Test 10

1. The following are true about aphakia: a. the refractive power of the eye is reduced b. the anterior focal length is increased c. an anterior chamber lens implant minimize the image size d. a posterior chamber lens implant does not alter the image size e. image magnification does not occur with contact lens correction of aphakia  answera.T   b.T   c.F   d.T   e.F The refractive power is reduced from 58D to 42D The anterior focal length is increased from 16.70mm (in a phakic eye )to 23.20mm Any lenses placed in front of the posterior chamber to correct aphakia will magnify the image. Spectacle will magnify it by about 33%, contact lens about 6% and anterior chamber lens by about 2-3%.     2. The following are true about biometry in a general population in the UK: a. 96% of axial lengths fall within the range 21.0 to 25.5mm  b. 98% of K readings fall within the range 40 to 48D c. the difference in the axial lengths between the two eyes of an individual is usually      less than 1 mm d. SRK/T formula is more accurate than SRK/II in the calculation of intraocular lens e. SRK/T formula is more accurate than the Holladay formula in intraocular lens      calculation answera.T   b.T   c.F   d.T   e.F 96% of axial lenghts fal within the range 21.0 to 25.5 mm and for 60% this is between 22.5 and 24.5 mm 98% K-readings fall within the range 40 to 48D and for 68% this is between 42 and 45D. An interocular difference in axial lengths of more than 0.3 mm or K readings which vary by more than one dioptre requires confirmation. SRK/T is more accurate than SRK/I or SRK/II but there is no statistical difference between SRK/T , Holladay and Hoffer Q in intraocular lens calculation.   3. In the calculation of the intraocular lens, the following will give an axial length which is shorter than expected: a. pseudophakia  b. silicone oil  c. corneal decompensation d. aphakia e. choroidal effusion answera.F   b.T   c.T   d.T   e.T Conditions that give a longer than expected readings are: pseudophakia eccentric staphyloma Conditions that give a shorter than expected readings are: aphakia silicone oil corneal decompensation retinal detachment  choroidal thickening     4. With regard to the back vertex distance (BVD): a. it is directly related to the back vertex power b. a hypermetrope can increase the effective lens power by slipping the glasses down      the nose c. it is increased in a patient with thyroid eye disease d. a +4D hypermetrope needs a stronger prescription if contact lenses were used e. the effective lens power of a concave lens is increased by moving the lens towards      the cornea answera.F   b.T   c.F   d.T   e.T Back vertex distance measures the distance between the posterior surface of a lens and the cornea. It is not affected by the back vertex power. In thyroid eye disease, exophthalmos decreases the distance between the lens and the cornea. The effective power of a convex (plus) lens is increased by moving it away from the eye and that of a concave (minus) lens is increased by moving it closer to the eyes. Therefore, correction of ametropia with contact lens requires a stronger prescription for a hypermetrope and a weaker one for a myope.   5. The following features of the eye reduce optical aberrations: a.  presence of pigment on the iris b. a flatter peripheral cornea c. relative insensitivity of the cones to oblique light d. presence of xanthophyll e. absorption of long wavelength by the crystalline lens answera.F   b.T   c.T   d.T   e.T Optical aberrations are reduced by: an aspheric cornea which is flatter peripherally absorption of shorter wavelength by the crystalline lens and the xanthophyll which would otherwise cause more light dispersion       Return to the main page