Tonometry Clinical Guide

Comparison of Tonometry Instruments

Infection Control & Instrument Maintenance

• GAT prism sterilisation: After each patient, clean the applanation prism with a fresh alcohol (isopropyl 70%) wipe and allow to air-dry for 5 minutes. Alternatively, use single-use disposable prism covers (e.g., Tonosafe). Full soaking in 1:10 sodium hypochlorite for 10 minutes is required for patients with known adenoviral conjunctivitis or suspected CJD. Rinse thoroughly with sterile saline after chemical disinfection to prevent corneal toxicity.
• iCare probes: Single-use disposable probes only. Never reuse. Check probe loading before each measurement.
• NCT: Non-contact; no direct corneal touch. Clean air puff nozzle and chin rest with disinfectant wipe between patients.
• Calibration: GAT should be verified against the calibration bar supplied with the instrument at the start of each clinical session. Replace the applanation prism if the calibration reading is outside ±0.5 mmHg.

General Preparation (All Methods)

1. Contact lens removal: Ask the patient to remove soft contact lenses before tonometry. Contact lenses alter corneal biomechanics and can be damaged or contaminated. Hard (RGP) lenses should also be removed and a 20–30 minute wait is ideal before measuring IOP.
2. Order of tests: Perform tonometry before corneal staining or slit lamp examination where possible to avoid epithelial disruption affecting the reading. If dilation is planned, measure IOP before instilling mydriatics.
3. Explain the procedure: Briefly describe what the patient will feel (a puff of air, light touch, or slight pressure) depending on the method. Patient cooperation and minimal blinking are essential for accuracy.
4. Seated position: For slit lamp GAT, ensure the patient is seated comfortably with the chin and forehead resting firmly in the headrest. The patient's outer canthus should be aligned with the canthus marker on the slit lamp.
5. Avoid lid squeezing: Instruct patient to keep eyes wide open and look at the fixation target. Eyelid squeezing artificially raises IOP. If necessary, gently hold the upper and lower lids apart with your fingers — but avoid applying pressure to the globe.

Topical Anaesthetic & Fluorescein (GAT)

1. Anaesthetic options:
   • Oxybuprocaine (benoxinate) 0.4%: Most commonly used for tonometry. Onset 30–60 seconds; duration ~15–20 minutes. Minimal stinging.
   • Proxymetacaine (proparacaine) 0.5%: Minimal sting on instillation. Preferred for patients who find oxybuprocaine uncomfortable.
   • Tetracaine 0.5%: Longer duration; more stinging on instillation. Generally not preferred for routine tonometry.
   • Minims (single-dose units): Use a fresh Minim for each patient to prevent contamination.
2. Fluorescein application: Use a fluorescein-impregnated strip (e.g., Fluoret) or combined fluorescein + anaesthetic drops (e.g., Minims Fluorescein + Benoxinate). Touch the moistened strip to the lower conjunctival fornix — do not touch the cornea. Instruct patient to blink twice to distribute the fluorescein.
3. Wait time: Allow 30–60 seconds after instillation for anaesthesia to take full effect before beginning GAT.
4. Fluorescein concentration: Excess fluorescein (bright orange, thick tears) causes the mires to appear too wide, giving a falsely high IOP reading. Insufficient fluorescein (thin mires) gives a falsely low reading. The correct endpoint is a clear, faint blue-green ring. If mires are too bright/wide, ask the patient to blink once or two before remeasuring.

Principle (Imbert-Fick Law)

GAT is based on the Imbert-Fick law: P = F / A, where P = IOP, F = the force required to flatten (applanate) a known area of the cornea, and A = the applanated area. The Goldmann prism applanates a 3.06 mm diameter circular area of the cornea. At this specific area, the surface tension of the fluorescein tear film (which would pull the prism towards the cornea) exactly balances the corneal rigidity (which resists applanation), theoretically cancelling both forces. The result is a direct measurement of IOP from the applied force alone.

Setup & Calibration

1. Calibrate the tonometer: Place the calibration bar over the prism holder. The pointer should deflect to each of the three positions (0, 2, 6 on the bar) when the drum is set to 0, 20, and 60 mmHg respectively. If outside ±0.5 mmHg, the instrument requires servicing.
2. Clean the prism: Wipe with 70% isopropyl alcohol wipe. Allow to dry completely — residual alcohol on the prism will sting the patient and affect the tear film.
3. Set the drum to 1 (10 mmHg): Starting at 10 mmHg avoids discomfort from excessive pressure if the IOP is low.
4. Align the prism: Set the prism to 0° (or 43° if astigmatism >3DC — see pearl below). The red mark on the prism holder aligns with the white mark at 0°.
5. Set cobalt blue filter and low illumination: Set the slit lamp illumination to maximum with the cobalt blue filter to excite fluorescein. Set the slit to a wide beam, no aperture. Angle the illumination approximately 60° from the observation axis.

