If you’ve read my other posts, you’ve probably guessed that I hate loose ends.  I really dislike it when a patient I’m monitoring for something disappears off the radar (I’m looking at you, corneal-paper-cut-person) but here I am, leaving a couple of my patients to go to another store.  The one I’m most sorry to leave behind is a lovely guy I’ve been fitting with a prosthetic contact lens.

I never heard the full story behind C’s phthisical right eye just that he was involved in an accident that not only left him blind in one eye but coping with a range of other health issues.  The damage to his eye was very noticeable given his skin tone and the dark brown colour of his left eye – it was blueish white and slightly shrunken.

One of the other optoms had ordered a prosthetic lens and fitted it years ago but C kept losing his lens.  It would fall or be blinked out without him noticing.  When the lens was in, it would move around his eye, getting caught on the lids or being pushed off to the side.  The lens, when off centre, drew more attention than the damaged eye so C had gotten into the habit of continually checking the centration of the lens – in a little mirror he carried (and I’m sure he got no end of ribbing from his friends about this), in shop windows and in the cutlery at restaurants.  He’d become a bit paranoid about the lens but, at the same time, was a little worried that if he mentioned these things, we would stop replacing them for him.

Given the movement of the lens, I assumed the fit was too flat.  Getting accurate K readings from the right eye was impossible as the ocular surface was rough and slightly misshapen.  I had a folder full of previous orders (ranging from a BC of 8.0 to 8.2) which I knew were too flat.  I popped a thin, low minus daily disposable in the right eye and tried to take Ks over it (thinking that the lens might drape over the surface and give the keratometer a better chance at getting a reading).  The mires were still too distorted, unfortunately.  I then thought about trying to work out Ks using a RGP trial set and FLUO patterns but, alas, this wasn’t immediately available to me (my previous store didn’t do any RGP fitting and very rarely saw RGP aftercares).  Finally, I decided to use bracketing.  The previous BC was 8.0 so I would order 7.4 then, if this was too tight the options were 7.6 and 7.8, if it was too flat then I would go to 6.8 and so on.

At this point, one of my colleagues pointed out that, if the eye was phthisical, surely a tight lens wouldn’t do much harm.  As long as C could remove it easily, it would be a benefit if it didn’t move much on the eye.

I called Cantor and Nissel and ordered a lens with the 7.4 base curve.  The colour and pupil were okay before, it was just the fit that I was adjusting. Unfortunately, the lens won’t arrive until the middle of January.  Fingers crossed that it works out.

Anyway, just in case you ever need to fit a prosthetic contact lens and don’t know where to start, here’s a rough guide:

1) Take Ks.  If this isn’t possible and you have your RGP fitting set to hand, find the best fit using your trial lenses and fluorescein. You will choose your base curve using the fitting guide from Cantor and Nissel after you find your total diameter.

2) Measure HVID (horizontal visible iris diameter) and order a TD 2-3mm larger.  You’ll also need to specify HVID to the lens manufacturers as they will be matching this.

3) Measure pupil size.  Now, this is difficult if you are only ordering for one eye as you want to have something that is close to the pupil diameter of the other eye in most lighting conditions.  If you order a 5mm pupil and the fellow eye goes between 3mm in bright light to 7mm in dim light then you are probably fine.  2mm of anisocoria is noticeable but it would be much worse if you ordered 7mm and they walked outside into sunlight.  You can ask your patient what lighting conditions they work/spend their time in so you can tailor it to “real life” rather than guessing from the lights on/lights off pupil diameters you measure with your Burton lamp.  Someone who works in a very bright environment (e.g. a shop) may be better with a smaller pupil.

4) Look at the colour of the iris.  Do this in the best light possible – if your patient isn’t too embarassed about being dragged out of your practice, I would have a look in natural light.  You can then match the colour and shade to the colour options Cantor and Nissel have.  Blue 1 would be the lightest blue and Brown 4 is the darkest brown.

5) Closed pupil or open pupil.  If the eye is blind then you want a closed (black) pupil but if the cosmetic lens is covering up damage to the iris, for example, and the eye is working, you want an open (clear) pupil.  In the case of a clear pupil, you can order the Rx required (remembering to adjust the spec Rx if it’s a high power).

6) Call the manufacturer and give them the specifications of the lens you want to order.  If you are unsure of which product to order or what’s available, here’s a link to the website:

http://www.cantor-nissel.co.uk/

I ordered a Nissel Naturals for C, if you were wondering.  The final spec was 7.4/14.00/plano with a 12mm iris, 6mm closed pupil in Brown 4.

These lenses are hydrogels and are designed to last.  A peroxide based cleaning system is recommended for them.

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