International Newsletter and Forum on Rigid Gas Permeable Contact Lenses, Corneal Shape, Health and Vision
 November 2014
In This Issue
Column
Respiratory Tract Complication
Care Solution
Anterior Segment Changes
I(n)-site-the-practice
Agenda
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Column
Going Blank

OK - let's take a(nother) look at the oxygen transmissibility of scleral lenses and the blanks used to manufacture them. Scleral lens fluid-filled clearance zones have been reported to, at least in theory, act as an additional filter. Based on these models, decreasing the clearance can help improve the Dk/t of the lens system in addition to increasing the Dk of the blank used and decreasing the thickness of the lens. The University of Montreal model found that the combination of a clearance of roughly 200 microns, a lens thickness of 250 microns and the highest-Dk blanks available (which is in the 150-170 units range) would just give adequate oxygen delivery to the cornea to meet the Holden-Mertz criteria to prevent any additional hypoxic stress on the cornea in open eye conditions. New studies from the University of Minho in Portugal by Compan et al published in the prestigious IOVS journal seem to confirm these findings, as they looked at both a modified theoretical model as well as clinical measurements of corneal edema. Based on a combination of these observations, they recommend the scleral lens blank to have a Dk of at least 125, with a thickness of no more than 200 microns, and a post-lens clearance of 150 microns to meet an oxygen tension of 55 mmHg. None of the lens combinations tested in their study with a clearance of 350 microns met the 55 mmHg criteria - which is considered to be the minimum critical barrier to avoid clinically significant hypoxia, according to the authors. It's an ongoing discussion - on the one hand is a theoretical model predicting that many scleral lenses scarcely provide the amount of oxygen needed, and on the other hand hypoxia with scleral lens wear is seldom seen clinically in contact lens practice. While all of these are very important things to consider, I still think we are missing something. But what exactly? Sorry to say... I am going blank here.
Eef van der Worp 
(R)GP Contact Lens Complications
Respiratory Tract Complication
 
In the series of (R)GP contact lens complications, this is kind of an unusual one. Many pros and cons of (R)GPs in relation to soft lenses can be discussed with patients. But while the literature is pretty much unanimous about the safety of (R)GP lenses compared to pretty much any other form of lens wear or modality, an interesting case was reported in the Dutch contact lens journal ContactlensInside by Jessica de Groot. A patient came to her office who was apparently 'terrified' to try (R)GPs. What was the case? This previously very successful, longtime (R)GP lens wearer occasionally placed a lens in her mouth to 'clean' it. On one occasion while she was doing this, a sudden coughing fit came over her... and she inhaled the lens. It sadly got stuck on the inside of her trachea, where it had to be surgically removed under full anesthesia. If it wasn't clear in the first place, this seems to be another good reason to instruct patients as to why they shouldn't put (R)GP lenses in their mouths.

De Groot - ContactlensInside, 2/2014 

Lens Care
A Problem to the Solution
 
Photo: P. Satjawatcharaphong

In Contact Lenses Today, Susan Gromacki describes a case of a 16-year-old high school student who was fitted with scleral gas permeable contact lenses. She was very successful, but at her one-month follow-up visit, she had developed a diffuse 1-2+ superficial punctate keratitis throughout her right cornea. When asked about her lens care regimen, it was clear that while the cleaning and disinfection were performed properly, she had resorted to filling the lenses with conditioning and disinfecting solution prior to application. What can be learned from this, according to the author, is: no matter how intelligent a patient, there is always the potential for noncompliance with the prescribed treatment regimen. Also, although there are many good options to disinfect scleral lens overnight, what a patient fills them in the morning upon application is critical - it needs to be a non-preserved solution. This topic is also covered in a case report as part of the fellowship of the scleral lens education society (SLS), presented in its newsletter. The case, described by Pam Satjawatcharaphong, emphasizes the fact that educating patients on correct cleaning of lenses and control of the contents of the fluid reservoir is critical in avoiding a toxic reaction on the cornea. This is particularly important in patients using scleral lenses for therapeutic purposes, as these corneas are already unhealthy and may be more susceptible to insult. Michael Ward's contribution to Contact Lens Spectrum is another reminder to be aware of Acanthamoeba infection and the risk involved when allowing tap water in our care regimen - also true in (R)GP lens wear, whether corneal, corneal reshaping or scleral. 

Corneal Involvement
Anterior Segment Changes
 
A study published in the Journal of Cataract and Refractive Surgery evaluated the changes occurring in the cornea, anterior segment anatomy, and intraocular pressure (IOP) in pregnant women. Sixty pregnant and 60 nonpregnant women were enrolled. Pregnancy was associated with greater corneal curvature and lower IOP. The corneal front steep keratometry value was statistically significantly higher in the pregnant group (44.8D versus 44.1D). No significant difference was found in corneal hysteresis, the corneal resistance factor, corneal posterior curvature, central corneal thickness and volume, anterior chamber depth and volume, or iridocorneal angle. A study published in Cornea investigated the association between diabetes mellitus (DM) and keratoconus. The study included 1377 patients with keratoconus and 4131 controls. The prevalence of type 2 DM was statistically significantly higher in patients with keratoconus (6.75%) than in matched controls (4.84%). The authors concluded that there may be a positive association between type 2 DM and the presence and severity of keratoconus.
I(n)-site-the-practice   
'What (about) the FOG' - part II
  

As so eloquently described by Dr. Maria Walker in the October edition of I(n)-site-the-practice, there is much to be investigated regarding the turbid clouding that can be a source of scleral lens dropout or a deterrent for scleral lens wear. In practice, without the ability to analyze the fluid and debris, we have encountered three types of debris. These three types can occur together in many cases, but depending on the type, we manage them in different ways. Mucus debris, Meibomian debris and fogging associated with atopic disease will all be discussed in this month's I(n)-site-the-practice. Understanding that the debris may be associated with ocular conditions can help reduce the problem when medical management is introduced. Also, manipulating the fit by improving scleral landing zone alignment can block the debris from entering the reservoir. Making every effort to reduce reservoir debris will minimize the risk for scleral lens dropout.

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I-site is an educational newsletter that is distributed on a monthly basis and provides an update on rigid gas permeable related topics (scientific research, case reports and other publications worldwide). I-site is objective and non-political. Its editor Eef van der Worp, optometrist, PhD, FAAO, FBCLA, FIACLE, FSLS is a lecturer for a variety of industry partners, but is not related to any specific company. Please contact us at: i-site@netherlens.com.