Measurement Procedure

1. Instil anaesthetic and fluorescein as described in Section 3. Wait 30–60 seconds.
2. Position the patient at the slit lamp with chin and forehead firmly in the headrest. Align outer canthus with the canthus marker. Ask the patient to fixate a target in the distance with the fellow eye if possible.
3. Advance the prism towards the cornea using the joystick. Approach from slightly below to avoid touching the upper lashes. Watch from the side initially to judge proximity — do not look through the eyepieces until close.
4. Make contact: As the prism touches the cornea, two blue-green semicircular mires will appear in the viewing field. The mires are the fluorescein rings formed at the upper and lower edges of the applanated tear film, bisected horizontally by the prism biprism optics.
5. Adjust the drum until the inner edges of the two mires just touch:
   • If the mires overlap (inner edges cross) → IOP reading is too high → reduce drum.
   • If the mires are separated (gap between inner edges) → IOP reading is too low → increase drum.
   • Correct endpoint: inner edges of the upper and lower mire just kiss — they touch but do not overlap.
6. Read the IOP: Read the number on the drum at the endpoint. Multiply by 10 to convert to mmHg (drum reads in units of 10 mmHg; e.g., drum reading of 1.5 = 15 mmHg).
7. Retract the prism immediately after reading. Do not leave the prism in contact with the cornea.
8. Take three readings per eye within the same session. If all three agree within 1 mmHg, average them. If any reading is more than 1–2 mmHg different from the others, take additional readings and use the median.
9. Measure right eye first, then left eye. Document time of measurement (relevant for diurnal variation monitoring).

Mire Troubleshooting

Principle

NCT uses a calibrated pulse of air to flatten (applanate) the cornea. An infrared optical monitoring system detects the exact moment of applanation — when the corneal surface is maximally flat — by measuring the reflectance of an aligned infrared beam. The time taken for the air pulse to flatten the cornea, or the pressure of the air jet at applanation, is converted to an IOP reading. No corneal contact is required, eliminating infection transmission and the need for topical anaesthesia.

Procedure

1. Clean the chin rest and forehead bar with a disinfectant wipe before the patient is seated.
2. Ask the patient to remove spectacles and contact lenses. Seat the patient comfortably with chin in the chin rest and forehead against the bar.
3. Align the patient's eye with the nozzle aperture using the joystick and chin rest height adjustment. The crosshair alignment target should be centred on the corneal reflex seen through the monitor.
4. Instruct the patient: "Look at the green dot/target inside the machine. The machine will puff air into your eye — try to keep your eye open and not blink."
5. Once the corneal reflex is centred and the alignment indicators confirm correct position, the machine will fire the air puff automatically (auto-trigger mode) or upon pressing the trigger button.
6. Take a minimum of 3 readings per eye. Most modern NCT instruments take 3 automatic readings and display the average. If readings are inconsistent (range >3 mmHg), take additional readings.
7. Record the average IOP and note that NCT was used (relevant for clinical interpretation and comparison with GAT values).

Limitations of NCT

• Less accurate than GAT, especially at high IOP (>21 mmHg) — NCT tends to overestimate at high IOP.
• More affected by corneal biomechanical properties (corneal hysteresis) than GAT.
• Requires patient cooperation and alignment — difficult in patients with dense media opacity, nystagmus, or poor fixation.
• Air puff can be startling; reflex blink and squeezing may occur. Some patients find it uncomfortable.
• NCT is a screening tool. Any abnormal or borderline NCT reading should be confirmed with GAT before clinical management decisions are made.

Principle

Rebound tonometry uses a lightweight, magnetised wire probe that is propelled towards the cornea by an electromagnetic coil. The probe makes momentary contact with the cornea and rebounds. The deceleration and rebound motion of the probe are analysed: a higher IOP results in faster deceleration and faster rebound. The device calculates IOP from the rebound parameters. No topical anaesthesia is required as the probe contact is so brief (milliseconds) that it is virtually imperceptible.

Procedure (iCare ic100 / ic200 / HOME)

1. Load a new single-use probe into the device. Confirm the probe is securely loaded and the device is ready (green LED).
2. Patient may be seated or standing. Ask patient to look straight ahead and keep the eye wide open.
3. Hold the device horizontally, parallel to the floor. The probe tip should be pointing directly at the corneal apex at a distance of approximately 4–8 mm from the corneal surface.
4. For the standard iCare ic100/ic200: rest the brow support (positioning device) against the patient's forehead to stabilise the device distance.
5. Press the measurement button. The device fires the probe 6 times automatically and discards the highest and lowest readings, displaying the mean of the remaining 4 measurements.
6. If the device displays an error (typically due to poor alignment or excessive eye movement), repeat the measurement sequence.
7. Record the result. Note that iCare readings are generally comparable to GAT at normal IOP ranges, but may overestimate at very low IOP and underestimate at very high IOP in some studies.

