|Fig.1: Vitreomacular traction as depicted by OCT.|
|Fig. 2: Macular defect in Stargardt Disease (A) is characterized by macular atrophy on OCT (B).|
|Age-matched control data as a normal reading on Diopsys NOVA-VEP (Fig. 3) and an abnormal reading which may be early evidence of glaucoma (Fig. 4).|
|A normal reading from Diopsys NOVA-ERG (Fig. 5) and an abnormal reading showing reduced response (Fig. 6).|
OCT, VEP, and ERG have greatly enhanced our abilities to diagnose early glaucoma and retinal degenerations.
Most clinicians dread the patient whose vision they can’t correct when the etiology is not apparent. Is it a retinal problem? Is it an optic nerve dysfunction? Is it a visual pathway problem? Is the patient malingering?
Today’s optometrists have access to technologies that assess structure and function to aid in the diagnosis of disease or the lack thereof. There are three technologies that optometry should be using to assess structural abnormalities of the eye and functional abnormalities of the retina, optic nerve, and visual pathways. Knowledge of these technologies is important so that the eyecare professional can decide when a particular test needs to be performed. Some technologies that are now state-of-the-art are quickly becoming standard-of-care.
When OCT was first introduced in 2002, it was an amazing new technology that enabled quantitative assessment of the retinal layers and retinal nerve fiber layer. The quantification of this data allowed for follow-up over time to assess change. The original OCTs, which were time-domain or TD-OCT, were slower and of reduced resolution, but they were often good enough to determine if there were structural defects in the retina.
In 2005, Optovue, Inc., introduced the SD-OCT, which acquired more data points much faster, such that eye movements did not interfere with the quality of the image, and had twice the resolution. Since then, many other companies have followed suit. Now, some details that were missed on TD-OCT are clearly seen in SD-OCT. The introduction of so many of these systems has allowed the price range to be more realistic for optometry and many OD offices have an OCT device.
CLINICAL USE OF OCT
The clinical applications of OCT include detection of macular abnormalities such as macular edema, vitreo-macular traction (Fig.1), macular degeneration, epiretinal membranes, macular holes, outer retinal degenerations, like retinitis pigmentosa, and Stargardt disease (Fig. 2).
FUNCTION: VEP AND PATTERN ERG
The average optometric practice has access to a perimeter which enables the detection and follow-up of field loss due to retinal disease, optic nerve disease, and visual pathway disorders. Results of visual field testing are great for patient care when the results are reliable. And the right visual field program must be performed so as not to miss a field defect caused by ocular disease.
However, fields can sometimes be difficult to interpret due to the subjective nature of the test and, unfortunately, not all patients are good field test takers. The need for objective assessment of retinal, optic nerve, and visual pathway function is the “holy grail” in ocular disease management. Now, with the advent of more user-friendly VEP and ERG systems to assess retinal, optic nerve, and visual pathway abnormalities, the realm of objective testing in the optometric office is now a clinical reality. Testing requires the easy application of electrodes on the skin and on the scalp using an electrode paste and specially formulated application discs.
ROLE OF VEP
The Visual Evoked Potential (VEP) is a response generated from the occipital cortex when a patient looks at a pattern consisting of a black-and-white checkerboard that reverses at a specific rate. In order for the potential or response to be generated, all parts of the visual pathway from the cornea to the occipital cortex must be intact. Any condition that causes a reduction in vision would be expected to cause a reduction in the VEP.
VEP systems generate patterns at both low contrast and high contrast. The latency and the amplitude of the response is generated and compared with age-matched control data (Fig. 3). Low contrast responses have been shown to be generated by the magnocellular pathways, which are affected early in glaucoma; therefore, an abnormal response to low-contrast stimulation with a normal response to high-contrast stimulation in a glaucoma suspect with a variable or unreliable visual field may be objective functional evidence of early glaucoma (Fig. 4).
USE OF pERG
In addition to VEP, another objective testing modality, pattern ERG (pERG), is also available on some systems and is believed to assess the functional integrity of the ganglion cell layer. Reductions in responses in the pERG in a glaucoma suspect may be objective evidence of disease.
An additional application of the pERG is in the differentiation of retinal/optic nerve head abnormality from visual pathway abnormality beyond the eye. For example, a patient with reduced vision who has a reduced VEP response with a normal pERG response might be expected to have a lesion in the pathways leading to the occipital cortex beyond the eye, whereas a patient with optic neuropathy would be expected to have reduced VEP and pERG responses (Figs. 5 and 6).
The clinical applications of performing VEPs and pERGs in the optometric office include the following:
- Objective assessment of visual acuity, which is valuable in non-verbal patients and in the confirmation of suspected malingering and hysterical blindness due to psychiatric disorders.
- Early determination of retinal and/or optic nerve dysfunction.
- Early detection of optic nerve disorders, including glaucoma.
- Differentiation of retinal and optic nerve disorders from disorders of the visual pathways beyond the eye to the occipital cortex.
- Prognosis of treatment in amblyopia.
Commercial systems to perform VEPs and ERGs have been available for years. However, systems that have made this type of testing more “user friendly” have enabled this to become more of a clinical reality for many practices that wish to embrace this technology. The Diopsys performs pattern and flash VEPs and pattern ERGs (Figs. 3-6). Konan Medical USA, Inc. recently introduced the EvokeDx, which features patented isolated check VEP tests (icVEP). The icVEP is designed to elicit responses on the magnocellular-ON pathway, which is thought to be affected by glaucoma early in the disease process.
Sherry Bass is a distinguished clinical professor at the State University of New York State College of Optometry in Manhattan.
WHERE TO FIND IT:
Diopsys, Inc. •973-244-0622 •diopsys.com
Konan Medical USA, Inc. •949-521-7730 •konan-usa.com
Optovue Inc. •866-344-8948 •optovue.com