Retinal nerve fibre layer and visual function loss in glaucoma: the tipping point

Area of Science:

• Ophthalmology
• Neuroscience
• Medical Imaging

Background:

• Retinal nerve fibre layer (RNFL) thickness is a key indicator of optic nerve health.
• Visual field (VF) tests assess functional vision, often used in glaucoma diagnosis.
• Early detection of glaucoma relies on correlating structural and functional changes.

Purpose of the Study:

• To identify the specific RNFL thickness threshold at which detectable visual field damage occurs.
• To establish the relationship between structural loss (RNFL thickness) and functional impairment (VF defects).

Main Methods:

• A prospective cross-sectional study involving 112 participants (72 healthy, 40 glaucoma).
• Spectral domain optical coherence tomography (SD-OCT) for RNFL thickness measurements.
• Humphrey visual field testing to assess functional vision.
• A 'broken stick' statistical model to determine the RNFL thickness tipping point.

Main Results:

• The mean RNFL thickness threshold for detectable VF loss was 75.3 micrometers, representing a 17.3% loss from normative values.
• A significant difference in the association slope between RNFL thickness and VF threshold was found below and above the tipping point (p<0.001).
• The slope was steeper below the tipping point, indicating a stronger correlation between structural loss and functional deficit.

Conclusions:

• Significant structural damage, approximately 17% RNFL loss, is required for functional deficits to be detected by current visual field testing methods.
• This finding highlights the sensitivity limitations of current functional tests in early glaucoma detection.
• Understanding this structural-functional relationship is crucial for refining diagnostic criteria and monitoring disease progression.