Visual & Optical Considerations with Contact Lenses

Visual Considerations with Contact Lenses

Learning Outcomes

• Describe the impact of optical factors on vision with contact lenses.
• Explore the relationship between corneal, lenticular, and residual astigmatism.
• Calculate the required contact lens power for a given spectacle prescription.
• Calculate spectacle magnification and prismatic effect caused by contact lenses.

Visual Considerations

• Visual Acuity
• Astigmatism
• Accommodation
• Convergence
• Field of View
• Anisometropia

Visual Acuity

• Myopes: Spectacle lenses decrease retinal image size compared to emmetropes. Fitting contact lenses (CLs) should improve acuity.
• Hypermetropes: Spectacle lenses magnify retinal image size, potentially reducing acuity in CLs.

Worked Example 1: Myope

• Spectacle Rx: -10.00DS at BVD 15mm.

• Calculate the change in spectacle magnification when transferring to contact lenses (assuming 3mm distance between cornea and entrance pupil).

• Spectacle Magnification Formula: $SM = \frac{1}{1-aF}$

• Spectacle Rx (Fs) = -10.00DS

• Spectacle Magnification: $SM = \frac{1}{1-(0.018 \times -10)} = \frac{1}{1.18} = 0.85$

• Spectacle Rx = -10.00DS, BVD = 15mm

• CL Rx = ?

• Ocular Refraction Formula: $F<em>{ocular} = \frac{F</em>s}{1-dF_s}$

• Ocular Refraction: $F_{ocular} = \frac{-10}{1-(0.015 \times -10)} = \frac{-10}{1.15} = -8.70D$

• CL Rx = -8.75D

• CL Rx = -8.75D

• Spectacle Magnification with CL: $SM = \frac{1}{1-(0.003 \times -8.75)} = \frac{1}{1.0625} = 0.97$

• Change in Spectacle Magnification: $0.97 - 0.85 = +0.12$

• A 12% increase in image size.

Worked Example 2: Hypermetrope

• Spectacle Rx: +10.00DS

• Spectacle Magnification Formula: $SM = \frac{1}{1-aF}$

• Spectacle Magnification: $SM = \frac{1}{1-(0.018 \times 10)} = \frac{1}{0.82} = 1.22$

• Spectacle Rx = +10.00DS, BVD = 15mm

• CL Rx = ?

• Ocular Ref: $F_{ocular} = \frac{10}{1-(0.015 \times 10)} = \frac{10}{0.85} = +11.76D$

• CL Rx = +11.75D

• CL Rx = +11.75D

• Spectacle Magnification with CL: $SM = \frac{1}{1-(0.003 \times 11.75)} = \frac{1}{0.965} = 1.04$

• Change in Spectacle Magnification: $1.04 - 1.22 = -0.18$

• An 18% decrease in image size.

Astigmatism: Corneal, Lenticular & Residual

• Primary refractive components: cornea and crystalline lens.
• Total astigmatism = Corneal astigmatism + Lenticular astigmatism.
• Spherical GP CLs can mask corneal astigmatism; spherical soft CLs do not correct it.
• Residual astigmatism is the astigmatism remaining after correction with a spherical contact lens.
• The origins of astigmatism are crucial for predicting vision with contact lenses and determining the best lens type.

Spherical Cornea / Spherical Rx

• Rx: -3.00DS
• Ks: 7.85mm spherical
• Vision should be equally good with soft or rigid contact lenses.

Spherical Cornea / Astigmatic Rx

• Rx: -2.00 / -1.75 x 90
• Ks: 7.90 @ 90 / 7.85 @ 180
• Astigmatism effectively of lenticular origin.
• Vision poor with either soft or GP spherical lens.
• Requires front surface toric contact lens to correct residual astigmatism.

Toric Cornea / Astigmatic Rx

• Rx: -2.00 / -1.75 x 180
• Ks: 7.80 @ 180 / 7.50 @ 90
• Astigmatism is predominantly of corneal origin.
• GP lens suitable – resultant liquid lens corrects 90% of astigmatism.
• A spherical soft lens will not correct corneal astigmatism – therefore may necessitate a soft toric lens.

