Cup-to-Disc Ratio: Assessment and Common Pitfalls

Cup-to-disc ratio describes the size of the central cup relative to the optic disc, and it is a basic measure in glaucoma assessment.

The number depends on the size of the disc, so a single value can mislead.

Therefore, interpret it together with the rim, the disc size and the other eye.

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What the Cup-to-Disc Ratio Measures

The cup is the pale central depression where the nerve fibers leave the eye.

The disc is the whole nerve head, including the rim of neural tissue.

Clinicians usually record the vertical ratio, because glaucoma tends to elongate the cup vertically.

Also, a value from 0.0 to 1.0 summarizes the shape in a single number.

Normal Cup-to-Disc Ratio Ranges

Most healthy eyes have a vertical ratio between 0.3 and 0.4.

The upper limit of normal is about 0.6 in most populations.

Armaly showed that cup size is largely inherited.

Therefore, a patient with a large cup may have a family with large cups and no disease.

Disc Size and the Cup-to-Disc Ratio

Large discs have larger cups, and small discs have small cups.

A ratio of 0.6 can be normal in a large disc and abnormal in a small one.

Garway-Heath and colleagues showed that relating the ratio to disc size improves the assessment.

Because disc size varies a lot, estimate it with the lens you use.

How to Examine the Disc

Dilate the pupil and use a slit lamp with a handheld lens such as a 78 or 90 diopter lens.

A stereoscopic view shows the contour of the cup and the rim.

Also, the direct ophthalmoscope gives a magnified but monocular view.

Record the cup and disc sizes in each eye, and note any notching or hemorrhage (see optic disc hemorrhage).

Asymmetry

A difference of more than 0.2 between the eyes raises suspicion.

Asymmetry can be physiologic when the discs differ in size.

Also, compare the rim and the field before you diagnose disease.

Therefore, asymmetry is a trigger for more tests and not a diagnosis.

Where the Cup-to-Disc Ratio Misleads

Observers differ a lot when they estimate the ratio.

Tilted discs and myopic discs distort the apparent cup (see tilted disc).

Also, glaucoma can damage the rim without enlarging a small cup much.

Because of this variability, follow the rim and photographs instead of a single number.

Cupping Versus Pallor

Glaucoma produces cupping with a loss of the rim.

Other optic neuropathies produce pallor with little cupping (see optic atrophy).

Also, compressive and ischemic lesions can produce cupping in some patients.

Therefore, look at the color of the rim as well as the shape.

Using the Cup-to-Disc Ratio in Follow-Up

Document the disc with photographs and optical coherence tomography.

Also, compare the images over time to detect change.

Because progression is what matters, the baseline is valuable (see primary open-angle glaucoma).

Photography and Imaging of the Cup

Disc photographs give a permanent record for comparison.

Optical coherence tomography adds objective measures of the rim and the retinal nerve fiber layer.

Also, stereo photographs allow a better view of the contour.

Therefore, store images at baseline and at regular intervals.

Rim Assessment

The rim shape matters more than the ratio.

Inferior and superior rim thinning, notching and baring of vessels signal glaucoma.

Also, the ISNT rule describes the normal rim pattern: inferior, superior, nasal, then temporal.

Because the rim reflects nerve fiber loss, examine it in each quadrant.

Race and Age

Average disc and cup size varies with ethnicity.

People of African descent have larger discs and larger cups on average.

Also, aging does not change cup size much in healthy eyes.

Therefore, apply norms with care.

Large Cups in Children

Large cups in children may reflect high pressure or congenital glaucoma.

Reversal of cupping can occur after pressure control.

Also, physiologic cups in children are small.

Therefore, take enlarged cups in children seriously.

Communicating the Finding

Explain the ratio in plain terms and state that it varies.

Avoid labeling a patient with a large cup as having glaucoma without more evidence.

Also, explain the plan for follow-up and tests.

Practical Tips at the Slit Lamp

Use a narrow beam and a stable lens, and adjust the focus to the contour of the cup.

Estimate the disc size first, and then judge the cup against it.

Also, look at the vessels, since the kink of a vessel marks the cup edge.

Because small details change the estimate, practice with a few photographs.

For the latest studies, see the PubMed results for cup-to-disc ratio.

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References

1. Armaly MF. Genetic determination of cup/disc ratio of the optic nerve. Arch Ophthalmol. 1967;78:35-43.
2. Lichter PR. Variability of expert observers in evaluating the optic disc. Trans Am Ophthalmol Soc. 1976;74:532-572.
3. Garway-Heath DF, Ruben ST, Viswanathan A, Hitchings RA. Vertical cup/disc ratio in relation to optic disc size: its value in the assessment of the glaucoma suspect. Br J Ophthalmol. 1998;82:1118-1124.
4. Weinreb RN, Aung T, Medeiros FA. The pathophysiology and treatment of glaucoma: a review. JAMA. 2014;311:1901-1911.