Sir,
I enjoyed reading the case report from Ogino et al.1 I agree that examining and treating patients with concurrent asteroid hyalosis and proliferative retinopathy is a significant challenge. I also agree that autofluorescence images and fluorescein angiography are much less affected by the presence of the vitreous opacities than fundoscopy and colour images.
The given reason that the wavelengths of light used to obtain these images are less affected by the asteroid bodies is however incorrect.
During biomicroscopy or colour photography light passes through the ocular media, reflects from the RPE/choroid, and exits through the ocular media into the imaging system. This is known as a double pass and is a multiplication not addition. The image quality is reduced by ‘media-squared’ not ‘media-doubled’.
In FFA and AF the light exiting the eye has its origin entirely in the posterior layers of the globe. The incident light is absorbed and molecule-bound electrons are raised into higher energy levels. The electrons make a transition to an intermediate energy level and new light is emitted with lower energy and longer wavelength. The new wavelength contains no information from the incident beam.
This light passes through the ocular media once and is therefore deemed ‘single pass’. The degradation of image quality is much less than in double pass.
In the case of asteroid hyalosis we see reflection of light and double pass with biomicroscope lenses or colour photography. With FFA and AF the asteroid hyalosis is much less apparent because the light originates in the retina and only passes out; any light reflected from the asteroid scatters back towards the retina and will not degrade our image.
It is good to understand the fundamental difference between single and double pass when examining the posterior segment; the quality of colour fundus images in the presence of media opacity can be poor but FFA images of the same eye much better.
The same knowledge can be useful when considering cataract surgery in a patient with concurrent macula disease. Our fundal view is double pass, whereas the patient sees a single pass. When I can see the retina clearly through a double pass of the cataract I tend to defer surgery.
References
Ogino K, Murakami T, Yoshimura N . Photocoagulation guided by wide-field fundus autofluorescence in eyes with asteroid hyalosis. Eye 2014; 28: 634–635.
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Davies, N. Comment on ‘Photocoagulation guided by wide-field fundus autofluorescence in eyes with asteroid hyalosis’: single and double pass of light in the ocular media. Eye 29, 149–150 (2015). https://doi.org/10.1038/eye.2014.215
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DOI: https://doi.org/10.1038/eye.2014.215