The iStent is one of the new generation medical devices which has recently become available in Australia for use during cataract surgery in patients with glaucoma. It forms part of the new trend towards minimally invasive glaucoma surgery (MIGS).
The stent facilitates intraocular pressure lowering thereby potentially either minimising glaucoma medication, or giving better intraocular pressure control in patients who are poorly controlled.
Whilst the device could theoretically be implanted in any patient with glaucoma, is is currently only TGA approved for use in the course of a cataract operation.
Intraocular pressure lowering in cataract surgery.
It is a well known that cataract surgery lowers intraocular pressure by as much as 30%. As such it is a good way to manage a patient who has concomitant glaucoma and cataract. As an alternative to a trabeculectomy, a cataract operation is a potentially good way to gain further lowering of pressure in people who perhaps are on maximal medical therapy and who have perphaps have also had laser treatment (SLT) but still do not have ideal pressure control.
The iStent, when implanted at the time of cataract surgery gives further lowering of intraocular pressure of around 16%. Whilst this may not seem much, it has to be remembered that every mmHg reduction in intraocular pressure in glaucoma patients results in a 19% reduction in risk of progression(1).
The implantation of the iStent is usually performed at the end of the cataract procedure. This is because the anterior chamber will be maximally deep as the lens has been removed. A gonio lens is required to visualise the trabecular meshwork. It increases surgical time only minimally. The stent comes pre-loaded on an introducer which grips the stent. The stent is then released once in place by depressing a button on the introducer.
There is no difference in the cataract procedure beyond this and no difference in the post operative management.
There is definitely a learning curve associated with its use and is probably best reserved for ophthalmic surgeons who are familiar with the anatomy of the trabecular meshwork.
Following surgery, the intraocular pressure is re-evaluated and it is sometimes possible to reduce the amount of glaucoma medication required.
The iStent works by bypassing the trabecular meshwork which is the site of outflow resistance in patients with glaucoma
The following illustration shows the path of aqueous within the eye and out through the iStent.
Flow of aqueous through the iStent
The decision to implant the iStent is usually made in the pre-operative period in patients who have glaucoma. The ability to visualise the trabecular meshwork during surgery is crucial and inability to do so would be the main reason not to implant it.
(1) Chauhan A. Acta Ophthalmol 2008;126:1030-1036
Lasik vs PRK is a common question that I am often asked.
So what is the difference between Lasik and PRK and which should you have?
This is an important question which is actually explained in details in other sections of this site but one that is worth summarising.
PRK stands for Photo Refractive Keratectomy and was one of the original corneal procedures for vision correction, first performed in Berlin in 1987.
In PRK, the epithelium of the cornea is removed by either alcohol or direct removal with a spatula. This of course is done with local anaesthetic. The epithelium is a layer of cells which is always reforming an is usually what is injured when someone inadvertently injures their eye, for example with a baby’s fingernail.
This exposes the stroma of the cornea where the laser will perform the correction. The stroma is the main bulk of the cornea. The epithelium which is removed grows back very quickly much like skin grows back after an abrasion.
Once the epithelium is removed, the laser is applied to the cornea to change its shape to correct vision. This is a very rapid procedure taking only about 30 seconds.
Following the procedure, a contact lens is placed in each eye to reduce the discomfort and to allow the epithelium to grow back. The contact lens is usually removed after about 36 hours.
Following surgery it is common to experience light sensitivity and discomfort for two to three days. Vision is better within days but is not at its best for several weeks to several months.
Due to the discomfort and the delay in achieving perfect vision, PRK became less popular and ultimately, largely replaced by Lasik.
Lasik stands for Laser Assisted In situ Keratomileusis and this is what most people think of when they think about laser vision correction.
In Lasik, a corneal flap is raised, much like lifting a lid and the laser correction is applied to the underlying stroma. Unlike PRK, the correction is applied at a deeper lever of the stroma and the only real difference is that in Lasik there is a flap and in PRK there isn’t.
The flap heals and eventually becomes difficult to even see.
The main advantages of Lasik over PRK is that there is absolutely no pain or discomfort and that excellent vision is almost immediate.
So which one?
So given that PRK is more uncomfortable and takes longer for vision to be excellent, why would anyone even consider it?
The main reasons why one would recommended PRK over Lasik are if you have thin corneas, the required correction is small or if the corneal topography scan is not entirely normal.
The advantage of PRK over Lasik is also one of structural integrity. A Lasik flap is typically 110-120 microns thick and contributes little to the final structural integrity of the cornea. Thus avoiding the flap leaves an additional 110-120 microns of untouched cornea.
Lasik flaps can (although) rarely develop problems such as epithelial ingrowth, folds (striae) or become dislodged. Therefore avoiding a flap in many ways is advantageous.
One cannot however get away from the fact that PRK has a slower recovery and is uncomfortable.
Long term there is no great difference in outcomes with both PRK and Lasik patients having similar visual outcomes.
