Method and apparatus for obtaining corrected intraocular pressure values

• US 20040260168A1
• Filed: 06/17/2003
• Published: 12/23/2004
• Est. Priority Date: 06/17/2003
• Status: Active Grant

• Current Assignee: Mitsugu Shimmyo
• Original Assignee: Mitsugu Shimmyo
• Inventors: Shimmyo, Mitsugu
• Granted Patent: US 7,204,806 B2
• Time in Patent Office: Days
• US Class Current: 600/399
• CPC Class Codes: A61B 3/107 for determining the shape or measuring the curvature of the cornea
• CPC Class Codes: A61B 3/16 for measuring intraocular pressure, e.g. tonometers

Abstract

The disclosed methods and systems use measured values from applanation tonometry and pachymetry, and corneal curvature values to produce corrected values of intraocular pressure to diagnose and treat pressure related eye diseases.

Claims

1. A method for determining interocular pressure comprising:

measuring interocular pressure by applanation tonometry A;

measuring central corneal thickness T by pachymetry; and

obtaining a corrected interocular pressure P in accordance with the relationship

P=A+(T_o−

T)/X

 

wherein T_o is a median corneal thickness value and wherein X is an empirically determined error ratio which declines in value for increasing A.

2. The method of claim 1, further comprising the step of measuring the curvature of the cornea and wherein the value P is further corrected for corneal curvature by adding to P a correction factor C related to corneal curvature.

3. The method of claim 2, wherein the corrected ocular pressure P is given by the equation

P=A+(T_o−

T)/X+C

where P is in mmHg, A is a Goldmannn applanation reading, R_o is 550μ

(microns), and T is central corneal thickness measured by ultrasonic pachymetry in microns.

4. The method of claim 3, wherein the correction factor C is given by the equation C=0.8·

(r−

7.85)

where r is a measured radius of the cornea in millimeters.

5. The method of claim 3, wherein corneal curvature is measured to obtain two keratomeric values K1 and K2 of an astigmatic cornea and wherein the correction factor C is given by the equation.

6. The method of claim 1 wherein X is a function of A and decreases non-linearly as A increases in value.

7. The method of claim 3, wherein X is a function of A and varies from about 17 to about 11 as A varies from about 5 to about 45 mmHg.

8. The method of claim 7, wherein X is given by the equation

X=18e⁻

0.005A

9. The method of claim 1, wherein X varies in accordance with the following table

10. A method for determining interocular pressures by correcting measured pressure values, A, obtained by applanation tonometry for patients, wherein the measured A values vary between at least 10 and 40 mmHg comprising

measuring interocular pressure A by applanation tonometry of the patients;

measuring total corneal thicknesses T by pachymetry for the patients; and

measuring at least one radius of the curvature r of the cornea of the patients;

obtaining a corrected interocular pressure P for each patient in accordance with the relationship.

P=A+f(T,X)+C(r)

 

where X is describable as a non-linear function of measured A values, where f(T,X) is a pressure correction term expressible as a function of T and X and wherein C (r) is a pressure correction term expressible as a function of r.

11. The method of claim 10, wherein

f(T,X)=(T_o−

T)/X

wherein T_o is a medium corneal thickness value, and wherein

C(r)=c₁*(r−

c₂)

where c₁ and c₂ are constants.

12. The method of claim 11 wherein X is an exponential functional of A.

13. The method of claim 11, wherein X is given by the equation

X=18e⁻

0.005A

14. A system for determining a corrected interocular pressure P comprising

means for inputting values for an applanation measurement A, for a central cornea thickness measurement T and for a measurement related in value to a radius of curvature of the cornea;

a processor for obtaining a value of X corresponding to a measured value of A and for calculating corrected interocular pressure P in accordance with the relationship

P=A+f(T,X)+C

 

where f(T,X) is a correction for corneal thickness and C is a correction determined from the measurement related value to curvature of the cornea; and

means for displaying the results of the calculation.

15. the system of claim 14 wherein C is determined as a function of radius, dioptric power or keratometric values of the cornea.

16. The system of claim 14, further comprising a Goldmann applanation measurement device for obtaining a value A in mmHg.

17. The system of claim 14, further comprising an ultrasonic pachymetry system for measuring central corneal thickness.

18. The system of claim 14, wherein the processor calculates the corrected interocular pressure P in accordance with the equation

P=A+(550−

T)/X+0.8 (337.5/D−

7.85)

wherein P and A are in mmHg, T is in microns and D is a measured dioptric value of the cornea.

19. The system of claim 14, wherein the processor calculates the corrected interocular pressure P in accordance with the equation

20. The system of claim 14, wherein

f(T,X)=(T_o−

T)/X

where T_o is a median central cornea thickness for the general population as measured by the thickness measuring technique and wherein X values decrease non-linearly with increase in the value of A.

21. The system of claim 14, wherein

f(T,X)=(T_o−

T)/X

where T_o is 550μ

where T is measured in microns and wherein X values given by an equation of the form

22. The system of claim 21, wherein

x₁ varies from about 0.24 to 0.08 for A in the range 0<

A<

60 and wherein x₂ varies from about 18.4 to about 14.4 for A in the range 0<

A<

60.

23. The system of claim 21, wherein X values are selected from the following values from the database memory

24. The system of claim 21 wherein X is in accordance with the equation

X=18e⁻

0.005A