Deriving "true" intraocular pressure through corneal indentation

Abstract

Purpose : Although Goldmann applanation tonometry is gold standard in measuring intraocular pressure (IOP), it is affected by central corneal thickness (CCT) and corneal biomechanics. Aim of this study was to derive IOP from corneal indentation.

Methods : According to the Imbert-Fick’s law, IOP is calculated by force (F) and area (A) during applanation.
IOP = F / A
Twenty freshly enucleated porcine eyes were obtained from a local abattoir. To confirm good condition of each eye, IOP in the anterior chamber was monitored by a pressure senor (IOP_true) and tested at perfusion rates from 4 to 12mL/min. At each stabilized IOP, F was measured during a 0.6mm corneal indentation using a 3.5mm diameter indenter. The equation from the Imbert-Fick’s law was rewritten during corneal indentation.
IOP₀ + △IOP = F / A_eq (Equation 1)
IOP₀: original IOP; △IOP: IOP change during indentation; A_eq: equivalent applanation area for a 3.5mm indenter

△IOP is a variable depending on CCT and IOP₀. A multiple regression model predicted △IOP as a function of CCT and IOP₀.
△IOP = -3.97 + 3.91 CCT + 0.1781 IOP₀ (Equation 2)

Equation 1 was rewritten to,
IOP₀ = F / A_eq - △IOP

Substituting Equation 2 gave,
IOP₀ + (3.91/1.1781)*CCT - (3.97/1.1781) = (1/1.1781*133.33*A_eq)*F
and was equivalent to
IOP₀ + 3.3189*CCT - 3.3698 = (F/1.1781*133.33)*(1/A_eq) (Equation 3)

CCT was measured in each eye using an ultrasound pachometer. A total of 74 data points was obtained from 20 porcine eyes. Half of them was used as training dataset to derive A_eq. The remaining half (testing dataset) was used to derive IOP₀, and compare it with IOP_true.

Results : Figure 1 is a plot of Equation 3, y = mx where slope m was 0.095093. A_eq (reciprocal of slope) was calculated as 10.52mm^2.

Rearranging Equation 3,
IOP₀ = (F/1.1781*133.33)*(1/A_eq) - 3.3189*CCT + 3.3698 (Equation 4)

Figure 2 shows a plot of predicted IOP₀ (from Equation 4) and the IOP from pressure sensor (IOP_true). The mean absolute error between the two IOP values was 1.84mmHg only.

Conclusions : A linear model is developed to correlate pressure change induced by indentation and the indentation force to the initial IOP. Using porcine eye data, corneal indentation can derive “true” IOP with good accuracy. We will validate this method in human eyes using dynamic contour tonometry to represent the “true” IOP

This abstract was presented at the 2023 ARVO Annual Meeting, held in New Orleans, LA, April 23-27, 2023.

 

Model described in Equation 3 and fitted to 37 data points (training dataset).

 

Model described in Equation 4 and fitted to 37 data point (testing dataset).