Test 4

1. Regarding accommodation: a. half of the amplitude should always remain in reserve to avoid fatigue  b. the accommodative amplitude decreases by about 1.5D every 4      years between the age of 40 and 48  c. accommodation decreases with age because the zonules become      less elastic  d. movement of the cornea occurs during accommodation  e. accommodation can not occur in the absence of convergence Answer1.    a.T       b.T    c.F    d.F    e.F Accommodation: occurs in response to blurred images can occur in the absence of convergence decreases with age and is believed to be caused by the reduced deformity and elasticity of the lens protein the iris, len, vitreous and retina move during accommodation but not the cornea   2. Stenopaeic slit: a. is used to determine the power of the sphere  b. can determine the best vision in a certain axis  c. can help determine the position of an iridotomy in corneal      scarring  d. can affect the visual acuity by the width due to the effect of      diffraction  e. in a patient with astigmatism can improve the vision Answer2.    a.F    b.T    c.T    d.T    e.T Stenopaeic slit: can be used to determine subjective astigmatic refractive error, especially when the retinoscopic reflex is dim. The power can then be deducted by adding spherical power to the best visual acuity can be used to locate the two meridians of astigmatism gives the best visual acuity when it is in the axis of the astigmatism and may be used to decide where iridotomy need to be performed in corneal scarring to allow light passage can be considered as an elongated pinhole and the narrower the slit width the more diffraction can occur which can affect the visual acuity 3. The following are true about Hertel's exophthalmometer: a. the measurement may be affected by the presence of strabismus  b. the foot plate should rest on the lateral canthus rather than the      orbital for accurate reading  c. patients of African origin has a higher normal reading than European  d. a difference in readings between the two eyes of up to 5 mm is      normal in up to 50% of the population  e. can be used to monitor the progress of Grave's eye disease Answer3.    .at    b.F    c.T    d.F    e.T Hertel's exophthalmometer: used to document and monitor the progress of proptosis measure the distance from the orbital rim to the corneal apex and therefore the foot plates should rest on the orbital rim in the presence of strabismus, the corneal apex becomes deviated and reading can become inaccurate in Caucasian, the normal value is 15 to 17mm with a range of 11 to 21 mm; in African, this may range from 12 to 24 mm a difference of 2 mm between the two eyes should be regarded as significant   4. The astronomical telecsope: a. has an eye piece which has minus power  b. the separation of the eye piece and the objective piece is the sum      of the focal lengths  c. the secondary focal point of the objective lens falls at the primary      focal point of the eyepiece  d. the image is erect  e. the emergent rays are parallel Answer4.    a.F    b.T    c.T    d.F    e.T Astronomical telescope: consists of two positive lenses (objective and eye pieces) the separation between the lenses are the sum of their focal lengths the image produced is inverted the focal point of the objective lens conincides with the primary focal point of the eyepiece the emergenct rays are parallel   5. Myopic shift occurs in: a. keratoconus  b. spasm of the ciliary body  c. staphyloma  d. lens subluxation  e. brittle diabetes Answer5.    a.T    b.T    c.T    d.T    e.T Myopic shift: occurs when the far point is moved forward occurs in keratoconus because the increased curvature of the cornea increase the refractive power spasm of the ciliary body increases accommodation which brings the far point forward staphyloma occurs in conditions such as myopia or rheumatoid arthritis and moves the retina away from the far point forward lens subluxation causes the far point to move forward poorly controlled diabetes mellitus can cause lens swelling resulting in increased refractive power

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