Goldmann Applanation Tonometry
Principle · Technique · Errors · Clinical Applications
Examination question · ~800 words
What is Goldmann applanation tonometry? Discuss the Imbert–Fick principle and Goldmann's modifications, the instrument components and technique, sources of error and their clinical correction, and the diagnostic and monitoring role of IOP measurement in glaucoma management.
Try to outline your answer mentally before expanding sections below.
Goldmann Applanation Tonometry (GAT) remains the gold standard for measuring intraocular pressure (IOP) in clinical practice, against which all other tonometers are validated. Introduced by Hans Goldmann in 1954, it is based on the modified Imbert–Fick principle and is integral to glaucoma diagnosis, monitoring, and management. A thorough understanding of its optical principle, technique, sources of error, and clinical interpretation is mandatory for the postgraduate ophthalmology candidate.
The Ideal Law and Goldmann's Modifications
Original Imbert–Fick Law: For an ideal, dry, infinitely thin, perfectly flexible sphere, the pressure inside equals the force required to flatten a given area divided by that area: P = F / A where P = internal pressure, F = applanating force, A = area of applanation.
The cornea, however, is not an ideal sphere — it has finite thickness and rigidity (which resists flattening) and a tear film that attracts the tonometer (surface tension forces). At an applanation diameter of 3.06 mm, Goldmann found empirically that these two opposing forces cancel each other out, rendering the measurement accurate.
| Force | Effect on IOP Reading | At 3.06 mm Diameter |
|---|---|---|
| Corneal rigidity | Overestimates IOP (resists applanation) | Cancels surface tension effect |
| Tear film surface tension | Underestimates IOP (pulls probe down) | Cancels corneal rigidity effect |
| Net error at 3.06 mm | Zero — forces cancel perfectly | GAT reading = true IOP |
The magic number 3.06 mm is not arbitrary — it is the diameter at which the net error due to corneal rigidity and surface tension is zero. At smaller diameters, surface tension dominates (underestimates); at larger diameters, rigidity dominates (overestimates).
The Goldmann tonometer tip is a biprism — a specially designed optical element that splits the circular contact area into two semicircles to facilitate measurement under the slit lamp.
- Plastic biprism: Material optimised for optical clarity and durability
- Offset orientation: Positioned at 60° from horizontal, so fluorescein mires are seen as two arcs
- Applanating surface: Outer diameter 7 mm; actual contact zone diameter 3.06 mm
- Counterbalanced arm: Allows smooth horizontal movement with calibrated dial reading (0–10 g force equivalent to 0–100 mmHg IOP)
- Scale conversion: Dial reading × 10 = IOP in mmHg (e.g., dial reading 1.6 = 16 mmHg)
Step-by-Step Examination Protocol
- Patient preparation: Instil topical anaesthetic (proparacaine 0.5% or benoxinate 0.4%) and sodium fluorescein dye (2% drops or fluorescein-impregnated strip moistened with saline)
- Slit lamp setup: Use cobalt blue filter; illuminate with broad, oblique beam from temporal side at ~60° angle
- Biprism orientation: White mark on tonometer should be at 180° (or at red mark at 43° for regular astigmatism >3 D to bisect the angle between principal meridians)
- Start at low force: Dial set to 1 g to avoid corneal indentation artefact
- Approach cornea: Advance biprism under slit-lamp guidance until it just touches cornea centrally; patient looks straight ahead
- Visualise mires: Two semicircular (D-shaped) arcs of fluorescein are visualised; inner edges should just touch for correct applanation
- Adjust dial: Rotate dial until inner margins of the two semicircles just kiss (overlap at single point); read dial and multiply by 10
- Record readings: Take minimum of 3 readings and average; note time of measurement; check both eyes
| Mire Appearance | Interpretation | Corrective Action |
|---|---|---|
| Mires too wide (large semicircles) | Excess fluorescein or excessive pressure applied | Widen spacing between mires; blot excess dye with tissue |
| Mires too narrow or incomplete | Insufficient fluorescein or inadequate contact | Add more fluorescein; reapply strip |
| Inner edges just touching / kissing | Correct applanation — endpoint reached | Read dial value and multiply by 10 |
| Pulsating mires (oscillating) | Ocular pulse present — normal physiological finding | Take reading at midpoint of pulsation; average peak and trough |
| Mires not level or misaligned | Incorrect biprism rotation or corneal astigmatism | Adjust biprism rotation; rotate to 43° position if astigmatism >3 D |
A. Central Corneal Thickness (CCT) — Most Clinically Significant Error
GAT is calibrated for a CCT of 520 µm. This is the most important source of systematic error in clinical practice, with direct implications for glaucoma diagnosis and risk stratification.
