Recently developed technologies enable more accurate measurements of choroidal thickness. In highly myopic eyes, the mean choroidal thickness measured histopathologically is 220 μm.
16 Enhanced depth imaging of SD-OCT, however, allows for clear detection of the choroid-retina and choroid-scleral interface and for the total choroidal thickness to be measured in vivo. Using this method, the mean choroidal thickness under the fovea was 308 μm in healthy volunteers.
17 Partial coherence interferometry technology is another useful modality for obtaining axial length measurements and intraocular calculations for cataract surgery (IOLMaster; Carl Zeiss Meditec, Inc.). If choroidal thickness is defined as the distance between the two peaks (P3 and P4), mean choroidal thickness is 307 μm.
18 There is still limited access to these technologies, and thus their wide application in observational studies remains challenging.
Commercial SD-OCT uses an 830-nm infrared light source, which has high reflectivity and scattering at the RPE/Bruch’s membrane interface. This fact makes observations of the deep choroid difficult because the signal is attenuated, thus limiting studies of the deep choroid with OCT. Highly myopic patients, however, usually have choroidal thinning, which facilitates detection of the scleral interface. Taking advantage of this characteristic, we attempted to measure choroidal thickness using the commercially available SD-OCT (Cirrus HD-OCT; Carl Zeiss Meditec, Inc.). Our experience suggests that the scleral interface is generally undetectable in eyes with a choroidal thickness greater than 300 μm. None of the study eyes in patients between the ages of 30 to 80, however, had such a thick choroid. Therefore, the results of the present study indicate that choroidal thickness is partly age dependent. Measuring the choroidal thickness in younger patients is difficult, however, because of signal loss. The new generation OCT with a 1-μm-wavelength band has greater penetration through the RPE–Bruch’s membrane interface,
19 20 and may therefore be capable of detecting the scleral interface in young healthy eyes.
SD-OCT was widely used to measure the retinal thickness in retinal thickening disease such as diabetic macular edema. The quantification of the retinal thickness facilitates the evaluation of the disease severity or comparison of the efficacy of treatment options.
21 Thus, retinal thickness OCT has become a routine examination in a clinical setting.
22 However, a single B-scan is still questionable for representing diseases of a whole macula. Automatic volume measurement by three-dimensional analysis that SD-OCT provides is advantageous in this point. The volume analysis of the choroid would be useful for quantitating the thickness or thinning of the whole choroidal; however, there is no algorithm available for automatic detection of the choroid–sclera interface.
In the present study, mean foveal retinal thickness was 201.2 μm. Another study reported a mean retinal thickness of 258 μm within the 500-μm circle of the fovea in normal subjects using the Cirrus HD-OCT.
23 We did not use the automated segmentation and thickness measurements of the Cirrus HD-OCT, but rather a manual method to measure the thickness specifically of the central fovea; thus, the foveal thickness in our study is thinner than that found in the other studies. Our study did not include normal subjects as the control, and therefore, we did not examine the difference between highly myopic eyes and nonmyopic eyes with our manual-based method.
Neither refractive error nor axial length was significantly associated with retinal thickness in our study. Previous studies revealed a significant negative correlation between axial length or refractive error and macular thickness on OCT images in adults
24 25 and children.
26 The correlation coefficient is low, however, ranging from 0.1 to 0.3. One reason for the disagreement between studies may be that we analyzed a smaller number of subjects. In addition, previous studies used the older version of the OCT, the Stratus, rather than the Cirrus. Because the Stratus is a time-domain OCT and has a different protocol for macular thickness analysis, the thickness is less than that obtained with the Cirrus-HD OCT.
23 Finally, previous studies used A-scan ultrasound for axial length measurements, and we used partial coherence interferometry. Ultrasound detects the inner surface of the retina as a peak, whereas partial coherence interferometry detects the RPE. These differences may have affected our results and may be why we failed to detect statistically significant correlations. Further studies with a larger number of subjects are needed to clarify this point.
