The non-contact tonometer is an applanation tonometer and works on the principle of a time interval. Measuring the time it takes from the initial generation of the puff of air to where the cornea is exactly flattened (in milliseconds) to the point where the timing device stops. It takes less time for the puff of air to flatten a soft eye than it does a hard eye.
Procedure for IOP Measurement
1.) Turn the power button to the on position and allow it to warm up for 30 seconds (usually once it is turned on it should be left on all day).
2.) Adjust the eyepiece until the reticle is in good focus.
3.) Push the button (D) to demonstrate to the patient what the air and sound are like. Have the patient hold their finger up in front of the instrument and trigger the air-pulse. At the same time note the reading on the display screen, for the NCT ii you should get a reading of 50 mmhg (+) or (-) 1 mmHg if the instrument is calibrated correctly.
4.) During the next three steps the operator should observe the patient and instrument from the side.
5.) Adjust the patient's height in the chin rest so that their outer canthus is aligned with the black mark. Make sure the patient's forehead activates the forehead switches and that the indicator light is on.
6.) Adjust the instrument height so that the light from the instrument objective shines in the center of the patient's pupil. Raise the safety lock, have the patient close their eyes and move the instrument forward until you see a donut shaped bright area with a shadowed center on the patient's lid, then release the safety lock and make sure the instrument will not move any farther forward.
7.) Have the patient open their eyes and again center the light so that it shines in the center of the patient's pupil. The patient should be able to see the red dot target clearly, if not, adjust the Rx wheel for the patients approximate spherical equivalent.
POWER WHEEL:
Silver dot towards you = Plano correction
Red dot towards you = -3 D correction
Next click = -10 D correction
Black dot towards you = +4 D correction
Next click = +14 D correction
8.) Have the patient open their eyes wide (look surprised) and look right at the red dot or target. Your emphasis should now be on alignment and focusing of the dancing red target within the black reticle. The target you see should have a white background with a central red dot. The red target should be moving if it is stationary then you are either too close or too far away from the patient's eye. You should have one hand on the height adjustment with your index finger over the air puff control (trigger) and the other hand on the joy stick for lateral movements. Once the red dot is inside the reticle and in focus depress the air puff trigger and you should get a reading on the display screen. This should be accompanied by a red light display located at the bottom of the display screen if the reading is accurate (NCT I only). The NCT II does not have this display light nor does it have a external fixation light.
You should take two more measurements to verify the readings accuracy (always allow 5 to 8 seconds between triggerings or the instrument will shut down). If you get readings of 18, 19, 22, and 18 the 22 reading should be disregarded. The 22 reading is most likely due to venous pulsation or lid blink. Venous pulsation can cause as much as a 4mmHg increase in IOP readings and a forceful blink can increase IOP read by as much as 10 mmHg. If you find a marked difference in readings between the two eyes and the higher readings are on the first eye you measured you should repeat those readings. It is not unusual for the first eye's measurements to be higher and this is mainly due to the anxiousness of the patient and the co-contraction of the extra ocular muscles to the feel and sound of the air puff or the patient squinting; see note in the handout under procedure. Remember, as always, a difference of 3mmHg between the two eyes should be considered suspect. Suspect: Glaucoma, iritis, iridocyclitis, or retinal detachment.
The front lens of the objective must be cleaned when the red dot you are trying to focus appears dim or unable to be focused. This objective lens unscrews from the instrument and the lens can be cleaned with a Q-TIP and tap water and then dried with a Q-TIP. The half silvered mirror that you can see once the objective has been removed will often need to be cleaned too. This is done in the same way only using alcohol on the Q-TIP and then allowing it to dry smoothly.
The only reason for using the over ride button would be for:
- 1.) Uncooperative young child
- 2.) Patients with nystagmus of poor vision
- 3.) Patients with irregular corneas or very dry eyes
- 4.) High degrees of corneal astigmatism
CHECKING OUT THE SYSTEM (NCT II)
1.) With the demonstration button depressed (D) you should get a reading of 50 (+) or (-) 1 if the instrument is calibrated.
2.) With the over ride button depressed you should get a reading of 99 indicating the air flow or pressure system is working correctly.
3.) With the ON button depressed you should get a reading of 65 indicating the electrical system is working correctly.
NCT I
POWER AND CONTROL SWITCH
1.) Demonstration reading will be 57 (+) or (-) 1 if the instrument is calibrated.
2.) Over ride reading will be the same 99 indicating the air flow or pressure system is working correctly.
3.) The ON position should be 68 indicating the electrical system is working correctly.
Suspect Readings For Both The NCT I & NCT II:
The digital readout is in mm's of Hg and for adults readings of 21 mmHg and higher should be rechecked with goldmann tonometry. For children, according to Dr. Meetz, 21 mmHg should be question, however, readings of 25 to 26 mmHg are usually the upper range and are more suspect. This is due mainly to anxiousness and fear of the instrument.
*Last minute note: There have been some reports that photorefractive corrective surgery, i.e., (PRK and LASIK) because of their thinning of the corneal reduces the ocular rigidity. This may result in IOP findings lower than their actual values. It's most likely the IOP's have not changed, but results secondary to the reduced ocular rigidity.
Latest information: Intraocular pressures (IOP) following Laser In Situ Keratomileusis (LASIK) and Photorefractive Keratectomy (PRK) are underestimated. It depends on the patient's Pre-Op refractive prescription and how much tissue needs to be removed. There is about 10 microns of tissue removed per diopter of refractive error. A patient with a refractive error of 3 diopters of myopia will have approximately 30 microns of tissue removed. For this patient the IOP findings would be underestimated by about 2mm Hg and more for higher refractive errors. This underestimation has been reported for Goldmann applanation tonometry and not air puff or non contact tometry. This makes some sense the Goldmann tonometer assumes a corneal thickness of approximately 545 microns, hence, the cornea is thinned in these procedures and ocular rigidity is changed. The thinner the cornea the less ocular rigidity and lower pressure readings.
