Assessment of reproducibility of measures of intraocular pressure and central corneal thickness in young white adults over a 16‐h time period

Abstract

These studies confirm that the reproducibility (repeatability) of NCT and pachometry are substantially different. It is suggested that this aspect of tonometry needs to be taken into account, especially where there are lower values, when considering the possible impact of corneal thickness on tonometry measures and their interpretation. These types of effects need to be further assessed in older patients with ocular hypertension, or other glaucoma suspects.

Effect of central corneal thickness on Goldmann applanation tonometry measures—a different result with different pachymeters

“…Such a result was obtained using either ultrasound or Orbscan pachymetry methods. Very similar results have also been obtained for Perkins applanation tonometry versus ultrasound pachymetry in elderly subjects [5], and also using non-contact tonometry (NCT) versus non-contact specular microscopy as an optical pachymetry method in young subjects [6]. Therefore, the use of an Orbscan to measure central zone corneal thickness (rather than ultrasound pachymetry) is not the reason for the failure to find a strong association between Goldmann tonometry values and the corneal thickness [8,9].…”

“…Numerous studies have indicated just how repeatable pachymetry measures are, largely regardless of the instrument used [4,6]. Therefore, when low or very low correlations between IOP and CCT measures are reported then this must, at least in part, be attributed to the variance in the tonometry measures [5,6]. In the present studies, three tonometer readings were taken in an effort to reduce the variance to its minimum value by using an averaged value for each IOP assessment.…”

“…Three readings were taken for both tonometry and pachymetry. This minimal value for variance is likely to be around ±10%, as compared to ±1% for pachymetry [6,14]. This unavoidable variance in tonometry measures is likely to be proportionately so much higher than for the pachymetry.…”

“…It remains to be established why such variance occurs in the IOP-CCT relationships found in different studies, but there are two inter-related issues that need to be considered. Numerous studies have indicated just how repeatable pachymetry measures are, largely regardless of the instrument used [4,6]. Therefore, when low or very low correlations between IOP and CCT measures are reported then this must, at least in part, be attributed to the variance in the tonometry measures [5,6].…”

“…The impact of CCT on the tonometry measures can be objectively assessed by examining whether the measured IOP values are predictably different with respect to the central corneal thickness values, i.e. by the IOP-CCT relationship [4][5][6].…”