Clinical examination of the optic nerve head

Pearls Manager: Andrew Tatham

Cup/disc ratio (CDR) depends on optic disc size. A large CDR in an eye with a normal large disc may be erroneously considered glaucomatous whereas a small CDR in an eye with a glaucomatous small disc may be erroneously considered normal. The use of CDR to classify patients is not recommended, instead attention should be focused on the neuroretinal rim.

The vertical dimeter of the optic disc can be measured at the slit lamp using a hand held high power convex lens. A narrow beam is used to measure the vertical disc diameter using the inner margin of the white Elschnig’s ring as the reference. A correction factor needs to be used depending on the magnification of the handheld lens.

EGS Guidelines, 4th edition, Page 54.

Reference: Healey PR, Mitchell P, Smith W, Wang JJ. Relationship between cup-disc ratio and optic disc diameter: the Blue Mountains Eye Study. Aus N Z J Ophthalmol 1997;25: Suppl 1:S99-101.