For successful diagnosis and management of patients with glaucoma, optometrists must understand the structure-function relationship as the disease progresses and be proficient at performing structural and functional exams using imaging devices that provide reproducible, objective measurements.
Structural changes in early glaucoma can be difficult to distinguish, and loss of visual function is often subtle and difficult for patients to appreciate or even notice. Additionally, there can be a significant structural change in the nerve fiber layer prior to a patient being able to detect if using a visual field test, explained Dr. Lonsberry, clinic director and professor, Pacific University College of Optometry, Forest Grove, OR.
The optic disc often changes before the visual fields, with change being relatively late in the disease process. The retinal nerve fiber layer (RNFL), however, usually changes before either the visual fields or the optic disc. This sequence of events makes an objective assessment of both the RNFL and the optic nerve head an important barometer for glaucoma assessment.
"What we're looking for is some type of objective assessment, something that's quantifiable, repeatable, easy to take, and doesn't take an hour to do," Dr. Lonsberry said.
Imaging devices such as SLPs (GDx Scanning Laser Polarimetry, Carl Zeiss Meditec Inc.) and OCTs (Cirrus and Stratus OCTs, Carl Zeiss Meditec Inc.) are automated tools for differentiating normal versus glaucoma-tous eyes before visual field loss is apparent. They provide quantitative measurement of the RNFL, with comparison to a normative database. Tests can often be performed without pupil dilation, and data are available immediately.
The basic principle of the SLP is that the amount of retardation of a polarized beam of light by the RNFL is directly proportional to the RNFL thickness. A thicker RNFL increases the retardation of polarized light. RNFL measurements are highly reproducible-within 2 u to 4 u-and are highly correlated with threshold perimetry.
There are two SLP models; both have features designed to address the fact that other structures in the eye have the same lamellar structure as RNFL and retard polarized light. When using the variable corneal compensator (VCC) model, a macular scan is performed first to eliminate corneal birefringence; therefore, what retardation of the polarized light that occurs in the second scan around the optic nerve is due to the RNFL.
The second SLP model (GDxPRO, Carl Zeiss Meditec Inc.) has enhanced corneal compensation (ECC). In eyes with poor signal-to-noise ratio, ECC images are less susceptible to measurement noises than VCC images. ECC also improves the measurement sensitivity in eyes with advanced glaucoma, potentially extending the dynamic range of the VCC system for tracking the disease, Dr. Lonsberry said.
The printout of results from these scans shows a variety of information. The most sensitive parameter for discriminating between normal and glaucomatous eyes is the nerve fiber index (NFI), Dr. Lonsberry said. This measurement will be raised on both focal and diffuse RNFL loss. A NFI of 0 to 30 is considered normal, 31 to 50 is considered suspicious, and above 50 is considered abnormal.
Clinical interpretation of SLP measurements is based on temporal, superior, nasal, inferior, temporal parameters. Summary measures are based on the calculation circle, and values outside normal are color-coded based on the probability of normality. Data include thickness values, standard deviation, and inter-eye symmetry.
Imaging with OCTs is an alternative to the SLP. In Dr. Lonsberry's view, the SLP potentially does a better job in analyzing the RNFL, but OCT has more versatility, such as scans of the optic nerve and the retina.
One OCT model (Cirrus) provides imaging and measurement of glaucoma and retinal disease; features include RNFL analysis, optic nerve head analysis, and macular thickness analysis.
GPA analysis is available on another OCT model (Cirrus HD-OCT, Carl Zeiss Meditec Inc.); the first two scans are used as a baseline, and subsequent exams will be compared to them. A color-coded sub-pixel map demonstrates change from baseline. The baseline can be changed, for example reset to the start of medication use so that the clinician can monitor the effects of treatment.
"This is the way we should be analyzing not only visual fields, but neuroretinal images. A comparison is made at every point with respect to the previous fields or scans, and any changes from the previous ones are highlighted," Dr. Lonsberry said.
This OCT model also provides change analysis with macular scans that utilize an automatic fovea finder. Macular thickness is compared to an age-matched normative database as indicated by a stoplight color code. The macular change analysis provides visual and quantitative comparison of two patient scans.
Dr. Lonsberry also pointed out that spectral-domain OCT (SD-OCT) is one of the devices now recommended for routine screening of chloroquine and hydroxychloroquine retinopathy. The American Academy of Ophthalmology recently released revised screening guidelines that reflect the development of new screening tools and new knowledge about the prevalence of toxicity.
The recommendations suggest that screening be performed with an objective test such as SD-OCT, multifocal electroretino-gram, or fundus autofluorescence, along with 10-2 automated visual fields. Dosing recommendations have also changed.1
1. Marmor, M, Kellner U, Lai TYY, Lyons JS, Mieler WF. Revised recommendations on screening for chloroquine and hydroxychloroquine retinopathy. Ophthalmology. Feb 2011;118(2):415-422.
Blair Lonsberry, OD, MEd, FAAOE-mail: email@example.com
Dr. Lonsberry is a lecturer for Alcon, Carl Zeiss Meditec, Inspire Pharmaceuticals, and Ista Pharmaceuticals.