As I proposed previously, size down any complications by sorting through the 5S system to break them down into simple components of visual impact and see your plan of action unfurl before your eyes so this so-called complex condition will logically and elegantly sort itself out into simplistic components that adversely affect vision and present themselves to be corrected in a single or staged fashion.
Consider, for example, the case of a patient with keratoconus with a corneal thickness of 450 µm and a documented cone on topography with low myopia (1 diopter or less) and high astigmatism (up to 5 D). In terms of the 5S system, we first want to make sure that he has potential to see, i.e., sight (visual acuity [VA]). Although the patient cannot tolerate hard contacts, the optometrist has noted that he has a VA of 20/25 with them.
Therefore, whatever we do, the patient should still have a VA of 20/25 or better. The patient has sight, there is no scar, and the site (center or periphery) is unaffected. Moreover, we know the patient has a relatively thin cornea (strength) and high astigmatism (shape). In terms of the 5S system, we must therefore correct for sight and shape.
Now using our Gulani-Nordan criteria for laser PRK candidacy, I would review that approach in this case and also categorize with patient using the following classification.