Posterior staphyloma is recognized as a protrusion of the posterior shell of the eye globe, and is a hallmark of high myopia. Several studies have demonstrated that posterior staphyloma formation is closely related to posterior retinal diseases, including macular hole retinal detachment and myopic foveoschisis.
27 28 29 This association is probably because deeper posterior staphyloma generates greater inward vector force to detach or split the neural retina, which is facilitated by vitreous cortex shrinkage, epiretinal membrane, or rigid internal limiting membrane.
30 31 32 Thus, posterior staphyloma is believed to cause various pathologies in high myopia-specific diseases.
These findings suggest that studies of choroidal disease are useful toward understanding the process of consequent atrophic changes. Further, the quantification of biometric changes is very helpful for monitoring the progression and estimating the relative risks for future development of the disease. In the present study, posterior staphyloma height was strongly correlated with choroidal thinning. Ultrasonography-based posterior staphyloma depth is well correlated with the degree of lacquer cracks, conus, RPE defect, and choroidal atrophy.
33 We demonstrated that posterior staphyloma height correlates well with axial length and refractive error. Thus, this newly developed parameter of posterior staphyloma height appears to be a good indicator for risk management of choroidal thinning and posterior staphyloma formation, but its correlation with myopia-specific disease must be further investigated.
Of interest, the pattern of choroidal thickness was unique, depending on its location. The nasal choroid was the thinnest, followed by the central foveal and inferior choroid, and then the temporal and superior choroid. One possible reason for this is the watershed zone of the choroid that is typically recognized as a hypofilling area of the choroid in angiography.
34 The watershed zone indicates the isolation of a choroidal capillary bed supplied by an independent posterior ciliary artery (PCA) that does not anastomose with another end artery, making it prone to ischemia. From one to five PCA branches arise from the ophthalmic artery.
34 There are many interindividual variations, but the posterior choroid is supplied by two major PCAs, the lateral and medial PCA, in approximately 90% of eyes.
35 A watershed zone between the medial and lateral PCA is observed vertically between the macula and optic nerve head in 60% of eyes.
34 Also, the lateral watershed zone is sometimes observed horizontally at the macula. Another possibility is an embryologic event. The optic fissure is closed at around 7 weeks of gestation.
36 This event may result in prominent thinning in the inferior part of myopic eye. If this is true, the inferior choroid must be thinner even in nonmyopic eyes. We must investigate the choroidal thinning in nonmyopic eyes first, which would provide a key for understanding of this phenomenon. Globe expansion may induce disproportional choroidal thinning, and a watershed zone at the central fovea may lead to more thinning. We believe that this is a major cause of progressive choroidal thinning at the nasal and subfoveal choroid. It remains uncertain why the inferior choroid was thinner than the superior choroid. A superior PCA is observed in 10% of eyes, which may be why the superior choroidal thickness was well preserved. Our data indicated that inferior posterior staphyloma height was significantly less than the superior height. Thus, posterior staphyloma expansion seems to be asymmetrical. Better knowledge of the mechanisms of thinning is crucial for understanding myopic disease. Further investigation is needed to clarify the mechanisms and pathogenesis of high myopia disease.
Another interesting point is that, after posterior staphyloma height, choroidal thinning correlated with age. Ramrattan et al.
37 investigated 95 normal eyes with a histologic approach and found a significantly negative correlation between age and choroidal thickness. Although the association was significant (
P < 0.01), the correlation coefficient was only 0.18, indicating a weak correlation. Similarly, in the present study, only borderline significance was detected between age and choroidal thickness in simple regression analysis. It is also noteworthy that choroidal thickness correlated strongly with posterior staphyloma height, but posterior staphyloma had no correlation with age. These observations lead us to hypothesize that choroidal thinning is dependent on staphyloma formation and aging, both of which are independent factors.