9.4  In Cases of Corneal Transplants, Corneal Edema or Scarring, Which Instrument Would Be Best to Use to Obtain Accurate IOP Measurements?

In eyes with corneal edema, significant surface irregularity, scarring, and recent corneal grafts, pneumotonometry, Tonopen, and DCT will give more accurate readings than Goldmann or other applanation tonometry. Pneumotonometry may be the most accurate way of measuring IOP in the operating room while the patient is supine. It has the added advantage of giving a printed recording of the real-time pulsatile IOP, the examination of which can confirm or question whether the reading is likely to be accurate. The Tonopen also may be useful in the operating room or at the bedside. If multiple readings are taken, the instrument records a standard deviation which is an indicator of consistency, and perhaps an indirect indicator of accuracy.

Summary for the Clinician

››Pneumotonometry may best estimate IOP in edematous or scarred corneas and in eyes with corneal transplants.

9.5  In Cases of Prosthetic Corneas How Can I Measure the IOP?

In eyes with totally distorted or calcified corneas and in those with corneal prostheses, or palpation, one of the transpalpebral tonometers may be the best way to estimate the IOP.

Summary for the Clinician

››No instrument estimate of the IOP will be accurate and reliable in the case of a prothetic cornea.

››Palpation can be used to estimate IOP as low, medium, or high.

9.6  Can I Convert the Readings

of One Instrument to Those

of Another?

While some have tried to apply a conversion factor to Goldmann readings, especially as they apply to corneal thickness, the lack of linearity in the relationship between corneal thickness and Goldmann readings, has made it impossible for any single conversion factor to take an inaccurate reading and make it accurate. The errors of the other instruments are also nonlinear, and therefore conversion cannot be made from one instrument to the next.

Summary for the Clinician

››Each instrument has its own non-linear margin of error.

››At the present time, we don’t know what conversion factor would have to be used between different instruments.

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