It’s hard being thrown in the deep end when working in laser refractive surgery. Moreover I know a lot of Optoms who are overawed when faced with trying to interpret scans.
I guess the main complication we all don’t to happen to our patients is that dreaded word ECTASIA following corneal laser refractive surgery.
Luckily there are a few telltale signs to look out for when you have a patients corneal topography scans in front of you during preoperative laser assessment or keratoconic screening,
Front & back surface topography are given the most weight in the keratoconus screening process.
All eyes that demonstrated any of the following topographic characteristics were deemed suspect keratoconus:
1) A cornea that is 510 microns or less is a signal to take a careful look at the topography. Central corneal thickness that varies by 30 microns or more between the two eyes is a sign often associated with keratoconus. This can easily be seen in the ‘Corneal thickness map.’
2) If the thinnest point on the cornea is more than 1.2mm away from the corneal apex then this is suspicious.
3) On the ‘sagittal/axial curvature map‘ Inferior steepening (>2.0 D inferior steepening), asymmetric bowtie, or skew bowtie are big signs of ectasia.
Inferior steepening on topography can be artificially indicated if the scan is misaligned. For this reason, topography should be repeated until the best possible examination is obtained.
3) Elevation maps are really important and work by placing a “best fit sphere” over the cornea (the radius of curvature that best matches the average curvature of the map).
The elevation map can clearly define areas of elevation or depression.
Red shading on the map indicates peaks or elevations in relation to the best-fit sphere
Conversely, colder blue colours signify areas of depression in relation to the sphere.
If there is eccentric or unusually high posterior elevation over the Best fit sphere (BFS) apex then this another sign of ectasia. (>+23microns is suspicious).
For the anterior elevation curve a difference of >+17microns is suspicious.
Looking out for these signs on the scans plus the obvious distorted ret reflex, decreased BCVA, increased higher order aberrations, asian ethnicity, history of eye rubbing etc should all be taken into consideration, to make the most accurate diagnosis for keratoconus, or to see how much risk there is of ectasia developing post laser refractive surgery.