Retinoscopy
Teaching images
81. Clinical Analogy - The Lighthouse Beam
Imagine shining a torch beam through a foggy window. If the glass is curved inward (concave, myopic eye), the beam converges and the reflected glow on the floor moves against your hand movement. If the glass bulges outward (convex, hyperopic eye), the reflected glow moves with your hand. When you place just the right corrective lens in front of that curved glass, all the reflected light suddenly fills the pupil uniformly - the 'reversal' point - because the far point of the eye is now at the peephole of your retinoscope (working distance). That instant of uniform glow is neutralisation, and the lens power at that moment tells you the refraction. Retinoscopy is nothing more than finding this lens.
2. Optical Principles of Retinoscopy
The Retinoscope - Instrument Overview
The retinoscope projects a streak (or spot) of light into the patient's eye and allows the examiner to observe the reflex from the patient's retina through the same instrument. Two types exist:
- Streak retinoscope (Copeland/Welch-Allyn): Projects a linear streak; preferred.
- Sleeve position depends on the retinoscope design: Copeland - sleeve up = plane-mirror effect; other retinoscopes - sleeve down = plane-mirror effect.
- Spot retinoscope: Projects a circular spot; largely obsolete.
- Sleeve position depends on the retinoscope design: Copeland - sleeve up = plane-mirror effect; non-Copeland retinoscopes - sleeve down = plane-mirror effect.
- With the retinoscope set to the plane-mirror effect, the reflex moves WITH the streak in hyperopia.
- The concave-mirror setting is the opposite sleeve position from the device-specific plane-mirror setting.
- In the concave-mirror setting, the reflex moves AGAINST in hyperopia.
- Exams always assume plane mirror (sleeve up) unless stated otherwise.
Far Point Concept - The Cornerstone
Teaching image
1- The far point (punctum remotum) is the point conjugate with the retina when accommodation is fully relaxed.
- Understanding its location relative to the retinoscope determines reflex movement:
| Refractive State | Far Point Location | Clinical Implication |
|---|---|---|
| • Emmetropia | • Infinity | • At infinity - parallel rays focus on retina |
| • Myopia | • Finite distance in front of cornea | • Far point between patient and retinoscope only when myopia exceeds the inverse of the working distance |
| • Hyperopia | • Virtual point behind retina | • Far point behind patient (virtual) |
| • Regular astigmatism | • Two different far points (one per principal meridian) | • Different reflex widths per axis |
| Refractive State | Far Point Location | Clinical Implication |
|---|---|---|
| • Emmetropia | • Infinity | • At infinity - parallel rays focus on retina |
| • Myopia | • Finite distance in front of cornea | • Far point between patient and retinoscope only when myopia exceeds the inverse of the working distance |
| • Hyperopia | • Virtual point behind retina | • Far point behind patient (virtual) |
| • Regular astigmatism | • Two different far points (one per principal meridian) | • Different reflex widths per axis |
Working Distance & Allowance
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1- The examiner works at a set distance - typically 67 cm (1.5 dioptres (D)) or 50 cm (2.0 dioptres (D)).
- Because the retinoscope is placed at the examiner's eye, the far point of a neutralised eye is at the retinoscope, not at infinity.
- A deduction (working distance allowance) must be made:
- At 67 cm: subtract 1.5 dioptres (D) from gross retinoscopy finding
- At 50 cm: subtract 2.0 dioptres (D) from gross retinoscopy finding
- At 1 metre: subtract 1.0 dioptres (D) - used in some Indian clinical setups
- Multiple-choice question (MCQ) Trap / Examiner Favourite
- Exam trap: A patient's retinoscopy shows neutralisation at +2.00 dioptres sphere (DS) at 67 centimetres (cm) working distance.
- The net refraction is +2.00 − 1.50 = +0.50 dioptres sphere (DS) (low hyperopia), NOT +2.00 dioptres (D).
- Always subtract working distance allowance.
- Forgetting this is the commonest multiple-choice question (MCQ) trap.
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Keep reading Retinoscopy
11 more sections are included with the Clinical plan.
- Reflex Characteristics & Interpretation
- Streak Retinoscopy Technique - Step by Step
- Retinoscopy in Astigmatism
- Cycloplegic Retinoscopy
- Retinoscopy in Special Scenarios
- Comparative Assessment Methods