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Ocular tonometry (How to measure intraocular pressure)

Definition of Tonometry:

Tonometry is the procedure eye care professionals perform to determine the intraocular pressure (IOP), the fluid pressure inside the eye. It is an important test in the evaluation of patients at risk from glaucoma. Most tonometers are calibrated to measure pressure in millimeters of mercury (mmHg).

Schiötz tonometry

If glaucoma is diagnosed early, treatment can then be given that may preserve vision. Although raised intraocular pressure (IOP) is not the only sign of glaucoma, the IOP should be checked routinely on all adults attending eye care facilities. Applanation tonometry is the most accurate method to measure IOP, but Schiötz tonometry is also a useful screening test. If Schiötz tonometry reveals a high IOP, this result should be checked and confirmed by applanation tonometry and the patient referred to the senior clinician at the eye clinic.

You will need

  • Schiötz tonometer, weights, and scale card
  • local anaesthetic drops
  • clean cotton wool or gauze swabs
  • isopropyl alcohol 70 per cent (methylated spirit) or impregnated ‘Mediswabs’.
Schiotz Tonometer
Schiotz Tonometer

Preparation

  • Test the tonometer using the spherical mould in the box and the 5.5 g weight. The pointer should immediately reach the ‘O’ marking.
  • Clean the plunger and disc of the tonometer with a gauze swab (or cotton wool) and the methylated spirit (or a Mediswab). Wipe dry with a clean dry gauze swab (or cotton wool).
  • Lie the patient flat with his or her head supported on a pillow.

Method

  • Wash and dry your hands.
  • Position yourself correctly: stand upright, behind the head of the patient, with your hands level with the patient’s head. Note the health worker’s posture need to be good, Bad posture can affect the tonometry reading.
  • Instil local anaesthetic eye drops and wait about 30 seconds.
  • Ask the patient to look at a fixed object (the patient’s own thumb or finger held directly in front of his or her eyes may work) and 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 the other hand, hold the tonometer (with the 5.5 g weight) between the thumb and index finger and place the plunger on the central cornea.
  • Allow the disc to lower gently onto the corneal surface.
  • Note the scale reading.
  • If the scale reading is ‘2’ or less, remove the tonometer, replace the 5 g weight with the 7.5 g weight, and repeat the procedure.
  • Note the scale reading again and remove the tonometer.
  • Tell the patient not to rub the eye – the anaesthetic will last for about five minutes.
  • Clean and dry the tonometer head.
  • Repeat the whole procedure for the other eye.
  • Clean and dry the tonometer again and store it safely in the box.
  • Using the scale card, convert the noted scale readings and record the IOP in the patient’s records.

About the author: Sue Stevens Nurse Advisor to the Community Eye Health Journal , International Centre for Eye Health, London School of Hygiene and Tropical Medicine, Keppel Street, London WC1E 7HT, UK.

Applanation tonometry

Unless there is a contraindication (e.g. trauma or corneal ulcer), all adults attending an eye unit should have their intraocular pressure (IOP) measured. Many people with glaucoma have no symptoms and do not know they have the condition. All children who have had cataract surgery should also have their IOP measured at every follow-up visit, if possible. Finding glaucoma early allows treatment to be given which will preserve sight. Although elevated IOP is not the only sign of glaucoma, measuring it is simple and quick to do. Applanation tonometry, using a Goldmann tonometer at a slit lamp, is the preferred method (the ‘gold standard’).

Equipment

  • Goldmann tonometer
  • Applanation prism
  • Disinfectant: isopropyl alcohol 70% or sodium hypochlorite 1%
  • Local anaesthetic drops
  • Fluorescein strips
  • Clean cotton wool or gauze swabs.

Preparation

  • Ensure the prism has been disinfected with isopropyl alcohol 70% or sodium hypochlorite 1%. The prism must be rinsed in sterile water and wiped dry with a clean swab. WARNING: residue of the disinfectant may cause a caustic burn on the cornea.
  • Check that the gradation marked ‘0’ on the measuring prism is aligned with the white marker point on the tonometer head.
  • Check that the calibrated dial of the tonometer is set around 10 mmHg.
  • Ensure that the patient is sitting comfortably at the slit lamp: at the correct height, with chin on the rest and forehead against the headband.
  • Set the magnification of the slit lamp at ×10.

Method

  • Instill the local anaesthetic drops and then the fluorescein. Only a very small amount of fluorescein is needed.
  • 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 blue filter is used to produce a blue beam.
  • Make sure the beam of light is as wide as possible, and that the light is as bright as possible. This makes visualising the fluorescein semi-circles easier (with the slit diaphragm fully open).
  • Ask the patient to look straight ahead, open both eyes wide, and keep perfectly still.
  • 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.
  • Note the reading on the dial and record it in the notes.
  • Withdraw the prism from the corneal surface and wipe its tip with a clean, dry swab.
  • Repeat the procedure for the other eye.
  • Wipe the prism with a clean, dry swab and replace the prism in the receptacle with just its tip touching the disinfectant.

About Authors:Sue Stevens Former Nurse Advisor to the Community Eye Health Journal: International Centre for Eye Health, London School of Hygiene and Tropical Medicine, London,UK.
Clare Gilbert Co-Director: International Centre for Eye Health, and Chief Medical Advisor: Sightsavers,UK.
Nick Astbury Consultant ophthalmic surgeon (part-time): Norfolk and Norwich University Hospital NHSTrust.

Source:  International Centre for Eye Health, London School of Hygiene & Tropical Medicine. All content is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License.

 

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