(The following are the answers to mock examination 3. To return to each question, click the number.)
a. Right suppression

b. Left suppression

c. Exotropia (crossed diplopia)

(Worth's four dot test tests the presence of binocular vision. It consists of four lights: two green, one red 
and one white. The patient is asked to view the dots wearing red and green filters (such that one eye sees
one red and one white light and the other sees the two green and one white lights) and report the number 
and colour of the dots he sees. Four dots indicate normal binocular vision; two red dots indicate 
suppression of the eye wearing the green filter; three green dots indicate suppression of the eye wearing 
the red filter; five dots (two red and three green) indicate diplopia which may result from exotropia or 

F =  principal point, O = object, I = image
(The centre of curvature of a concave mirror is equal to the radius of the concave mirror whereas 
the principal point of the concave mirror is equal to half the radius of the concave mirror (r/2).
When the object is between the centre of curvature and the principal focus, the image is real, inverted and magnified. With the object inside the principal focus, the image moves to the other side of the mirror and becomes virtual, erect and magnified.)
a. +2.50 / -1.50 X 80 = +1.75 (spherical equivalent)

b. -4.00 / + 6.00 X 90 = -1.00 (spherical equivalent)

c. +1.50 / -3.50 X 45 = -0.25 (spherical equivalent)

(To draw the power cross remember that the power of the cylinder is 90 degrees to the axis.
The spherical equivalent is calculated by adding the value of the sphere and half the value of the cylinder.)

a.  The induced prism in each eye is 5 X 1cm = 5 dioptres base up.
(Prentice's rule states that the prismatic effect is equal to the point from the optical centre in cm multiply by the the dioptric power of the lens)
b.  1cm  X  5D  X  0.2m / 1.0m = 1cm displacement for each eye.
(This is calculated by remembering that 1 prism dioptre deviates an object placed 1 m away by 1 cm.)
c. Below it.
(The image is displaced downward by a a base-up prism)
a. Coronal section.

b. Enlargement of the muscles in the right eyes especially the inferior rectus.

c. Right thyroid eye disease.

d. Any of the following abnormalities may occur :

      • lid lag
      • restricted eye movement in all directions especially upgaze
a.  The retina and optic nerve are stimulated with a shifting checkerboard pattern. This external visual stimulus causes measurable electrical activity in neurons within the visual pathways. This is called the visual evoked response (VER) and is recorded by EEG electrodes located over the occiput. Using special computer techniques, the evoked responses measured over multiple trials are amplified and averaged.
(With pattern-shift VER, the waveform normally appears as a straight line with a single positive peak (100 msec after stimulus presentation). Abnormalities in this characteristic waveform may be seen in a variety of pathologic processes involving the optic nerve and its radiations. Pattern-shift VER is a highly sensitive means of documenting lesions in the visual system. It is especially useful when the disease process is subclinical for example ophthalmologic exam is normal and patient lacks visual symptoms.)
b. Left eye which shows a significant prolonged latency.

c. Left retrobulbar neuritis.

a. The visual field shows bilateral altitudinal field defect. 
    Possible causes include:
  •     bilateral ischaemic optic neuropathy (arteritic or non-arteritic
  •     bilateral superior hemi-retinal artery occlusion
  •     bilateral superior hemi-retinal vein occlusion

  • b. The visual field shows bilateral constricted visual fields.
        Possible causes include:

  •     retinitis pigmentosa
  •     bilateral dense laser pan-photocoagulation
  •     advanced glaucoma

  • c. The visual field shows a left congruous horizontal  wedge-shaped field 
        defect. It is seen in lesion of the right lateral geniculate nucleus such as
        cerebrovascular accident.

    a. The corneal topography shows with-the-rule astigmatism.
        ( The astigmatism is shown by the typical bow-tie appearance and since the steepest part of the 
            cornea is at 90 degrees the astigmatism is with-the-rule)
             b. Cutting the cornea superiorly will flatten the cornea and therefore reduces
                 the astigmatism.
                    (provided the tension of the sutures are equally distributed and not tightened)

             c. A temporal cornea approach with small incision surgery has minimal effect on the
                 corneal topography. 
                   (Due to the small incision and the further distance between the temporal peripheral cornea and the centre 
                    of the cornea.)

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