| CCT Range | Effect on GAT | Clinical Implication |
|---|---|---|
| Thick (>560 µm) | Overestimates IOP (stiff cornea resists applanation) | May falsely label patient as glaucoma suspect / ocular hypertension |
| Thin (<480 µm) | Underestimates IOP (less mechanical resistance) | May miss true glaucoma; NTG patient may actually have higher-than-measured IOP |
| Normal (520 µm) | No systematic error | GAT reading most accurate |
| Post-LASIK/PRK | Significantly underestimates IOP (altered biomechanics) | Use non-contact methods or CCT-corrected nomograms |
Correction rule of thumb: For every 10 µm deviation from 520 µm, IOP changes by approximately 0.5 mmHg (Ehlers nomogram) or ~0.7 mmHg (Doughty & Zaman). A thin cornea indicates true IOP is higher than the GAT reading; a thick cornea indicates true IOP is lower than the GAT reading.
B. Corneal Curvature
- Steep corneas (small radius of curvature) → overestimate IOP
- Flat corneas → underestimate IOP
- Usually a minor error compared to CCT, but cumulative when combined with other factors
C. Corneal Astigmatism
- Regular astigmatism >3 D: Creates elliptical rather than circular contact area → IOP measurement error
- Correction technique: Rotate biprism to 43° position (theoretical midpoint between two principal meridians)
- Residual error: Even after this correction, approximately 1 mmHg underestimation per 4 D of astigmatism persists
D. Fluorescein Concentration
- Excess fluorescein: Wide mires → overestimation of IOP (endpoint appears earlier)
- Insufficient fluorescein: Thin or missing mires → underestimation
- Ideal mire width: Each arc should occupy approximately 10% of the mire diameter
E. Scleral / Ocular Rigidity
- High rigidity (nanophthalmos, older patients, myopic shift with age): Overestimates IOP
- Low rigidity (high myopia, large eyes): Underestimates IOP
- Important consideration when interpreting tonography findings and in post-LASIK patients
F. Valsalva Manoeuvre / Breath-holding / Tight Collar
- Venous congestion: Transient IOP elevation → artificial overestimation
- Clinical practice: Always measure IOP with patient relaxed; ensure collar/necktie is loose
- Timing: Avoid measurement immediately after straining or coughing
G. Calibration Error
- GAT should be recalibrated regularly using the calibration weight provided
- If arm rests at positive (+) end when unloaded: Spring force too high → overestimates IOP
- If rests at negative (−) end: Spring force too low → underestimates IOP
- Recommended frequency: Calibrate before each clinic session or whenever accuracy is in doubt
- A 5–10% error in calibration results in ±2 mmHg IOP measurement error
H. Corneal Disease & Pathology
- Oedematous cornea (Fuchs endothelial corneal dystrophy, acute angle closure): Softened cornea → underestimates IOP
- Corneal scarring / opacity: May prevent proper applanation; measurement unreliable
- Post-keratoplasty: Graft-host interface irregularity may affect measurement
| Overestimation of IOP | Underestimation of IOP |
|---|---|
| Thick CCT (>520 µm) | Thin CCT (<520 µm) |
| Steep corneal curvature | Flat corneal curvature |
| Excess fluorescein | Insufficient fluorescein |
| High scleral rigidity | Low scleral rigidity (high myopia) |
| Valsalva / tight collar / straining | Post-refractive surgery (LASIK/PRK) |
| Calibration error (spring too strong) | Corneal oedema (FECD, acute ACG) |
| Excessive pressure applied by examiner | Breath-holding or neck compression |
| Tonometer Type | Principle | Key Advantages | Limitations |
|---|---|---|---|
| Goldmann AT (Gold Standard) | Applanation (Imbert–Fick) | Most accurate; widely validated; clinical reference standard | Requires slit lamp; CCT-dependent; contact method (need anaesthetic) |
| Non-contact (Air puff) | Applanation via air jet | No anaesthetic required; rapid screening; patient-friendly | Less accurate than GAT; overestimates in corneal irregularity |
| Perkins AT | Same as GAT | Portable; handheld; useful in supine patients, children, OR | Observer-dependent; less precise than slit-lamp GAT |
| iCare (Rebound) | Rebound tonometry; probe velocity | No anaesthetic; portable; self-tonometry possible | Less accurate in extreme IOP ranges; CCT-dependent |
| Pneumotonometer | Air-jet with silicon tip | Useful in irregular corneas (post-graft); soft contact lenses | Requires calibration; limited availability |
| Tono-Pen | Electronic applanation | Portable; works on irregular surfaces | Variable accuracy; requires multiple readings for reliability |
| Pascal DCT (Dynamic Contour) | Dynamic contour tonometry | Theoretically CCT-independent; measures ocular pulse amplitude (OPA) | Expensive; less widely available; requires validation |
| ORA (Ocular Response Analyser) | Air-jet + corneal hysteresis measurement | Assesses corneal biomechanics; useful post-LASIK | Expensive; research tool; limited clinical utility |
Normal IOP Range
Normal IOP: 10–21 mmHg (population mean ~15.5 mmHg). The statistical upper limit of normal is 21 mmHg (2 SD above mean). However, this is a statistical construct, not a pathological threshold — glaucomatous damage can occur below 21 mmHg.
