A non-contact tonometer (NCT) is a device that measures intraocular pressure without directly touching the eye. It flattens the cornea with an air puff and calculates the intraocular pressure from the time or pressure required.
Due to the following features, it has become the most widely used screening tonometer in ophthalmology clinics, health checkups, and comprehensive medical examinations in Japan.
Normal intraocular pressure (IOP) is considered to be between 10 and 21 mmHg1). The average IOP in Japanese individuals is approximately 14–15 mmHg. IOP is determined by the balance between aqueous humor production and outflow resistance.
On the other hand, the Goldmann applanation tonometer (GAT) is the gold standard for IOP measurement, and the accuracy of NCT is inferior to that of GAT1). In cases requiring precise IOP measurement, GAT should be selected.
NCT is a tonometer intended for screening and is less accurate than GAT. It tends to measure lower, especially in the high IOP range. If NCT shows 22 mmHg or higher, re-examination with GAT is necessary. If high IOP is confirmed on re-examination, a detailed glaucoma examination (visual field test, OCT) will be performed.
NCT measurements are interpreted according to the following criteria.
| IOP (mmHg) | Evaluation | Action |
|---|---|---|
| 10–21 | Normal range | Follow-up |
| 22–24 | Suspected ocular hypertension | Repeat measurement → Recheck with GAT |
| ≥ 25 | Ocular hypertension | Recheck with GAT and detailed examination |
| ≤ 9 | Suspected ocular hypotension | Repeat measurement and detailed examination |
Ocular hypertension is a condition in which intraocular pressure exceeds the statistically normal upper limit (21 mmHg) but no abnormalities are found in the optic nerve or visual field1).
NCT measurement tendency varies depending on the intraocular pressure range.
If the measurement error exceeds 3 mmHg, remeasurement is necessary. Poor fixation or poor eyelid opening can also cause errors.
The theoretical basis of NCT is the Imbert-Fick principle. When an infinitely thin sphere with no wall rigidity is flattened by a plane, the relationship W = A × Pt holds between the flattening force W, the flattened area A, and the internal pressure Pt of the sphere.
GAT realizes this principle using a flattening prism with a diameter of 3.06 mm. NCT realizes the same principle using an air puff.
The measurement process of NCT is as follows:
The measurement time is very short, 1 to 3 ms (milliseconds). Therefore, it has the characteristic of being easily affected by the pulse wave (periodic fluctuation of intraocular pressure due to heartbeat).
Central corneal thickness (CCT) is a parameter that affects the accuracy of many tonometers2).
The reference CCT is approximately 520 μm. However, an increase in thickness due to corneal edema is an exception; when edema is present, the cornea becomes softer, resulting in underestimation.
Thin CCT is associated with an increased risk of progression from ocular hypertension to glaucoma2). On the other hand, no generally accepted CCT correction formula has been established, and applying correction factors to individual measurements is not recommended2).
All tonometers that applanate the cornea are affected by corneal biomechanical properties2). Devices that applanate the cornea quickly with an air puff, such as NCT, are more affected2).
It is the opposite. When the cornea is thin, intraocular pressure is measured lower (underestimated) than the true value. With a thick cornea, it is measured higher (overestimated) than the true value. This occurs not only with GAT but also with NCT. Since no correction formula for CCT has been established, it is not recommended to individually adjust correction values. In cases where CCT deviates significantly from the normal value, consider using a more accurate tonometer.
To obtain appropriate measurements, preparation is required on both the patient and examiner sides.
Preparation
Relaxation: Tension can cause intraocular pressure to be measured higher. Ensure the patient is sufficiently relaxed before measurement.
Prevent blinking: Instruct the patient not to blink.
Assist eyelid opening: Assist in opening the eyelids, being careful not to press on the eyeball, so that the eyelids and eyelashes do not obstruct the optical path.
Measurement
Alignment: Operate the joystick to align the cornea on the monitor.
Air puff: Press the switch to release a puff of compressed air. In automatic models, the puff is triggered automatically when proper alignment is achieved.
Repeat measurements: Repeat the measurement at least 3 times, ensuring that the measurement error is within 3 mmHg.
Adopt the value: Use the average or median of the measurement results.
If NCT shows high intraocular pressure of 22 mmHg or higher, confirm with repeated measurements and then perform re-examination with GAT1). If high intraocular pressure is confirmed by GAT, perform a detailed glaucoma examination (visual field test, evaluation of optic nerve and retinal nerve fiber layer by OCT)1).
If glaucomatous changes (visual field abnormalities, optic disc cupping enlargement, etc.) are observed even though NCT is within the normal range, suspect normal-tension glaucoma. To evaluate diurnal and nocturnal variations in intraocular pressure, consider measuring intraocular pressure at multiple times or using home iCare HOME for 24-hour intraocular pressure monitoring1).
In cases with corneal opacity, corneal edema, or difficulty opening the eyelids, accurate measurement with NCT is difficult. Consider switching to GAT or a rebound tonometer (iCare).
NCT is suitable for intraocular pressure screening in health checkups and comprehensive medical examinations. Because it requires no contact, no anesthesia, and can be performed in a short time, it can be efficiently administered to many examinees.
For follow-up of the same patient, it is desirable to use the same tonometer 2). Measurement values from different models cannot be directly compared.
Intraocular pressure measurements fluctuate due to various factors. The main influencing factors are summarized below.
| Factor | Direction of effect | Remarks |
|---|---|---|
| Thin cornea (CCT < 520 μm) | Underestimation | Approximately 0.2 mmHg variation per 10 μm of CCT 2) |
| Thick cornea (CCT > 520 μm) | Overestimation | Same as above |
| Corneal edema | Underestimation | Underestimation despite increased thickness |
| Corneal epithelial disorder | Decreased measurement accuracy | Surface scattering disturbs reflected light pattern |
| After refractive surgery (LASIK/PRK) | Underestimation | Measured 0.3–0.4 mmHg lower per 10 μm ablation2) |
| Supine position | Overestimation | 3–5 mmHg higher than sitting position |
| Winter | Overestimation | Seasonal variation occurs1) |
| After exercise / alcohol consumption | Tendency to underestimate | Causes a decrease in intraocular pressure |
| Straining / Valsalva maneuver | Overestimation | Relaxation is important |
Corneal hysteresis (CH) is an indicator of corneal viscoelasticity. It can be measured with the Ocular Response Analyzer (ORA) and has been reported as an independent predictor of glaucoma risk2). NCT is easily affected by this because it flattens the cornea quickly2).
After refractive surgery such as LASIK or PRK, the cornea becomes thinner and flatter, causing intraocular pressure to be measured lower than actual2). It has been reported that for every 10 μm of corneal ablation, the measured pressure is 0.3–0.4 mmHg lower2). Careful visual field and OCT monitoring are necessary for glaucoma evaluation after surgery.
Even in healthy individuals, intraocular pressure fluctuates by 4–5 mmHg over the course of a day. In glaucoma patients, it may fluctuate even more. Generally, it tends to be higher in the early morning and decreases from afternoon to night, but there is considerable individual variation.
Conventional NCT has limitations as it is easily affected by central corneal thickness and corneal curvature. In response, tonometers that consider the viscoelastic properties of the cornea have been developed.
These are said to provide IOP values with reduced influence of CCT, but further investigation is ongoing regarding their correspondence with GAT.
In recent years, an increasing number of NCT models have a function that measures CCT simultaneously with the air puff and displays an IOP value corrected for the influence of CCT. Since the correction algorithm varies by model, it is important to understand the characteristics of the measuring device before use.
