Gas permeable contact lenses, also known as GP or rigid gas permeable (RGP) lenses
, are severely underrated. Of the various types of gas permeable lenses, the most underutilized and underrated are small-diameter corneal GP lenses.
These lenses, compared to their more popular soft lens
counterparts, are more conducive to good eye health (smaller size, better oxygen permeability, more resistance to bacteria and deposits) and provide crisper vision (due to its firm material, the lens maintains a smooth, curved surface).
In this article, I will state my case for corneal GP lenses—they are an essential part of my toolkit and I hope you will be encouraged to add them to yours.
Corneal GP lens types and indications for use
GP lenses can be used for the correction of myopia
, hyperopia, astigmatism, and presbyopia. There are several different designs that can be utilized. There are many different proprietary designs available within these categories. I would recommend using your favorite contact lens lab and getting familiar with their designs for the different patient cases that you may encounter.
Figure 1 highlights a corneal GP lens on the eye.
Figure 1: Courtesy of Noha Seif, OD
Table of GP lens types and features:
|Type of GP Lens
|Same power/curvature used all along the surface of the lens.
|Varying powers/curvatures across the surface of the lens. Often used to reduce spherical aberrations and used in multifocal designs.
|The back surface of the GP is toric while the front is spherical. Used when the refractive cylinder is 33 to 50% greater than the corneal toricity, or when the corneal toricity does not allow a spherical base curve to center on the eye.
|Both surfaces of the GP are toric. Bitoric lenses are typically indicated when the corneal cylinder is >2.50D.
|The front surface of the GP is toric while the back surface is spherical. Used when the amount of refractive astigmatism is significantly different from the corneal astigmatism and when there is no need for a toric back surface.
|A multifocal GP where the power is separated into zones, similar to a traditional spectacle bifocal.
|Specialty GP lens
|There are a variety of specialty GP lenses available to correct for eyes that have mild keratoconus, corneal scarring, or who have undergone a corneal transplant. Specialty GP designs are also utilized in orthokeratology.
Table 1: Courtesy of Noha Seif, OD
Download the Caring for GP Lenses patient handout here and check out a preview at the end of the article!
The advantages and disadvantages of corneal GP lenses
When it comes to initial comfort, it’s inarguable that soft lenses excel comparable to GP lenses. Don’t let the adaptation period scare you off from prescribing them, though. Once the patient has adapted to the lenses, a process that takes an average of 2 weeks, the comfort is comparable to soft lenses
Moreover, the vision is crisper, the lenses are more durable and less expensive over the long run, and most importantly, they are healthier due to their superior oxygen permeability and smaller size. Be the doctor that puts the patient’s long-term eye health at the forefront rather than convenience.
Takeaway: The advantages of GP lenses include superior vision, oxygen permeability, durability, and value compared to soft lenses. They also provide long-term comfort, resistance to infection and deposits, and easier handling.
Conversely, the disadvantages of GP lenses are the initial adaptation period, the potential increased risk of dust/debris particles in dusty environments, and the potential increased risk of dislodge during vigorous activities.
How to fit a corneal GP: empirical vs. diagnostic fitting
In optometry school, we were required to calculate GP powers in clinic. Although it was good practice, my enthusiasm for gas permeable lenses was immediately dampened. It wasn’t until a few years later that I learned there was another way to order GP lenses! Maybe a day will come when I have the hankering to pull out my calculator again, but for now efficiency and practicality are my priorities.
Clinically speaking, I would recommend the following fitting tips for the different types of GP designs:
- For refractive error such as myopia, hyperopia, regular astigmatism, and aspheric multifocals, empirical fitting will save you and the patient time and provide the patient with a great first lens, especially from a vision quality standpoint.
- To order a lens empirically, you'll want to provide your lab of choice with the following at an absolute minimum: the latest glasses prescription and keratometry readings. To order empirically, provide your lab with refraction and K’s.
- Other useful information to keep in mind: pupil size, horizontal visible iris diameter (HVID), information on upper and lower lid landing in relation to the limbus, eyelid laxity, and palpebral aperture size.
Performing diagnostic fitting for gas permeable lenses
With this information alone, and for the vast majority of regular corneas, a diagnostic fit set will not be necessary. However, I absolutely do recommend performing a diagnostic fitting
for segmented multifocals and for patients with irregular corneas (keratoconus, corneal scarring, post-LASIK
, or post-corneal transplant).
With a segmented multifocal
, seeing pupil placement and how a patient’s lids interact with the lens is immediately useful. With irregular corneas, a patient’s spectacle and keratometry readings are of minimal benefit—the best way to efficiently dial in the fit in these cases is to observe a lens on the eye before you place the order.
