01Purpose and principlesWhat the assessment is for and the core concepts behind it.
The optic disc is the visible exit point of retinal ganglion-cell axons. Its central cup is normal anatomy; glaucoma is identified through loss or change of neural tissue, supported by functional assessment and the broader examination. A numerical cup-to-disc ratio is a description, not a diagnosis. It can obscure focal rim loss in a small disc or make a large healthy disc appear falsely alarming.
Perimetry asks how well the visual system detects stimuli at selected locations. It is a functional test influenced by both disease and the way the person performs it. Optical coherence tomography provides structural measurements that are also vulnerable to acquisition and interpretation errors. Reliable diagnosis depends on the relationship among clinical disc examination, imaging, fields and longitudinal change. Agreement strengthens an interpretation; disagreement should lead to a reasoned check rather than automatic acceptance of the most striking printout.
Key points
- Assess the neuroretinal rim and disc size, not a cup-to-disc ratio in isolation; large normal discs can have large cups.
- A glaucomatous field pattern should make anatomical sense alongside the disc and retinal nerve-fibre findings.
- Read the test quality and raw results before accepting an OCT colour code or a visual-field summary index.
- Repeat uncertain perimetry when appropriate, correcting practical problems and considering learning, fatigue and refractive factors.
- Once a defect is established, use a consistent field-testing strategy to support comparison over time.
- Disproportionate pallor, impaired colour vision, unexpectedly poor acuity or a vertical-meridian field pattern should raise concern for another optic nerve or visual pathway disorder.
- Referral can be justified by convincing nerve or field damage even when Goldmann pressure is below 24 mmHg.
02Indications, selection and cautionsWhen it is useful, when urgency changes and important limitations.
Focal thinning or notching, a nerve-fibre defect or a disc haemorrhage may be significant even without a very large overall cup; document the location for comparison.
A broad cup in a large disc with a preserved rim and stable tests may represent normal anatomy; compare disc dimensions before interpreting inter-eye cup asymmetry.
Arcuate loss, a nasal step or paracentral loss can correspond to affected nerve-fibre bundles; inferior disc injury generally relates to superior field loss and vice versa.
Rim pallor greater than cupping, poor colour discrimination, an unusual vertical boundary or a rapid time course should prompt assessment for non-glaucomatous disease.
03Method and interpretationA systematic approach to the test and its findings.
Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.
- 01
Stereoscopic optic nerve examination with baseline imagingFirst step - Why
- Describe disc size, rim configuration, haemorrhage and the surrounding nerve-fibre layer.
- Interpretation and limitations
- Use a good clinical view and photographs or OCT where available; judge change against previous images rather than a single population-based cup threshold.
- 02
Standard automated threshold perimetry - Why
- Map light-detection sensitivity and establish the distribution of functional loss.
- Interpretation and limitations
- Inspect reliability, pointwise findings and the pattern across repeat tests; a single depressed global index cannot establish the cause or location of disease.
- 03
OCT with inspection of acquisition and segmentation - Why
- Measure nerve-fibre and ganglion-cell structure as an adjunct to examination.
- Interpretation and limitations
- Review scan quality, centration, segmentation and anatomical context before interpreting flagged thinning, especially with high myopia, tilted discs or concurrent retinal disease.
- 04
Pressure, gonioscopy and corneal thickness - Why
- Complete the glaucoma assessment and define pressure-related and anatomical context.
- Interpretation and limitations
- Damage may occur within usual pressure ranges; these tests identify risk and mechanism but do not convert an unreliable field or scan into a confirmed diagnosis.
- 05
Directed neuro-ophthalmic assessment when findings are atypical - Why
- Investigate an alternative optic nerve or visual pathway disorder.
- Interpretation and limitations
- History, colour vision, pupil responses and an appropriate neurological examination guide specialist-directed imaging or other tests; routine brain imaging is not needed for every large optic cup.
04Clinical next stepsHow the result changes management or prompts escalation.
01Structural assessmentDescribe what the optic nerve showsFirst stepAn optic disc appears cupped, asymmetric or otherwise suspicious during examination.+
- 1Establish the quality of the view and record disc size, focal rim shape, pallor, haemorrhage and relevant nerve-fibre findings in each eye.
- 2Obtain baseline photography or OCT when available and compare with previous images, checking that apparent differences are not simply changes in image scale or quality.
