Operational Definition of Myopia
Myopia is operationally defined as a spherical equivalent refractive error of ≤ -0.50 diopters (D), though this threshold carries significant classification bias risks. 1
Standard Clinical Definitions
The consensus operational definition for clinical and epidemiologic research establishes the following quantitative thresholds 1:
- Myopia (all myopia): Spherical equivalent refractive error ≤ -0.50 D 1
- High myopia: Spherical equivalent refractive error ≤ -6.00 D 2, 1
- Pathologic myopia: The categorical term for adverse structural complications of myopia, distinct from the refractive measurement 1
Underlying Pathophysiology
Myopia is fundamentally a refractive anomaly primarily caused by an increase in axial length of the eyeball 3. This distinguishes:
- Axial myopia: Myopia resulting from increased axial eye length 1
- Refractive myopia: Myopia resulting from excessive refractive power of the optical components 1
Important Clinical Caveats
The -0.50 D threshold, while widely accepted, has known limitations. The consensus acknowledges this cutoff carries significant risks of classification bias, meaning individuals near this threshold may be misclassified 1. In clinical practice, this means borderline cases require careful longitudinal monitoring rather than relying solely on a single measurement.
The operational definition should distinguish secondary myopia (myopia resulting from other ocular or systemic conditions) from primary myopia 1. In children with high myopia, syndromic causes must be explored 3.
Pre-Myopia Classification
For prevention research purposes, the condition of "pre-myopia" has been defined to provide a framework for studying myopia prevention strategies, though specific refractive thresholds for this category require clinical context 1.