic map from OCT is very limiting because it does not have a normative database, making HRT technology, with its three iterations, a better choice.
HRT instrumentation uses scanning diode laser light to reflect light back and is based on the speed of recovery of the reflected light to generate a topographic map.
"We know that this instrument can discover pre-perimetric glaucoma-there are numerous publications," he said.
The latest HRT generation has a large normative database that includes specific ethnic data as well as Moorfields regression analysis and glaucoma probability score software.
Imaging of the RNFL
The RNFL can be measured using scanning laser polarimetry (SLP) (GDx, Carl Zeiss Meditec) and time-domain OCT (TD-OCT) (Stratus, Carl Zeiss Meditec). RNFL measuring devices also are very useful for detecting pre-perimetric glaucoma, but are rather limited for detecting progression.
OCT4, which is spectral-domain OCT (SD-OCT), is manufactured by about six companies, some with normative databases, Dr. Rhee said. They have a faster acquisition time that should decrease variability and improve its ability to detect anatomic progression.