What are the classifications and ocular changes of myopia, including low and high myopia definitions, clinical criteria, and typical anatomical changes?

Myopia: Classifications and Ocular Changes

Definitions and Classifications

Myopia is classified as low myopia (spherical equivalent refractive error ≤ -0.50 to > -6.00 diopters) and high myopia (spherical equivalent refractive error ≤ -6.00 diopters), with pathologic myopia defined by the presence of structural complications rather than refractive error alone. 1

The International Myopia Institute (IMI) has established standardized definitions to facilitate consistent research and clinical management:

• Myopia: Spherical equivalent refractive error ≤ -0.50 diopters
• Low Myopia: Spherical equivalent refractive error ≤ -0.50 to > -6.00 diopters
• High Myopia: Spherical equivalent refractive error ≤ -6.00 diopters 1

It's important to distinguish between high myopia and pathologic myopia, which are often incorrectly used interchangeably. Pathologic myopia is specifically defined by the presence of typical complications in the fundus, particularly posterior staphyloma or myopic maculopathy equal to or more serious than diffuse choroidal atrophy. While pathologic myopia frequently occurs in eyes with high myopia, these complications can also develop in eyes without high myopia. 2

Anatomical Changes in Myopic Eyes

Axial Elongation

The primary anatomical change in myopia is axial elongation of the eyeball:

• High myopia is associated with eye elongation exceeding 26-26.5 mm 3
• This elongation creates structural changes throughout the eye, particularly in the posterior segment

Posterior Staphyloma

Posterior staphyloma is the key differentiating factor between high myopia and pathologic myopia:

• Defined as an outward bulging of the posterior eye wall
• Most common finding in patients with pathologic myopia
• Responsible for many complications including retinal layer splitting and foveoschisis 3
• Can be visualized using wide-field optical coherence tomography 2

Retinal and Choroidal Changes

Myopic maculopathy encompasses several changes:

• Diffuse choroidal atrophy
• Retinal layer splitting leading to myopic foveoschisis
• Macular holes with or without retinal detachment
• Myopic traction maculopathy
• Myopic macular neovascularization 3, 2

Impact on RNFL and GCL

High myopia affects the retinal nerve fiber layer (RNFL) and ganglion cell layer (GCL) in several ways:

• Mechanical stretching of the retina due to axial elongation leads to thinning of the RNFL and GCL
• These changes can complicate the diagnosis of glaucoma, as they may mimic glaucomatous damage
• Studies have shown that high myopia (exceeding -6D) is significantly associated with higher prevalence of glaucomatous optic neuropathy compared to lower degrees of myopia 4
• The Beijing Eye Study demonstrated that marked to high myopia (exceeding -6D) may be a risk factor for glaucomatous optic neuropathy, independent of intraocular pressure 4

Clinical Implications and Complications

Myopia, particularly high and pathologic myopia, is associated with significant complications:

• Retinal detachment
• Retinal neovascularization
• Early cataracts
• Glaucoma 5
• Myopic traction maculopathy
• Dome-shaped macula 2

The risk of these complications correlates with the severity of myopia, making early detection and monitoring essential. In children with high myopia, clinicians should consider the possibility of syndromic myopia and conduct appropriate investigations. 5

Clinical Management Considerations

For accurate diagnosis and monitoring:

• Use standardized classification systems to define disease stage
• Employ optical coherence tomography to identify novel lesions like dome-shaped macula and myopic traction maculopathy
• Consider wide-field optical coherence tomography for visualizing large staphylomas 2
• Monitor for development of complications, particularly in patients with high myopia

Treatment options for myopia control include:

• Outdoor activities
• Defocusing corrective lenses
• Defocusing contact lenses
• Orthokeratology
• Pharmacological treatments with low-dose atropine 5

For complications of pathologic myopia:

• Anti-VEGF therapies for myopic macular neovascularization
• Vitreoretinal surgery for myopic traction maculopathy 2