Refraction Principles and Case History

Refraction Principles

• Principles of refraction remain constant, regardless of visual impairment.
• Comprehensive case history is crucial, including medical, ophthalmic, visual, social, educational, and employment factors.
• Task analysis is essential, asking patients to describe their problems.
• Objective and subjective elements of refraction must be considered.
• 8-10% of new low vision patients only require an updated prescription.

Case History

• Comprehensive case history is fundamental to providing appropriate low vision aids and advice.
• Patients must feel comfortable and at ease during the consultation.
• The practitioner should demonstrate genuine interest and expertise in visual rehabilitation.
• Psychological aspects of sight loss and practitioner-patient interaction must be considered (e.g., Dodds et al studies).

Medical History Questions

• Questions should focus on relevant disabilities likely to affect daily function (e.g., rheumatoid arthritis, Parkinson's, multiple sclerosis).
• Medical conditions affecting motivation (e.g., diabetes, terminal illness) should be addressed.
• Ophthalmic history focuses on ocular health aspects related to visual impairment (e.g., cataracts, macular degeneration).
• Educational, employment, and social issues related to visual impairment need to be discussed.
• Tasks and rehabilitation should be addressed.

Visual Function Questions

• Questions should directly relate to the assessment of vision.
• Questions need to be specific to prescribing and rehabilitation advice.

Current Optical Corrections

• Existing eyeglasses should be evaluated and prescriptions documented.
• Patients must express their opinions about the usefulness of existing glasses.
• Idiosyncratic responses should be documented and addressed.

Retinoscopy

• Accurate retinoscopy is essential for efficiency.
• Factors affecting accuracy include media clarity, eye movements, and patient cooperation.
• Using a rechargeable, halogen bulb retinoscope is recommended.
• Reducing or eliminating background illumination can be helpful in challenging cases.
• "Radical retinoscopy" with reduced working distance can improve image clarity.

Keratometry/Corneal Topography

• This is rarely used in routine low vision assessment, but can help confirm astigmatism (especially irregular astigmatism).

Subjective Refraction

• Trial frames should be comfortable and fitted like final glasses.
• Spherical lenses are placed in the back of the trial frame; cylindrical lenses are placed in the front.
• Fully-apertured trial lenses should be used to assess eye movements and fixations.
• Refraction over existing glasses can clarify if changes are significant (using a Halberg trial clip).

Distance Vision

• Chart selection depends on visual acuity.
• Patients with acuity under 1.0 LogMAR (6/60 Snellen) need 1-3 meters charts and 0.6-1.0 LogMAR (6/24-6/60 Snellen) should use 3-4 meters charts. Six-meter charts are for acuities better than 0.6 LogMAR.

Near Acuity

• Near acuity tests should be recorded using existing glasses and a standard +4.00 near addition.
• Practical consideration is given to working distance.
• The working distance may be 25cm for certain individuals.
• Consider factors like lighting and patient comfort.
• Use of fogging lenses is not generally helpful in low vision.

Binocularity

• Binocular vision is not a typical aspect for most visually impaired patients, but can be considered.
• If binocularity is present in both eyes, base-in prisms may be used.
• Maximum achievable addition for binocularity is +12.00 diopters.
• Occlusion tests are recommended for patients lacking binocular vision.
• Consider using different lens options that include options such as Bifocals.

Spectacles Prescribing

• Visual impairment or medial opacities can lead to unstable refraction.
• Patients with high refractive error may be offered contact lenses.
• Contact lenses may increase field expansion, particularly in aphakia, but magnifi cation may be reduced.
• Consider factors including cost, discomfort, and ability to function with the lenses.
• Refractive surgery considerations need careful consideration and discussion with a surgical team.