Gas Permeable Contact Lenses:
A Primer on their History and Fitting
I have been wearing gas permeable contact lenses (GP’s) for many years. Initially, I started wearing hard polymethylmethacrylate (PMMA) lenses and then switched to GP’s. As a wearer, I have found there to be many admirable traits to gas permeable contact lenses. Even though the gas permeable contact lens market is a small one, it is indeed a viable market for the astute practitioner. The following article will cover the history of GP’s, comparisons of GP’s vs. soft contact lenses, fitting considerations, practical considerations, and finish with an intriguing case history of a keratoconus patient.
Even though soft contact lenses dominate the market, there is still a small, but significant market in 2010 for GP’s compared to soft contact lenses (SCL’s). In present-day optical, optometric, and ophthalmology offices - safety, convenience, comfort, practicality, and efficiency all drive the contact lens markets. In this atmosphere the patient will frequently wonder which modality is more comfortable, especially in the absence of any promotion of gas permeable lenses. Have you seen any marketing or advertising for gas permeable contact lenses in the news, print, or TV media? Probably not. Yet millions of dollars are spent by CIBA, Bausch +Lomb, CooperVision, and Vistakon to promote current and new SCL’s. According to the website, www.linkedin.com/pub/jason-rake/6/919/a53, “a territory manager for Vistakon in the San Diego area managed an advertising budget of $240,000.”
Gas permeable contact lenses were introduced to the marketplace in the late 1970’s. Most gas permeable contact lenses are manufactured with silicone which makes them more flexible and more oxygen permeable than the hard PMMA contact lenses that predated the GP’s. Because gas permeable contact lenses do not contain water, protein and lipid materials in the tear film do not adhere to gas permeable contact lenses as readily as they do for soft contact lenses. This leads to greater comfort and durability for this modality in the long term.
So why should ECPs fit rigid gas permeable contact lenses when the market is being driven by soft contact lenses, soft toric contact lenses, and soft bifocal contact lenses? There are many patients that have been fit with gas permeable contact lenses and do not want to change their eye care routine. They do not want their crisp vision being affected by a different type of contact lens. They may simply be totally happy with their gas permeable contact lenses and wish to continue. The risk of microbial keratitis and serious corneal infections is much less with gas permeable contact lenses compared to soft contact lenses. There are also those patients that will definitely benefit by using gas permeable contact lenses, including those patients with steep corneas, patients suffering from keratoconus, patients with irregular astigmatism, and those patients that have been unsuccessful with soft contact lenses. Gas permeable contact lenses can provide UV protection and the Dk values range from a low of 18 to a phenomenal high of 151.
Various colors such as blue, green, grey, crystal blue, brown, ocean blue, forest green, aqua, ice blue, and electric blue are available for color enhancement properties. Many ECPs still enjoy the in-office benefits of finishing or polishing gas permeable contact lenses. Prescription changes can be done within an office setting and edges can be polished for comfort while a patient has a minimum of waiting time. Gas permeable contact lenses will provide good vision, are durable, and are more deposit resistant than soft contact lenses. Some eye care professionals are using gas permeable contact lenses to reshape the cornea and to reduce or eliminate nearsightedness, a process called orthokeratology. Orthokeratology is being positioned successfully as an alternate – a reversible alternate – to LASIK procedures.
The Fitting Process
Every contact lens evaluation must include a thorough eye examination with an emphasis on ocular health. Corneal integrity and corneal health must be thoroughly evaluated through a biomicroscopic examination. One must be sure that the cornea is free of scars, infections, and dryness. Any of these problems must be addressed before considering a contact lens procedure. The positions of the eyelids and the eyelashes must be determined. Ptosis, blepharochalasis, trichiasis, ectropion, entropion, and blepharitis need to be evaluated. The bulbar and palpebral conjunctiva must be evaluated for redness, dryness, pingueculas, pterygiums, and allergic or infectious conjunctivitis.
