(The following are answers to mock examination 5. To return to the question click on the number.)
(Remember that : 
                            i) the power of the cylinder is 900 to the axis 
                           ii) 2/3 metre is equivalent to -1.50 which has to be taken off) 



b. OD  -1.00 / -1.50 X 90
    OS  -0.50 / -1.50  X 120


i. Accommodation spasm. When the symptoms occurs the patient is likely to have constricted
    pupils and esotropia. The spasm can be broken with cyloplegia.

ii. The following treatment is useful:

  •     Frequent looking up from work or 
  •     Prescription of reading glasses  

b. The patient is pre-presbyopic. Myopes require less accommodation with glasses than
    contact lenses. In addition, the prismatic effect (base-in prism) offered by the concave 
    glasses assist convergence during reading.


a. There is a 4D difference between the two meridians. Therefore, the required cylinder 
     correction is either:
      - 4.00 X 40 or
      + 4.00 X 140

b. The tight suture is at 1400 and need to be removed.


a. More myopic.

b. Dislocation of the implant. Astigmatism can occur due to tilting of the lens. 


    a = 17 mm
    b = 17 mm
    c = 22.5 mm
    total lens power = + 60 D
    refractive index of the eye = 1.33

b. The nodal point is 17 mm anterior to the retina and all refraction occurs at the plane through 
     the nodal point. The size of the retina image can be calculated as:

     Object height / retinal image height = distance from nodal point / 17 mm
     retinal image height = object height  X 17 mm/ distance from nodal point
     retinal image height = 200 X 17 / 1000
     retinal image height = 3.4 mm


a. The CT scan shows a right subdural haematoma. (The haematoma appears as a lenticular 
     lesion. In this scan, the lesion has a lower density than the brain suggesting that the 
     haematoma is more than 2 weeks old.) 

b. The angiography shows a left internal carotid artery aneurysm in the region of the cavernous 
    sinus. (Aneurysm in this region can compress on the cranial nerves supplying the extra-ocular movement 
     causing motility problem.) 

c. The MRI scan shows a lesion arising from the dorsal mid-brain. (This can lead to raised 
    intracranial pressure and Parinaud's syndrome ie. up-gaze palsy, pupillary light-near dissociation 
    and lid retraction) 


a. Optic disc drusen. (The ultrasound shows an area of high echo density at the optic nerve head, this is 
    typical of disc drusen.) 

b. Vitreous haemrrhage. 

c. There is a raised lesion arising from the choroid. Malignant melanoma should be top of the 
    differential diagnosis.

(This is calculated using the formula 

 U + D = V   U = object vergence in dioptres 
                     D = lens power in dioptres 
                     V = image vergence in dioptres)
i) The image is 0.333 m to the left of the lens and is inverted.
 ( U = 1/50 cm = 1/0.5 m = - 2D, the sign is negative because the vergence of the object is divergence 
   U + D = -2 + 5 = +3 
   V = 3 
    and therefore the image vergence = 1/3 = 0.333m) 

ii) The image is at infinity and erect.
    ( U = 1/0.2 m = -5 D , negative sign because the vergence of the object is diveregence 
      U + V  = -5 + 5 = 0 
      zero vergence means that the image is at the infinity.) 


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