In our study on eyes with pathologic myopia, WF-OCT with a scan region of 16 × 14 mm and a depth of 5 mm revealed the morphology of posterior staphylomas in highly myopic eyes. Except for two eyes, the staphylomas were visible by WF-OCT in their full extent. When the detectability of posterior staphylomas by WF-OCT and by 3D-MRI was compared, WF-OCT was superior to 3D-MRI in all but two eyes in which the width of the staphyloma was longer than the scan length of the WF-OCT. In general, both techniques allowed the shape-based differentiation of the staphylomas in a similar manner. In addition to cost, one of the major advantages of WF-OCT over 3D-MRI was the ability to visualize the tissues in a markedly higher resolution and allowed the differentiation among vitreous, retina, choroid, and sclera. In addition, 3D-MRI showed only the outer surface of the vitreous cavity because, using T2-weighted images, it visualized the shape of intraocular fluid. In contrast, WF-OCT allowed visualization of the structures of the ocular wall. In eyes with an abnormal retinal surface, such as in myopic retinoschisis, 3D-MRI, in contrast to WF-OCT, may thus not validly show the contour of the sclera.
Morphologic features of posterior staphyloma as examined by WF-OCT included a gradual thinning of the choroid from the periphery toward the edge of the staphyloma and a gradual rethickening of the choroid in direction toward the posterior pole, as well as a gradual thickening and inward protrusion of the sclera at the staphyloma edge. Thinning of subfoveal choroid is a well-known feature of myopic eyes in general
22,23; however, such gradual thinning of the choroid from the periphery toward the staphyloma edge as shown in the present study has not been previously reported. It was not previously possible to visualize the choroid and sclera in a wide range of the posterior ocular segment. The findings obtained in the present study agree with the observations made on smaller posterior staphylomas (such as peripapillary staphyloma
14 or inferior staphyloma due to tilted disc syndrome
24) in previous studies in which conventional OCT devices with a shorter scan line length were used. These investigations suggested that the OCT features of a gradual scleral thickening and a gradual choroidal thinning at the staphyloma edge might be a consistent and useful marker to detect the edge of any type of staphylomas.
Because our study had a cross-sectional design, it has been unclear whether the morphologic features at the staphyloma edge occurred before staphyloma formation or whether they were the consequences of the development of a staphyloma. It also remains unclear whether the choroid, sclera, or potentially another tissue (such as Bruch's membrane) is primarily affected and responsible for subsequent staphyloma formation. In addition to choroid and sclera, it has recently been discussed that Bruch's membrane may be an important structure leading to axial elongation, caused by a growth of Bruch's membrane in the midperipheral region and pushing the posterior Bruch's membrane backward.
25 It would lead to a compression of the macular choroid and a passive elongation and thinning of the sclera, most marked at the posterior pole. The integrity of Bruch's membrane at the staphyloma edge needs to be addressed in future studies by using a device with better resolution.
Future studies may quantify the dimensions of the posterior staphylomas measuring their minimal and maximal diameters, their depth, and their shape and location in the spatial relationship to landmarks such as the optic nerve head and the macula, to provide data for biomechanical calculations and models describing the development of pathologic myopia. The formation of a staphyloma includes visually important tissues, such as the optic nerve head and the macular retina, which can significantly affect the visual prognosis of highly myopic patients.
1,11 The quantitative assessment of staphylomas could thus be a step to establish treatments targeting the development of staphylomas before blinding complications occur. In addition, the relationship between a staphyloma and other tissues that could influence the scleral shape (such as vitreous, intrascleral/episcleral vessels) may be an interesting theme to be examined.
When discussing the findings obtained in our study, its limitations may be taken into account. First, this study examined highly myopic patients who visited a third referral center. The results may therefore not represent the general population of highly myopic patients. Second, a quantitative histomorphometric analyses of the dimensions and shape of the choroid and the sclera were not performed, so that the findings were described in a qualitative manner. Third, due to the technical limitations of the OCT method, the reconstructed 3D-OCT images might have been imprecise in the periphery of the fundus. Fourth, despite the increased width and depth of the OCT images using the new WF-OCT technology, very large staphylomas could not be visualized by WF-OCT. An even longer scan line may be necessary to visualize all staphylomas regardless of their size and location. Future studies applying a new prototype of WF-OCT with a 20-mm scan length may address that limitation of the current investigation.
In conclusion, WF-OCT provided images of posterior staphylomas in highly myopic eyes in a resolution and wide field of view previously unachievable. WF-OCT may replace 3D-MRI in assessing posterior staphylomas, which are a hallmark of pathologic myopia.