Does the cornea change daily?

[russell s]

I'm intrigued by this as over the last few years I've noticed my vision varies quite a lot throughout the day with contacts. Also, when my optician attempted to fit me with Kerasoft IC lenses he get completely different readings for my prescription (from machine that prints out supermarket type receipt) from one week to the next.

I've recently been diagnosed with a dystrophy - map dot fingerprint - in both eyes, compounding my KC. As I understand it the map dot causes the outer layer of the cornea to become unstuck from the layer below in affected areas. I wonder if the weight of contact lens may be adding to the problem - pulling the top layer away?

[GarethB]

.... when my optician attempted to fit me with Kerasoft IC lenses he get completely different readings for my prescription (from machine that prints out supermarket type receipt) from one week to the next.

A probable reason for this is that these machines are designed for a 'normal eye' and ours are anything but and as far as I am aware these are designed for glasses prescriptions which are very different to contact lens prescriprions.

To fit a Kerasoft IC lens and any other for that matter, the topography is only part of the story because that only looks at the centre of the cornea and large RGP lenses and soft lenses go beyond this region so if the fitter fails to take these in to account, they will probably always get a variable result. I have found it is important to observe the edges of the cornea and the workshops I have attended, the optoms are encouraged to rotate the slit lamp so they observe the cornea from the side rather in addition to straight on. That way you get a lens that fits better outside the optical zone to give lens stability and so they have a stable optical region to correct the visual acquity.

Lens designs regardless of the material are far more advanced than they were even 5 years ago and as such require different techniques to fit but unfortunatly many optoms are stuck in their ways or never have the oppertunity to learn how to fit and develop the skills needed for the newer lenses.

If I am critical about my corrected vision, it does vary daya to day and throughout the day but on average I still get 6/5 corrected in both eyes with only a subtle change in sharpness which mainly depends on how long I have been reading a book or looking at a computer screen as your blink rate will decrease so causing the eyes to dry out slightly and so the lenses. For me now a really bad day because I have been wearing lenses for nearly 20 hours per day for a week or so (which can happen when I travel with work) the vision can drop to 6/6.

[Lynn White]

Hi munster,

You bring up a very important point about keratoconus and before I go on to explain in full, let me say right now, I have patients with the same issues, older than you, and CXL has resolved them.

First: The KC cornea is around 60% softer than normal corneas. This means that any pressure on the cornea causes bio-mechanical changes. This can include rubbing your eyes and sleeping with your eye pressed against a pillow or bedding.

Even for "normal" patients, RGP lenses cause moulding effects that cause the cornea to change shape when they are removed and spectacles are worn. This phenomenon is aptly called "spectacle blur" and is something all optometrists used to learn about back in the days when most of the normal population wore rigid lenses. You don't hear much about it now because most people wear soft lenses, which cause hardly any moulding effect.

So, for "normals", the time it takes for the cornea to stop changing when you stop wearing rigid lenses varies from around 2-3 weeks to 4-5 months. The MOST change takes place within the first couple of days and definitely you can see the difference within hours. The single factor that determines how long it takes to settle after discontinuing wear is how many years the person has been wearing rigid lenses. The more years, the longer it takes for a cornea to stabilise. For people with normal corneas, this IS an issue if they want refractive surgery, as they have to let all of this settle down before they can go ahead with it.

In the case of KC, this issue is exaggerated. Therefore it is entirely normal for you to find your vision is variable in the way you describe. Also, Russell, this explains why your optometrist was getting different readings. Changing into a soft lens from a rigid one has to take into account this settling period. Once the "demoulding effect" has sorted itself, then it is much easier to fit into soft lenses.

However...... as someone who has fitted a LOT of soft lenses for keratoconus and followed patients over ten years or more, I have noticed an intriguing fact. Some patients, who are not actively progressing in any way, nevertheless have very unstable corneas. By this I mean the corneal shape can fluctuate significantly over relatively short periods of time.

These are patients who are wearing soft lenses or even spectacles, so it is not down to rigid lenses. Some of these are patients in their late 30s and 40s, where received wisdom says they should be "naturally crosslinked".

One patient of mine, almost 50, was having this issue. He had come to see me because he felt his KeraSoft lenses were not fitting correctly as his vision was still poor. However, the issue was, really, that by the time he got his new lenses, his prescription had changed again. He had been wearing RGP lenses before, so we gave this adequate time to settle - except it never did. I started monitoring him really closely and found his cornea was literally steepening and flattening like a trampoline over a matter of weeks.

Finally, through mutual discussion between ourselves and a corneal surgeon, he went for cross linking.

At the first follow up of the first cross linked eye, my patient arrived at the practice cautiously excited. "Tell me, it it normal for your vision to be the same at the end of a day as it is when you wake up?" he asked.

"Of course it is.... " I replied, then realised the implication of what he was saying. Within a week of the cross linking, his usual swing in vision during the day had slowed and now, round 6 weeks later, it had stopped. This pattern was repeated with the other eye and now he has stable vision only a few months after CXL. His ghosting issues have also significantly reduced, which I find is a real benefit of CXL.

Many centres are being cautious about CXL and saying you don't need it if you are "older". This is not based on studies or research, this is based on the assumption that KC is active for around ten years and then "stabilises". We do not know, really, what happens to KC corneas as patients get older and there has not been any long term study examining what happens to ANY KC corneas if they are simply left alone without any contact lens wear at any age. This is for the purely practical reason you cant leave people without vision.

Some KC corneas do stabilise, others absolutely do not. There are also cases where people appear to develop KC when they are older, confounding the "natural crosslinking" theory.

Finally, to sum up,if your vision is very variable in the way munster describes, then CXL at any age can be beneficial.

Lynn

[munster]

However...... as someone who has fitted a LOT of soft lenses for keratoconus and followed patients over ten years or more, I have noticed an intriguing fact. Some patients, who are not actively progressing in any way, nevertheless have very unstable corneas. By this I mean the corneal shape can fluctuate significantly over relatively short periods of time.

These are patients who are wearing soft lenses or even spectacles, so it is not down to rigid lenses. Some of these are patients in their late 30s and 40s, where received wisdom says they should be "naturally crosslinked".

One patient of mine, almost 50, was having this issue. He had come to see me because he felt his KeraSoft lenses were not fitting correctly as his vision was still poor. However, the issue was, really, that by the time he got his new lenses, his prescription had changed again. He had been wearing RGP lenses before, so we gave this adequate time to settle - except it never did. I started monitoring him really closely and found his cornea was literally steepening and flattening like a trampoline over a matter of weeks.

That pretty much describes me. I use Kerasoft lenses and im 38 years old.

I often put in my lenses, 1st thing in a morning and it takes a good few hours for my vision to sharpen and focus.

Like I've already mention previously, first thing in a morning my vision can be ok with just glasses. Then after a few hours, my vision gets blurry and my same glasses can become useless.

I did remember 1 day I woke up and my vision was perfect with glasses, no blurring, ghosting, smearing. It was like that all day. I could have just used glasses alone, but that was just a very rare 1 day in seven years that happened.

Its just so frustrating when it fluctuates during the day I pray for the day of 20mp bionic eyes

[Lynn White]

Hi munster,

OK, then its your eyes and not lenses. It actually might be complicated by the fact that the lens inside your eye is also trying to cope with your distorted cornea and is constantly refocussing.

I would certainly look into the possibility of CXL.

Lynn

[munster]

Thx, Lynn. Will look into it