Under pressure

You may have heard about the impact that scleral lenses potentially may have on the intraocular pressure (IOP) of the eye. Is this true? And how harmful is it? Let’s see what the Contact Lens Evidence-based Academic Reports (CLEAR) papers by the BCLA (the British Contact Lens Association) have to say about this. These papers were published in the April issue of Contact Lens and Anterior Eye (open access) and consist of eleven different reports ranging from ocular anatomy and physiology to all types of lens modalities, including a special report on scleral lens fitting. The biggest hurdle for the team in writing the paper, and what generated the most discussion, was in fact IOP with regard to finding a well-balanced and evidence-based answer to the question raised above. By Eef van der Worp

GLAUCOMA

First things first: there is a beautiful but delicate balance between the amount of fluid that is produced by the ciliary body (which is located at the base of the crystalline lens) and the amount of drainage at the trabecular meshwork (located at the base of the iris anteriorly, in the corner that the iris forms with the cornea). The exact location of that trabecular meshwork and outflow is important, but we will get to that later. If the balance between the production and the drainage is ‘off’, then the pressure in the anterior chamber of the eye rises. This could lead to irreversible damage to the optic nerve head at the back of the eye – this condition is called glaucoma. The visual field of glaucoma patients can gradually decrease, which is usually symptomless until it is too late and only tunnel vision remains.

THE PRESSURE IS ON

So, the pressure is on eye care practitioners worldwide to monitor this on a routine basis, really. The IOP as such can be measured quite easily using an air-puff (non-contact IOPmeasurement) or by using a small probe that is placed on the cornea (after applying an anaesthetic) called Goldmann applanation tonometry. Basically, the amount of pressure needed to flatten the cornea to a plane state is converted to a number that is used in practice. The units used are millimeters of mercury (mm/Hg) pressure. Values under 21 mm/Hg are generally considered a safe level, although it can be a bit more complex than that in reality, but that is beside the point here.

ANCIENT SCLERALS

Ever since the very first introduction of (glass) scleral lenses, there has been talk about the potential effect that sclerals could have on IOP. One mechanism behind that could be that the scleral lenses exert force on the area where the trabecular meshwork is located and could prevent drainage. This indeed is the area where the scleral lens typically lands and rests. Another theory is that the ‘seal’ caused by sclerals may influence flow, and outflow particularly, of fluids in the anterior chamber. So why not just measure IOP during scleral lens wear and get on

with it? The problem is that it is hard to measure IOP during scleral lens wear because the scleral lens is literally in the way. The air forces or probe simply cannot be applied to the cornea during scleral lens wear. So, the first set of analyses of IOP in scleral lens wear always consisted of removing the lens and measuring IOP directly after lens removal. This has the obvious advantage that the lens is not in place at the time of measurement. Using this method, some found small but statistically significant increases in IOP.

THROUGH THE EYELID

To measure IOP with the lens in-vivo, it makes sense to measure the IOP not on the cornea but on the sclera just outside of the scleral lens borders. There are devices that indeed can measure IOP on the sclera or through the eyelid (no anaesthetic needed). The biggest pushback that studies with these instruments get is that the measurements are not the same as ‘corneal IOP’ and that the instruments are found to not be particularly reliable. Even so, using trans-palpebral IOP (measurement through the eyelid), two studies reported an average increase of 5.0 mm/ Hg after 1 or 4.5h of lens wear, while one study reported a 1 mm/Hg increase in IOP after 6h of lens wear.

THE HOLE STORY

A good new addition to the IOP-scleral story is a study in which investigators drilled a large hole in the center of the scleral lens and measured IOP through it. This does eliminate some of the other disadvantages. The lens is still in place, and we measure the corneal IOP values. They did report an average increase of 3.8 mm/Hg. An elevation in the measured IOP can occur quickly, as an increase was seen 1−2 min after lens application. A limitation of this method, though, is that the ‘seal’ that a scleral lens typically creates is broken. This model could be very viable though for fenestrated scleral lenses.

OPTIC NERVE HEAD OBSERVATION

The best thing to do, really, is to look at the optic nerve head directly and to measure the minimum rim width (MRW). Here, you can directly see and measure the effect of any potential IOP increase, if there is any. So far, two studies have looked at this. Maria Walker and colleagues in Houston (USA) have done some important groundwork in the field, and found a minimal effect on optic nerve head morphology in healthy adult eyes, using the non-scleral-lens-wearing eye as a control. Conversely, Samaha et al. from the University of Montreal (CA) observed a small but significant reduction in Bruch’s membrane opening minimum rim width after complete lens settling.

TAKE HOME MESSAGE

The take home message for clinical practice is that scleral lenses could alter IOP to some degree. But, the jury is still out, and definite conclusions cannot be drawn at this point based on the current evidence, the CLEAR paper states. A couple of considerations for clinical practice: the values discussed are all averages, and relatively short-term effects were investigated. Potentially, the eye adapts to the new situation in the long run. Plus, there are huge inter-subject differences, and we cannot predict who will spike and who will remain stable, as Joseph Shovlin put it in a recent column in Review of Optometry. To add to the complexity, IOP measurement in keratoconus patients (a major indication for sclerals of course) is complex in itself, because the corneal parameters and rigidity are completely different from the normal cornea. Whether lens design, including non-symmetrical lens design options, is beneficial and could alleviate the problem is still to be seen. Avoiding scleral lenses in patients with progressive glaucoma seems to make sense but is not always easy. Sclerals are typically not worn ‘for fun’: there is usually a major indication to wear these lenses, and the benefits should be outweighed against the risks – while monitoring visual fields and optic nerve head changes. But in some other cases, alternatives, including corneal GPs, hybrids or soft custom lenses, can be considered potentially. n

Sources - Barnett M, Courey C, Fadel D, Lee K, Michaud L, Montani G, van der Worp

E, Vincent SJ, Walker M, Bilkhu P, Morgan PB. CLEAR - Scleral lenses.

Cont Lens Anterior Eye. 2021 Apr;44(2):270-288 - Cheung SY, Collins MJ, Vincent SJ. The impact of short-term fenestrated scleral lens wear on intraocular pressure. Contact Lens Anterior Eye 2020;43(6):585–8. - Fogt JS, Nau CB, Schornack M, Shorter E, Nau A, Harthan JS. Comparison of pneumatonometry and transpalpebral tonometry measurements of intraocular pressure during scleral Lens Wear. Optom Vis Sci 2020;97(9):711. - Huggert A. Increase of the intraocular pressure when using contact glasses. Acta Ophthalmol (Copenh) 1951;29(4). - Samaha D, Michaud L. Bruch membrane opening minimum rim width changes during scleral Lens Wear. Eye Contact Lens 2021 May 1;47(5):295-300 - Shovlin. Pressure Cooker. Review of Optometry. 2021 Aug 15th: p74. - Walker MK, Pardon LP, Redfern R, Patel N. IOP and optic nerve head morphology during scleral Lens Wear. Optom Vis Sci 2020;97(9):661–8.

Eef van der Worp, BOptom, PhD, FAAO, FIACLE, FBCLA, FSLS is an educator and researcher. He received his optometry degree from the Hogeschool van Utrecht in the Netherlands (NL) and has served as a head of the contact lens department at the school for over eight years. He received his PhD from the University of Maastricht (NL) in 2008. He is a fellow of the AAO, IACLE, BCLA and the SLS. He is currently adjunct Professor at the University of Montreal University College of Optometry (CA) and adjunct assistant Professor at Pacific University College of Optometry (Oregon, USA). He lectures extensively worldwide and is a guest lecturer at a number of Universities in the US and Europe.