Advantages & Limitations

Principle

The Tono-Pen is a handheld micro-applanation tonometer. A small, flat probe tip is applied to the cornea. A strain-gauge transducer within the probe detects the force required to applanate a very small area of the cornea. Multiple measurements are taken and averaged electronically. Because it applanates a much smaller area than GAT (1.02 mm vs 3.06 mm), it is less affected by corneal curvature but more affected by corneal rigidity.

Procedure

1. Instil topical anaesthetic. A disposable latex cover (Ocu-Film tip cover) must be placed over the probe tip — this is essential for infection control and is required for each patient.
2. Calibrate the Tono-Pen by holding it vertically downward and pressing the activation switch until a beep is heard and the display shows "Cal Good."
3. Ask patient to look straight ahead. Hold the device like a pen, perpendicular to the corneal surface.
4. Lightly and briefly touch the central cornea with the probe tip in a series of gentle taps — do not press. Each successful reading produces a single beep.
5. The device requires 4 acceptable readings, after which it produces a double beep and displays the IOP with a confidence interval indicator (5%, 10%, 20% — lower percentage = more consistent readings).
6. Accept readings with ≤5% or ≤10% confidence interval. Repeat if >20% as readings are inconsistent.
7. Remove and dispose of the probe tip cover. Clean the probe with an alcohol wipe.

When to Use Tono-Pen

• Irregular or oedematous corneas: Corneal oedema, bullous keratopathy, Fuchs' endothelial dystrophy — GAT mires are distorted; Tono-Pen small tip can find a clear corneal area.
• Post-corneal transplant: Irregular topography makes GAT unreliable; Tono-Pen provides a usable estimate.
• Keratoconus: Steep apex and thin central cornea make GAT difficult. Tono-Pen can measure at the periphery to avoid the thin apex.
• Patients unable to use the slit lamp: Wheelchair-bound, head positioning problems, severe lid abnormalities — the handheld Tono-Pen provides flexibility.
• Paediatric examination under anaesthesia (EUA): Tono-Pen and Perkins are both appropriate for supine measurements under GA.

Why CCT Matters

CCT Measurement — Pachymetry

• Ultrasound pachymetry (contact): Gold standard for CCT. Probe placed perpendicular to the central cornea after topical anaesthesia. Takes 5–10 readings; displays the average. Measurements accurate to ±5 µm.
• Optical coherence tomography (OCT): Non-contact; measures CCT as part of anterior segment imaging. Comparable accuracy to ultrasound.
• Scheimpflug imaging (Pentacam, Corvis ST): Provides full corneal thickness map, not just central CCT. Preferred for keratoconus screening and pre-refractive surgery assessment.
• Specular microscopy: Can estimate CCT but not the primary purpose of the instrument.

IOP Correction for CCT — Ehlers Correction Table

Multiple correction formulae exist (Ehlers, Shah, Doughty-Zaman). The most commonly used in clinical practice applies approximately +/−0.7 mmHg per 10 µm deviation from 520 µm(Doughty-Zaman). Note: these corrections are population-derived estimates and should be used as guides to clinical risk stratification, not as precise mathematical corrections.

Correction values based on Doughty MJ & Zaman ML (2000). Use as clinical guide only — individual corneal biomechanics (hysteresis, stiffness) vary independently of CCT and affect accuracy. Biomechanically corrected IOP (ORA or Corvis ST) reduces CCT-related error.

Key Interpretation Principles

• IOP is not a diagnosis: Elevated IOP alone does not mean glaucoma. IOP must be interpreted alongside optic disc appearance, retinal nerve fibre layer (RNFL) thickness (OCT), visual fields, and angle anatomy (gonioscopy).
• Asymmetry between eyes: An IOP difference of ≥4 mmHg between the two eyes is clinically significant and should prompt further investigation even if both absolute values are within the normal range. Asymmetric IOP is a risk factor for glaucoma.
• Diurnal variation: IOP varies throughout the day — typically highest in the early morning. A single IOP measurement captures only one point in this cycle. In glaucoma suspects with borderline IOP, phased IOP measurements (measuring at different times of day) or 24-hour monitoring may be indicated.
• IOP fluctuation: Wide IOP fluctuation (>6 mmHg over time) is an independent risk factor for glaucoma progression, even when mean IOP is within target range.
• Target IOP: In established glaucoma, a "target IOP" is set — typically a 20–30% reduction from baseline, or an absolute target (often ≤18 mmHg for moderate glaucoma, ≤15 mmHg for advanced). Whether IOP is being consistently maintained at or below target is the key monitoring question.
• Normal tension glaucoma (NTG): IOP consistently ≤21 mmHg despite glaucomatous disc and field changes. NTG represents ~30–40% of glaucoma in European populations and up to 90% in some Asian populations. Vascular risk factors, nocturnal hypotension, and disc haemorrhages are particularly important in this group.

