Overview
This lecture covers the definitions, significance, and measurement methods for the range and amplitude of accommodation in the human eye, focusing on practical tests and related concepts.
Accommodation: Definition and Importance
- Accommodation is the process by which the eye adjusts its optical power to maintain clear focus on near objects.
- It occurs via contraction of the ciliary muscle, causing the lens to become more globular and increase refractive power.
- Accommodation allows for clear vision at varying distances, particularly near tasks.
Key Terms: Near Point and Far Point
- Far Point (Punctum Remotum): The farthest distance at which small objects can be seen clearly; varies with the eye's refractive status.
- Near Point (Punctum Proximum): The closest distance at which small objects can be seen clearly; recedes with age.
- In emmetropic eyes, the far point is at infinity; in hypermetropic eyes, it is virtual (behind the retina); in myopic eyes, it is real and in front of the retina.
Range and Amplitude of Accommodation
- Range of accommodation = distance between near point and far point (in cm or meters).
- Amplitude of accommodation = dioptric difference between near point and far point powers.
- Formula: Amplitude = 1/near point (m) - 1/far point (m); for corrected patients, far point = infinity, so amplitude = 1/near point (m).
- Normal near point varies with age: at 10 years = 7 cm; at 40 years = 25 cm; at 45 years = 33 cm.
- Hofstetter's formulas estimate age-related values for amplitude of accommodation:
- Maximum = 25 - 0.4 × age
- Minimum = 15 - 0.25 × age
- Average = 18.5 - 0.3 × age
Methods for Measuring Amplitude of Accommodation
- Four main methods: three subjective (push-up, pull-away, minus lens), one objective (dynamic retinoscopy).
- Push-up: Move a near target closer until first sustained blur is reported.
- Pull-away: Move a near target away until the patient can read it clearly.
- Minus lens: Add minus lenses to stimulate accommodation until sustained blur occurs; total amplitude = lens power tolerated + accommodative demand for fixed distance.
- Dynamic retinoscopy: Objectively observes accommodative response using a retinoscope as near stimulus is moved closer.
- Push-up method overestimates amplitude due to retinal image size increase, depth of focus, and proximal cues.
- Minus lens method typically yields 2 diopters less than push-up due to lack of these factors.
RAF Rule and Reading Distances
- RAF (Royal Air Force) rule is a graduated scale with sliding targets for measuring near points.
- Harmon's distance: Distance from elbow to middle knuckle, used as typical reading distance in children.
- Amplitude can be measured monocularly (pure accommodation) or binocularly (includes convergence); binocular readings are typically 0.2–0.6 D higher.
Key Terms & Definitions
- Accommodation — Eye's ability to change optical power for near focus.
- Far Point (Punctum Remotum) — Farthest point of clear vision.
- Near Point (Punctum Proximum) — Nearest point of clear vision.
- Range of Accommodation — Distance between near point and far point.
- Amplitude of Accommodation — Dioptric difference between near and far points.
- RAF Rule — Instrument for measuring near points using a sliding target.
- Harmon's Distance — Elbow-to-knuckle distance, used for reading tests.
- Push-up Test — Subjective test moving target towards the patient for near point.
- Pull-away Test — Subjective test moving target away for clarity.
- Minus Lens Method — Subjective test adding minus lenses to stimulate accommodation.
- Dynamic Retinoscopy — Objective method observing reflex during near focus.
Action Items / Next Steps
- Review the video and concepts of retinoscopy before the next lecture on accommodative response and lead/lag of accommodation.