Normal Tension Glaucoma (NTG): Approximately 25–40% of POAG patients have NTG (IOP ≤21 mmHg) in Western populations, commonly taught as 'approximately one-third'. The earlier literature citing 5–10% is a significant underestimate and is inconsistent with the Collaborative NTG Study, Baltimore Eye Survey, and current textbook consensus.
Diurnal Variation of IOP
- Peak IOP: Typically occurs in early morning (6–8 AM); troughs in evening/night
- Normal fluctuation: <5 mmHg over 24 hours is acceptable; >8 mmHg fluctuation is suspicious for glaucoma
- Clinical relevance: In glaucoma suspects, IOP phasing (measurement every 2–4 hours over 24 hours) reveals true peak pressure
- Post-LASIK consideration: Nocturnal IOP measured in prone position may be significantly higher than daytime sitting GAT reading; affects interpretation of post-operative IOP control
Always measure IOP at the same time of day in glaucoma follow-up to eliminate diurnal bias and permit accurate longitudinal comparison. A reading at 9 AM cannot be directly compared to a follow-up reading at 5 PM in the same patient.
The GAT must be calibrated regularly using the calibration check weight provided with the instrument. This weight is a standard reference used to verify correct mechanical balance of the tonometer arm.
| Test Weight Applied | Expected Arm Position | Interpretation if Abnormal |
|---|---|---|
| 0 g (no weight) | Arm should rest at the 0 mark | If displaced → abnormal spring tension |
| 1 g weight | Arm should move to the +/− check mark | If not — calibration error present |
| 2 g weight | Arm should move to second reference mark | If failed → tonometer should be replaced or serviced |
- Recommended frequency: Calibrate before each clinic session or whenever accuracy is in doubt
- Clinical impact: A 5–10% error in calibration can result in ±2 mmHg IOP error
- Maintenance: Contact manufacturer if calibration cannot be achieved
The tonometer biprism tip must be disinfected between patients to prevent transmission of adenovirus, herpes simplex, HIV, hepatitis B, and theoretically prions (CJD — Creutzfeldt-Jakob disease).
Accepted Disinfection Methods
- 3% hydrogen peroxide for 5–10 minutes: Effective, inexpensive, widely used
- 70% isopropyl alcohol wipe: Rapid disinfection; rinse with saline before use
- 0.5% sodium hypochlorite: Effective but may damage biprism over prolonged time
- Autoclaving: NOT recommended — high temperature damages the cement bonding the biprism to the arm
Special Considerations
- Disposable tonometer tips: Gold standard for infection control, particularly during epidemic keratoconjunctivitis (EKC) outbreaks
- CJD/Prion suspected: If CJD risk is suspected, use sodium hypochlorite or autoclaving (despite damage risk). A dedicated tonometer tip should be used and destroyed after patient contact
- Standard alcohol wipes: Are NOT prionocidal (do not inactivate prions) — inadequate for CJD risk
| Clinical Scenario | IOP Interpretation & Management |
|---|---|
| Glaucoma screening | IOP >21 mmHg on 2+ occasions → warrants full glaucoma workup (optic disc assessment, visual field, OCT RNFL, CCT measurement, gonioscopy) |
| Ocular hypertension (OHT) | IOP >21 mmHg with normal optic disc, normal visual field, normal CCT; OHTS trial guides treatment decision based on risk factors |
| Normal tension glaucoma (NTG) | IOP ≤21 mmHg with documented glaucomatous damage; exclude secondary causes; often associated with thin CCT |
| Acute angle closure glaucoma | IOP often 40–70 mmHg; corneal oedema → underestimation of true IOP by GAT; emergency treatment required |
| Post-trabeculectomy monitoring | Target IOP usually ≤12–15 mmHg; lower targets in advanced glaucoma |
| Uveitic glaucoma | Paradoxically LOW IOP during active uveitis (ciliary body shutdown); high IOP may indicate steroid response or angle closure |
| Post-LASIK | True IOP is higher than GAT reading indicates; use Goldmann + ORA (OCT-based or other CCT-independent methods) for accuracy |
| Paediatric / uncooperative patients | Perkins tonometer, Tono-Pen, or iCare without anaesthetic; general anaesthesia if necessary |
Trap 1 — FALSE
“The Imbert–Fick law was designed specifically for the cornea.”
