Meta-Analysis of Tonometer Correction Formulae to Determine True Normal Intraocular Pressures Across a Diverse Range of Species.

Abstract

OBJECTIVE: To utilize previously published tonometer correction formulae to investigate whether true intraocular pressure (IOP) is relatively conserved across a diverse range of vertebrate species. ANIMALS STUDIED: Tonometer correction formulae for 21 species. PROCEDURES: A literature search was performed to identify prior studies containing correction formulae for non-invasive tonometers used in normal animal and human subjects. In all cases, direct manometry was compared to non-invasive tonometry values. Where necessary, multiple correction regression formulae were combined to generate a single representative formula using a meta-analysis generic inverse variance method. A subsequent literature search was then performed to collect uncorrected IOP values of normal animals and humans using a variety of non-invasive tonometers. These IOP values were then corrected using the relevant correction formula. RESULTS: Sixty-five tonometer correction formulae and 104 studies containing IOP values in normal animals and humans acquired using non-invasive tonometry met the inclusion criteria. Corrected IOP values were calculated for 21 species using 13 types of non-invasive tonometers. Overall corrected and uncorrected mean IOP values were typically within a range of 10 to 25 mmHg (corrected: 16/21 species, uncorrected: 14/21 species). Differences between overall mean corrected and uncorrected IOP values ranged from 0.2 to 15.3 mmHg with an average of 6.1 mmHg difference. CONCLUSIONS: True normal IOP is relatively conserved across a diverse range of vertebrate species despite uncorrected values from individual tonometers suggesting otherwise. Maintaining true IOP between 10 and 25 mmHg appears to be important in ocular physiology in many vertebrates.