Magnification alters retinal vessel appearance in myopia

Paper title: The Optical Nature of Myopic Changes in Retinal Vessel Caliber
Authors: Yii F (1,2), Strang N (3), Moulson C (3), Dhillon B (1,2,4), Bernabeu MO (5), MacGillivray T (1,2)

1. Centre for Clinical Brain Sciences, University of Edinburgh, Edinburgh, UK
2. Anne Rowling Regenerative Neurology Clinic, University of Edinburgh, Edinburgh, UK
3. Department of Vision Sciences, Glasgow Caledonian University, Glasgow, UK
4. Royal Infirmary of Edinburgh, Edinburgh, UK
5. School of Informatics, University of Edinburgh, Edinburgh, UK

Date: Published online October 10, 2024
Reference: Ophthalmol Sci. 2024 Oct 10;5(1):100631. eCollection 2025 Jan-Feb.

[Link to open access paper]

Retinal vessel calibre measurements, such as central retinal arteriolar and venular equivalents (CRAE and CRVE), are commonly used in both clinical practice and systemic disease research. These measurements can be affected by ocular magnification, which varies with axial length and refractive error. Some previous studies have reported narrower vessels in myopic eyes even after applying magnification correction, prompting speculation that reduced perfusion may accompany axial elongation.1,2    However, these observations may reflect measurement artefacts introduced by inaccuracies in the correction method, rather than true biological change. One source of potential error is the assumption that all imaging systems are telecentric. Telecentric fundus cameras produce a fixed image size regardless of axial length, but this is not true of all devices. This study aimed to determine whether applying magnification correction removes the apparent link between axial length (AL) and vessel narrowing. 

Eighty-two healthy right eyes from young adults aged 16–31 years at Glasgow Caledonian University were analysed. Retinal vessel calibre was measured in pixels from fundus photographs using semiautomated analysis, and associations with AL were assessed before and after applying correction formulas.

Key findings were as follows.

• Before correction, each 1 mm increase in AL was linked to a 0.49-pixel reduction in CRAE and a 0.91-pixel reduction in CRVE.
• After applying magnification correction, these associations disappeared.
• Additional analyses showed that incorrectly assuming the camera was telecentric introduced a false association, making vessels appear wider in longer eyes.
• The results support that observed vessel narrowing in myopia is an optical artefact caused by magnification, not a biological effect of axial elongation.

This study provides clarification for interpreting retinal vessel calibre in myopic eyes. When vessel narrowing is observed in longer eyes, it is often assumed to reflect vascular remodelling or reduced ocular perfusion. However, this research demonstrates that such associations are primarily optical artifacts caused by reduced ocular magnification, not biological changes. Crucially, when axial length–related magnification is properly corrected, vessel narrowing disappears—regardless of the magnification formula used.

For clinicians, this has implications when evaluating retinal photographs for systemic risk markers such as arteriolar narrowing or venular widening, particularly in young or myopic patients. Without applying magnification correction, or if the fundus camera’s optical design is mischaracterised, clinicians may over- or underestimate vessel dimensions, potentially leading to inaccurate inferences about vascular health.

In clinical practice imaging, most analysis tools do not apply magnification correction by default. Practitioners should be aware of which formula (if any) is used and how it relates to the patient's axial length or refractive error. Understanding whether the imaging system is telecentric is also critical, as incorrect assumptions can reverse the direction of apparent changes.

A limitation of this study is the use of a relatively small sample of healthy young adults, meaning the findings may not be applicable to other groups, such as children, older adults, or individuals with ocular or systemic disease.

The design of the imaging system is another consideration. Because the telecentricity of the fundus camera couldn’t be confirmed, the correction factor had to be estimated, which could have affected accuracy. All of the magnification correction methods also made assumptions such as symmetrical eye shape and small-angle optics, which may not hold true in all eyes.

This study looked at vessel dimensions based on imaging, not vascular function. So, while vessel narrowing in myopia appears to be an optical effect rather than a biological one, this doesn’t rule out the possibility of physiological differences in ocular blood flow. Further research using direct measures of perfusion will be needed to explore this.

Purpose: Dimensional measures of retinal features are subject to the optical influence of ocular magnification. We examined the impact of ocular magnification on the association between axial length (AL) and measurements of retinal vessel caliber in fundus photographs.

Design: Cross-sectional study.

Participants: Eighty-two normal right eyes from healthy participants aged 16 to 31 years.

Methods: Central retinal arteriolar and venular equivalents (CRAE and CRVE) were derived from color fundus photographs using semiautomated software. Ordinary least squares linear regression was used to assess the influence of AL (independent variable) on CRAE and CRVE, controlling for age, sex, and ethnicity, both before and after magnification correction using different formulae. These formulae estimate magnification based on different ocular parameters: AL only (Bennett’s formula), refractive error only (Bengtsson’s formula), and refractive error combined with keratometry (Littmann’s formula). Previous research has primarily relied on Bengtsson’s formula, which is less accurate than Bennett’s formula. We also examined the impact of treating the nontelecentric fundus camera used in this study as telecentric when applying these magnification correction formulae.

Main outcome measures: Central retinal arteriolar and venular equivalents (in pixels).

Results: Before magnification correction, increasing AL was associated with decreasing CRAE (β: −0.49, 95% confidence intervals: −0.89 to −0.09, P = 0.02) and CRVE (β: −0.91, 95% confidence intervals: −1.62 to −0.20, P = 0.01). After magnification correction, this observation was no longer evident, regardless of the correction formula applied. When inappropriately assuming the fundus camera to be telecentric, we observed a bias toward increasing magnification-corrected CRAE and CRVE with increasing AL (β coefficients were positive or became more positive), reaching statistical significance (P < 0.05) for CRAE corrected using Bennett’s or Littmann’s formula, and for CRVE corrected using Bennett’s formula.

Conclusions: Failing to correct for ocular magnification results in apparent narrowing of vessels in longer eyes, while inappropriate assumptions about telecentricity during magnification correction introduce an optical artifact that causes apparent widening of vessels. These findings suggest that myopic changes in retinal vessel caliber are optical (not biological) in nature. Proper correction of this effect to accurately derive dimensional measures is a crucial—yet often overlooked—methodological consideration in “oculomics” research investigating retinal biomarkers of systemic conditions.

[Link to open access paper]