Optic Disc, Optic Cup, and Normal Cup-to-Disc Ratio
The normal cup-to-disc ratio (CDR) is approximately 0.3 to 0.4, with values of 0.7 or less considered within normal limits, though this must be interpreted in the context of optic disc size. 1
Anatomical Definitions
The Optic Disc
- The optic disc is the visible portion of the optic nerve head where retinal nerve fibers exit the eye. 2
- It represents the anatomical structure through which all visual information travels from the retina to the brain 2
- The vertical disc diameter typically measures approximately 1.8 mm in emmetropic eyes 3
The Optic Cup
- The optic cup is the central depression or excavation within the optic disc where there are no nerve fibers. 4, 1
- This cup represents the area where the retinal vessels enter and exit the eye 1
- The cup size is physiologically related to disc size and pathologically increases with glaucomatous damage 5
Normal Cup-to-Disc Ratio Values
Standard Reference Values
- Using biomicroscopic examination (Hruby lens), the mean CDR in normal eyes is 0.38 1
- Using direct ophthalmoscopy, the mean CDR is slightly lower at 0.25 1
- Almost all normal eyes have CDR values of 0.7 or less, regardless of examination method 1
Critical Thresholds for Suspicion
- Any CDR of 0.7 or greater should be viewed with suspicion for glaucoma 1
- Any vertical CDR larger than the horizontal CDR warrants concern 1
- Only 9% of normal eyes have ovoid cups, and vertical predominance is rare 1
Clinical Significance and Context
Disc Size Dependency
- CDR must be interpreted relative to optic disc size, as larger discs physiologically have larger cups 2, 5
- The relationship follows the equation: CDR = (-1.31 + (1.194 × DD))/DD, where DD is disc diameter 5
- Small discs with seemingly "normal" CDR values may actually harbor glaucomatous damage 5
Risk Assessment
- A larger cup-to-disc ratio is an established risk factor for developing primary open-angle glaucoma 2
- Risk calculators for glaucoma progression incorporate vertical CDR as a key variable alongside age, pattern standard deviation, central corneal thickness, and IOP 2
- Progressive enlargement of the CDR indicates glaucomatous optic nerve damage and conversion from suspect to definite glaucoma 2
Clinical Pitfalls to Avoid
Examination Method Matters
- Different examination methods yield different CDR values—biomicroscopy consistently gives slightly larger values than direct ophthalmoscopy 1
- Clinicians should document which method they use, as apparent changes in CDR may simply reflect different measurement techniques rather than true progression 6
- Any disparity of more than 0.2 between direct ophthalmoscopy and biomicroscopy estimates should raise suspicion 1
Age Considerations
- There is a tendency toward increasing CDR with advancing age in normal individuals 1
- This physiologic increase must be distinguished from pathologic enlargement due to glaucoma 1