Refractive Surprise (Not)

‘Refractive surprise’ is a quaint term coined by eye surgeons to denote an unexpected, and generally unwelcome, refractive and visual outcome following an ophthalmic procedure designed to correct vision – usually intraocular lens implantation or laser refractive surgery. The refractive surprise that I was about to experience was perhaps not all that unexpected.

Upon waking on the Monday following my right eye cataract surgery, which was performed three days earlier, I was surprised to find that everything was clear in the distance out of my highly myopic left eye. I suddenly realised I had mistakenly slept in my contact lens! I put this down to post-operative confusion in a new visual world. Everything would have looked reasonably clear the night before, because I could see moderately well in the distance out of my mildly myopic pseudophakic right eye, leading me to forget that I had a contact lens in my left eye.

While gazing into the mirror after removing my contact lens, I noticed that my right pupil was miotic. I thought I had better consult the literature, and sure enough, I discovered that diabetic patients (such as me) can develop transient pupil size anomalies following intra-ocular (IOL) surgery, perhaps due to neuropathic changes to the sympathetic nervous system.¹

Tracking My Refraction

Later that morning I headed to the eye clinic for my post-surgical check-up. The tech examined my eye and confirmed the miotic right pupil. My right eye was recorded as having distance vision 6/9; reading vision N6 (slow), subjective refraction -1.25/-0.50 x 70, corrected distance visual acuity 6/5^-1 and near acuity N5 (slow) with a +2.25D add. Intra ocular pressures were normal at R&L 11mmHg.

Mike then examined my eyes with the slit lamp and Volk lens and declared all was OK. He suggested that refractive change can occur – typically in the myopic direction – for up to two months after surgery, as the IOL and capsule settle. It was only three days after surgery, so I’d just have to be patient and wait for
the final refractive outcome.

Anxious to keep track of my progress, I resorted to having either Suzanne do a refraction on me, or measuring my own refraction in my lab at work, every week or so. To my horror, the astigmatism in my right eye kept increasing over the next few weeks, peaking at -1.75D at one stage! However, the refraction eventually stabilised to -1.25/-1.00 x 75, with distance visual acuity of 6/5.

But of course, the intended outcome of the surgery in my right eye was that this would be my ‘reading eye’, perfectly corrected for computer screen distance. However, I noticed a significant improvement when looking at a computer screen through the above prescription in a trail frame. And my flying angel of death (a small blind spot near my right macula) didn’t help the situation. Does this mean I will forever need to wear glasses for computer work?

Hunting the Source of My Refractive Surprise

My refractive surprise of 1.00D astigmatism in my right eye was not especially welcome. And in a sense, this wasn’t really a surprise, either. You may recall from a previous blog that Suzanne and I suspected that the toric power of the IOL inserted into my right eye was too high, and it seems we were right. So, I had to find out what went wrong.

Bill very graciously gave me full access to my clinical records, so Suzanne and I went into the eye clinic, sat down with a pot of strong coffee, and proceeded to forensically examine the files. It didn’t take long to track down the source of my unwanted astigmatism.

Essentially, the problem was that multiple measurements of my corneal curvature (power) using various techniques gave conflicting results. We listed all corneal measurements obtained over the previous four year period undertaken at both Suzanne’s optometry practice and Bill’s clinic, using instruments including a Humphrey Instruments Autorefractor, Zeiss IOL Master, hand-held keratometer and Medmont corneal topographer. Recorded cylinder powers inexplicably varied between 1.00D to 2.00D, although the minus cylinder axis was constantly between 170° to 180°.

We determined from the records that a cylinder power of 2.00D was used for calculating the desired IOL power. Now, it is quite some time since I have done a refraction on anyone in anger, but I clearly recall the optometric principle of always erring, if in doubt, on the side of choosing a lower cylinder power for incorporating in spectacles. Based on the conflicting corneal topography findings, I would have suggested a ‘compromise’ reduced cylinder power of perhaps 1.50D upon which to base IOL calculations, had I had the courage to made that call prior to IOL surgery. But as they say, it’s always easier in hindsight…

Another issue that Suzanne and I unearthed is that Bill allowed 0.50D for surgically-induced astigmatism. Based on a comparison of corneal topography before and after IOL surgery, it emerged that only 0.20D of astigmatism was induced. So, this can account for 0.30D of my ‘refractive surprise’. Of course, it is impossible to accurately predict the amount of surgically-induced astigmatism, but had we based the IOL calculation on 1.50D (instead of 2.00D) of astigmatism and 0.20D (instead of 0.50D) of surgically-induced astigmatism, I probably would have ended up with the desired outcome of very close to -1.75D sphere.

Yag Laser Capsulotomy

When I visited the eye clinic for my one-month post-surgical follow-up visit, Mike examined my eyes on the slit lamp, and observed some fibrosis and wrinkling of posterior capsule. Bill then entered the room, took a look, confirmed Mike’s observation and suggested we perform a Nd:YAG (Neodymium-doped yttrium aluminium garnet) laser capsulotomy there and then. Some say you should wait six months after IOL surgery before having a YAG procedure, but apparently Bill often does it sooner. He added that he was confident that my quality of vision would improve following this procedure.

I was taken off to sign consent forms, and then lead into a tiny, non-descript room, with what looked like an ordinary slit lamp. But this was no ordinary slit lamp. It was the YAG laser set up. The tech carefully positioned me behind the instrument and adjusted various switches and dials to the desired settings. Bill entered the room, checked the settings, and within about 20 seconds he had lined up the laser, and started firing away. I could hear many tiny clicks, each of which was accompanied by a bright red light flash. Presumably each click corresponded to a laser blast.

I asked “What if I blink while you shoot a laser beam… will I end up with a hole in my eyelid?”. Bill said there is no harm if the laser strikes the eyelid during a blink. Just to be safe, I concentrated on keeping my eye open throughout the procedure.
As well, Bill occasionally paused and told me to take a few blinks and then open my eyes wide before firing some more.

When my flying angel of death came up in discussion, Bill asked hopefully “So that still hasn’t gone away yet?”. As an aside; I am always trying to find out as much as I can about my flying angel of death, and a few months after this visit, came across a new instrument at a conference trade display in the USA that can image the retina at a cellular level – the rtx1 Adaptive Optics Retinal Camera (Imagine Eyes, Orsay, France). We captured an image of the macular area of my right eye (Figure 1). Truly stunning! Individual receptor cells can be clearly observed. But there was nothing apparently untoward that could explain my small paramacular defect.

Over the next couple of days, I noticed a few small random floaters. Presumably this was due to bits of tissue debris left over from the laser blast. After a few days those floaters were gone, most likely because the bits of tissue dissolved away, or were lysed or phagocytosed.

Contrary to Bill’s optimistic prediction, my 1.00D astigmatism was unaltered by the YAG laser capsulotomy, and my uncorrected near vision was still degraded to the same extent. My thoughts now began to focus – with some trepidation due to the previous problematic vitrectomy surgery in my right eye – towards the next stage of my ophthalmic surgery… a left eye vitrectomy.

Professor Nathan Efron is a researcher at the Institute of Health and Biomedical Innovation and School of Optometry and Vision Science, Queensland University of Technology. He is currently president of the Australian College of Optometry and vice-president of the International Society for Contact Lens Research.

Reference

1. Mirza SA, Alexandridou A, Marshall T, Stavrou P. Surgically induced miosis during phacoemulsification in patients with diabetes mellitus. Eye 2003; 17: 194–199.