Tonometry

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Tonometry

 

the measurement of pressure—usually referring specifically to intraocular pressure. All tonometric methods are based on measurement of the eyeball’s capacity for deformation.

The approximate magnitude of intraocular pressure can be estimated by digital tonometry—the examiner making a subjective judgment of the degree of firmness of the eye by pressing it with the fingers. Greater accuracy in tonometry is achieved by the use of tonometers of varying design. Impression tonometry measures the degree of indentation of the eyeball produced by the tonometer rod and translates the reading into measures of intraocular pressure. Applanation tonometry measures the extent to which the eye can be flattened; the imprint (or tonogram) produced by a cylinder’s pressure on the eye is in the shape of a small circle, whose dimensions are converted into an index of pressure.

References in periodicals archive ?
Results of an earlier study revealed a comparable rise in IOP with age in common kestrels, Muscovy ducks (Cairina moschata), and greylag geese (Anser anser) measured by applanation tonometry, (9) whereas the same could not be proven for eastern screech owls.
GAT, or any other form of applanation tonometry, produces a gross underestimation of IOP when the cornea is highly oedematous due to the soft cornea being 'moulded' or 'squashed' giving the appearance of applanation, even though the underlying IOP plays no part in the measurement.
If pressure is above 21mmHg in either eye and there is a suspicious visual field defect, then the pressure should be repeated initially with applanation tonometry (ideally immediately following the sight test).
GAT and its many non-contact mimics rely on the 'Imbert Fick' principle, not a law but an explanation for applanation tonometry where none of the assumptions are true.
Salvetat ML, et al (2011) Comparison of iCare tonometer and Goldmann applanation tonometry in normal corneas and in eyes with automated lamellar and penetrating keratoplasty.
In a patient with a thicker than average CCT, applanation tonometry will overestimate the true IOP while, conversely, in a patient with a thinner than average CCT, applanation tonometry will underestimate the true IOP.
28mmHg with Goldmann applanation tonometry (GAT), for every 10pm increase in CCT.
The joint letter, sent from the College of Optometrists and the Royal College of Ophthalmologists outlines their belief that a commissioned service to community optometrists, for repeating pressure measurements from a Goldmann-style Contact Applanation Tonometry, would significantly reduce the number of positive referrals.