- 3Assess pressure, angle and functional testing, then refer or monitor through the appropriate pathway according to the combined findings and clinical urgency.
02Field interpretationCheck quality before identifying a patternA visual-field test contains an abnormal result or appears worse than an earlier test.+
- 1Confirm the correct eye, test strategy, refractive correction and clinical context, then inspect reliability and any technician observations about performance.
- 2Examine the distribution of depressed points and relate it to disc, nerve-fibre and retinal findings rather than relying only on the grey scale or mean deviation.
- 3AlternativeEscalationRepeat a doubtful non-urgent result with improved instruction or an appropriate alternative strategy, while escalating promptly if the overall evidence suggests serious or progressive disease.
03Discordant findingsResolve uncertainty without losing urgent diseaseThe scan, disc examination and field results fail to agree or show an atypical pattern.+
- 1Look for optical and acquisition explanations, including cataract, incorrect correction, eyelid obstruction, poor fixation and OCT segmentation error.
- 2AlternativeReassess the clinical history and examine for alternative retinal or optic nerve disease, considering neuro-ophthalmic input when pallor, colour loss or neurological features are disproportionate.
- 3Record what remains uncertain and arrange a defined repeat or specialist assessment; do not label unresolved contradictory data as stable glaucoma by default.
05Risks, monitoring and follow-upComplications, safety checks and further assessment.
- Use the same field measurement strategy for comparison after an established defect, recording any necessary change of machine or test pattern that affects interpretation.
- Compare serial structure and function over several reliable visits where possible; one unexpected test may need confirmation, whereas consistent deterioration requires treatment review.
- Repeat baseline imaging if a clinically important disc change is detected, preserving earlier images so the direction and timing of change remain visible.
- Adapt testing for concentration, mobility, language, hearing or cognitive difficulties; document limitations and select a clinically useful alternative rather than repeatedly obtaining unusable results.
- Escalate suspected progression near fixation or in an only-seeing eye promptly, even when the measured pressure appears acceptable and central acuity remains good.
06Special situationsVariants, exceptions and circumstances that change the usual approach.
The rim is tissue and the cup is space
A cup ratio compresses a three-dimensional structure into one number. Two discs with similar ratios may have very different amounts and distributions of remaining neural tissue. Describing a focal inferior notch and its corresponding field defect is more informative than reporting only a large cup.
Understand what threshold testing measures
The perimeter estimates the dimmest stimulus detected at each location. The person should fixate centrally, respond only when a light is seen and know that some stimuli are deliberately difficult to detect. Reassurance, comfortable positioning and an opportunity to pause can improve the usefulness of the result without coaching the answers.
Reliability flags require interpretation
Frequent false-positive responses can make sensitivity appear better than it is, while fatigue or inconsistent responses can produce misleading depression. False negatives can also increase with genuine advanced disease. Reliability numbers therefore support clinical judgement; they should not function as a universal automatic acceptance or rejection rule.
Global indices can hide local harm
Mean deviation summarises overall sensitivity loss, which may be influenced by diffuse optical effects such as cataract. A small paracentral defect can be important despite a relatively modest global change. Read the location of damage and its relationship to fixation, and ask whether the test pattern samples the threatened area sufficiently.
OCT colours are comparisons with a database
A statistically unusual thickness is not synonymous with disease, and a result within the reference range does not guarantee health. Myopic anatomy, segmentation faults and the choice of normative population can affect classification. Inspecting the actual scan and following a credible within-person trend reduces dependence on a coloured summary.
Advanced disease changes the measurement problem
When substantial nerve-fibre loss is already present, structural measurements may have less room to show further thinning. Functional assessment and the remaining central field become especially valuable. Conversely, early structural change can precede a reproducible field defect, so absence of perimetric loss should not erase convincing documented nerve change.
07Common pitfallsFrequent interpretation and management errors.
- 01
Diagnosing glaucoma from a large cup-to-disc ratio without considering disc size and rim tissue.
- 02
Accepting a red OCT sector as disease without examining the scan or considering myopic anatomy.
- 03
Calling a poor-quality single field progression and escalating treatment before checking the result and clinical risk.
- 04
Ignoring a reproducible focal defect because the global field index or visual acuity remains relatively good.
- 05
Reassuring on the basis of normal pressure despite convincing optic nerve or visual-field abnormalities.