CLs OSCE

Question 1

What are the stations of the OSCE?

Answer 1

Keratometry, pupil measurement, HVID measurement, CL prescription calculation 20% of final mark

Question 2

What is HVID?

Answer 2

Horizontal visible iris diameter. Limbus damage should be avoided by adding 2mm to the result.

Question 3

How do you measure pupil size?

Answer 3

Measure in normal lighting and compare to pupil gage on pen torch, and in dim lightin using a burton lamp from a distance (not too close) using the UV setting.

Question 4

What should the lens diameter be in comparison to the HVID measurement?

Answer 4

HVID+ 2mm to avoid limbus area.

Question 5

How do you calculate contact lens prescription?

Answer 5

Fc=Fs/(1-dFs) where Fs is spectacle power, d= distance in meters and Fc= contact lens power.

Question 6

How do you calculate Fc when there is an astigmatism present?

Answer 6

Divide cyl by two then add to the spherical (only when under 1.00Dc)

Question 7

What is BVD and what is it used to calculate?

Answer 7

Back vertex distance. Calculate the power of the lens (is d in Fc=Fs/(1-dFs)

Question 8

Calculate the contact lens prescription when the BVD is 8mm and the current Rx is +10.50D

Answer 8

8mm= 0.008m +10.5/(1-10.5x0.008)= +11.46 so Fc= +11.50

Question 9

When would you prescribe a spherical contact lens in a patient with an astigmatism?

Answer 9

If they have less than 1.00Dc. If more, a toric Cl will need to be prescribed.

Question 10

How do you calculate the Rx power if the Px Fs is +5.00 with an astigmatism of 0.50cyl?

Answer 10

Divide the cyl by 2 to +0.25cyl then add to the spherical Cl Rx so +5.25sph. This is the BVS. (then calculate Fc using the equation).

Question 11

Whats the rule for Rx below 4D?

Answer 11

The contact lens power and spectacle power (prescription) are the same.

Question 12

When recording ketatometer results, how do you record the axis?

Answer 12

Using @ instead of x

Question 13

Whats the average range of K values?

Answer 13

7.25mm and 8.50mm

Question 14

What is with the rule astigmatism?

Answer 14

Larger (flattest) meridian is horizontal. Axis will be around 180 in minus cyl form

Question 15

What is against the rule astigmatism?

Answer 15

The largest (flattest) k meridian is vertical. With minus cyl around 90 degrees.

Question 16

What is oblique astigmatism?

Answer 16

Where principle meridians are between 30 and 60 degrees and 120 and 150 degrees.

Question 17

What is the equation to calculate corneal astigmatism?

Answer 17

F= (n’-1)x1000/r where n’ is 1.3375

Question 18

What should you do before aligning the patient for keratometry?

Answer 18

Focus the eyepiece.

Question 19

What meridian for the K reading do you write down first? The horizontal or vertical?

Answer 19

The horizontal meridian first

Question 20

Whats the difference between a one position and a two position keratometer?

Answer 20

One position takes both H and V k readings at once whereas two position has to be rotated to the other meridian.

Question 21

How do you calculate contact lens selection from k values?

Answer 21

Add around 0.8-1 to the flattest K (largest number).

Question 22

What do you use the align the keratometer to the patient’s eye?

Answer 22

A pentorch whilst their eyelid is closed

Question 23

What does the patient need to be looking at when measuring pupil diameter

Answer 23

At a fixed point in the distance.

Question 24

How do you convert mm to m?

Answer 24

Divide by 1000 e.g 1mm is 0.001m

Question 25

What do you need to ensure to do whilst conducting keratometry and measurements?

Answer 25

Explain to the patient what you will be doing (i.e explain the tests and what they're for).