Lens · 13 of 24
What are the zonules?
The zonules are the suspensory ligament of the lens: hundreds of fine, strong fibres that run from the ciliary processes to the lens capsule around its equator, holding the lens centred behind the pupil and passing the ciliary muscle's pull to it. They are the mechanism of accommodation, and when they fail the lens moves.
- Made of
- Fibrillin microfibrils, the same protein that is faulty in Marfan syndrome
- Run from
- The valleys between the ciliary processes
- Attach to
- The lens capsule at and around the equator, front and back
How they focus the eye
The zonules are under tension when the ciliary muscle is relaxed, and that tension keeps the lens flat for distance. When the ciliary muscle contracts its ring narrows, the zonules go slack, and the lens's own elastic capsule rounds it into a stronger lens for near. Relax the muscle and the zonules pull it flat again. Nothing else in the body focuses this way.
The zonules are also a barrier: with the lens they separate the aqueous in front from the vitreous behind, which matters when the lens is removed and the vitreous can come forward.
When they give way
Weak zonules let the lens subluxate. In Marfan syndrome the fibrillin is abnormal from birth and the lens drifts up and out; in homocystinuria it goes down and in. Blunt trauma tears zonules in a segment, so the lens tilts. Pseudoexfoliation, a common deposit of grey material on the lens and iris in older people, weakens zonules slowly and makes cataract surgery riskier because the lens can drop during the operation.
At the slit lamp the signs are an iris that wobbles with eye movement, a lens that trembles, and after dilation a visible lens edge or the zonules themselves stretched across the pupil.
Examine it
Weak zonules (trauma, pseudoexfoliation, Marfan syndrome) let the lens subluxate.
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