What is Tonometry
Tonometry is the procedure that is performed by eye care professionals to measure the intraocular pressure (IOP), the fluid pressure inside the eye. This procedure is one of the important tests that help in evaluating the risk of Glaucoma. IOP is measured in millimetres of mercury (mmHg).
Tonometer: The instrument which is used for measuring Intraocular Pressure is known as a tonometer.
Types of Tonometry
- Indentation tonometry
- Applanation tonometry
Indentation tonometry measures the Intraocular pressure by pressing the cornea by the small plunger having known weight. The higher the IOP the plunger required more weights to push against and indent the cornea. Schiotz tonometer is used in the Indentation tonometry procedure. Due to its simplicity, portable, reliability, cheaper and relative accuracy this tonometer is commonly being used since 1905. In developing and underdeveloped countries this tonometer is used in screening camps, glaucoma screening campaigns etc.
Parts of Schiotz Tonometer
- Lever Arm
Things needed for Schiotz Tonometry
- Schiotz tonometer with additional weights, and scale card
- Topical Anesthesia drops such as Lignocaine or proparacaine ophthalmic solution.
- Clean cotton wool or Gauze swabs
- Alcohol swab or Spirit swab
Before measuring IOP you have to calibrate the tonometer first, you can use spherical mould in the box and 5.5 gram weight for calibration, if the tonometer is in good condition needle reading will be reached ‘0″ on the scale immediately. Clean the plunger and disc of the tonometer by using alcohol swabs. Make it dry with the help of dry cotton before touching the cornea.
Follow these steps for measuring IOP
- Instruct patient to lie on the bed (Supine position). Wash your hands properly and make them dry.
- Stand upright behind the head of patients in a comfortable position for measuring IOP.
- Instil local anaesthetic eye drops (Lignocaine or proparacaine ophthalmic solution) and wait for 30 seconds.
- Instruct the patient to look at a fixed object or ceiling and ask to keep absolutely still.
- With the thumb and index finger of one hand, gently hold open the patient’s eyelids, taking care not to put any pressure on the eye.
- With another hand gently rests the footplate of the tonometer vertically on the centre of the cornea.
- Record the scale reading.
If Reading is 2 or less, you have to remove the 5.5-gram weight and replace that weight with 7.5 gram and repeat the procedure again. Record the scale reading.
- Remove the tonometer and install antibiotics eye drops on the patient eyes.
- Use the conversional chart and convert it from Scale reading to mm of Hg.
Schiotz tonometry is usually done for the screening test. Sometimes it doesn’t provide accurate results if it shows high IOP results in this need to reconfirmed by using the most accurate applanation tonometer.
The applanation tonometry concept introduced by Goldmann in 1954. This tonometer is based on p. Its principle is based on the Imbert–Fick law, which states that:
The pressure inside a sphere (P) is equal to External force (F) needed to flatten a portion of a sphere divided by the area of sphere (A).
P = F/A
P= Pressure F= Force A= Area
Based on Imbert Fick Law Commonly used applanation tonometers are as follows
- Goldmann applanation tonometer
- Perkin’s applanation tonometer
- Pneumatic tonometer
Goldmann Applanation Tonometer
Goldmann applanation tonometry (GAT) is considered the standard method for measuring intraocular pressure. It is the gold standard method for measuring IOP. Goldmann Applanation tonometer consists of a double prism mounted on a standard slit lamp. This is more accurate than schiotz tonometer because this tonometer only applanate a small area of 3.06mm diameter of the cornea and ocular rigidity doesn’t interfere with readings.
Corneal irregularities and Central corneal thickness (CCT) influence the result of Goldmann applanation tonometry.
Parts of Applanation Tonometer
- Goldmann tonometer
- Applanation prism
- Disinfectant: isopropyl alcohol 70% or alcohol swab
- Local anaesthetic drops
- Fluorescein strips
- Clean cotton wool or gauze swabs.
First of all, prism needs to be disinfected with isopropyl alcohol 70%. Make it dry by wiping prism with a dry clean swab. Check that the gradation marked ‘0’ on the measuring prism is aligned with the white marker point on the tonometer head remember that adjusting the tonometer prism is done on patients with significant astigmatism. Check that the calibrated dial of the tonometer is set around 10 mmHg. Set the magnification of the slit lamp at 10X. The Goldmann tonometer should be regularly calibrated.
Procedure to Measure Intraocular Pressure
- Instil the local anaesthetic drops eg Lignocaine 4% or Proparacaine
- Apply fluorescein strip
- For measuring the IOP in the right eye, make sure the slit beam is shining onto the tonometer head from the patient’s right side; for the left eye, the beam should come from the patient’s left side.
- Move the filters so that the cobalt blue filter.
- Make wide beam and full brightness to make it easy for visualising the fluorescein semi-circles easier
- Instruct the patient to look straight ahead, open both eyes wide, and ask not to move eyes in any direction.
- With the thumb, gently hold up the patient’s top eyelid, taking care not to put any pressure on the eye
- Direct the blue light from the slit lamp onto the prism head. Make sure that the tonometer head is perpendicular to the eye.
- Move the tonometer forward slowly until the prism rests gently on the centre of the patient’s cornea.
- With the other hand, turn the calibrated dial on the tonometer forward until the two fluorescein semi-circles in the prism head are seen to meet and form a horizontal ‘S’ shape. The correct end point is when the inner edges of the two fluorescein semi-circle images just touch as shown in the image below.
Note the reading on the dial and record it in the notes. The recorded intraocular pressure
is determined by multiplying the dial reading by ten.
Central Corneal Thickness and Tonometry Reading
Recent studies have suggested that in patients with thin corneas the intraocular pressure may be underestimated whilst in those with a thick cornea the pressure will be overestimated.
Contraindication of tonometry
- Ruptured globe
- Conjunctivitis and corneal ulcers