High myopia, defined as spherical equivalent > 6 diopters (D) or axial length greater than 26.5 mm, is a major cause of visual impairment and blindness.^{1,​2,​3,​4} In high myopia, progressive elongation and deformation of the posterior segment may lead to the development of myopic macular lesions, including lacquer cracks, posterior staphyloma, choroidal neovascularization (CNV), macular hole retinal detachment, and chorioretinal atrophy. Vision-threatening pathology such as dome-shaped maculopathy and tilted-disc syndrome are also associated with high myopia.

Spectral-domain optical coherence tomography (SD-OCT) is widely used for noninvasive diagnosis and monitoring of abnormal chorioretinal morphology such as subretinal fluid associated with CNV, posterior staphyloma, and photoreceptor damage caused by chorioretinal atrophy.⁵ Swept-source OCT (SS-OCT) has further improved the imaging beneath the retinal pigment epithelium (RPE) such as the choroid and the sclera by using a longer wavelength.^6,​7 Fluorescein angiography (FA) is also an important diagnostic tool to evaluate myopic CNV in clinical practice. Leakage of the dye in the late phase is used to identify the presence and activity of the CNV. Indocyanine green angiography (ICGA) is another useful tool to evaluate the lacquer crack formation, choroidal blood flow, and extent of CNV lesion. Although those instruments are essential for the management of high myopia–related macular pathologies, the visualization of small retinal/choroidal vessels and the extent of CNV are sometimes difficult for the precise assessment.

OCT angiography (OCTA) is a new imaging modality that allows simultaneous visualization of vascular patterns of the choroidal and retinal vessels without the use of exogenous dyes.^{8,​9,​10,​11} The layer-specific imaging capabilities of OCTA have the potential to visualize both superficial and deep retinal capillary plexus, and the choriocapillaris by segmentation of each layer. Because it is difficult to visualize layer-specific vascular patterns by FA or ICGA, OCTA may provide a better understanding of pathogenesis and evaluation of treatment options for high myopia–related macular pathologies.

This chapter presents an overview of the OCTA findings in high myopia–related diseases and demonstrates the characteristics and clinical relevance of OCTA in high myopia.

13.2 Optical Coherence Tomography in High Myopia

▶ Fig. 13.1 shows OCTA images of the normal myopic macula taken by the RTVue XR Avanti (Optovue Inc, Fremont, CA) with AngioVue software. The 6 × 6 mm ( ▶ Fig. 13.1c–f) and 3 × 3 mm ( ▶ Fig. 13.1g–j) scanning areas centered on the fovea were obtained. The four en face images can be visualized on AngioVue mode, that is, superficial retina layer, deep retina layer, outer retina layer, and choriocapillaris layer. En face OCTA images show larger retinal vessels and capillaries in the superficial layer compared to the deep layer. As in healthy eyes without high myopia, the vascular density in the deep retinal layer in the highly myopic eyes is much higher than that in the superficial layer. In normal eyes, the outer retinal layer is devoid of blood vessels and flow signal is absent. Choriocapillaris layer shows dense microvascular structure within the choriocapillaris below the RPE.

13.3 Imaging Artifacts in High Myopia

The understanding of the imaging artifacts is important for accurate interpretation of the OCTA images.¹² First, the occasional eye motion causes motion artifacts on the OCTA image. Second, large superficial retinal vessels may result in lower signal below the vessels, which causes the shadowlike artifacts in the deep retinal layer, outer retinal layer, and the choriocapillaris layer ( ▶ Fig. 13.1d–f). The shadow artifacts are not part of the vasculature and may limit OCTA imaging on deeper structures. Third, projection artifacts, that is, superficial retinal vessels seen below, in deeper layers,¹² are often seen in highly myopic eyes compared with nonmyopic eyes ( ▶ Fig. 13.1j). The projection artifacts may limit the accurate quantitative assessment of vascular density in the deeper layer such as the choriocapillaris layer.

13.4 Myopic Choroidal Neovascularization

CNV is a major cause of severe visual loss in patients with pathologic myopia.¹³ Myopic CNV occurs in 4 to 11% of patients with high myopia, with most of the eyes progressing to 20/200 or worse within 5 to 10 years after onset.^14,​15 FA is the gold standard for diagnosing CNV; however, small CNV is sometimes obscured by transmitted hyperfluorescence due to surrounding chorioretinal atrophy and may limit identification of the active CNV lesion. In OCTA using RTVue XR Avanti with AngioVue software,¹⁶ the CNV flow can be detected at the outer retinal layer between Bruch’s membrane and the inner nuclear layer/outer plexiform layer junction ( ▶ Fig. 13.2). Because the outer retinal layer is devoid of blood vessels in normal subjects, flow signals at this layer strongly suggests CNV originating from the choroid ( ▶ Fig. 13.2c, h). The signal of the CNV network is more prominent in OCTA compared with FA or ICGA ( ▶ Fig. 13.2).

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