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

“Near triad”: accommodation, pupillary constriction, convergence

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)

Adie’s pupil: damage to ciliary ganglion may cause loss of accommodation

## 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

Lens capsule antigens are similar to those in the glomerular basement membrane

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

Muscarinic agonists (eg pilocarpine) act on the ciliary muscle to produce accommodation, anterior pull on the choroid and traction on the trabecular meshwork fibres via the scleral spur to aid aqueous outflow

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

Homocystinuria causes lens dislocation because zonules are cysteine rich