*topic 7 crystalline lens part 2 Flashcards
what is accommodation?
When a near object is to be focused on the retina, an increase in refractive
power is needed. The increase in power is called accommodation.
what happens during accomodation?
Contraction of ciliary muscle (stimulation of the 3rd CN)—– relaxes the tension in the zonules—lens compressed at the equator
lens thickness increases anterior to posterior →anterior lens surface moves forward (anterior chamber becomes shallower) →anterior curvature steepens (no change in the position of the posterior pole)
This causes the change in lens power–Accommodation (also accompanied by miosis & convergence)
what is the dioptric power of the lens?
what is the decrease in range of power at 40 and 60 years old?
The dioptric power of the lens is about +15.00D (+10 to +20D)
The range of the power decreases with age
About 8 D at 40 years
~1 D by 60 years
what is presbyopia? what causes it?
The loss of accommodation (reduced amplitude of accommodation) is presbyopia.
As the lens ages and its cells population increases the cells become more tightly packed ( density ↑) within the lens capsule.
The lens becomes less flexible and there is ↓ability of contraction and relaxation of ciliary muscle
Hence more difficult to accommodate and also to restore the lens to its unaccommodated state. This makes it more difficult to read small prints at near and noticing blur when looking up from close work.
what are the 9 ways to manage presbyopia?
- Separate glasses for distance and near
- Bifocals
- Multifocals/PALs
- Contact lenses
- Combination of Spectacles &/or CLs
- Monovision
- Refractive surgery
- Increased lighting
- Increase font size for digital devices/print material
what is the age that cataracts usually occur?
80% of human beings have some alterations in lens transparency (cataract) after 60-65 years of age.
what are the 3 types of cataracts and their locations?
- nuclear cataract (lens nucleus)
- cortical cataract (anterior or posterior lens cortex)
- posterior sub capsular cataract (next to posterior capsule)
what are the 4 causes/mechanism of cataracts? (PEDR)
- Protein aggregation which results in membrane disruption
- enzymes may be damaged by oxidation- electrolyte imbalance.
- Disruption of membrane lipid which contributes to membrane permeability
- Reduction in glutathione synthesis ( reduces protection against oxidation damage)
what is nuclear cataract? what are 4 of its symptoms? (BMGD)
Hardening of the lens nucleus. Due to oxidative damage to lens proteins and lipids. Worsened by a lack of antioxidants in the diet and cumulative exposure to UV radiation.
4 symptoms:
- blurred vision for far
* *2. myopic shift - glare
- desaturation of colours
what is cortical cataract? what are 3 of its symptoms? (BHG)
Opacity in the anterior or
posterior lens cortex. ***Seen in periphery in a spoke-like manner
Due to imbalance of electrolytes causing over-hydration of cortex. Affects fibre cells on surface of the lens
3 symptoms:
- Blurred vision
- ***Hyperopic shift
- Glare in dim light/night time
what is posterior subcapsular cataract? what are 2 of its symptoms?
Opacity next to the posterior capsule. Migration of cells from equatorial region to replace the lens fiber nuclei loss. These cells accumulate at the posterior pole forming an opacity.
2 symptoms:
- Reduced near vision
- Glare in bright light
what are the 7 managements for cataract?
- Prescribe corrective glasses
- Avoid UV exposure (wear hats/visors, umbrellas, sunglasses, tinted lenses)
- Improved light conditions to enhance contrast
- Antioxidant diet
- Lifestyle changes (stop smoking, alcohol consumption, exercise more)
- Systemic health check (keep blood pressure and blood sugar in control)
- Cataract surgery (intra ocular lens)