FALSE. The Imbert–Fick law is a general physical law for ideal spheres. Goldmann applied and modified it for corneal tonometry by identifying the 3.06 mm applanation diameter where errors cancel.
Trap 2 — FALSE
“At 3.06 mm applanation diameter, corneal rigidity overestimates and surface tension underestimates IOP — and they cancel each other.”
TRUE. This is the fundamental rationale for selecting 3.06 mm as the applanation diameter.
Trap 3 — FALSE
“A patient with CCT 560 µm has a GAT reading of 24 mmHg. The true IOP is higher.”
FALSE. Thick cornea (>520 µm) → GAT overestimates → true IOP is lower than 24 mmHg. This patient likely has normal true IOP.
Trap 4 — FALSE
“Excess fluorescein causes underestimation of IOP.”
FALSE. Excess fluorescein causes overestimation — wider mires make the endpoint (inner edges touching) appear earlier, yielding higher dial reading.
Trap 5 — FALSE
“Post-LASIK, GAT typically overestimates IOP.”
FALSE. Post-LASIK cornea is thinner and has altered biomechanics → GAT underestimates true IOP.
Trap 6 — FALSE
“Perkins tonometer requires a slit lamp.”
FALSE. Perkins is hand-held and portable — used in children, OR, and supine patients without a slit lamp.
Trap 7 — FALSE
“A dial reading of 1.6 corresponds to IOP of 1.6 mmHg.”
FALSE. Dial reading × 10 = IOP in mmHg. A reading of 1.6 = 16 mmHg.
Trap 8 — FALSE
“In corneal oedema, GAT overestimates IOP.”
FALSE. Oedematous cornea is softer → underestimates IOP. Particularly relevant in FECD or acute angle closure glaucoma.
Q: Why 3.06 mm and not 4 mm or 2 mm as the applanation diameter?
A: The 3.06 mm diameter is the unique point where corneal rigidity (which overestimates) and surface tension (which underestimates) forces cancel exactly. At smaller diameters, surface tension dominates; at larger, rigidity dominates. This empirical discovery by Goldmann is the foundation of accurate applanation tonometry.
Q: A patient post-LASIK has GAT of 10 mmHg but complaining of halos and blurred vision. How do you evaluate true IOP?
A: Post-LASIK cornea is thinner and has altered biomechanics → GAT significantly underestimates true IOP. Evaluate with: (1) Dynamic Contour Tonometry (DCT) or ORA (corneal biomechanics-corrected); (2) CCT measurement to apply Ehlers/Doughty–Zaman correction; (3) AS-OCT to assess corneal thickness; (4) Pentacam if available. True IOP may be much higher than 10 mmHg.
Q: What are the units of the GAT dial, and how is IOP derived from the dial reading?
A: The dial is marked 0–10, representing grams of force (0–10 g). IOP (mmHg) = Dial reading × 10. This simple relationship comes from the fact that at the 3.06 mm applanation diameter with proper biprism design, the mechanical applanating force in grams numerically equals IOP/10 in mmHg.
Q: How would you modify GAT technique in a patient with >4 D of regular corneal astigmatism?
A: Rotate the biprism to the 43° position (marked red on the tonometer) — this is the theoretical bisector of the two principal meridians. This reduces the elliptical distortion of the contact area. Even with this adjustment, residual underestimation of approximately 1 mmHg per 4 D of astigmatism persists; consider tonometry in both meridians separately for higher accuracy.
Q: Name two tonometers that are independent of CCT. How do they overcome CCT bias?
A: Dynamic Contour Tonometry (DCT) — uses a contoured probe matching corneal shape; directly measures ocular perfusion pressure rather than assuming a sphere. ORA (Ocular Response Analyser) — uses air-jet to deform cornea + measures corneal hysteresis (elastic recoil); calculates IOP correcting for individual corneal biomechanics. Both are theoretically CCT-independent but require validation.
Q: What is ocular pulse amplitude (OPA), and which tonometer can measure it?
A: Ocular pulse amplitude is the difference between peak and trough IOP during the cardiac cycle (normally 0.5–3 mmHg). Pascal DCT (Dynamic Contour Tonometer) can measure OPA — it displays both mean IOP and the ocular pulse waveform. High OPA correlates with better ocular perfusion and may have prognostic value in glaucoma.
