Test III

1. The following are true about astigmatism: a. total ocular astigmatism is equivalent to corneal astigmatism plus lenticular      astigmatism b. when a patient with corneal astigmatism is fitted with a soft contact lens, most of the      astigmatism is transferred to the anterior surface of the contact lens c. hard spherical contact lens is better than soft spherical contact lens in correcting      corneal astigmatism d. lenticular astigmatism cannot be corrected with contact lens e. lenticular astigmatism can be corrected with cataract surgery answera.T   b.T   c.T   d.F   e.T Hard spherical contact lens can neutralize up to 90% of the corneal astigmatism. Lenticular astigmatism can be corrected with toric contact lens.     2. The following are true in a patient with the following findings: Spectacle prescription:  -3.00D Keratometry reading:    7.80mm (43.25D) along 1800                                            7.50mm (45.00D) along 900   a. the spectacle prescription is wrong b. there is a 1.75D with the rule corneal astigmatism c. there is no lenticular astigmatism  d. a spherical hard contact lens is useful in correcting this patient's vision e. a spherical soft contact lens can fully correct this patient's vision answera.F   b.T   c.F   d.F   e.T The above prescription shows that the patient has both corneal and lenticular astigmatism. There is a 1.75D with-the-rule corneal astigmatism and 1.75D against-the-rule astigmatism which neutralize each other. A spherical hard contact lens will neutralize the corneal astigmatism leaving behind a residual cylinder of  -1.75 at 900 (+1.75 at 1800) A spherical soft contact lens on the other hand transfer all the corneal astigmatism to its front surface without optically neutralizing it and therefore will still be cancelled out by the lenticular astigmatism.   3. The following are true about astigmatism: a. oblique astigmatism occurs when the principal meridian is not at 900 to each other b. a right corneal suture superiorly causes a with-the-rule astigmatism c. compound myopic astigmatism occurs when both meridian focus light in front of the      eye d. simple myopic astigmatism occurs when one meridian is emmetropic and the other is      myopic e. mixed astigmatism occurs when one meridian is myopic and the other hypermetropic   answera.F   b.T   c.T   d.T   e.T Oblique astigmatism occurs when the principal meridian lies more than 200 from the horizontal or vertical meridian. Compound hypermetropic astigmatism occurs when both meridian focus light behind the eye. Simple hypermetropic astigmatism occurs when one meridian is emmetropic and the other is hypermetropic   4. With regard to accommodation: a. the near point for a myope is closer without wearing correcting glasses b. measurement of the near point of accommodation with a RAF rule is more accurately      performed monocularly than binocularly c. 'method of spheres' is a useful technique for measuring the amplitude of      accommodation in young hypermetropes. d. accommodation is induced with concave lenses e. an uncorrected myope (-2D) with a near point of 0.1m has an amplitude of      accommodation of 8D. answera.T   b.T   c.F   d.T   e.T With binocular measurement, the accommodation measurement may be inaccurate due to poor convergence. Method of spheres may be inaccurate in young hypermetropes due to failure of relaxing the accommodation Amplitude of accommodation = (1/near point) + refractive error = (1/0.1)+ (-2)D = 6D     5. The following are true about colour blindness: a. protanopia is caused by abnormal red cones b. tritan is caused by abnormal blue cones c. deutan is caused by abnormal green cones d. deuteranomalous trichromatism is the most common type of colour blindness e. blue cone monochromatism occurs in patients who lose the function of blue cones answera.T   b.T   c.T   d.T   e.F Protan (red)  Deutan (green) Tritan (blue) Blue cone monochromatism occurs in patients who have only blue cones.     6. 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%.     7. 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.   8. 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     9. 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.   10. 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