Eyes with high myopia are significantly larger in volume than nonmyopic eyes, and this enlargement occurs independently of the presence of posterior staphylomas, according to new research.
Using three-dimensional magnetic resonance imaging (3D MRI), researchers from the Department of Ophthalmology and Visual Science at the Institute of Science Tokyo evaluated 370 eyes with high myopia (HM) and 44 non-HM eyes to investigate how ocular volume correlates with axial length, eye shape, and myopic complications.
HM was defined as a refractive error worse than −8 D or an axial length of at least 26.5 mm. MRI-based volumetric analysis allowed for objective evaluation of globe shape and the presence of posterior staphylomas from multiple viewing angles.
The investigators found that HM eyes had higher mean eye volume (12.42 ± 2.40 mL vs 9.67 ± 1.41 mL in non-HM eyes) and were generally larger in volume by approximately 1.3 times (P<.001). The maximum recorded volume was 24.95 mL in HM eyes, which was nearly 3 times greater than the non-HM mean.
Axial length in HM eyes was also longer (30.17 ± 2.35 mm) than non-HM eyes (24.73 ± 1.08 mm, P<.001). Lateral length of HM eyes was significant, but differences in vertical length were not.
Five eye shape categories were defined from 3D MRI images: spherical, cylindrical, nasally distorted, temporally distorted, and barrel-shaped. Barrel-shaped eyes were most common in both HM and non-HM eyes, while temporally distorted shape was seen only in HM eyes.
Volume by shape (in HM eyes):
- Barrel: 13.12 ± 2.13 mL
- Cylindrical: 11.56 ± 2.04 mL (significantly smaller)
- Temporal: 12.95 ± 3.48 mL
- Spherical: 12.1 ± 1.53 mL
The researchers noted that axial length was shorter in spherical and cylindrical-shaped eyes, compared with the other shapes. “These findings suggested that the smallest eye volume in cylindrical-shaped eyes was not only because the posterior scleral pole was conical but also because the axial and vertical lengths were shorter. However, this conclusion needs further investigations, including longitudinal observations,” they added.
Posterior staphylomas were identified in 74.4% of HM eyes but none of the non-HM eyes. Interestingly, the presence of a staphyloma was not associated with a larger eye volume; however, axial length was significantly longer in eyes without staphyloma (30.42 mm vs 29.42 mm, P=.006). Multivariate regression analysis also showed that eye volume was negatively associated with age.
Volume and axial length increased with the severity of myopic maculopathy, based on the META-PM classification system. However, complications such as myopic traction maculopathy, myopic neovascularization, or myopic glaucoma-like optic neuropathy were not significantly correlated with ocular volume.
Because this was a single-center study, there may have been selection bias for the participants, which limited the generalizability of the findings, the authors noted. Among their other limitations, some staphyloma cases may have been missed because of the resolution of the 3D MRI, and variance in ocular volume measurements was possible because of the manual nature of that step in the process.
The researchers concluded that HM eyes are significantly larger in volume than non-HM eyes and that this enlargement is primarily due to equatorial and axial expansion—not necessarily the formation of a staphyloma. “However,” the authors noted, “the relationship between the volume of an eye and the presence of posterior staphyloma has not been investigated yet in large case cohorts [and] posterior staphyloma formation was correlated with reduced eye volume, which was possibly due to flattening of scleral curvature anteriorly in staphyloma.”
These insights may aid clinicians in understanding disease mechanisms in pathologic myopia.
A full list of author disclosures can be found in the published research.