Accomodation

• Zonules control accommodative movement of the ciliary muscle upon the lens

• Ciliary muscle comprises three groups of smooth muscle fibre bundles which function with the zonules and the elastic lens capsule to alter the refractive power of the lens

• Not a true reflex
• Accommodation is the slowest part

• Note: if accommodation is prevented (eg. using a refracting lens) convergence will still be associated with pupillary constriction

Development

• Accommodation begins to develop at 2 months of age
• Well developed by 8 months
• 2 year old: 20 dioptres
• 20 year old: 10 dioptres
• 60 year old: no or little accommodation (<2 dioptres)

Lens component

• Anterior radius of curvature (10mm) is larger than the posterior (6mm) 
• These values change as the lens enlarges with age.

• Accommodative power decreases with age: approximately 14 dioptres at 10 years becomes 10 dioptres at 20 years

• The nucleus has greater refractive index (1.406) than the cortex (1.386)
• Lens capsule contracts during accommodation
• Anterior pole moves anteriorly
• Axial width increases
• Equatorial diameter decreases
• Lens moves with gravity
• Lens capsule
• Variable thickness: poles are thinner than the equator
• Elastic: changes shape passively in response to CB contraction
• Diabetic patients have a more fragile capsule

Ciliary muscle component

• Smooth muscle
• Contracts and reduces tension on the zonular fibres

• Mostly longitudinally arranged fibres forming a circle with some passing to anterior choroid

• Oblique fibres attach to the scleral spur

AC:A ratio

• The number of prism dioptres of convergence induced by each dioptre of accommodation
• Normal range is 3:1 to 5:1
• Can be altered
• Optically: glasses
• Medially
• Surgically
• Heterophoria method
• AC:A = IPD in cm + (Near PCT)-(Distance PCT)Near fixation distance in dioptres

• Note: PCT values are positive for esodeviations (convergences) and negative for exodeviations (divergences)

• Uses the changes in deviation for near versus distance
• Gradient method
• Uses measurements at the same distance but induces accommodative changes using lenses
• Does not require the IPD

• AC:A = Deviation when accommodation greatest - Deviation when accommodation leastAmount of accommodation in dioptres

• Distance-near comparison
• Simpler, quicker, no calculations
• Compare the deviation at distance with that for near
• If the near deviation is >10PD more, the AC:A is likely abnormally high

• Fully refractive/accommodative esotropia can be eliminated by glasses wear.

• If an esotropia is reduced but not eliminated by hypermetropic correction then squint surgery may be needed in addition to glasses

• See Part 2 package for full chapters on strabismus

Zonular fibres

• Radially arranged zonules of Zinn
• Attached 2mm anterior and 1mm posterior to lens equator
• Bundles of fine fibres composed of microfibrils
• Composed principally of fibrillin (cysteine-rich glycoprotein)
• Fibrillins form the structural scaffold of extensive microfibrils
• Provide strength and elasticity to ocular connective tissues
• Synthesised by non-pigmented ciliary epithelium
• Emerge from basal lamina of epithelium and insert beneath the lens capsule
• Pass from ciliary processes to lens and do not cross each other
• Others form part of the vitreous base