Toric Cornea / Spherical Rx

• Rx: -3.00DS
• Ks: 7.80 @ 180 / 7.50 @ 90
• Approx 1.50 WTR corneal astigmatism evident
• Cancelled by 1.50D ATR lenticular astigmatism.
• Spherical GP lens would correct corneal astigmatism leaving unacceptable 1.50D of residual astigmatism.
• A soft lens should be employed.

Accommodation

• Myopes need more accommodation when switching from spectacles to CLs.
• Hypermetropes need less accommodation when switching from spectacles to CLs.
• This effect begins when Rx exceeds +5.00D.
• Particularly significant for myopic patients over 40.
• May precipitate presbyopic symptoms earlier than with spectacles.
• Assess near vision with trial lenses of correct power in situ.

Convergence

Hypermetrope

• Hypermetropes converge less with CLs than with spectacles.
• Base out prismatic effect induced by spectacles forces more convergence.

Myope

• Myopes converge more with CLs than with spectacles.
• Base in prismatic effect induced by spectacles results in less convergence.

Field of View

Hypermetrope

• Hypermetropes experience ring scotoma effect with spectacles.
• CLs move with the eye, so there are no such limitations.

Myope

• Myopes experience ring diplopia effect with spectacles.
• CLs move with the eye, so there are no such limitations.

Anisometropia

• Refractive anisometropia (e.g., post-cataract surgery) results in different spectacle magnification between eyes.
• Patients often benefit from improved binocular status with CLs.
• Axial anisometropia is theoretically better corrected with spectacles.
• It is not obvious to the practitioner if the anisometropia is refractive or axial.
• In practice, axial anisometropes often benefit from CLs due to reduced differential prismatic effects, especially for near tasks.

Worked Example 3: Refractive Anisometropia & Spectacle Magnification

• Determine the difference in spectacle magnification between the two eyes when transferring from spectacles to contact lenses.
• RE: +2.00DS, LE: +10.00DS
• BVD = 12mm
• Cornea – Entrance Pupil = 3mm
• Spectacle Magnification Formula: $SM = \frac{1}{1-aF}$

Worked Example 3: Spectacles

• Right Eye (FS = +2.00DS):
  • $Spectacle Magnification = SM = \frac{1}{1-(0.012 \times 2)} = \frac{1}{0.976} = 1.025$
• Left Eye (FS = +10.00DS):
  • $Spectacle Magnification = SM = \frac{1}{1-(0.012 \times 10)} = \frac{1}{0.88} = 1.136$
• Difference in SM (Spectacles) = 1.136 - 1.025 = 0.111

Worked Example 3: Contact Lenses

• Right Eye (FCL = +2.00DS):
  • $Spectacle Magnification = SM = \frac{1}{1-(0.003 \times 2)} = \frac{1}{0.994} = 1.006$
• Left Eye (FCL = +11.25D):
  • $Spectacle Magnification = SM = \frac{1}{1-(0.003 \times 11.25)} = \frac{1}{0.966} = 1.035$
• Difference in SM (Contact Lenses) = 1.035 - 1.006 = 0.029

Worked Example 4: Anisometropia & Prismatic Effect

• Rx: RE -4.00DS, LE +1.00DS
• Vertical prismatic effect on depressing the eyes 10mm below the optical centres to look at an object 25cm in front of the spectacle plane: $P = cF$
• P = Prismatic effect, c = displacement (cm), F = BVP
• Determine the differential prismatic effect when corrected with spectacles and contact lenses.

Worked Example 4: Anisometropia & Prismatic Effect Spectacles

• Right Eye: $P_{RE} = 1 \times 4 = 4$ Prism Dioptres Base Down
• Left Eye: $P_{LE} = 1 \times 1 = 1$ Prism Dioptre Base Up
• Differential Effect (Spectacles) = 5 Prism Dioptres
• Assuming contact lenses remain centred, the vertical differential would be negated (c = 0, therefore P = 0).

Why Can’t I See In My CLs?

• Visual Acuity
• Astigmatism
• Accommodation
• Convergence
• Field of View
• Anisometropia

Further Reading

• Refer to previous visual optics notes from OPT101 Clinical Skills 1 & OPT102 Practical Optics.
• Gasson A & Morris J (2010) The Contact Lens Manual 4th Edition. (Chapter 5). Butterworth Heinemann