Rediscovering the Retina

5 Key Takeaways on the Retina’s Anatomy and Physiology

By Anne-Marie Lahr, OD 

“Never stop learning because life never stops teaching.”

I recently came across this quote and it perfectly ties into my overall approach to life and recent experiences. I’m an academic at heart – although I didn’t realize it when I first began my higher education! I received a BS in Biology from University of Akron, and then earned my Doctor of Optometry degree from The Ohio State University College of Optometry in 1991. I then completed an International Teaching Fellowship in Optometric Education as well as a Primary Care Residency at Salus University (Pennsylvania College of Optometry) in the mid 1990s and that was it – I found my passion in eye care and have loved sharing my knowledge through education. From being an assistant professor at Salus University to my position as Director of Education and Professional Relations at Hoya Vision Care, I’ve obtained a wide breadth and depth of experience in the eye care industry.

After 24 years of educating others about ophthalmic optics and lens design, I was aware that I would need to review and reacquaint myself with the anatomy and physiology of the retina when I joined MacuLogix as the Associate Director of Professional Relations. And, despite my years involved in all things eyecare-related, I still have much to learn from this constantly evolving field. After all, you never truly stop learning and willingness to continue your education – or revisit the basics – is what keeps us at our very best. As I began the process of brushing up on the retina, I quickly became aware that what we know about the retina and macula has evolved over that past 20 years! Below are my key takeaways as I dove into the research and rediscovered the retina:

1. Rods Outnumber the Cones in the Macula

Sometimes we eye care professionals think in generalities, recalling the rods are found in the periphery and cones dominate the macular area.  However, rods actually outnumber cones in the macula – in fact, there are 9 rods for every cone. Of course, the fovea is cone dominated, but the larger parafovea is rod-dominated. 

2. There is the Misconception that Visible Drusen are the Primary Lesions in AMD

In fact, the primary lesions associated with age-related macular degeneration (AMD) occurs within Bruch’s Membrane and the retinal pigment epithelium (RPE). These lesions are not visible with clinical imaging or research imaging in vivo. 

                        Figure 9. Layers of AMD pathology. (from Pikuleva and Curcio, 2014) 

3. The Primary Lesions in AMD Are Made of Cholesterol

And this is the same type of cholesterol that is found in the arteries of those with atherosclerosis. The morphology of the lesions is the same in the two diseases. The panmacular cholesterol – or, more specifically, basal laminar and basal linear deposits (BLamD and BLimD respectively) -– accumulates in Bruch’s Membrane and between the basal surface of the RPE cells and Bruch’s Membrane. 

4. Drusen Are All Connected

Really the visible drusen are just the tip of the iceberg. Histopathological studies show that drusen are an accumulation of the panmacular cholesterol deposits. As the cholesterol builds up, drusen become clinically visible. And even though they appear as individual spots on the macula, they are in fact just peaks of the invisible cholesterol layer. Just like an iceberg lurking under the water, there are plenty more drusen below what can be seen on the surface.

                                     Figure 2. Two small soft drusen connected by basal linear deposit. (from Spaide and Curcio, 2010) 

5. Night Blindness is the First Symptom of AMD

Cholesterol deposits cause localized Vitamin A deficiency that leads to impaired dark adaptation – difficulty adjusting from bright light to darkness. These “invisible” lesions of cholesterol cause Bruch’s Membrane to thicken, creating oxidative stress and inflammation that impedes the normal transport of nutrients from the choriocapillaris, including that of vitamin A. That, in turn, affects photoreceptor health and creates a localized vitamin A deficiency.

Research shows that dark adaptation function is compromised from the earliest stages of AMD and impairment increases as the disease progresses. In fact, impaired dark adaptation indicates the presence of AMD at least three years before drusen are visible upon clinical examination – making dark adaptation the canary in the coal mine for the detection of AMD. By understanding impaired dark adaptation and its connection to AMD, we will be much better equipped to tackle this progressive eye disease.

Learn more about dark adaptation now

These five takeaways are the cornerstones to understanding the retina and how it relates to age-related macular degeneration. Throughout my learning journey, it occurred to me that this was knowledge I had already obtained but had to brush up on to sharpen my skills. In short, every piece of knowledge acquired in optometry school is essential and being willing to continue your education as a professional – and revisit old topics – is the key to success.

Cheers,