An observational study published in Clinical Ophthalmology examines the association between Goldmann applanation tonometer-calculated intraocular pressure (GAT-IOP) values and central corneal thickness (CCT) measurements taken with either ultrasonic instruments, such as pachymetry, or optical instruments, such as anterior segment optical coherence tomography (AS-OCT). 

Noteworthy findings include an affirmation for the impact of CCT on IOP values gathered with GAT. “Central corneal thickness could affect the GAT-measured IOP values,” according to the investigators.

Although previous studies reported that OCT underestimated CCT in comparison with an ultrasonic pachymeter, this analysis demonstrates a “strong positive association” between CCT measurements with both optical AS-OCT and corneal ultrasonic pachymetry. 


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Researchers say they identified “a significant positive correlation associating GAT-IOP values and the measured central corneal thickness both by optical and ultrasonic devices (P =.01 and P =.03, respectively).”

Ophthalmologic instruments estimate CCT at an average of about 520 µm to 550 µm, and clinicians typically use the adjustment factors to reach a corrected IOP value. Investigators found no significant difference when comparing GAT-IOP and CCT-corrected IOP value with the use of ultrasonic (P =.47) or optical AS-OCT (P =.06) methods. 

IOP was positively correlated to central corneal thickness-corrected pressure (IOPcc). This correlation was greater for those in the optically-measured CCT group (r=0.54; P =.001) but still highly significant in the ultrasonically measured set (r=0.46; P =.004).

Faculty of medicine researchers in the ophthalmology department at Minia University in Minia, Egypt, examined the GAT-IOP values first, and then took CCT measurements of 46 healthy patients (46 eyes). Patients with glaucoma, corneal opacities, or prior laser refractive surgeries were excluded. Participants ranged from 35 to 76 years of age, with a mean age of 48.37 plus or minus 10.8 years. The Ehlers formula was used to determine corrected IOP values.

Limitations of the study include a moderately small sample size of exclusively healthy patients in a nonrandomized design and the use of only 2 types of CCT-measuring tools. 

This investigation adds important knowledge to the often-discussed topic of adjusting IOP value based on CCT, as well as which instruments are available and reliable for determining corneal thickness.

Reference

Ahmed MAA, Abdelhalim AS. Corrected intraocular pressure variability with central corneal thickness measurement. Clin Ophthalmol. 2020;14:4501-4506. doi:10.2147/OPTH.S288391