Capturing a wide field of view when measuring corneal topography

To optimize corneal topography image capture your aim is to image as much of the corneal surface as you can, which can be difficult as eyelids and eyelashes get in the way. Here it is worth pointing out that the image quality scores that some topographers provide can be misleading in this regard. It is possible to gain what might be considered a high accuracy score with your patient’s eyes screwed up to the point that only a tiny amount of the corneal surface is imaged as shown below. This is because these instrument scores tend to describe instrument focus rather than image quality, and you really need both to help you in your quest for OrthoK fitting stardom.

good vs narrow aperture

The fix is to get your patients to open their eyes as wide as possible while you are taking the measurements. Firstly, assure them that while the instrument might get close to the eye, it isn’t going to touch the eye. Once the instrument is aligned ask your patient to take a couple of blinks and then stare wide. You just watch and capture the image at the time when you consider their eye to be at its widest. How much of an area you need to capture will depend on the lens design you are using, but a range of at least 4.5mm in all directions from the center should be sufficient.

If despite your best efforts, you find it difficult to obtain sufficient cover, there are a couple of tricks that you can try to improve the situation. Some topographers have the option to stitch together multiple maps to make a composite, but if this isn’t an option then you can try to alter where your patient fixates.

Composite maps

Some corneal topography software offers the function of stitching together multiple maps to create a composite map that has a larger surface area (see below). Typically, they require you to capture an image with the patient fixating centrally, and then in up, down, left and right gaze, and selecting these images to create the composite. The principals of good image capture described above still apply, but this can approach can be useful if you are finding it difficult to work with narrow palpebral aperture. If available to you I advise you consider creating composite at least at baseline image capture to give you the most data to work from at the initial lens fit stage.

Composite maps

Changing patient fixation

The problem you are most likely to face is insufficient corneal curvature information in the temporal quadrant due to the line of sight in most people being nasal to the corneal apex. So, when reviewing the maps, you have just captured you will have enough corneal curvature data to reach the nasal landing point of the lens from the apex, but the data will end before you reach the temporal lens landing point. If this is the case, and composite maps aren’t an option, try getting the patient to fixate one or two rings to the temporal side of center, recapture and reassess the maps. If still not enough get them to fixate a further ring along, and if you have gone too far to now leave a shortage of data on the nasal side, get the patient to fixate a ring closer towards the center of the instrument.

About Paul

Dr Paul Gifford is a co-founder of Eyefit, an information resource to assist contact lens practitioners in all modes of practice. Learn more about him here.