Overview
This lecture explains the concept of accommodation in the eye, its anatomical basis, the mechanism, key parameters, and calculations related to accommodation and its clinical relevance.
Structure and Refractive Power of the Eye
- The main refractive surfaces of the eye are the cornea (+45 diopters) and the crystalline lens (+15 diopters), totaling about +60 diopters.
- The eye acts as a converging lens, focusing parallel 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 curvature (convexity) of the anterior surface of the lens, making it fatter.
Anatomy Related to Accommodation
- The lens is located behind the iris and is connected to the ciliary body via suspensory ligaments (zonules).
- The ciliary body contains the ciliary muscle, which controls lens shape during accommodation.
Mechanism of Accommodation
- For distant vision, ciliary muscle is relaxed, suspensory ligaments are taut, and the lens is flat.
- For near vision, ciliary muscle contracts, suspensory ligaments relax, and the lens becomes more convex (globular), increasing its power.
Key Accommodation Points
- Near point of accommodation (punctum proximum) is the closest distance at which small objects can be focused; it recedes (increases) with age.
- Far point of the eye (punctum remotum) is the farthest point that can be focused, at infinity for normal eyes, in front of the retina for myopia, and behind for hypermetropia.
- Range of accommodation is the distance between near and far points.
Measuring Accommodation
- The RAF rule (Royal Air Force rule) is used to measure near and far points of accommodation.
Amplitude of Accommodation
- Amplitude is the difference in refractive power (in diopters) between the accommodated (near) and unaccommodated (far) state.
- It is calculated as: amplitude = (1/near point in meters) - (1/far point in meters).
- Amplitude of accommodation decreases with age, causing the near point to move farther away.
Calculation Examples
- A 10-year-old child with a near point at 8 cm has an amplitude of 12.5 diopters.
- Myopic child (–2D) needs less accommodation; hypermetropic child (+3D) needs more, using sign conventions for calculation.
- The Donder’s curve illustrates age-related decline in amplitude of accommodation.
Key Terms & Definitions
- Accommodation — ability of the eye to change its refractive power to focus on near objects.
- Diopter — unit of measurement for the optical power of a lens.
- Ciliary muscle — muscle that changes the shape of the lens during accommodation.
- Suspensory ligaments (zonules) — fibers connecting lens to ciliary body.
- Punctum proximum — nearest point of clear vision.
- Punctum remotum — farthest point of clear vision.
- Amplitude of accommodation — difference in refractive power between near and far focus states.
- RAF rule — instrument to measure range of accommodation.
Action Items / Next Steps
- Practice calculating amplitude of accommodation for different ages and refractive errors.
- Review anatomy of the lens, ciliary body, and suspensory ligaments.
- Understand sign conventions for power calculations in myopia and hypermetropia.