A trial frame refraction, keratometry measurements, a visual field test, biomicroscopy, intraocular pressures, and a dilated fundus examination will provide good information on the patient’s overall ocular health and provide clues as to how successful a patient will be with contact lenses.
Contact lens fittings can be done either as a part of the ocular examination or can be scheduled for another time. The goal of a contact lens fitting is to find the most appropriate contact lens that can be worn for optimal comfort, vision, safety, convenience, practicality and efficiency. Because there are many options to consider between the patients’s needs and with the current contact lens market, care must be taken to insure that the most appropriate lens is chosen initially. If the practitioner is recommending a gas permeable lens then base curves must be calculated, and prescriptions, thicknesses, diameters, and peripheral curves must also be determined.
Educational and information processes can include viewing DVD videos on the contact lens fitting, insertion, and removal processes. Many websites can be recommended to patients to be viewed in the office or at home for instructional or educational support. Properly trained contact lens technicians can be involved in this process. Handouts and written information should always be provided to every patient. Patients should have written documentation concerning their initial wearing schedules, solutions to use, the need for follow-up care, and phone numbers to call in case of an emergency. These facts should be well documented in a patient’s file in order to limit liability issues.
The patient or parent in the case of a minor must sign an informed consent and disclosure form being made aware of the risks of wearing contact lenses. In my practice, we ask the patients or their parent to sign a form, “understanding the importance of adhering to the proper lens care procedures and instructions that they have received. They understand that they are responsible for scheduling and keeping their appointments and that failure to do so could result in damage to their eyes.” The law requires that a contact lens prescription must be provided to the patient.
Ordering Gas Permeable Contact Lenses
When ordering gas permeable contact lenses from optical laboratories, it is necessary to provide as much information as possible to ensure that the product ordered is specific to the patient’s visual needs. When ordering a gas permeable contact lens, the material must be determined. There are many, and include Boston XO, Boston XO2, Paragon HDS, Boston ES, SGPII, Fluoroperm 60, and Fluoroperm 30. Refraction results and keratometry measurements should be included unless the eye care professional is calculating the base curve measurements and contact lens prescription themselves. The color of the contact lens must be included in the order. Base curves, powers, diameters, optical diameters, thickness, and peripheral curves can be calculated by the eye care professional or they can be calculated by the optical laboratory. It depends upon what is most appropriate for the patient and how reliable the optical laboratory is at manufacturing a specific product with only a refraction and keratometry numbers. Many companies will provide the eye care professional with a manual or fitting guide for determining the fitting parameters for specific brands of lenses.
Most eye care professionals have years of ordering experience and know what works and what does not. ECPs who have not fit many gas permeable contact lenses may want to consult with the optical laboratories until they develop their own “comfort zone” with the fitting process as well as with a specific contact lens optical company. Knowledgeable colleagues can provide you with some added information based on their years of fitting experiences. The consultants that I have spoken to are always willing to help and provide insight into how to fit a gas permeable contact lens. Personally, I am lucky to have several optometric colleagues that I speak to regularly to discuss the field of optometry as well as to discuss the world of contact lenses that is ever-changing.
Many contact lens companies have keratoconus gas permeable contact lenses that can be made to order. The ABBA-Kone lens by ABBA Optical is a Paragon HDS lens with a Dk of 58 and is available in blue or green. Art Optical makes the AKS lens and is available in “various GP materials” according to Tyler’s Quarterly. The Dk values are available from 18-151, depending upon the material. Blanchard Optical makes the Rose K lens and is available in any Boston material. Conforma Labs makes the Conforma-K available in Boston XO with a Dk of 100 or Boston ES material with a Dk of 18. These lenses are available in the color blue. Quite often, gas permeable contact lenses will be able to provide keratoconus patients with good vision and good comfort, but they are always a fitting challenge.
