Refraction in Optometry

1. The Goal of Refraction

2. Types of Refractive Error You Are Measuring For

• Guyton and Hall Textbook of Medical Physiology, p. 625

3. Steps in the Refraction Procedure

Step 1 - Visual Acuity Baseline

Step 2 - Objective Refraction

• The gold standard objective method, especially for children and non-verbal patients.
• A retinoscope projects a streak or spot of light into the eye. You observe the reflex in the pupil as you sweep the light beam:
  • "With" motion (reflex moves same direction as beam) → add plus lenses to neutralize → indicates hyperopia or low myopia
  • "Against" motion (reflex moves opposite to beam) → add minus lenses → indicates myopia
• Neutralization point: when the reflex appears to fill the entire pupil with no movement - this is your working distance correction
• A working distance correction of -1.50 D (for 67 cm) or -1.00 D (for 100 cm) is subtracted from the gross retinoscopy result to get the net refractive error

• An automated instrument (autorefractor) projects an infrared target onto the retina and calculates the refractive error electronically
• Fast and reproducible, but not definitive - it is a starting point for subjective refinement
• Can be unreliable in patients with irregular corneas, cataracts, or poor fixation

Step 3 - Cycloplegic Refraction (when indicated)

• Cyclopentolate 1% - one drop, repeated after 5 minutes; maximum cycloplegia within 30 minutes; accommodation recovers in 2-3 hours; preferred for most children
• Atropine 1% - stronger cycloplegia; used for heavy hyperopia, darkly pigmented irides, or when cyclopentolate is insufficient; instilled at home twice daily for 1-3 days before the exam; vision effects last up to 2 weeks; watch for systemic toxicity (flushing, fever, restlessness)
• Topical anesthetic (e.g., proxymetacaine) before cycloplegic drops reduces irritation and reflex tearing, ensuring better drug retention

• Kanski's Clinical Ophthalmology, p. (Strabismus chapter - Cycloplegia section)

Step 4 - Subjective Refraction

• Start from the objective refraction
• Fogging technique: over-plus the patient to relax accommodation, then reduce plus (or increase minus) until best acuity
• Duochrome test: uses chromatic aberration - red and green wavelengths focus at slightly different points. When letters on the green side look sharper → over-minused (add plus). When letters on the red side look sharper → under-minused (add minus). End point: both sides appear equally clear

• A Jackson Cross Cylinder (JCC) is a combined +0.25/-0.50 D crossed cylinder lens on a handle
• Axis check: flip the JCC around the axis of the trial cylinder - the patient picks which orientation gives clearer/sharper lines on the astigmatism chart
• Power check: flip JCC with its axes at 45° to the trial cylinder - patient picks which gives clearer vision; add or reduce cylinder power accordingly
• Repeat axis and power checks iteratively until the patient cannot distinguish between the two JCC positions

• Once monocular refraction is complete, balance the two eyes so accommodation is equal bilaterally
• Methods: fogging one eye and testing the other, or alternating occlusion with Risley prisms

• For patients over ~40 years with reduced near vision
• Start with an age-appropriate add (typically +1.00 D at 40 years, increasing ~+0.25 D per 2 years)
• Refine by testing near acuity at the patient's habitual reading distance

4. Special Considerations

• Kanski's Clinical Ophthalmology, p. (Strabismus/Amblyopia chapter)

5. The Final Prescription

• -2.50 D sphere (myopic)
• -1.00 D cylinder at axis 180° (astigmatism)

Summary Flow

Uncorrected VA
    ↓
Objective refraction (retinoscopy / autorefractor)
    ↓
Cycloplegia if needed (children, latent hyperopia)
    ↓
Subjective refraction (fogging → duochrome → JCC)
    ↓
Binocular balance
    ↓
Near addition (if presbyopic)
    ↓
Final prescription