L5 - Baseline Measurements + Keratometry Flashcards
Baseline measurements
Pupillary Distance HVID and VVID (palpebral aperture) Pupil diameter (minimum, habitual, maximum) Blink rate Lid tension Lid angle Corneoscleral profile (CSP)
Small and large VVID (palpebral aperture)
Small <9mm
Large >11mm
Photopic and Mesopic pupil
Photopic 3-4mm
Mesopic 4-5mm
Blink rate
10-12 per minute
1 blink per 5-6 seconds
Normal lid angle
1/5 of cornea covered by upper lid
High lower lid position
Lower lid covers significant part of cornea
Small and Large palpebral aperture
Small - both lids cover cornea
Large - neither lids cover cornea
Keratometry equation
Rn = (2ay’) / (y-y’)
Keratometry - distorted mires/ no stable coincidence mires
Corneal scars
Dry eye
Patient didn’t blink (high tear film evaporation)
Unstable tear film
Keratometry- mires incomplete
Eyes not open wide enough, cable Infront of mires
Keratometry - very small radius
Patient not fixated properly (measured too far outside)
Opposite eye not covered
Keratometry - central radius (flat cornea and normal range)
Flat cornea = r>8.2mm
Normal range = 7.4
Cornealscleral profile (CSP) and CL fit
Flatter the sclera in comparison to the cornea, flatter the CL needs to be
The difference between central radii…
Indicates toricity of the cornea (resulting in corneal astigmatism)
Spherical cornea, small CA, normal CA, high CA, very high CA
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
Total Astigmatism (TA)
The sum of Corneal and Internal Astigmatism (CA+IA)
Internal Astigmatism (IA) occurs due to…
Occurs due to posterior lens surface of the cornea and tilting or shape of the crystalline lens or refractive index change of optical components
Corneal Astigmatism (CA)
When cornea has unequal curvature or diaptic power of the anterior lens surface
TA = IA + CA within a range of…
∆A (IA - CA) of 20degrees
Power meridian
Axis meridian
Power meridian is the meridian of greatest power (r(mm)@(degrees))
Axis meridian is the meridian of least optical power (cyl(D)x(degrees))
TA < IA contact lens suggestion
SCL
TA > IA contact lens suggestion
RGP
How to calculate CA
Find the difference between the mm of the meridians in Keratometry
0.05mm=0.25D
Difference between meridians in D = CA
With the rule astigmatism
Steepest meridian (lowest mm) is on 90degrees
Against the rule astigmatism
Steepest meridian (lowest mm) is on 180 degrees
If the prescription doesn’t coincide with CA…
Then there is lenticular astigmatism
Assessment of corneal shape
Keratometry
Corneal topographer
Orbscan
Pentacam