A Keratoconus Case Study
This case study involves a challenging keratoconus patient who has been a patient for many years. “Sandra” is a 50 YO WF and had been a soft contact lens wearer for many years. Her vision began to change in 2009 and I became suspicious of keratoconus in her left eye due to those changes. She was referred to a noted local corneal specialist in order to have her corneas evaluated with a corneal topographer. Due to some issues with corneal dryness, punctual plugs were placed in both eyes in the superior and inferior puncta.
In 2009, her refraction was
OD: -3.00-0.50 X 50 Va 20/20 ADD +2.00
OS: -4.75-2.25 X 100 Va 20/40 ADD +2.25
Keratometry measurements were:
OD: 45.25/45.62 180/90 degrees, mires clear and regular
OS: 49.87/47.25 20/110 degrees, mires irregular and oblong
She was using Refresh artificial tears, 1-2 gtts OU four times a day and was placed on HydroEyes nutritional supplements. HydroEyes has nutritional components that treat the mucin, aqueous, and oily layers of the pre-corneal tear film and can be a very effective adjunctive tool in treating dry eyes. At this time, she was using her CooperVision soft contact lenses infrequently. She was using a Cooper Proclear lens OD: 8.6 -3.25 14.2 and a Frequency Toric OS: 8.7 -4.50-2.25 X 100 for distance use and using a pair of +2.00 reading glasses.
In 2010, her vision in her left eye became worse and her refraction was:
OD: -2.75-0.75 X 70 Va 20/20
OS: -4.50-6.00 X 100 Va 20/100
Her new keratometry measurements were:
OD: 45.37/45.75 180/90 degrees, mires clear and regular
OS: 51.37/46.00 20/110 mires irregular and oblong
With such a big change in corneal curvature, it was now time to consider a gas permeable contact lens for keratoconus. All information was discussed with the patient with the goal of investigating if I could improve her vision in her left eye. A gas permeable contact lens fitting was completed and instructions were provided concerning removal, insertion, wearing times, solution use, and the need for follow-up care.
The parameters that I ordered were:
Paragon HDS material with a Dk of 58 in a blue color
OD: Base curve 7.42 Diameter 9.1 mm Rx: -3.25 Optical Zone 7.7 Center Thickness 0.13 mm
OS: Base Curve 7.45 Diameter 9.3 mm
Rx: -3.00-2.25 X 180 Optical Zone 7.7
Center Thickness 0.24 mm
Peripheral Curve 1.25 Prism, Front toric special design lens
Upon evaluation, both lenses were centered well and moved well. Visual acuities were OD: 20/20 and OS: 20/40
She continues to use the +2.00 for reading. After dispensing these new lenses with a wearing schedule for 2 weeks, she was provided written and verbal instructions and solutions. She left my office happy that her vision in her left eye was at an acceptable level, for now. She returned for a follow-up visit which yielded the same results as the initial consultation. I gave her further instructions concerning vision changes, sensitivity problems, movement of the contact lenses, and the need for follow-ups annually or as needed if she experienced any discomfort or fluctuations in her visual acuity.
I discussed the options with her that if this was not a satisfactory outcome, then a corneal transplant may be an option down the road. Another contact lens option would be a keratoconus gas permeable lens. Other future options for this patient may include; Intacs or corneal inserts which received FDA approval for treating keratoconus in 2004. Tiny plastic inserts are placed in the periphery of the cornea that can help reshape the cornea for clearer vision. Corneal cross-linking is a non-invasive procedure that strengthens cornea tissue in keratoconus patients. FDA clinical trials began in 2008. The outer epithelial portion of the cornea is removed and riboflavin is then activated with UV light.
New technology, new plastics, and future research discoveries will add to our improved options for keratoconus patients as well as for those who simply want contact lenses in any form. The advances that have occurred over the past twenty years have been very helpful for those patients in need. It will be interesting to see what develops over the next 5-10 years. It is one of the many reasons that I love being an optometrist. Keeping up with the latest technological advances as well as furthering one’s educational aptitude allows all of the eye care professions to be dynamic and wonderful professions. For those of us blessed to pursue these outstanding professions, we are truly fortunate.