High Myopia

Introduction

High myopia, defined as a spherical equivalent equal or higher than −6 diopters (D) or an axial length ≥26 mm, is one of the leading causes of low vision worldwide. The higher the myopia, the more likely complications can occur, so vision can be damaged. There is an increased in myopic retinopathy from 3.8% to 89.6%, in eyes with a refractive error of <−4 and at least −10 D, respectively. Choroidal neovascularization, retinal detachment, macular hole, foveoschisis, epiretinal membrane, vitreomacular traction syndrome, and chorioretinal atrophy are some complications that are often in these patients. The choroid plays a principal role in most of high myopia diseases.

Choroidal thickness in high myopia has been described thinner comparing with controls eyes in studies using spectral-domain optical coherence tomography (OCT). The choroidal thickness profile is completely different than controls. High myopic eyes, with no history of ocular surgery or treatment, have a thicker choroid in the temporal area, decreasing at each location further nasal toward the optic nerve, which differs with control profile where the greater thickness occurs in the subfoveal area, then the temporal and the thinnest in the nasal section. Axial length, spherical equivalent, posterior staphyloma height, and age have been demonstrated to correlate with choroidal thickness in high myopia. A decrease of 1.27 µm ⁵ or 1.9 µm ² of choroidal thickness per year, a decrease of 8.7 µm or 9.39 µm per negative diopter, and a decrease of 25.91 µm ² per each millimeter of axial length have been described ( Fig. 11.1 ). Choroidal thickness in high myopic eyes with history of choroidal neovascularization is significantly thinner. Wang et al . suggested that macular choroidal thickness predicts better the severity of the myopic maculopathy than axial length or spherical equivalent, and there is a strong association between lacquer cracks and macular choroidal thinning, finding stellate lacquer cracks in thinner choroids comparing with linear lacquer cracks.

A decrease in visual acuity in high myopic eyes can be attributed to numerous diseases affecting the surface of the retina, the inner or outer retina, the retinal pigment epithelium (RPE), or the choroid. They can be related to the natural history of high myopia or due to any complication, which disturb the retinal anatomy. An inexplicable low vision in myopic patients with apparent no macular alteration has been a subject of study for some investigators. Highly myopic patients with no macular disease have been studied with spectral-domain OCT, to correlate visual acuity with retinal and choroidal thickness, resulting in a positive correlation between visual acuity and outer retinal thickness (corresponding from the external limiting membrane to RPE-Bruch membrane complex) and choroidal thickness. So, a thicker outer retina and choroid should indicate a better visual acuity.

Myopic Choroidal Neovascularization

Myopic neovascular membrane is a disease secondary to high myopia, etiology of which remains unknown. OCT has become a useful tool for diagnosing and monitoring this pathology. In its active phase, the neovascular membrane appears as a dome-shape elevation above the RPE, with a minimum amount of subretinal fluid ( Fig. 11.2 ). In the cicatricial phase, a hyperreflective area is observed in the surface of the membrane due to a hyperpigmentation of the damaged RPE, and there is a marked attenuation of OCT signal in deeper layers ( Fig. 11.3 ). In the atrophic phase, there is a flattening of the neovascular membrane with a higher reflectivity at the choroid level due to atrophy of the retina and choriocapillaris ( Fig. 11.4 ).

In commercial OCT devices, there are not standard values for retinal layers density in high myopia because they can be influenced by axial length and also due to the distortion in the scan induced by the axial length. The software of the devices doesn’t show quantitative data, and it must be supplied by manual measurements.

Keane et al . developed a new tool in the software (OCTOR), trying to increase the quality of the retina thickness measurements and also to quantify the morphological features of the neovascular membrane in high myopic patients. This tool permits a manual position of the preestablished limits of the OCT image and then, quantifies the studied area. This software could be useful in the quantitative analysis of the anatomic results after the treatment and in the possible correlation with final visual acuity.

The limitations of the OCT in the study of neovascular myopic membranes are as follows: There are no standard values for layers thicknesses, the low quality of the images obtained with the different devices, there is no possibility to differentiate blood from fibrosis and RPE proliferation and subretinal fluid, which usually is very low. All these limitations in the study of neovascular myopic membranes generate the need to take into account the patient’s symptoms and the features of fluorescein angiography. The combination of spectral-domain OCT and fluorescein angiography has demonstrated to be more sensitive diagnosing neovascular myopic membranes, than each one alone.

On the other hand, OCT permits the diagnosis of concomitant disease that occurs in high myopia, as epiretinal membrane, lamellar or full-thickness macular hole, and myopic tractional maculopathy, which could affect the final visual acuity.

OCT angiography (OCT-A) is a novel and noninvasive tool that generates en-face images of dynamic blood perfusion. On OCT-A, neovascular myopic membrane appears as a large hyperintense vascular network, resulting in two types of myopic membranes: “interlacing” and “tangled” vascular networks. The interlacing type is usually associated with an active status of the membrane, and it has an intense well-defined vascular network on OCT-A; on the other hand, the tangled type is often associated with an absence of myopic neovascular membrane activity, determined by spectral-domain OCT and/or fluorescein angiography, and is observed as a loosely laced vascular network on OCT-A. OCT-A in neovascular myopic membrane has low quality nowadays, probably due to a worse quality of the images in larger eyes, or due to tilted images of myopic patients ( Fig. 11.5 ).