Test 33

1. The following are true about presbyopia:

a. it tends to occur earlier in myopia than hypermetropia

b. it tends to occur earlier in uncorrected than corrected 
    hypermetropia

c. it tends to occur earlier in the tropics than in temperate 
    countries

d. it occurs in patients whose amplitude of 
    accommodation is less than 4 D

e. in order for an emmetrope with a remaining amplitude 
   of accommodation of 3 D to read clearly at 25cm, the 
   appropriate glassed should be 2 D

 
Answera.F   b.T  c.T   d.T  e.T
Presbyopia than to occur earlier in hypermetropia than myopia. For comfortable reading, 1/3 of accommodation should be kept in reserve. In an emmetrope with 3D, 1D should be kept in reserve leaving 2D for close work. 4D is needed for reading at 25cm, thus 2D plus glasses are needed in this patient.

2. The following are true about intraocular lens implant:

a. a forward movement of the implant will make the eye 
    more hypermetropic

b. a forward movement of the implant will increase the 
    effectivity of the lens 

c. the implant is displaced inferiorly if there is inferior 
    zonulysis

d. PMMA cannot be used to make foldable lens

e. PMMA lens is thicker than silicone lens for the same 
    refractive correction

 

Answera.F  b.T  c.F  d.T  e.T
A forward movement of the implant will increase the effectivity of the lens and make the eye more myopic. The implant is displaced superiorly if there is inferior zonulysis. PMMA is rigid and cannot be used to make foldable lens. PMMA lens has a lower refractive index than silicone and therefore thinner lens for the same refractive correction.

 
 

3. With regard to hydrogel intraocular lens:
 

a. it has the same refractive index as silicone

b. it can be used to make foldable lens

c. it increases in size with hydration

d. it is more susceptible to damage from YAG laser 
    than PMMA lens

e. it can be sterilized using heat

 
Answer
a.T   b.T  c.T  d.T  e.T
Both hydrogel and silicone has a refractive index of 1.43.

 

. 4. Regarding anti-reflection coatings on the glasses:

a. it is made up of a thin film of crystalline substances

b. it works by constructive interference of reflected lights

c. coating can be applied to either the anterior or 
    posterior surface only

d. wavelengths at the ends of the visible spectrum are 
    only partly reflected 

e. it can be achieved by dipping process

Answer
a.T  b.F  c.F  d.T  e.F
Coatings are applied by vacuum evaporation and deposition technique. It is made up of a thin film of crystalline substances and are applied to both the anterior and posterior surfaces. Anti-reflection works by destructive interference of reflected lights. It is maximal for a given wavelength. 

5. With regard to contact lens:

a. when changing from glasses to contact lenses, 
    the myope need a weaker lens

b. when changing from glasses to contact lenses, 
    the hypermetrope needs a stronger lens

c. the near point of accommodation is reduced in 
    a myope wearing contact lens

d. the near point of accommodation is increased in
    a hypermetrope wearing contact lens

e. contact lens increases the field of vision compared
    to glasses.

 
Answer
a.T   b.T  c.T  d.T  e.F
When changing from spectacles to contact lenses, the 
myope needs a weaker lens and the hyperope needs
a stronger lens. The myope who has spectacles as well 
as contact lenses, each correcting distance vision, will find that the near point of accommodation is more remote with the contact lenses than with the glasses; the hyperope will find the reverse.

 

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