Imaging helps to identify retinal disease

September 15, 2020

Fundus autofluorescence may hold potential in diagnosing SARS-CoV-2 infection

ODs have several technologies available to assist in the diagnosis and management of patients’ ocular and visual conditions during the current novel coronavirus pandemic. One of these technologies is fundus autofluorescence (FAF). This article demonstrates the utility of using FAF technology to identify numerous fundus conditions and speculates on FAF use as a potential future tool for identifying SARS-CoV-2 infection in the eye.

Retinal imaging technology has taken great strides over the last several decades, and eyecare practitioners have the benefit of many different technologies to help diagnose and manage posterior segment eye disease. One of these technologies that has evolved over this period of time is fundus autofluorescence (FAF).

Since the introduction of human retinal fluorescein angiography in 1959 by Harold Novotny, MD, and David Alvis, MD, the ability to study, diagnose, and manage numerous retinal pathologies has expanded greatly. Fluorescein angiography (FA) requires intravenous injection of fluorescein sodium dye and uses special filters in the retinal camera to enhance the dye fluorescence.

Fluorescence consists of emission of longer wavelength of light or radiation from a substance as a result of a shorter wavelength light or radiation striking that substance. In the case of fluorescein angiography, the excitation wavelengths (that pass through the camera’s excitation filter) are in the ultraviolet/blue spectrum, and the emission wavelengths (that return through the camera’s barrier filter) are in the blue/yellow spectrum. Over the years these camera filters, optical systems, and sensors have become more refined to maximize the fluorescence effect for clinical use.1

A fortuitous side benefit of fluorescein angiography was noted even before injection of fluorescein dye: certain fundus structures demonstrated natural fluorescence through the filters without the need for an external dye (Figure 1). This autofluorescence is due to natural molecules, called fluorophores, within certain tissues. While fluorescein (and a related dye, indocyanine green) are exogenous (“outside of the body”) markers, fluorophores are endogenous (“inside the body”) markers that may represent normal physiology or pathology within the fundus tissue.

Macular degeneration

An important fluorophore in the retina is N-retinylidene-N-retinyl-ethanolamine (A2E), which is present in lipofuscin. Lipofuscin accumulation, in the form of retinal drusen, is a predominant sign of macular degeneration and several other retinal diseases.2

While normal retinal pigment epithelial (RPE) cells contain a moderate amount of lipofuscin (appearing uniform grey on an autofluorescence image), a metabolically-stressed RPE cell may produce much larger amounts of lipofuscin in the form of drusen. This drusen appears hyper-fluorescent (bright white) on an autofluorescence image. When an RPE cell reaches a point of atrophy and death, the result is no lipofuscin present, and the RPE atrophy area will appear hypo-fluorescent (dark black) on an autofluorescence image (Figure 2).

Autofluorescence is therefore especially valuable in diagnosing and managing both non-exudative macular degeneration consisting primarily of macular drusen, as well as the atrophic form of macular degeneration consisting of RPE atrophy regions.

RPE

While macular degeneration is a key eye disease that can be visualized with fundus autofluorescence, several other fundus pathologies may present with distinctive characteristics seen only with this imaging mode. Some examples include hereditary retinal diseases such as retinitis pigmentosa and Stargardt disease, toxic retinopathies from systemic medications such as hydroxychloroquine (Plaquenil, Sanofi Aventis) and pentosan polysulfate sodium (Elmiron, Janssen Pharmaceuticals, Inc.; Figure 3), and optic disc drusen (Figure 4).

Aside from optic disc drusen (which usually hyperfluoresces due to the hyaline-like calcifications that comprise the drusen bodies), almost all autofluorescence pathology in the retina focuses on changes in the RPE. An interesting similarity between fundus autofluorescence and the green (“red free”) filter used in fundus photography is that both imaging types do not penetrate deeper than the RPE.

For example, a choroidal nevus would be invisible in the photo using either imaging type. This phenomenon with either of these methods requires a normal, intact RPE. The RPE masks deeper structures due to the wavelength absorption of its intracellular contents. Compromise or loss of the RPE may therefore reveal deeper choroidal tissue, using either imaging method.

White-dot syndromes

An evolving use for fundus autofluorescence is in the diagnosis of subclinical (also termed “occult”) retinal conditions—that is, conditions that are not noted by readily-identified signs or symptoms. A category of occult retinal conditions that may present in the exam chair are termed the “white-dot” syndromes.

The white-dot syndromes are a group of multifocal chorioretinitis diseases that may present with white dots on the fundus as a key feature. The cause is often considered idiopathic, although a virus trigger is thought to be implicated.