To perform a diagnostic fitting, place a diagnostic lens from your fitting set of choice on the patient’s eye. Directly evaluate the fit of the lens, perform an over-refraction, and provide your preferred lab with your findings to proceed with an order. For difficult fits
, I find it helpful to provide the lab with pictures and/or videos.
Figure 2 demonstrates a keratoconus design GP lens fitting in progress. Placing a diagnostic GP lens on an eye with keratoconus allows the practitioner to efficiently evaluate the fit and vision potential.
Figure 2: Courtesy of Noha Seif, OD
Figure 3 shows a segmented GP lens. Immediately evident is the rotation and pupil placement in relation to the reading zone.
Figure 3: Courtesy of Noha Seif, OD
Case report 1: Choosing a GP lens for an active teen patient
A 14-year-old male presented to our clinic interested in wearing contact lenses for sports. He was prescribed soft lenses at a different provider's office but struggled with insertion and removal
. His spectacle prescription was: +4.25-3.50x020 and +5.25-4.50x166.
Although this patient's family came to me seeking to retry soft contact lenses, I quickly steered them toward GP lenses for the following reasons:
- With his prescription, vision through a traditional soft contact lens would always be sub-optimal.
- This correction would be available via a custom soft lens, but the thickness of the material would provide low oxygen permeability and be prone to heavy deposits. I do not recommend lenses made of this material for kids or teens from a health perspective. Additionally, the optics will still be sub-optimal.
- The small diameter and rigid material of a GP would make it much easier to handle from an insertion and removal standpoint. A soft lens with such a high hyperopic toric prescription will always be a bit tricky to insert.
Interestingly, the primary selling point to the patient was the ease of insertion and removal after having failures with the soft lens application process. These lenses were fit empirically as a bitoric lens with no additional adjustments needed to the prescription.
Figure 4 highlights from left to right: a commercially available soft toric contact lens, a custom soft toric lens, and a bitoric GP lens diameter side-by-side comparison. The small diameter GP, which holds its shape, proved much easier to insert and remove.
Figure 4: Courtesy of Noha Seif, OD
Case #2: Selecting a GP lens for a patient with corneal scarring
A 40-year-old male was referred to our clinic for an updated glasses prescription. He had suffered multiple strokes that impaired several functions, including his ability to blink, fine motor skills, and speech. Although his facial paralysis had resolved, his vision was down in one eye not because of a prescription shift, but due to a corneal scar
with neovascularization that had formed during his episode of paralysis.
His vision with glasses was correctable to 20/50. I knew that a GP lens could do better. One of the beautiful characteristics of GP lenses is that they can help mask things like irregular astigmatism induced by a corneal scar. I proceeded with a diagnostic fitting to evaluate his vision potential. With only a diagnostic lens, we were able to achieve 20/20.
In this instance, the reasoning for proceeding with a GP lens over a soft lens was straightforward. A soft lens will provide minimal benefit due to surface irregularities, whereas an appropriately fit GP lens can potentially vault the corneal insult noted in the image allowing for a smooth refractive surface to improve vision.
With this thought in mind, care must be taken during the fitting process to avoid inducing further hypoxia which could possibly exacerbate neovascular progression and scar tissue development. Finally, the reason we proceeded with a small-diameter corneal GP lens over a scleral lens
in this case, however, was ease of insertion.
Figure 5 illustrates a GP lens on an eye with corneal scarring and neovascularization. The VA went from 20/50 in spectacles to 20/20 with a GP lens.
Figure 5: Courtesy of Noha Seif, OD
Although the patient's motor skills were continuously improving, there was still instability in his hands and difficulty with fine motor control. The tools used for scleral lens
application and removal would have required too much dexterity and the patient was keen on regaining independence and avoiding the need for a caretaker. Application and removal is much easier with a small GP lens. It was generally achievable for the patient and, as a bonus, is straightforward enough for friends or family to do if necessary.
Takeaway: A scleral lens is not necessary when a GP lens will do the job—or do the job better! The patient tolerated the lens well, had zero issues with adaptation, and was thrilled with his regained vision.
Gas permeable lenses in a nutshell
GP lenses, especially small-diameter corneal gas permeable lenses, are a great alternative to soft contact lenses for patients with increased vision and eye health needs.
I believe they should also be considered for young contact lens wearers
, patients who have difficulty inserting and removing contact lenses, and even patients with financial constraints (GP lenses often come out to be cheaper than soft lenses over the long run). With the numerous benefits they offer, I encourage you to consider them the next time you are prescribing contact lenses in-clinic.
Below is a preview of the downloadable patient handout. Click on the heading or image for access to the cheat sheet!