Topic 7: (Part 2) Presbyopia & Cataract

Question 1

explain the process of accommodation

Answer 1

for near fixation, more refractive power is needed.

accommodation is this increase in refractive power by modification of the lens shape.

the ciliary muscle contracts which relaxes the tension of suspensory ligaments & the lens is compressed at its equator.

lens size increases anterior to posterior where the anterior lens surface moves forwards and the posterior surface stays put. (anterior surface steepened, anterior chamber shallower)

This causes the lens to be more convex, increasing the refractive power - accommodation (accompanied by miosis and convergence)

Question 2

explain what presbyopia is

Answer 2

presbyopia is the loss of accommodation with age (usually starts at age 40).

this is because the as the lens ages & its cell population increases, the cells become more tightly packed (density increases) within the lens capsule.

now the lens is more rigid & less flexible

with a more rigid lens, there is less contraction of the ciliary muscle (does not relax the zonules as much) as well as less relaxation of the ciliary muscle. Hence it is not as easy to accommodate or restore the lens to its unaccommodated state.

thus, presbyopes will have difficulty with near tasks

Question 3

define cataract

Answer 3

alterations in lens (cortex and/or nucleus) transparency which occurs in 80% of people after 60-65 years of age

Question 4

explain causes/mechanisms of cataract

Answer 4

• protein modification/aggregation that results in membrane disruption
• disruption of membrane lipid which contributes to membrane permeability
• reduction in glutathione synthesis (polypeptide protecting lens from oxidation)
• enzymes may be damaged by oxidation- electrolyte imbalance

Question 5

state the age related cataracts and its locations

Answer 5

nuclear cataract (central region)

cortical cataract (edges)

posterior subcapsular cataract (posterior)

Question 6

explain nuclear cataract

Answer 6

(1) it is the hardening of the NUCLEUS, causing a myopic shift. (2) due to oxidation damage to lens proteins and lipids. (3) worsened by lack of antioxidants in diet & cumulative exposure to UV radiation.

SYMPTOMS: blurred far vision, desaturated colours, glare

Question 7

explain cortical cataract

Answer 7

(1) it is the opacity in the anterior or posterior lens CORTEX, causing hyperopic shift. (2) due to imbalance of electrolytes- over hydration of cortex. (3) affects fibre cells on the surface of the lens

SYMPTOMS: blurred vision, glare in dim light

Question 8

explain posterior subcapsular cataract

Answer 8

(1) it is the opacity next to the posterior capsule. (2) due to accumulation of migrating cells (from equatorial region to replace lens fibre nuclei loss) at the posterior pole

SYMPTOMS: reduced near vision, glare in bright light