Most corneal topographers in clinical practice use Placido disk image capture technology and typically come in two different versions: either bowl type like the Oculus Keratograph; or cone type like the Medmont E300. There are other approaches to measuring topography, like slit scanning (B&L Orbscan) and Scheimpflug (Oculus Pentacam) systems, each with their own differences in methodology but essentially, they all do the same, that is map corneal shape. When you dig down into it they measure reflections from the tear film overlaying the cornea, so topographers really measure the shape of the anterior surface of the tear film.
If your patients tear film is poor, the topography image you are going to capture is also going to be poor. Remember bad data in (poor image) leads to bad data out (measurements that you intend to use for OrthoK lens fitting). This means that the most important thing you need to do before measuring corneal topography is to assess the tear film. If tear break up time is low or there are artifacts like mucin globules these will affect the instruments pattern that is reflected from the cornea.
It’s usually quite easy to see if the tear film is a problem just by assessing the topography image while you are taking the measurement. Dry spots will cause the mire images to lose sharpness and artifacts will cause bumps in the mire images. Sometimes you can solve these problems by asking the patient to take a couple of blinks before capturing a measurement but if this doesn’t work you need to take more invasive action, which actually isn’t that invasive, you just need to instil a small amount of non-viscous wetting drops and wait for around 30 seconds for the tear film to stabilise before taking measurements.
My personal approach is to always check the cornea with the slit-lamp before measuring corneal topography, which gives an opportunity to make a quick assessment of the tear film. The advantage is that a lubricating drop can then be instilled if needed before putting the patient on the topographer. If you look closely you should be able to detect if the tear film is breaking down too quickly without using fluorescein. You’re not after a diagnostic assessment here just a decision on whether to instill a lubricating drop, however, there’s no problem using fluorescein if you prefer, just try to use a small amount to not drown the eye.