As optical coherence tomography (OCT) uses optical principles to evaluate the retinal and optic nerve head (ONH) structures, special care must be taken when interpreting the OCT scans of eyes with high refractive errors. The prevalence of myopia is dramatically increasing in younger generations, especially in East and Southeast Asian countries. Given that myopia is an important risk factor for development of glaucoma, clinicians have to face challenging OCT cases in distinguishing glaucomatous changes in eyes with high myopia. The structural deformation common in highly myopic eyes, such as tilted disc, peripapillary atrophy, or posterior staphyloma, makes it difficult not only to obtain a clear image of an OCT scan but also to specify glaucomatous change from physiologically healthy status. The first half of this chapter describes the causes of OCT scan errors (i.e., ocular magnification errors, scan-circle diameter and location, insufficiency of normative database population, segmentation errors) that are incurred due to the anatomical characteristics of high myopia. The second half of this chapter introduces the new parameters additional to peripapillary retinal nerve fiber layer (RNFL) thickness (i.e., macular ganglion cell structure, Bruch’s membrane opening (BMO)-based neuroretinal rim thickness) that are used for improved accuracy of glaucoma diagnosis in highly myopic eyes.
Key wordshigh myopia – ocular magnification error – red disease – temporal raphe sign – Bruch’s membrane opening-based minimum rim width – three-dimensional neuroretinal rim thickness – three-dimensional wide-field map
11 Special Considerations: High Refractive Errors
Refractive errors, including myopia and hyperopia, prevent light from reaching the retina with precision focus. This affects visual acuity, certainly, but it can also impact the results of optical coherence tomography (OCT) imaging, which uses optical principles to observe the structure of the retina and optic nerve head (ONH). Changes in the ocular structure associated with refractive errors can influence the risk of developing glaucoma. Hyperopic eyes are associated with a narrow angle, which increases the risk of primary angle-closure glaucoma (PACG). On the other hand, myopia is one of the major risk factors for development of primary open-angle glaucoma (POAG). When interpreting OCT images of eyes with high refractive error for diagnosis and monitoring of glaucoma, differing attention for myopic and hyperopic eyes is needed, in the following aspects: (1) careful anterior segment examination is more meaningful for PACG risk assessment in hyperopic eyes (discussed in Chapter 9); (2) structural anomalies of the posterior segment common to highly myopic eyes, such as tilted disc, chorioretinal and peripapillary atrophy, or posterior staphyloma, not only mimic glaucomatous change but also affect OCT scan characteristics.
Half of the world’s population (approximately 5 billion people) is projected to be myopic by 2050, with as much as 10% highly myopic. 1 In developed countries in East and Southeast Asia, there is currently a myopia epidemic, the prevalence of myopia increasing more markedly at younger ages, reaching 80 to 90%. In the future, given that the risk of glaucoma increases as these generations age, clinicians will be challenged to interpret those eyes’ OCT scans to distinguish glaucoma from myopia. This chapter summarizes the considerations for interpreting OCT scans in highly myopic eyes and provides practical tips for using OCT scans to improve the accuracy of glaucoma diagnosis in such eyes. The first half of this chapter discusses the causes of OCT scan errors due to the anatomical characteristics of high myopia as well as the cautions to be heeded in interpreting them; the second half introduces the new parameters for improved diagnosis of glaucoma in highly myopic eyes.
11.2 Technical Issues of OCT that Need to be Considered in High Myopia
11.2.1 Scan-Circle Size: Ocular Magnification Error
The commercially available OCT devices provide peripapillary retinal nerve fiber layer (RNFL) thickness data as measured at a certain distance from the center of the optic disc. In order to derive the true size of an object on the fundus, it is necessary to take into account the curvature of the cornea, the anterior chamber depth, and the axial length, all of which contribute to the characteristics of the reflected image on the fundus. The relationship between the measurement of the OCT image and the actual fundus size can be expressed as t=p ×q ×s\ t=p\ \times q\ \times s, where t is the actual fundus size, p is the magnification factor related to the camera of the OCT imaging system (known as 3.382 in the Cirrus HD-OCT system), q is the magnification factor related to the eye, and s is the measurement on OCT. 2 The ocular magnification factor q (in millimeters per degree) can be determined with the formula q=0.01306 ×(x-1.82)\ q=0.01306\ \times (x-1.82), where x is the axial length. In the case of Cirrus HD-OCT (Carl Zeiss Meditec, Dublin, CA, USA), the default axial length is 24.46 mm, and the scanning radius for the peripapillary RNFL scanning protocol is set to 1.73 mm. If the axial length of the eye is greater than the default (24.46 mm), which is a common condition in highly myopic eyes, the scanning radius is magnified (greater than 1.73 mm), and this effect, the so-called “magnification error,” can skew the measurement of RNFL thickness.
The actual average RNFL thickness at the 1.73-mm-radius circle can be estimated according to the assumptions that (1) all of the retinal nerve fibers emerging from the retinal ganglion cells (RGCs) exit the eyeball through the ONH; (2) if the same number of retinal nerve fibers cross the different-sized circles with the same center, the larger scan circle will have a lower average RNFL thickness, and (3) the total cross-sectional areas of the retinal nerve fibers under the different circles will be equal, because the same number of retinal nerve fibers cross the circles.
The total cross-sectional area of the RNFL and the magnified radius of the scan circle can be estimated in the following equations: