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Tonometry

Goldmann Applanation Tonometer

The word applanate means to flatten. The purpose of applanation tonometry is to find ocular pressure. This follows the formaula Pressure = Force/Area. The applanation tonometer flattens a known area (3.06mm) with a given force to give pressure.

Clean

The Goldmann Applanation Tonometer may be cleaned by wiping with soap and water. While cleaning don’t bend the arm of the tonometer and make sure that the readings on the tonometer head are set to zero and level when finished.

Disinfect

The tonometer tip should be disinfected between each patient. The tonometer tip may be disinfected by putting it in a 10 minute soak of 3% hydrogen peroxide of wiping with alcohol for 10 seconds. Allow the alcohol to dry from the surface of the tip before pressing it against the patients eye. If the alcohol has not dried the patient will get a chemical burn on the cornea.

Calibrate

The tonometer is not actually calibrated in the clinic. If calibration is needed then it is sent to professionals. We do have to verify that the instrument is calibrated correctly. To do this we use a calibration bar. This bar with of the marks for the 0,2,and 6 position. This corresponds to the pressures of 0, 20, and 60 mm Hg. Here are the calibration steps:

  1. Set the tonometer to 0. The arm should be in free motion. If the dial is turned backward a small distance(-.50mm Hg) then the arm should fall backward, away from the patient, if the dial is turned forward a small distance (.50mm Hg) then the arm should fall toward the patient.
  2. Attach the calibration bar.
  3. Set the bar at the 2 mark and see of the bar falls forward or backward with a small movement in either direction.
  4. Set the bar at the 6 mark and see of the bar falls forward or backward with a small movement in either direction.

Measuring

IOP (intraocular pressure) is measured in mmHg. Normal eye pressure is between 10-22mmHg. When checking eye pressure make sure that the inside of the mires line up.

Each line on the tonometer dial corresponds to 2 mmHg. You will see the numbers 1,2,3,4 etc. on the tonometer dial these numbers are in increments of 10 mmHg. If the dial says 1 then the pressure is 10 mmHg. If the dial is at 1 plus 2 lines then the press is 14 mmHg.

If the mires are not aligned while measuring pressure pull away from the patient, realign then place the tonometer back on the patients eye. If you try to realign while the tonometer tip is still in contact with the patients eye then you will damage the cornea.

Complications

Two times when you should avoid contact tonometry.

  1. If the patient has an eye related infectious disease such as a bacterial or viral conjunctivitis then the contact tonometry may spread an infection from on eye to the other, previously non infected, eye. The most common complication of contact tonometry is conjunctivitis.
  2. Foreign bodies. The patient already has a foreign body in the eye. Don’t push it in deeper with the tonometer.

Three things that can result in incorrectly high IOP readings.

  1. Holding breath. When a patient is holding his breath his IOP will increase. Ask the patient to breath in and out normally.
  2. Straining. If a patient is straining to lean up to the slit lamp then the IOP will be high. Always make sure the patient is comfortable and not straining.
  3. Squeezing  eyelids. No one likes to keep their eye open while something is coming at it. Some patients really really don’t like it and they will close their little eyes nice and tight so there is no way for you to get to the eye. When this happens you will have to open the eyelids and do your best to get a good reading while they are, unintentionally, trying to close their eye.

Other Tonometry Methods

Three other methods for obtaining an IOP reading.

  1. Schiotz tonometry. This is a funny ice cream scoop looking instrument. When using this instrument a chart is needed to convert to mmHG from the reading the the needle points to. When collecting a reading the patient must lay back. This is different from other types of tonometry.
  2. TonoPen. This is the fastest way to get an IOP reading. All you do is put a cover over the tip. Push the button and tap the eye. This is less accurate than the Goldman.
  3. Noncontact tonometry “air puff” (pneumotonometer or pneumo for short). This is appropriate for patients that are present for glasses. This is the least accurate way to get eye pressure.

Intraocular Pressure Dynamics

For this section memorize the flow of aqueous humor.

  1. Aqueous humor enters the posterior chamber via the production of the ciliary body
  2. Aqueous flows through the pupil
  3. Aqueous enters the posterior chamber
  4. Aqueous leaves the posterior chamber via the trabecular meshwork

IOP is determined by the rate aqueous enters and exits the eye. If the aqueous is not getting absorbed by the trabecular meshwork as fast as it is being produced by the ciliary body then IOP will increase. It is believed that high IOP in open-angle glaucoma patients is caused  by the decreased function of the cells in the trabecular meshwork.

Three things that increase IOP other than glaucoma.

  1. Thick cornea – The best explanation of this comes from Ophthobook. I am using his analogy here. When we check IOP we press on the eye and measure the force exerted by the eye, but corneal thickness needs to be taken into account. For example, if  you were to kick the car tire then kick a bike tire which tire would you say had a high pressure? Well, you may think that the car tire has a higher pressure because it is more ridged when you kick it. It turns out that they have approximately the same pressure. There reason that the car tire has a higher pressure is that the car tire is thicker. The same is true for the eye. A thicker cornea will give a higher eye pressure.
  2. Steroid use – Patients who use steroids have higher IOPs.
  3. Time. Getting your IOP checked early in the morning. This may sound crazy, but it is true. During the day you will have higher eye pressures in the morning than you do a night. This can carry up to 5-10mmHg.

Check out my New Study Guide Study For the Certified Ophthalmic Assistant Exam