At the end of the day, the decision as to which one to have will be decided by your eye surgeon based on which is most suitable.
If you have an adequately thick cornea with a normal scan then you will probably have Lasik.
If you have a thin cornea, a small correction or an unusual corneal scan then you will have PRK
I have just recently returned from the Australian Cataract and Refractive Surgery annual conference which this year was in Port Douglas. The focus of this meeting is as the name implies, on new developments in the area of cataract surgery and vision correction.
This year there were many new items which are worthy of discussing.
Implantable Collamer Lens (ICL)
I have written about this before and so it was timely that Visian should produce a new lens. This concept is not new but the current lens is new and improved.
One of the main changes is the incorporation of a small central hole in the lens. The reason for this is to minimise the risk of pupil block and subsequent angle closure glaucoma. Under normal circumstances, aqueous is formed by the ciliary body which is situated behind the iris. It then flows via the pupil to the trabecular meshwork. This normal physiology is interfered with by the presence of the ICL which sits just behind the pupil. Aqueous is still able to get past the lens as the seal between iris and lens is not watertight. Under certain circumstances it can become watertight and therefore lead to angle closure. In this scenario, fluid behind the lens cannot get past, increasing the pressure and causing the iris to be pushed forward. This is called iris bombe. The iris then in turn is pushed hard against the trabecular meshwork obstructing it.
The central hole in the new lens eliminates this risk as aqueous can flow through it making angle closure unlikely.
Finevision trifocal lens
This is another area which is developing and which in my opinion is now good enough to be worth considering. The holy grail in refractive and in cataract surgery is the problem of not being able to deliver vision at all distances. That is to say, we can deliver good distance vision but not simultaneously near vision. Or the exact opposite, in that we can deliver good near but at a cost of distance. This is because modern intraocular lenses cannot focus. Our own natural lens also loses its ability to focus at around age 45. This is why reading glasses become necessary. This is due to a stiffening of the lens preventing it from changing shape. Attempts to circumvent this problem using intraocular lenses have been around for some time but all suffer from numerous problems. Halos, glare and loss of contrast sensitivity are the main problems. These mulifocal intraocular lenses work by having a diffractive pattern on the lenses which result in two images, one for near and one for far. The problems stems from the diffractive pattern which lead to the glare and haloes. Further, as the slight is split for near and far, it means that only a small percentage of the available light is use at any time. Many patients are not bothered by this but those who are highly critical are. Also truck drivers, Taxi drivers and pilots in particularly are not good candidates due to the haloes and glare special at night time.
The FineVision lens which has just been released by Bausch and Lomb will hopefully address some of these problems. Instead of having a distance portion and a near portion, rather it has a near, intermediate and far portion. This will therefore translate into the ability to see to drive, use the computer and read. It still has the problems of other multifocal lenses but to a much lesser extent. This lens is probably still not for those whose lively hood depends on driving at night or flying but most everyone else. It is also not for those who are highly critical of their vision. Patients who have macular pathology such as macular degeneration or prior macula hole surgery are also best to avoid this lens.
My approach to this problem has been for a long time to implement monovision but this too has limitations.
This is another promising technology which addresses the problem of presbyopia. Little is known about this lens to date except that it is probably based on some form of multifocal platform. It is due to be announced at the forthcoming European Cataract and Refractive Conference in London in September.
A New Zealand based surgeon has implanted 20 of these lenses as part of a trial. He was not at liberty to discuss the lens in any detail but did present his own findings. Essentially he found that patients had excellent distance and intermediate vision and good near.
It is unknown when this lens will become available or wether it lives up to the hype. My guess is that it may be available sometime in 2015 in Australia, probably in the second half
SMILE is Small Incision Lenticule Extraction. It is an alternative to LASIK or PRK. In the SMILE procedure only a small opening is created in the cornea and a lenticule is formed by the laser in the stoma of the cornea. The lenticule is then removed. No flap is created. This has been referred to as “Bladeless Lasik”.
Lasik by contrast creates a large corneal flap and laser is applied to the exposed corneal surface and the flap is deposited.
The main advantage of SMILE is because only a small incision (2-4mm) is created, there is less severing of corneal nerves and therefore less corneal anaesthesia. This translates into less post procedure dry eye.
Our laser centre is at present evaluating this technology and we may be implementing it in the near future.
Some time ago I wrote about the use of the SMI system for cataract surgery. The SMI system is a computer assisted method of guiding cataract surgery. Essentially it allowed visualisation of intraocular lens alignment, wound location and capsulorhexis size.
Whilst this system was certainly better than “eyeballing” it, it tended to be unpredictable and erratic.
Alcon subsequently bought the SMI system and used it as the basis for their new Verion system. Essentially it is SMI on steroids.
Today I had the opportunity to give it a trial run and I was quite impressed.
