Norm of Tonometry Test for Glaucoma
10–22 mm Hg; mean 16 mm Hg with standard deviation of 3 mm Hg.
Warning: 22–28 mm Hg. More testing required.
Normal values of 21 mm Hg or less can occur in a condition known as normal or low-tension glaucoma.
Major concern: >38 mm Hg.
Panic levels: There is lack of definitive intraocular pressure cutoff level for glaucoma.
Usage of Tonometry Test for Glaucoma
Screening for glaucoma. Ongoing monitoring for clients with glaucoma.
Description of Tonometry Test for Glaucoma
In glaucoma, the intraocular pressure increases either because of blocked drainage or because of excessive production of aqueous humor. Tonometry testing involves measurement of intraocular pressure using a tonometer, an instrument that is lightly pressed directly against the anesthetized eye. The tonometry test can be conducted in any of three ways. In contact tonometry, in which the instrument touches the eye, the Schiøtz method, the Goldmann applanation method (commonly known as the “blue light” test), or a handheld method with a small penlike tonometer may be used. Contact tonometers make an indentation in the eye with a specific amount of weight and record the amount of resistance to the indentation, which is then converted to an intraocular pressure. In noncontact (indentation) tonometry, pneumotonometry or “air” tonometry may be used. Noncontact tonometry measures eye pressure indirectly. Noncontact tonometry has been found to improve compliance with testing in children, and there has been improvement in the accuracy of the handheld units. Pneumotonometry is the gentlest method and preferred for clients after LASIK surgery. Overall, tonometry is superior to digital tension, which tends to underestimate pressure, for obtaining intraocular pressure in young children. The Goldmann applanation method is considered the criterion standard for this procedure.
Professional Considerations of Tonometry Test for Glaucoma
Consent form NOT required.
Corneal abrasion or infection.
Corneal infection or ulcer, unless absolutely necessary. It is also contraindicated in clients who may be unable to hold very still during the test (that is, those with persistent coughing or sneezing).
- Remove contact lenses and loosen any jewelry or clothing (e.g., tie, tight collar, necklace) around the neck area.
- For all methods except the noncontact methods, anesthetic eye drops are instilled bilaterally.
- Schiøtz method: Obtain sterile tonofilms for the contact tonometer.
- Schiøtz method:
- a. The client is positioned supine.
- b. One eye is tested at a time. After the tonometer is zeroed, the eyelids are held open as the tonometer is placed against the eyeball. The tonometer is pressed against the eye with a specific amount of weight, and the tonometer scale reflects a number that is converted to millimeters of mercury (mm Hg) for an intraocular pressure (IOP) reading.
- Goldmann applanation method:
- a. A fluorescein-stained paper is touched against the surface of the eye and removed.
- b. The slit-lamp is advanced until the tonometer touches the eye surface.
- c. A digital measurement of the pressure required to flatten a small portion of the eye surface is recorded.
- Handheld tonometer method:
- a. A small, penlike handheld tonometer is lightly pressed against the surface of the eye and a digital measurement of the eye pressure is taken.
- Noncontact “air puff” method (indentation tonometry):
- a. The client's head is positioned on a chin rest and the equipment is aligned to the eye. As a calibrated puff of air is expelled from the equipment against the eye surface, this causes an indentation in the eye surface and a photoelectric cell measures the amount of corneal deformity indicated by changing reflections back to the light source from the corneas.
- Noncontact pneumotonometry method:
- a. A handheld pneumotonometer connected to a long tube is placed against the cornea. A stream of air flows through the tube and is directed into the sensing tip, where ocular pressure, standard deviation, and ocular pulse pressure are measured and displayed on a screen.
- Eyeglasses may be worn in place of contact lenses.
- See Client and Family Teaching.
Client and Family Teaching
- Provide a thorough explanation of the procedure, emphasizing that the client must cooperate by keeping the eyes open during testing.
- Contact lenses must be removed. Bring eyeglasses to wear, if needed, after the test.
- Avoid rubbing the eyes or replacing contact lenses until the local anesthetic has worn off (about 2 hours). Rubbing the eyes before this time can cause corneal abrasion, which is painful and takes several days to heal.
Factors That Affect Results
- It may be necessary to adjust the weight placed against the eyeball to obtain a consistent pressure reading.
- Clients who have undergone laser eye surgery may test normal, yet still have increased intraocular pressure, because laser eye surgery results in thinner corneas. In these clients the Pachymetry test for corneal thickness is recommended.
- IOP evaluation in the detection of glaucoma has approximately 50% specificity.
- Pneumotonometry is another specialized method of measuring IOP and is used in cases of irregular corneas or after keratoplasty when the applanation tonometer cannot be used.
- For abnormal findings, the client should have a full ophthalmologic examination, including cup-to-disk ratio and field studies.