L5 - Baseline Measurements + Keratometry Flashcards

1
Q

Baseline measurements

A
Pupillary Distance 
HVID and VVID (palpebral aperture)
Pupil diameter (minimum, habitual, maximum) 
Blink rate 
Lid tension 
Lid angle 
Corneoscleral profile (CSP)
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2
Q

Small and large VVID (palpebral aperture)

A

Small <9mm

Large >11mm

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3
Q

Photopic and Mesopic pupil

A

Photopic 3-4mm

Mesopic 4-5mm

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4
Q

Blink rate

A

10-12 per minute

1 blink per 5-6 seconds

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5
Q

Normal lid angle

A

1/5 of cornea covered by upper lid

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6
Q

High lower lid position

A

Lower lid covers significant part of cornea

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7
Q

Small and Large palpebral aperture

A

Small - both lids cover cornea

Large - neither lids cover cornea

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8
Q

Keratometry equation

A

Rn = (2ay’) / (y-y’)

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9
Q

Keratometry - distorted mires/ no stable coincidence mires

A

Corneal scars
Dry eye
Patient didn’t blink (high tear film evaporation)
Unstable tear film

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10
Q

Keratometry- mires incomplete

A

Eyes not open wide enough, cable Infront of mires

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11
Q

Keratometry - very small radius

A

Patient not fixated properly (measured too far outside)

Opposite eye not covered

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12
Q

Keratometry - central radius (flat cornea and normal range)

A

Flat cornea = r>8.2mm

Normal range = 7.4

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13
Q

Cornealscleral profile (CSP) and CL fit

A

Flatter the sclera in comparison to the cornea, flatter the CL needs to be

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14
Q

The difference between central radii…

A

Indicates toricity of the cornea (resulting in corneal astigmatism)

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15
Q

Spherical cornea, small CA, normal CA, high CA, very high CA

A
Spherical cornea = ∆r = 0.0mm
Small CA = ∆r<0.1mm
Normal CA = 0.1<∆r<0.3mm
High CA = ∆r>0.3mm
Very high CA = ∆r>0.4mm
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16
Q

Total Astigmatism (TA)

A

The sum of Corneal and Internal Astigmatism (CA+IA)

17
Q

Internal Astigmatism (IA) occurs due to…

A

Occurs due to posterior lens surface of the cornea and tilting or shape of the crystalline lens or refractive index change of optical components

18
Q

Corneal Astigmatism (CA)

A

When cornea has unequal curvature or diaptic power of the anterior lens surface

19
Q

TA = IA + CA within a range of…

A

∆A (IA - CA) of 20degrees

20
Q

Power meridian

Axis meridian

A

Power meridian is the meridian of greatest power (r(mm)@(degrees))

Axis meridian is the meridian of least optical power (cyl(D)x(degrees))

21
Q

TA < IA contact lens suggestion

22
Q

TA > IA contact lens suggestion

23
Q

How to calculate CA

A

Find the difference between the mm of the meridians in Keratometry
0.05mm=0.25D
Difference between meridians in D = CA

24
Q

With the rule astigmatism

A

Steepest meridian (lowest mm) is on 90degrees

25
Against the rule astigmatism
Steepest meridian (lowest mm) is on 180 degrees
26
If the prescription doesn’t coincide with CA…
Then there is lenticular astigmatism
27
Assessment of corneal shape
Keratometry Corneal topographer Orbscan Pentacam