From the Guidelines
To calculate the cup-disc ratio on fundoscopy, first identify the optic disc and cup during examination of the retina, and then determine the ratio by comparing the vertical diameter of the optic cup to the vertical diameter of the entire optic disc, typically using a slit lamp biomicroscope with a 78D or 90D lens, as described in the primary open-angle glaucoma suspect preferred practice pattern 1. When performing this assessment, it is crucial to ensure proper pupillary dilation for optimal visualization. The cup-disc ratio is determined by mentally measuring the vertical height of the pale central depression (cup) and dividing it by the total vertical height of the optic disc.
- A normal cup-disc ratio typically ranges from 0.3 to 0.4, with ratios greater than 0.5 potentially indicating glaucomatous changes, as evidenced by the primary open-angle glaucoma suspect preferred practice pattern 1.
- Other features such as disc hemorrhages, neuroretinal rim thinning, and asymmetry between eyes (a difference greater than 0.2 is suspicious) should also be noted during the examination. The cup-disc ratio is clinically significant because progressive enlargement of the cup relative to the disc suggests ongoing optic nerve damage, which is characteristic of glaucoma and requires further investigation and possible treatment, as highlighted in the study on primary open-angle glaucoma suspect preferred practice pattern 1.
From the Research
Calculating Cup Disc Ratio on Fundoscopy
To calculate the cup disc ratio on fundoscopy, the following steps can be taken:
- Measure the vertical cup-to-disc ratio (VCDR) by comparing the vertical diameter of the cup to the vertical diameter of the disc 2, 3
- Measure the horizontal cup-to-disc ratio (HCDR) by comparing the horizontal diameter of the cup to the horizontal diameter of the disc 2, 3
- Compare the measurements to established norms or baseline values to assess for any abnormalities or changes 2, 3
Factors to Consider
When calculating the cup disc ratio, consider the following factors:
- The method of measurement, as different methods (e.g. slit-lamp funduscopy, Heidelberg Retina Tomography II, stereoscopic optic nerve photos) may yield different results 2
- The presence of glaucomatous or ocular hypertensive conditions, as these may affect the accuracy of measurements 2, 3
- The use of optical coherence tomography (OCT) imaging and visual field testing to relate structure-function mapping and improve disease diagnosis and monitoring 4, 5
Clinical Applications
The calculation of cup disc ratio has clinical applications in:
- Diagnosing and monitoring glaucoma, including the use of structure-function mapping to relate OCT measurements to visual field test locations 4, 5
- Assessing the appearance of the optic disc on fundus photographs and classifying eyes based on the presence of signs suggestive of glaucomatous optic neuropathy 6
- Relating clinical features seen on fundus photographs to glaucomatous damage detected on visual fields and OCT scans 6