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Test 21

1. A 50 year-old man has refractive errors of  Right eye -3.00DS   Add +2.00 Left  eye -3.50 DS   Add +2.00     He likes to wear contact lenses so that one eye is used for distance      and the other for reading ie. monovision correction. The following      are true: a. the dominant eye is usually selected for near vision b. if the right eye is chosen for reading, the contact lens     power should be around +1.00Ds c. if the left is chosen for reading, the contact lens power     should be around -1.50 DS d. his stereopsis will be reduced e. his contrast sensitivity will be reduced   Answer a. F   b.F   c.T   d.T   e.T The dominant eye is usually selected for distant vision. The correct lenses for reading should be about -1.00 DS for the right eye and -1.50 DS for the left eye. The binocular function will be reduced with monovision correction as is the contrast sensitivity. 2. The following are true about contact lenses: a. both regular and irregular astigmatism can be corrected      by rigid gas permeable contact lens b. contact lens correction of hypermetropia increases the      size of the retinal image c. symptoms of presbyopia are increased in contact lens      correction of high myopia d. aniseikonia is reduced if unilateral refractive ametropia      is corrected with contact lens e. aniseikonia is reduced if unilateral axial ametropia is      corrected with contact lens   Answer a.T   b.F   c.T   d.T   e.F The rigid lens can maintain its shape and trap tear  behind its lens. This smoothes the surface of the cornea and hence any corneal astigmatism. Contact lens  correction of hypermetropia decreases the size of the  retinal image when compared with spectacle correction. The reverse is true with myopic correction. Spectacle correction of myopia with its prismatic effect help to reduce the symptoms of presbyopia. Spectacle correction of unilateral ametropia causes aniseikonia in the case of refractive ametropia but not in axial ametropia. 3. The following are true about tilting of the intraocular lens: a. it is the major cause of post-operative astigmatism b. it causes an increase in the spherical power c. a tilt of the lens around its vertical axis causes      against-the-rule astigmatism d. a 20 degrees tilt of the lens induces 4D of astigmatism e. it is commoner with anterior than posterior chamber      lens implant Answer a.F   b.T  c.T   d.F   e.F Although lens tilt can cause significant refractive changes, most tilts are small and do not contribute to significant changes.  A tilt can change both the spherical and astigmatic power. A tilt around the vertical axis causes against-the-rule astigmatism. A tilt of 10 degrees of a 20D induces 0.5D cylinder, a 20 degrees tilt induces 2.0D and a 30 degrees tilt induces 5D of cylinder. The iris prevent significant tilting of an anterior chamber lens. 4. Foldable intraocular lenses can be made from: a. acrylic  b. silicone c. PMMA d. hydrogel e. poly-HEMA   Answer a.T   b.T   c.F   d.T   e.T PMMA is rigid and can not be folded. The other name for hydrogel is poly-HEMA.   5. Regarding spherical aberration: a. spherical aberration of a spherical lens occurs due to      stronger refraction of paraaxial rays than peripheral rays b. aspherical lens reduces spherical aberration c. planoconvex lens reduces spherical aberration d. bull's eye retinoscopic reflex in children is the result      of spherical aberration e. dilatation of pupil increases spherical aberration   Answer a.F   b.T   c.T   d.T   e.T In spherical aberration, the spherical lens refracts the peripheral rays more strongly than the para-axial rays. This occurs because the peripheral lens has a different refractive power from the central lens. This aberration can be reduced with aspherical or planoconvex lenses. Bull's eye retinoscopic reflex is the result of spherical aberration. In this reflex, the peripheral reflex shows against movement while the central reflex has been neutralized by trial lenses. Dilatation of pupil increases spherical aberration.   Return to the main page