Which prescription is worse, -1.75 x 90 or -1.25 x 70, in terms of severity of nearsightedness (myopia)?

Comparing Myopia Prescriptions: -1.75 x 90 vs -1.25 x 70

The prescription -1.75 x 90 represents a more severe degree of myopia than -1.25 x 70, as it has a higher spherical power (-1.75 diopters vs -1.25 diopters) regardless of the axis orientation.

Understanding Myopia Prescriptions

Myopia prescriptions consist of three components:

• Sphere power (the first number): Indicates the degree of nearsightedness
• Cylinder power (not explicitly shown in these prescriptions but implied)
• Axis (the number after "x"): Indicates the orientation of astigmatism correction

Key Factors in Comparing Severity

1. Spherical Power:

   • The primary determinant of myopia severity is the spherical power
   • Higher negative numbers indicate stronger prescriptions
   • -1.75 is more negative than -1.25, making it the stronger prescription

2. Axis Differences:

   • The axis (90° vs 70°) only indicates the orientation of astigmatism correction
   • The axis itself doesn't affect the overall strength of the myopia correction
   • Axis numbers merely show the direction of the astigmatic component

Clinical Implications

The difference of 0.50 diopters between these prescriptions is clinically significant:

• Visual acuity: A person with -1.75D will have more blurred distance vision without correction than someone with -1.25D
• Functional impact: The -1.75D prescription would cause more difficulty with distance tasks when uncorrected
• Progression risk: Higher myopia is associated with greater risk of progression to pathologic myopia

Risk Factors for Myopia Progression

According to ophthalmology guidelines, patients with higher myopia should be monitored for:

• Risk of further progression, especially in children and adolescents
• Potential development of high myopia (typically defined as -5.00D or greater)
• Associated risks with higher myopia, including:
  • Increased risk of retinal detachment
  • Higher risk of myopic maculopathy in severe cases
  • Greater likelihood of early cataract development
  • Increased glaucoma risk

Special Considerations

The presence of high myopia has been identified as a risk factor for hydroxychloroquine retinopathy 1, with one study showing an odds ratio of 5.03 (95% CI 1.29 to 24.79) for retinopathy in patients with high myopia compared to those without.

Conclusion

When comparing these two prescriptions, the -1.75 x 90 prescription represents a more severe degree of myopia than -1.25 x 70 based on the higher spherical power, which is the primary determinant of myopia severity and its associated visual and health implications.