Acute Angle-Closure — Recognising the Emergency

Ocular Hypertension (OHT)

• IOP consistently ≥22 mmHg on at least two separate occasions, with no glaucomatous optic disc changes and no visual field defects.
• Lifetime risk of converting to glaucoma: approximately 10% over 5 years (OHTS data). The 5-year risk can be calculated using the OHTS/EGPS risk calculator (IOP, CCT, disc size, age, pattern standard deviation).
• High-risk OHT features: IOP >26 mmHg, CCT <555 µm, large CDR, family history of glaucoma, age >65, African or Afro-Caribbean ethnicity.
• Low-risk OHT (NICE guidelines, UK): IOP 22–25 mmHg, CCT >590 µm, no disc changes, low OHTS risk score → observation only with annual review may be appropriate.

Glaucoma Suspect

• A patient with one or more risk factors for glaucoma but no definitive diagnosis: OHT, suspicious disc appearance (large CDR, disc haemorrhage, RNFL defect), suspicious visual field, family history of glaucoma, thin CCT, or narrow angles.
• Requires comprehensive glaucoma workup: IOP (multiple readings, both eyes), CCT, disc photography, OCT RNFL, gonioscopy, Humphrey visual fields (24-2 or 30-2).
• Frequency of monitoring depends on risk level. High-risk: 3–6 monthly. Moderate: 6–12 monthly.

Hypotony

• IOP <10 mmHg. May cause maculopathy, choroidal effusion, and hypotony maculopathy (retinal folds).
• Causes: Post-surgical wound leak, over-drainage bleb, cyclodialysis cleft, uveitis with ciliary body shutdown, trauma, excessive glaucoma medication effect.
• Requires prompt ophthalmology referral if symptomatic or associated with structural changes.

Children

• Normal IOP in children: 10–21 mmHg (same range as adults; may be slightly lower in infants).
• GAT is difficult in uncooperative children. iCare (no anaesthetic) and NCT are preferred for awake children.
• Paediatric glaucoma (congenital, infantile, juvenile): presents with corneal enlargement (buphthalmos), Haab's striae (horizontal breaks in Descemet's membrane), corneal oedema, high myopia, photophobia, and epiphora. Urgent ophthalmology referral.
• Examination under anaesthesia (EUA) with Perkins or Tono-Pen used when awake assessment is impossible.
• Children's corneas are thinner and more elastic — IOP may be underestimated by GAT.

Post-Refractive Surgery (LASIK / PRK / SMILE)

• LASIK and PRK reduce CCT and alter corneal biomechanics. GAT will significantly underestimate IOP post-procedure — the degree of underestimation correlates with the amount of tissue removed.
• A correction of approximately +1 mmHg per 10 µm of stromal ablation is a rough guide, but not precise.
• Biomechanically corrected IOP (ORA's IOPcc or Corvis ST bIOP) is more reliable post-refractive surgery.
• Always document pre-operative IOP in refractive surgery candidates. Pre-existing glaucoma is a contraindication to LASIK.
• iCare rebound tonometry is also less affected by corneal ablation than GAT.

High Myopia & Keratoconus

• High myopia: Often associated with thin CCT and lower corneal hysteresis → GAT underestimates IOP. Annual IOP monitoring is particularly important as high myopia is itself a risk factor for glaucoma (especially NTG).
• Keratoconus: Very thin, irregular central cornea causes significant GAT underestimation. NCT is similarly unreliable. Tono-Pen (peripheral corneal measurement) or iCare are preferred. Biomechanically corrected IOP essential if glaucoma is suspected.

Steroid Use (Ocular & Systemic)

• Steroid-induced ocular hypertension: Topical, periocular, intravitreal, and systemic corticosteroids can raise IOP by reducing trabecular outflow facility. Risk is greatest with potent topical steroids (dexamethasone, prednisolone acetate) used long-term.
• IOP rise typically occurs 2–6 weeks after starting steroids. Steroid responders (genetic predisposition) may show IOP rises >10 mmHg. Patients with POAG and glaucoma suspects are at highest risk.
• Management: Discontinue or change to lower-potency steroid (fluorometholone, loteprednol) where clinically possible. Add IOP-lowering medication if IOP remains elevated. IOP usually returns to baseline 2–4 weeks after cessation.
• Any patient commenced on long-term topical steroids (e.g., post-LASIK, post-cataract, uveitis) should have IOP monitored at 4–6 weeks and regularly thereafter.