Normal Cup-to-Disc Ratio
The normal cup-to-disc ratio in a healthy eye averages approximately 0.3-0.4, with almost all normal eyes having a ratio of 0.7 or less. 1
Specific Normal Values by Measurement Method
- Hruby lens/biomicroscopic examination: Mean cup-to-disc ratio of 0.38 1
- Direct ophthalmoscopy: Mean cup-to-disc ratio of 0.25 1
- Upper limit of normal: Cup-to-disc ratios of 0.7 or greater should be viewed with suspicion for glaucomatous damage 1
The Hruby lens or biomicroscopic lens consistently provides slightly larger measurements than direct ophthalmoscopy (typically 0.1-0.2 difference), but should be used to determine the fullest extent of cup contour. 1
Critical Adjustment for Disc Size
Cup-to-disc ratio must be interpreted in the context of optic disc diameter, as disc size significantly affects what constitutes "normal." 2
- Small discs (1.1-1.3 mm): Median cup-to-disc ratio of 0.35, with 97.5th percentile at 0.59 2
- Large discs (1.8-2.0 mm): Median cup-to-disc ratio of 0.55, with 97.5th percentile at 0.74 2
- This represents a 0.2 increase in both median and 97.5th percentile values from small to large discs 2
One quarter of all optic discs fall into small or large categories, making disc size estimation essential for accurate interpretation. 2 A small cup in a small disc can indicate advanced glaucomatous damage, while a large cup in a large disc may be entirely normal. 3
Age Considerations
There is a tendency toward increasing cup-to-disc ratio with advancing age, though no strong linear relationship exists. 1
Suspicious Features Requiring Further Evaluation
Any of the following warrant glaucoma evaluation:
- Cup-to-disc ratio ≥0.7 1
- Vertical cup-to-disc ratio larger than horizontal ratio (present in only 10% of normal eyes, and rarely differs by ≥0.2) 1
- Disparity >0.2 between direct ophthalmoscope and biomicroscopic lens measurements 1
- Oval cups (present in only 9% of normal eyes) 1
Clinical Context: When Small Cup-to-Disc Ratio Matters
Patients with idiopathic intracranial hypertension have significantly smaller cup-to-disc ratios (average 0.13-0.14) compared to normal populations, which may lower the threshold for developing optic disc edema. 4 Similarly, a small cup-to-disc ratio is an established risk factor for non-arteritic anterior ischemic optic neuropathy. 4
Risk Factor for Glaucoma
A larger cup-to-disc ratio is an established independent risk factor for developing primary open-angle glaucoma in at-risk patients. 5 Progressive enlargement over time, particularly with elevated intraocular pressure, suggests glaucomatous cupping and requires immediate assessment including IOP measurement, gonioscopy, and evaluation for disc hemorrhages. 6