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7 Corneal Power Distribution display
by Ina Conrad-Hengerer, MD
7.1 Visual acuity impairment during nighttime driving with distance
spectacles – nocturnal myopia?
A driver had been wearing distance spectacles with the following refraction values for 2 years:
OD: sph -0.25 cyl -0.50 A 170° VA 20/20
OS: sph -1.00 cyl -0.25 A 27° VA 20/20
Slit lamp microscopy showed clear, refracting media bilaterally with no corneal scarring or other
abnormalities. Fundoscopy was unremarkable. Mesopic pupil diameter was 3.00 – 3.50 mm. The
above refraction values were found to be confirmed in the Pentacam® refraction map. The possibility
of keratoconus was excluded (Figure 26, Figure 27).
With the Corneal Power Distribution display covering a diameter zone from 1.0 to 8.0 mm the
Pentacam® calculated right-eye total cornea refractive power (TCRP) as having an almost constant
astigmatism at around 1.00 – 1.10 D from the centre up to 5.0 mm peripherally, which then rose
from 1.30 D at 6.0 mm to 1.60 D at 7 mm and further to 2.10 D at 8 mm (Figure 28). For the left eye
the Corneal Power Distribution display showed an astigmatism of 0.30 D from the center up to 2.0
mm which then rose towards the periphery, reaching 0.60 D at 3.0 mm, 0.90 D at 4.0 mm, 1.10 D at
5.0 mm, 1.20 D at 6.0 mm, 1.40 D at 7.0 mm and 1.70 D at 8.0 mm (Figure 29).
It was therefore decided to determine the correction needed for nighttime driving by subjective
testing. A satifacory outcome was achieved by increasing left-eye astigmatic correction by 0.75 D
(giving a refraction of sph -1.00 cyl -1.00 A 30°), and a pair of nighttime driving spectacles were
fitted accordingly.
7 Corneal Power Distribution display
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