Examples of white-dot syndrome diseases include acute posterior multifocal placoid pigment epitheliopathy (APMPPE), serpiginous choroiditis (Figure 5), birdshot retinopathy, multiple evanescent white-dot syndrome (MEWDS), presumed ocular histoplasmosis syndrome (POHS, Figure 6), multifocal choroiditis and panuveitis (MCP), punctate inner choroidopathy (PIC), and diffuse subretinal fibrosis (DSF).

Many white-dot syndrome diseases can manifest in young, otherwise heathy adults. While a common demographic for some conditions (such as MEWDS) is a young adult Caucasian myopic female, these syndromes can cross age, sex, and ethnicities (Figure 7).

Recent flu-like symptoms (fever, malaise, headaches) may be associated with certain white-dot syndromes and should be asked about in the patient history. This flu connection implicates a viral trigger to the chorioretinal signs and accompanying visual symptoms.3

While the primary virus suspected in some of the white-dot syndromes and other occult retinal conditions is of the herpes simplex or varicella zoster categories, other more exotic viruses have been shown to manifest chorioretinal pathologies that present with distinct autofluorescence patterns. Examples in the literature include human papilloma virus, cytomegalovirus, West Nile virus, Zika virus, and Ebola virus.4-6

Could the novel coronavirus (SARS-CoV-2) responsible for COVID-19 also be identified through retinal autofluorescence imaging?

COVID-19 and FAF

Currently conjunctivitis is known as a potential ocular finding from SARS-CoV-2 infection; however, the prevalence of this finding in comparison to other systemic signs and symptoms is quite low.7

Animal studies have shown retinal disease from coronaviruses,8 and a study published in May 2020 described cotton wool spots, microhemorrhage, and hyperreflective lesions in the inner nuclear layer and ganglion cell layer of patients with COVID19, identified using optical coherence tomography (OCT).9

These findings attest to the microvascular and neural disease processes that the virus has been shown to cause systemically. However, at the time of writing of this article, no distinctive chorioretinal autofluorescence patterns due to the novel coronavirus have been discovered.

The lack of any distinctive retinal autofluorescence biomarkers for SARS-CoV-2 infection does not mean that none exist. We are still early on, in the both the spread and clinical examination and testing of this virus on infected patients, and there is likely a limited amount of data gathered at this time.

Another strong reason for the limitation of new findings is the significant social distancing and protective measures both healthcare practitioners and society in general have implemented to prevent person-to-person viral transmission.10

It is also possible that retinal changes may occur during an asymptomatic phase in the course of the disease when the patient does not typically present for an eye examination.

Finally, not every eyecare practitioner currently utilizes retinal autofluorescence as part of their routine, or even medical-based, eye examinations.

Perhaps optometrists and ophthalmologists can serve an important role in screening for novel coronavirus infection through retinal autofluorescence imaging in the future, should distinct identifiable patterns emerge over time.

As more and more clinical data becomes available, eyecare practitioners may play a larger role on both diagnosis and management of this virus. Retinal autofluorescence could be a technology that assists to this end, and incorporating this as part of regular testing regimen may potentially yield useful information.

As eyecare professionals concerned about the visual, ocular, and overall well-being of our patients, optometrists should stay curious and vigilant to discover new biomarkers using the best technologies available.

References

1. Hurley BR, Regillo D. (2009) Fluorescein Angiography: General Principles and Interpretation. In: Arevalo JF. (eds) Retinal Angiography and Optical Coherence Tomography. Springer, New York, NY

2. Yung M, Klufas MA, Sarraf D. Clinical applications of fundus autofluorescence in retinal disease. Int J Retina Vitreous. 2016 Apr 8;2:12.

3. Wong E, Nivison-Smith L, Assaad NN, Kalloniatis M. OCT and Fundus Autofluorescence Enhances Visualization of White Dot Syndromes. Optom Vis Sci. 2015 May;92(5):642-53.

4. Yeh S, Forooghian F, Faia LJ, Weichel ED, Wong WT, Sen HN, Chan-Kai BT, Witherspoon SR, Lauer AK, Chew EY, Nussenblatt RB. Fundus autofluorescence changes in cytomegalovirus retinitis. Retina. 2010 Jan;30(1):42-50.

5. Manangeeswaran M, Kielczewski JL, Sen HN, Xu BC, Ireland DDC, McWilliams IL, Chan CC, Caspi RR, Verthelyi D. ZIKA virus infection causes persistent chorioretinal lesions. Emerg Microbes Infect. 2018 May 25;7(1):96.

6. Steptoe PJ, Scott JT, Baxter JM, Parkes CK, Dwivedi R, Czanner G, Vandy MJ, Momorie F, Fornah AD, Komba P, Richards J, Sahr F, Beare NAV, Semple MG. Novel Retinal Lesion in Ebola Survivors, Sierra Leone, 2016. Emerg Infect Dis. 2017 Jul;23(7):1102-1109.

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