Overview
This lecture introduces the concept of accommodation in the eye, explaining its physiology, underlying anatomy, and related key terms and calculations.
Refractive Power of the Eye
- The cornea provides about +45 diopters (D) of refractive power.
- The crystalline lens provides about +15 D, for a total ocular power of approximately +60 D.
- The eye acts as a converging lens to focus parallel light rays from distant objects onto the retina.
Concept of Accommodation
- Accommodation is the process by which the eye increases its refractive power to focus diverging rays from near objects onto the retina.
- This is achieved by increasing the convexity (curvature) of the anterior surface of the lens, making the lens "fatter."
- When looking at distant objects, the lens is flatter with a 10 mm anterior radius of curvature and 6 mm posterior; for near objects, the anterior radius reduces to 6 mm, increasing power.
Anatomy Involved in Accommodation
- The lens is suspended behind the iris by suspensory ligaments attached to the ciliary body.
- The ciliary muscle in the ciliary body is key to accommodation.
- When looking at distant objects, the ciliary muscle relaxes, ligaments are taut, and the lens is flat.
- For near vision, the ciliary muscle contracts, ligaments loosen, and the lens becomes more convex and globular.
Mechanism of Accommodation
- Contraction of the ciliary muscle relaxes suspensory ligaments.
- Relaxed ligaments reduce tension on the lens capsule, allowing the elastic lens to become more convex.
- Increased lens convexity boosts its refractive (converging) power, focusing diverging rays onto the retina.
Near and Far Points
- The near point (punctum proximum) is the closest distance at which small objects can be clearly focused; it increases with age.
- The far point (punctum remotum) is the farthest point the eye can see clearly; in normal eyes it is at infinity.
- The range of accommodation is the difference between the far and near points.
Measuring and Calculating Accommodation
- The RAF (Royal Air Force) rule measures near and far points of accommodation.
- Amplitude of accommodation is the difference in dioptric power between the eye's accommodated (near) and unaccommodated (far) states.
- Power (D) is calculated as the reciprocal of viewing distance in meters (Power = 1/Distance in meters).
- Amplitude = (1/Near Point in meters) – (1/Far Point in meters).
Age-related Changes and Clinical Relevance
- Amplitude of accommodation decreases with age (Donders' curve), raising the near point.
- Loss of accommodation with age leads to presbyopia.
- Hypermetropia and myopia affect the far point; sign conventions are key for calculation.
Key Terms & Definitions
- Accommodation — The process by which the eye changes optical power to maintain focus on objects at different distances.
- Diopter (D) — Unit of measurement for the optical power of a lens.
- Ciliary Muscle — Muscle in the eye responsible for changing lens shape during accommodation.
- Suspensory Ligaments — Fibers connecting the ciliary body to the lens, regulating lens shape.
- Near Point (Punctum Proximum) — Nearest point at which the eye can focus.
- Far Point (Punctum Remotum) — Farthest point the eye can focus on clearly.
- Amplitude of Accommodation — The difference in refractive power between accommodated and unaccommodated states.
- RAF Rule — Device used to measure near and far points of accommodation.
Action Items / Next Steps
- Review anatomy of the eye, especially structures involved in accommodation.
- Practice calculating amplitude of accommodation for different ages and refractive errors.
- Study age-related changes in accommodation (presbyopia) and their clinical implications.