Bill Harvey: Optic disc assessment advice
Author: Bill Harvey
This week I have mostly been thinking about… optic disc measurement (Fast Show reference there, for the more mature reader).
Last week I helped run a workshop on retinal photography. Attendees had to interpret a number of images and then decide how the image might have been improved upon, either by changes to the initial camera settings or by image enhancement after capture.
Most people know that you can adjust the flash setting, useful where media is less than transparent, and the background light setting, helpful for darker pigmented retinae for example. For younger, large-pupiled patients, a reduced flash is preferred. Fewer were aware of how images themselves can be enhanced by altering intensity and colour channels. All image software offers this.
Fewer still were aware of the many tools available in most image software, such as help with measuring CD ratios or outlining lesions to help assess change over time. Bearing in mind continued evidence for inter-practitioner variation in the assessment of CD ratios, and knowing that photography is often a delegated function for which the buck stops with the practitioner, it is well worth playing with our cameras to make sure we are using them to their best capacity.
And talking about disc assessment, a new paper in last month’s American Journal of Ophthalmology has questioned the validity of the ISNT rule in deciding upon the health of a disc (whereby in a healthy disc the inferior neuroretinal rim appears thickest, then superior, then nasal then temporal).
‘The ISNT rule is only valid for about a third of disc photos and less than half of RNFL measurements in normal patients. ISNT rule variants, such as the IST and IS rule, may be considered, as they are valid in over 70% of patients.’ Until OCTs are ubiquitous, we should be measuring disc heights and remember Spaeth’s grading rules.