Q: A patient with FECD has GAT reading of 8 mmHg. What is the likely true IOP and why?
A: FECD (Fuchs Endothelial Corneal Dystrophy) causes corneal oedema → softened cornea → underestimates IOP on GAT. The true IOP is likely significantly higher than 8 mmHg. The cornea's reduced resistance to applanation falsely lowers the dial reading. Use non-contact tonometry, DCT, or ORA for more accurate measurement.
Q: Why does post-trabeculectomy hypotony cause choroidal detachment? What IOP threshold triggers intervention?
A: Severe hypotony (<6 mmHg) allows inward collapse of sclera → uveal-scleral detachment (choroidal detachment). The mechanism involves loss of aqueous volume and anterior chamber shallowing, causing mechanical separation. Choroidal detachment risk peaks at IOP <6 mmHg persistently. Intervention is considered when IOP <4 mmHg or if complications occur; some surgeons intervene at 5 mmHg to prevent long-term sequelae (hypotony maculopathy).
Q: How does the Valsalva manoeuvre affect GAT readings? What clinical scenario is this relevant to?
A: Valsalva → increased venous pressure → venous congestion → transient IOP elevation → artificial overestimation on GAT. Clinically important when: (1) anxious patients straining during measurement; (2) tight collar/necktie; (3) breath-holding; (4) coughing. Practice point: Instruct patient to relax, breathe normally, loosen collar. Re-measure if suspicious values obtained. This is relevant in both screening and glaucoma monitoring to avoid false elevation.
Q: What is the OHTS trial and what was its key conclusion regarding CCT and glaucoma risk?
A: OHTS (Ocular Hypertension Treatment Study) — landmark randomised trial (1997–2007, n~1600) comparing treated vs untreated OHT patients. Key finding: CCT is an independent risk factor for conversion to POAG (not merely a measurement confounder). Thick CCT (>588 µm) conferred lower risk of glaucoma conversion (RR ~0.3); thin CCT (<555 µm) conferred higher risk (RR ~1.3). This means CCT has dual effects: (1) measurement bias (thick cornea overestimates GAT) AND (2) biological protection (stiff cornea resists optic nerve head damage). Clinical implication: Thick cornea OHT patients are at genuinely lower risk — both the measurement is overestimated AND they are biologically protected.
A 52-year-old patient presents with bilateral IOP of 28 mmHg on GAT with CCT of 620 µm bilaterally. The optic discs and visual fields are entirely normal. The patient is anxious about glaucoma. How would you counsel this patient regarding their true IOP, glaucoma risk, and need for treatment? Which trial evidence guides your decision, and what is the calculated Goldmann-corrected IOP?
Answer
With a CCT of 620 µm (100 µm above the standard 520 µm), the GAT systematically overestimates IOP due to increased corneal rigidity. Using the Ehlers correction (~0.5 mmHg per 10 µm), the correction is approximately +5 mmHg overestimation, yielding corrected IOP ≈ 28 − 5 = 23 mmHg. Using the Doughty & Zaman formula (~0.7 mmHg/10 µm), correction would be 7 mmHg, yielding corrected IOP of ~21 mmHg (approximately at the statistical upper limit of normal). The Ocular Hypertension Treatment Study (OHTS) is the key evidence guiding this case. OHTS identified CCT as an independent risk factor for glaucoma conversion — thick CCT (>588 µm) conferred significantly lower risk of conversion to POAG (RR ~0.3), even after adjusting for baseline IOP. This patient has thick cornea (protective factor), normal optic discs, and normal visual fields — risk stratification strongly favours observation over treatment. Counselling approach: (1) Reassure the patient that the IOP reading is likely an overestimate due to corneal thickness; (2) Explain the concept of CCT and its dual effects on tonometry (measurement overestimation) and on glaucoma biology (mechanical protection); (3) Initiate 6-monthly monitoring with: IOP measurement (ideally at consistent time), optic disc assessment (photographs/OCT), OCT RNFL imaging, and visual fields (automated 24-2) annually; (4) Avoid the diagnostic label 'ocular hypertension' unless confirmed on repeated, CCT-corrected measurements; (5) Counsel on eye health: avoid eye trauma, maintain systemic BP control, healthy lifestyle. Treatment recommendation: No pharmacological treatment is indicated at this stage in the absence of structural (disc cupping, RNFL defect) or functional (VF loss) glaucomatous damage. The patient is at genuinely lower risk due to thick cornea — both the measurement is overestimated AND they have a protective biological phenotype.
References
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