Comparison of Two Formulas Used to Calculate Summarized Retinal Vessel Calibers

Area of Science:

• Ophthalmology
• Medical Imaging
• Biometry

Background:

• Accurate measurement of retinal vessel caliber is crucial for assessing ocular and systemic health.
• Existing formulas like Parr-Hubbard and Knudtson are used to calculate central retinal arterial equivalent (CRAE) and central retinal venular equivalent (CRVE).
• The impact of vessel selection on these calculations is not fully understood.

Purpose of the Study:

• To compare the Parr-Hubbard and Knudtson formulas for calculating retinal vessel calibers.
• To evaluate the effect of excluding specific retinal vessels on the calculated CRAE and CRVE.
• To assess the consistency of arteriovenous ratio (AVR) under different vessel inclusion scenarios.

Main Methods:

• Central retinal arterial equivalent (CRAE) and central retinal venular equivalent (CRVE) were calculated using both Parr-Hubbard and Knudtson formulas.
• Six large retinal arterioles and venules near the optic nerve head were included in the initial analysis.
• Vessels were subsequently omitted individually (one artery, one vein, or both) to re-evaluate parameters.

Main Results:

• Both methods demonstrated good correlation for CRAE (r²=0.58) and CRVE (r²=0.84).
• Significant differences in absolute CRAE and CRVE values were observed between the Parr-Hubbard and Knudtson methods (p < 0.000001).
• Excluding even one vessel significantly altered CRAE and CRVE values (p < 0.000001), while AVR remained stable when an equal number of arterioles and venules were analyzed.

Conclusions:

• Absolute CRAE and CRVE values are sensitive to the number of vessels included in the calculation.
• The arteriovenous ratio (AVR) is a more robust measure, showing comparability between methods as long as the same number of arterioles and venules are analyzed.