9.2.7  Trans-Palpebral Tonometers

Recently instruments have been developed that measure IOP through the eyelids; examples include the Proview and the TGDc-01 [29–31]. While these instruments generally are not accurate enough for regular clinical use, they may have some value in approximating IOP when ordinary tonometry is not possible, such as with corneal prostheses and totally scarred corneas.

Summary for the Clinician

››No instrument currently measures IOP accurately under all conditions.

››The Goldmann tonometer is quite accurate in eyes with average corneal thickness.

››In eyes with very thin corneas, either naturally or after LASIK, DCT is the best source of accurate IOP measurement.

››Pneumotonometry also provides reasonably accurate IOP readings across the range of corneal thicknesses and corneal pathology.

››Readings taken with the Goldmann tonometer in eyes with corneal thickness less than 530 mm should be regarded as underestimating the true pressure. The thinner the cornea, the more suspicious one should be about underestimating the IOP with the Goldmann or other applanation devices.

9.3  If Goldmann Applanation is not Available During an Exam Under Anesthesia, What Instrument

is the Next Most Preferred for IOP Measurement?

The Goldmann tonometer cannot measure IOP when the subject is supine. The most accurate way of measuring IOP in the supine position (as would be true under general anesthesia, under sedation, or at the bedside) is with a Perkins or Draeger tonometer (modified Goldmann-type devices) as long as the cornea is of average thickness and curvature. If the cornea is edematous, thin, thick, or irregular, pneumotonometry would be the most accurate. This is the device used in our own operating room as it works well even if the cornea is edematous [32] (Fig. 9.3a, b). Somewhat less accurate but still relatively usable is the Tonopen.

Summary for the Clinician

››A portable Perkins or Draeger tonometer is best when measuring average corneas in the supine position.

››Pneumotonometry is best when the cornea is abnormal.

››The Tonopen is practical but does not provide the most accurate IOP estimate.

Fig. 9.3  (a) The pneumotonometer probe on the eye. It floats on a cushion of gas. (b) The pneumotonometer readout includes a digital average intraocular pressure in mm Hg. The paper tracing

also indicates the puslatile nature of intraocular pressure and when the pulses are present and the intraocular pressure is steady, indicates a satisfactory reading