Measurement of IOP in a clinical setting requires a force that indents or flattens the eye. Applanation tonometry is the method used most widely. It is based on the Imbert-Fick principle, which states that the pressure inside an ideal dry, thin-walled sphere equals the force necessary to flatten its surface divided by the area of the flattening:
P = F/A
where P = pressure, F = force, and A = area. In applanation tonometry, the cornea is
flattened, and IOP is determined by measuring the applanating force and the area flattened (Fig 2-4).
The Goldmann applanation tonometer measures the force necessary to flatten an area of the cornea of 3.06 mm diameter. At this diameter, the resistance of the cornea to flattening is counterbalanced by the capillary attraction of the tear film meniscus for the tonometer head. Furthermore, the IOP (in mm Hg) equals the flattening force (in grams) multiplied by 10. A split-image prism allows the examiner to determine the flattened area with great accuracy. Fluorescein in the tear film is used to outline the area of flattening. The semicircles move with the ocular pulse, and the endpoint is reached when the inner edges of the semicircles touch each other at the midpoint of their excursion (Fig 2-5).
Applanation measurements are safe, easy to perform, and relatively accurate in most clinical situations. Of the currently available devices, the Goldmann applanation tonometer is the most valid and reliable. Because applanation does not displace much fluid (approximately 0.5 µL) or substantially increase the pressure in the eye, this method is relatively unaffected by ocular rigidity. Table 2-3 lists possible sources of error in tonometry.
An excessive amount of fluorescein results in wide mires and an inaccurately high reading, whereas an inadequate amount of fluorescein leads to artificially low readings.
Marked corneal astigmatism causes an elliptical fluorescein pattern. To obtain an accurate reading, the clinician should rotate the prism so the red mark on the prism holder is set at the least curved meridian of the cornea (along the negative axis). Alternatively, 2 pressure readings taken 90° apart can be averaged.
The accuracy of applanation tonometry is reduced in certain situations. Corneal edema predisposes to inaccurate low readings, whereas pressure measurements taken over a corneal scar will be falsely high. Tonometry performed over a soft contact lens gives falsely low values. Alterations in scleral rigidity may compromise the accuracy of measurements; for example, applanation readings that follow scleral buckling procedures may be inaccurately low.
Applanation tonometry measurements are also affected by the central corneal thickness (CCT). Recently, the importance of CCT and its effect on the accuracy of IOP measurement has become better understood. The Goldmann tonometer is most accurate, with a CCT of 520 µm; however, population studies have shown a wide range of normal, with mean CCT between 537 and 554 µm.
Increased CCT may give an artificially high IOP measurement; decreased CCT, an artificially low reading. IOP measured after photorefractive keratectomy (PRK) and laser in situ keratomileusis (LASIK) may be reduced because of changes in the corneal thickness induced by these and other refractive procedures. As a rough guide, using an overview of published studies, it can be estimated that for every 10-µm difference in CCT from the population mean (approximately 542 µm), there is a 0.5 mm Hg difference between actual IOP and the IOP measured with a Goldmann tonometer. However, because the relationship of measured IOP and CCT is not linear, it is important to remember that such correction factors as this are only estimates at best. In addition, the biomechanical properties of an individual cornea may vary, resulting in changes of the relative stiffness or rigidity of the cornea and altering the measurement. The Goldmann tonometer, Perkins tonometer, pneumatonometer, noncontact tonometer, and Tono-Pen are all affected by CCT. Currently, there is no validated correction factor for the effect of CCT on applanation tonometers; therefore, clinical application of any of the proposed correction methods should be avoided.
The Ocular Hypertension Treatment Study (OHTS) found that a thinner central cornea was a strong predictive factor for the development of glaucoma in subjects with ocular hypertension. Subjects with a corneal thickness of 555 µm or less had a threefold greater risk of developing POAG compared with participants who had a corneal thickness of more than 588 µm. Whether this increased risk of glaucoma is due to underestimating actual IOP in patients with thinner corneas or whether thin corneas are a risk factor independent of IOP measurement has not been completely determined, but the OHTS found CCT to be a risk factor for progression independent of IOP level.
The Perkins tonometer is a counterbalanced applanation tonometer that is portable and can be used with the patient either upright or supine. It is similar to the Goldmann tonometer in using a split-image device and fluorescein staining of the tears.