So what does Verion do? Well it is technology that images the eye on the day of surgery and is able to capture information about the cornea such as keratometry and vessel landmarks.
This information is then displayed as a Heads Up Display (HUD) in the surgeons left eye. Thus when looking through the operating microscope, the display is visible and appears as superimposed on the patients eye.
By use of a foot pedal one can sequentially change the view to allow various different parameters to be displayed. In my trial, I initially was able to see a cursor that showed me where to place my main wound. I was then able to call up the capsulorhexis which was set to 4.8 mm but can be set to whatever is desired. Finally an alignment mark is able to be called up to show where the axis of the toric lens should be placed. In my Femto cataract cases, the main wound and rhexis had already been performed so it was used to align the toric lens.
My usual procedure is to mark the eye with a texta colour with the patient sitting up prior to surgery at the twelve and 3 o’clock positions. I then mark the toric axis with the patient on the table. Patients usually don’t like being marked as it is irritating so this saves on that.
Unlike the SMI, I found the Verion to be quicker in acquisition and more stable in displaying the information. I found that the SMI would often incorrectly identify the limbus, instead locking on to the the pupil. This problem seems to have been fixed.
No it is not intraoperative aberrometry but it is certainly a step forward in better alignment and easier surgery
Further to my recent comments about the Visian Collamer lens, there has been a study looking at this lens.
This was recently also mentioned in Mivision.
Basically in this study, 41 eyes were followed for 8 years, looking at quality of vision, endothelial cell loss, stability of refraction, complications and cataract formation.
Cataract was found in just less than 5% and is known to be a potential risk. The patients that developed cataract however, had uneventful cataract surgery with good vision afterwards. No one lost vision. Endothelial cell loss was only 6.2% at 8 years.
The article highlighted how the ICL is a good option for patients instead of LASIK with good outcomes and few complications.
Last year I wrote a post lamenting the loss of the Alcon Cachet lens and giving my opinion on the Implantable Contact Lens (ICL).
I stated that this was not a good option as it led to potential complications of which cataract was one of them, and said that I would not implant these. Like many things in life I have come to change my stance on this. Why? Well, because I have come across many satisfied patients that had the ICL implanted and that would have been otherwise turned away and told that there were no options available to them. I have also attended a number of conferences in which many cases were presented of patients with ICL’s. The reality is, that if done well, ICL’s are a very good option for some patients. I personally know a surgeon who has implanted hundreds without a single complication. Lets therefore explore what an ICL is and who it is good for and who it is not good for.
What is an ICL
The Visian ICL is a lens which is implanted behind the iris and in front of the eye’s natural lens. It is not unlike a regular contact lens except that it is IN the eye rather than ON the eye. The term ICL actually stands for Implantable Collamer Lens so named because of the material of which it is made of.
Fig 1. An Implantable contact lens behind the iris
The lens is biocompatible and does not react with the eye
Implanting the ICL is a very straightforward procedure. It is usually done under a local anaesthetic but in some cases can be done under a full general anaesthetic. The pupil is initially dilated with dilating drops. The lens is introduced through a 2.4 mm clear corneal incision and manipulated under the iris through two side ports. This of course is done under viscoelastic which is removed at the end of the case. The pupil is then constricted at the end of the case with Miochol. That is it!
Who is this for?
The ideal candidate is a high myope but can also be a hyperope providing that they have an adequate anterior chamber depth.
Thus, any patient who has a refraction outside the range for Lasik ( >10 diopters for myopes or > 4 diopters for hyperopes). These patients, if under the age of 45 would normally be given no options as they are outside the laser range and too young for refractive lens exchange.
The key requirement, as stated above is that there is an anterior chamber depth of at least 3mm. This may not be the case in hyperopes in particular. If this requirement is not met then even the ICL is not an option
Who is this NOT for?
Young patients (>21 and < 45) who has a refraction within the scope of LASIK and who adequate corneal thickness are better of with LASIK or PRK.
Patients over the age of 45 (presbyopic) are better of with a refractive lens exchange (but may be OK for LASIK).
Anyone with an anterior chamber depth of less than 3mm is not suitable.
Keratocconus is contentious and I think may be best with NO treatment..
Eye rubbers are out! Why? Due to the risk of the ICL contacting the natural lens and causing trauma to it.
What are the risks of the ICL?
Firstly it is an intraocular procedure and therefore like all such procedures it can lead to problems. This however is a consideration for every cataract patient that I see and in 2014 the risk is acceptably small.
Secondly as the naysayers will point out, it can lead to cataract formation if the ICL comes in contact with the natural lens. Whilst this is true, the risk is low. I would argue that a 15 diopter myope who develops a cataract as a consequence of having an ICL implanted and has cataract operation, will still be significantly happier after their cataract operation than they were with their original glasses or contact lenses. Yes they will lose their accommodation but with monovision or multifocal intraocular lenses, this is no comparison to the problems with glasses or contacts.