Dr Ben Wild
Overview |
Most of the focusing power of the eye comes from the cornea, the clear tissue in front of the colored iris, and the intra-ocular lens, a clear tissue located through the pupil. Normally, when the eye is focused on a distant target, the muscles that control the intra-ocular lens's shape are relaxed and when the eye is focused on a near target, the muscles are tensed. This change in intra-ocular lens shape is what enables the eye to see a distant target clearly and then switch to see a near target clearly.
A sagittal view of light from a distant object focusing on the retina (top) and light from a near target focusing on the retina (bottom).
Unfortunately, the intra-ocular lens never stops growing. Presbyopia occurs when the intra-ocular lens itself becomes sufficiently large and sufficiently stiff that the muscles that control its shape are no longer strong enough to sufficiently change its shape. This leads to the inability to focus both distant and near targets without the use of 2 different prescriptions.
A sagittal view of a lens unable to focus the light from a near object on the retina. Reading glasses would be needed to focus this light back on the retina but would then also make a distant object out of focus.
Presbyopia usually starts between the late 30s and mid 40s and progresses until the lens becomes completely static between the early and late 50s. It does not progress beyond this point.
Signs and Symptoms |
Signs
Inability to see up close when the eyes are properly corrected for distance, squinting.
Symptoms
Eyestrain, headaches, difficulty switching between distance and near targets.
Causes and Risk Factors |
Causes
The intra-ocular lens grows sufficiently large and becomes sufficiently hard that it can no longer change shape. Changing shape is needed for changing focus.
Risk Factors
Age. Presbyopia occurs to everybody between late 30s and mid 40s.
Prevention and Treatment |
Prevention
There are no known preventative measures.
Treatments
· Reading glasses.
· Bifocal or trifocal glasses.
· Progressive glasses.
· Monovision laser surgery, lens replacement surgery or contact lenses.
· Multifocal lens replacement surgery or contact lenses.
· Pilocarpine eye drops.
Prognosis |
Presbyopia occurs in everybody. It is not a disease and does not increase risk of developing any eye disease. It is a normal part of ageing.