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Wide-field autofluorescence imaging – a game changing lifesaver

13/04/2018By Lewis Williams PhD
A UK expert on ultra-widefield imaging delivered an Optos-sponsored Australian webinar recently, explaining some of the finer details of autofluorescence imaging. LEWIS WILLIAMS reports.

In his ‘Autofluorescence Imaging: The Game Changer’ webinar, UK optometrist and early adopter of ultra-widefield (UWF) imaging Mr Simon Browning detailed the anatomy, physiology, and the biochemistry/photochemistry of the normal retina. He also described the effects of common retinal pathologies and age changes on the foregoing, and the changes in fundal appearance that result.

Bedford-based Browning has practised optometry for more than 30 years and has held positions with the UK’s College of Optometrists and Association of Optometrists. He is also an experienced lecturer on UWF imaging technologies, and in a previous webinar had showed that autofluorescence discloses and monitors changes in drusen over time, including those changes attributable to dietary and lifestyle changes.

This webinar presented the diagnostic and practice development aspects of UWF, especially autofluorescence. He described the autofluorescence feature, available in Optos UWF devices such as the Daytona, as a “practice game changer and lifesaver – literally.”


"The autofluorescence feature, available in Optos UWF devices such as the Daytona, is a practice game changer and lifesaver – literally."
Simon Browning, UK optometrist and early adopter of ultra-widefield (UWF) imaging

That claim is based on their ability to reveal early signs of retinal pathologies that first appear in the extreme periphery of the fundus, a retinal area particularly difficult to view using conventional instruments.

Traditional approaches use ophthalmoscopy, usually a binocular indirect instrument or a so-called wide-field fundus camera (45–60°) with or without the assistance of scleral indentation. However, subject to the occasional anatomical limitation (big noses, overhanging orbit margins, pathology, etc.), Optos devices offer one-take views of up to 200°.

As that coverage can reach past the fundal landmarks of the vortex veins, UWF imaging bears little resemblance to traditional imaging methods, is convenient, and reduces the need to create photomontages to offer wider coverage; although this is a feature of the device.

The live webinar was presented as an online lecture consisting of text slides supporting Browning’s voice component, along with numerous UWF autofluorescence images supporting the cases referred to. The key biochemical to autofluorescence is lipofuscin – present as fine yellowish-brown pigment granules that are composed of lipid-based residues of lysosomal digestion, they are electron-dense, and autofluoresce.

While they are present in many organs, in a retinal context at least, they are viewed as a cumulative barometer of retinal ‘wear and tear’ – an ‘ageing’ pigment, more apparent in those over 40 years of age. Bright autofluorescence is indicative of excess lipofuscin, whereas dull or absent autofluorescence indicates reduced or absent lipofuscin.

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The continual replacement of the photoreceptors’ active outer-segment discs (approximately an 11-day cycle) results in the breaking down of ‘used’ discs in the RPE (by phagocytosis). This leads to a local accumulation if the normal, active process of the passage of ions, water, and metabolic by-products towards the choroid is affected adversely.

The RPE is responsible for maintaining the neurosensory retina, as well as the retina-blood barrier by virtue of its tight intercellular junctions. Anything affecting the RPE, such as excess oxygen and ageing, can increase autofluorescence by an increase in lipofuscin.

Importantly, natural ageing leads to a natural increase in background autofluorescence (by 70 years of age, the RPE’s cell cytoplasm has reached about 70% of its lipofuscin capacity). Lipofuscin accumulation in the RPE can be debilitating thereby affecting its defence systems to the extent that AMD can result.

Optically, lipofuscin is best excited by light between 300–600 nm and it fluoresces as wavelengths between 480–880 nm.

Poor fundal autofluorescence (FAF) is suggestive of a compromised RPE (less lipofuscin) that can be due to geographic atrophy or an inherited retinal dystrophy. Enhanced FAF can be due to increased lipofuscin or a disease such as Stargardt disease or Best disease (vitelliform macular degeneration).

Essentially, FAF monitors RPE integrity but differences exist, e.g., FAF in diabetic retinopathy is different from that of AMD. Other conditions mentioned were rod-cone dystrophy (hypo-fluorescence), Plaquenil toxicity, Alzheimer’s and Parkinson’s diseases, schisis at the optic nerve head (OCT can confirm), central serous retinopathy (CSR), and vitreous traction (increased autofluorescence).

If lipofuscin is apparently ‘on’ the retina, a ‘leaking forward’ scenario, such as a tumour, needs to be considered.

The ease of ‘attending’ a webinar, the smooth presentation that results from editing the slide set beforehand, and the availability of the presenter to answer questions from the ‘audience’ at the end of their presentation, means that more such initiatives are likely to be offered by Optos and others.

WEBINAR VIDEO


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