The Clinical Significance of Rod Intercept in the Management of AMD
By Timothy Earley, OD
As a primary care profession, optometry has long been keenly aware of the importance of monitoring trends in our patients and how they inform our management and treatment plans. For decades, optometrists have closely followed our patients’ intraocular pressure (IOP) and the pattern standard deviation (PSD) in visual field assessments in our management of glaucoma. Corneal thickness is closely followed in patients with form fruste or frank keratoconus with serial pachymetry readings. Closely following trends over time in a patient’s refraction allows the optometrist to pick up changes in eyeglass prescriptions that could indicate cataracts, macular edema, or diabetes-induced blood sugar changes. Needless to say, a great deal of our professional decision-making is centered around the analysis and interpretation of serial measurements obtained by diagnostic testing.
I believe that it is of utmost importance that optometry begins to look closely at functional testing that allows us to monitor changes in patients diagnosed with or at-risk for age-related macular degeneration (AMD). For many years, optometrists have followed the progression of AMD by describing changes in clinical signs that are able to be imaged. RPE changes, drusen, focal atrophy, and macular scars are observable with slit-lamp examination through a condensing lens. Fundus photography and a dilated fundus examination have been the primary means of assessing the health of the macula for many years. More recently, the use of OCT to evaluate the outer retinal layers has become more mainstream. These high-quality images allow observation of subtle clinical signs never before seen with standard imaging. However, neither of these devices, as useful as they may be, are able to analyze the functioning of the macular photoreceptors. They allow us to take a high-resolution look at the structure of the macular tissue, but they do not allow us any insight into its functioning.
In the autumn of 2018, I was fortunate to have the ability to add dark adaptometry testing to my primary care practice. Macular assessment by dark adaptometry allows me to evaluate the function of the rod photoreceptors in my patients. As we all know, the rods outnumber the cones in our macula by 9:1, and it has been well documented that rod photoreceptors are damaged earlier and more severely in AMD. Evaluating rod-mediated dark adaptation is now straightforward and easy to perform in the office: A bright light is presented to a defined area of the macula just superior to fixation. The bright light “bleaches” the specified macular tissue and creates an after image. In a healthy macula, the photoreceptors “bleached” by the light will recover from the bleaching event and return to normal sensitivity within 6.5 minutes. This normal sensitivity is representative of the rod’s ability to regenerate rhodopsin, which relies on the metabolic exchange of vitamin A from the choriocapillaris to the photoreceptors. Lipid deposits within Bruch’s membrane and between Bruch’s membrane and the RPE which can precede the appearance of clinically evident drusen, act as a barrier to the transport of vitamin A, slowing down the regeneration of rhodopsin resulting in delayed dark adaptation.
Here is where the measurement of rod-mediated dark adaptation, as characterized by the Rod Intercept (RI®), has clinical utility. As well as being shown to be the earliest biomarker of AMD, a prolonged RI has been found to be over 90% specific and sensitive for clinically evident AMD. In patients with an RI greater than 6.5, further testing and evaluation is scheduled.
Having a quantifiable measure (RI) and a highly specific and sensitive diagnostic tool have completely changed our approach to AMD management. In our practice, we’ve seen many patients with little to no clinically evident disease (RPE changes or drusen), yet they have very delayed dark adaptation times.
"Having a quantifiable measure (RI) and a highly specific and sensitive diagnostic tool have completely changed our approach to AMD management."
Our goal with all of these pre-clinical and early cases is to stabilize, or possibly reduce RI. In fact, we have seen dozens of patients whose RI times have improved 12-18 months after beginning a high-quality carotenoid supplement.
Why do we do this? It’s simple:
Early diagnosis and early intervention are extremely important. We aim to slow the progression of this potentially blinding disease in every patient who walks through our door.
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Practical Implementation of Dark Adaptation in Optometric Practice
This resource goes beyond explaining why dark adaptation is so important in optometric practice. Having already established the reasons why functional testing is essential, this year’s report outlines the practical strategies for how to make routine testing a reality in your own eye care practice.
About the Author
Dr. Earley joined Medina Vision Centre in 1998. He was born and raised in Honesdale, PA and earned his Bachelor’s degree in Biology/Pre-Med from the University of Scranton. He earned his Bachelor’s degree in Visual Science and his Doctorate in Optometry from the Pennsylvania College of Optometry. Dr. Earley is a national speaker and consultant on age-related macular degeneration, specialty contacts and practice management.