Test 9

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.   Return to the main page