Degenerative myopia, also referred to as pathological myopia, is a severe and progressive eye condition that goes beyond simple nearsightedness. It is defined by a high degree of myopia and specific structural changes that damage the back of the eye. This distinction is important because only a subset of people with high myopia develops these sight-threatening pathological changes. Early and accurate identification of these changes allows for timely intervention and monitoring to preserve vision.
Quantifying the Nearsightedness
The diagnostic process begins by establishing the quantitative threshold of high nearsightedness, which acts as the foundation for the condition. This quantitative assessment primarily involves measuring the refractive error, which is the degree to which the eye fails to focus light correctly. High myopia is generally defined as a spherical equivalent of -6.00 Diopters (D) or more. This measurement is typically obtained through standard subjective refraction tests performed by an eye care professional.
A second, more objective measurement is the axial length, which is the distance from the front to the back of the eyeball. In eyes with high myopia, this length is significantly elongated due to excessive growth. An axial length of 26.0 millimeters or greater is considered a strong indicator that the eye may be at risk for degenerative changes. This measurement is taken using a non-invasive technique called biometry, often an A-scan ultrasound or optical coherence biometry, which provides precise documentation of the eye’s physical size.
Clinical Examination for Structural Changes
Once high nearsightedness is confirmed, the diagnosis shifts to detecting the characteristic structural damage that defines the degenerative nature of the condition. A comprehensive dilated fundus examination is performed, where the ophthalmologist uses a specialized lens to directly inspect the retina, choroid, and optic nerve. The extreme elongation of the eyeball forces the inner layers to stretch and thin, revealing telltale signs of pathology.
The most notable sign a clinician looks for is a posterior staphyloma, which is a localized, outward bulging of the sclera at the back of the eye. This mechanical deformation is a hallmark finding and directly correlates with the stretching of the ocular tissues. The doctor also looks for myopic maculopathy, which describes a range of degenerative changes in the macula, the central part of the retina responsible for sharp vision.
Signs of Pathological Thinning
Pathological thinning of the eye layers manifests as chorioretinal atrophy, which can be diffuse (widespread) or patchy (discrete, sharply demarcated areas). These areas show a loss of the retinal pigment epithelium and underlying choroidal blood vessels. Another specific finding is the presence of lacquer cracks, which are fine, irregular breaks in Bruch’s membrane. These cracks indicate severe mechanical strain and are a precursor for more serious complications. The clinician may also identify Fuchs spots, which are pigmented scars resulting from a healed episode of choroidal neovascularization.
Advanced Imaging Confirmation
Sophisticated imaging technology confirms pathological signs, maps their extent, and establishes a baseline for monitoring progression. Optical Coherence Tomography (OCT) is a standard tool that provides high-resolution, cross-sectional views of the retina and the layers beneath it. OCT is invaluable for visualizing subtle changes, such as the separation of retinal layers known as myopic foveoschisis.
The OCT allows for precise measurement of choroidal thickness, which is typically thinned in degenerative myopia, and helps detect fluid accumulation or swelling. Specialized OCT angiography (OCTA) visualizes the blood flow within the retina and choroid without injectable dye. This is useful for the early detection of myopic choroidal neovascularization (CNV), where new, fragile blood vessels grow under the retina.
Wide-field fundus photography captures a broad image of the back of the eye, documenting the full extent of the pathology. This imaging is effective for recording the size and shape of the posterior staphyloma and the area of peripheral atrophy. If the view of the retina is obscured by a cataract or vitreous hemorrhage, B-scan ultrasonography can be used. This ultrasound technique provides a clear image of the internal eye structures, confirming the staphyloma when direct visualization is not possible.