CONTINUOUS WEAR CLS
Continuous Wear Contact Lenses
Will your patients succeed in silicone hydrogel contact lenses? This article will help you find the answer.
By James Boucher, OD, MS, FAAO, and Gary Poteet, OD, MS, FAAO
After the initial approval of the extended wear contact lens category, many doctors and clinicians observed hypoxia-related adverse responses. But we now have more than one good option to redeem our clinical future with extended wear. At present, the FDA has approved three contact lenses for 30 days of continuous wear (CW):
1. Focus Night & Day (CIBA Vision)
2. PureVision (Bausch & Lomb)
3. Menicon Z (Menicon)
In a marketing conflict, a patent court ordered Bausch & Lomb to remove its product from the U.S. market, leaving us with one soft silicone hydrogel CW contact lens and one GP CW contact lens.
With two CW lenses to choose from, how do we select the right candidates?
Patients Who Seek Continuous Wear Good candidates for CW include motivated patients who desire "continuous vision." A good example is someone who possesses a high myopic prescription and is unable to read the alarm clock at night without vision correction. Also, patients who have a difficult time maintaining adequate hygiene could benefit from this wearing schedule because their handling time would decrease.
Further, we could include the candidate whose continuous vision is a plus to his job. This category includes policemen, firemen and anyone considered "on call" and required to be ready at a moment's notice. In some of these cases, CW contact lenses might be more of a necessity than a convenience.
Patients Who Have Difficulty with Regular Wear Another group of candidates experiences a difficult time with the ritual associated with contact lens wear. Here, we encounter the spectrum of patients that range from those who just have a difficult time with lens application and removal to those who have anatomical issues. (For example, some people have small apertures and they encounter difficulty when applying all contact lenses.)
Also, consider patients who have a physical impairment, such as absence of usable digits or severely arthritic hands. CW technology may provide these patients with freedom not previously available to them.
Weeding Out Unsuitable Candidates
We're always striving to supply the appropriate contact lens to the appropriate patient. CW candidates need to understand the importance of, and be willing to demonstrate, appropriate skills and hygiene during application and removal. They also need to exhibit an appropriate health status with a "friendly" ocular environment.
In the ideal ocular environment, the patient would possess a well-balanced tear chemistry. You can assess tear makeup by using vital stains such as lissamine green and rose bengal. Other tests used to assess the tear film are tear break-up time (TBUT), Shirmer strips and biomicroscopy of the eyelids, tears and ocular surface. It's imperative to verify an absence of infection or inflammatory concerns. The patient can't exhibit any signs of active blepharitis, conjunctivitis or meibomianitis. Systemic concerns are an important consideration in appropriate CW contact lens patients as well. Examples of unsatisfactory CW patients are immunocompromised individuals, patients who have compromised vascular systems and can't supply the needed oxygen and long-term smokers.
Silicone Hydrogel Lenses The ideal-fitting silicone hydrogel contact lens should fit similar to any well-fit hydrogel lens. Patients may experience a higher level of initial awareness with silicone hydrogel lenses such as with Focus Night & Day because of its higher modulus of elasticity. The higher modulus of the material lessens the degree to which the lens flexes and conforms itself to the corneal surface, as compared to a standard hydrogel lens. Therefore, the lens is less forgiving and the fitting relationship between the lens and the corneal surface must be more precise. Silicone hydrogel lenses may display a tendency to "flute" at the edges if the lens-to-cornea relationship isn't correct.
Note: Our office was a clinical site for both PureVision and Focus Night & Day FDA clinical trials for 30-day continuous wear. During the trials for Focus Night & Day, only the 8.6mm base curve lens was available and we did notice edge fluting. Since it's been approved, the lens is also available in an 8.4mm base curve, which has solved the edge fluting problem in most cases.
Take keratometry readings and choose a base curve. If the Ks are steeper than 43.25 diopters, then we usually start with the 8.4mm base curve. Allow 10 minutes for the lens to equilibrate. The lens should center with good corneal coverage and no lens edge stand off. On primary gaze the lens should move between 0.2mm and 0.6mm. On superior gaze, the lens should exhibit about 0.8mm of movement. On a "push up" test, the lens should move freely. Silicone hydrogel lenses tend to move more than hydrogel lenses and give the impression that the lens is slightly loose. Adequate tear exchange should occur for debris and bacteria removal. You can use high molecular weight fluorescein to evaluate the fit, but it's rarely needed.
GP Continuous Wear Contact Lenses The Menicon Z GP contact lens is available in three different designs. The alpha -1 design is best used when added mass, weight or thickness is desirable to minimize lid interaction. The thin design helps increase comfort for patients who experience inferior decentration or excessive post blink movement. The aspheric is most useful with moderate with-the-rule astigmats, soft toric candidates or in cases where centration is not ideal with a spherical design.
Each Menicon Z contact lens design has a suggested fitting criteria, which Con-Cise Contact Lens Company (one of Menicon's U.S. distributors) provides. The standard criteria is based on the mean keratometric reading, amount of corneal astigmatism and desired overall diameter.
One useful recommendation is to lightly polish the lens edges and soak them in Claris (Menicon) for a few hours before dispensing. The polish helps reduce any tendency the lens may have to seal off initially.
Closely monitor patients who've been fit with silicone hydrogel lenses. If they're currently wearing hydrogel lenses on a daily wear basis, then start silicone hydrogel lens wear on a similar daily wear schedule before starting overnight wear. You should see every patient, regardless of his past lens experience, the following morning after his first night of wearing the lenses. Follow a schedule similar to that of patients wearing regular hydrogel lenses on an extended wear basis for silicone hydrogels. Perform a slit lamp exam with fluorescein instillation at each follow-up visit and look for any palpebral conjunctival changes by everting the lids. Don't be surprised to observe less myopia over time on patients who you've switched from hydrogel extended wear to silicone hydrogel CW. This is because the higher oxygen permeability reduces hypoxia-induced edema.
Many times microcysts, which you may have detected with extended wear of hydrogel lenses, come to the surface of the cornea in greater numbers when a patient is refit with higher Dk/t lenses. The microcysts then gradually disappear over a few months. Neovascularization, striae and microcysts (aside from those "purged" from prior extended wear of hydrogel lenses) are rare with silicone hydrogel wear.
Looking Out for Potential Complications
Even though silicone hydrogel lenses reduce adverse events related to hypoxia, other complications that we see with regular hydrogel lenses, such as contact lens peripheral ulcers, contact lens acute red eye and infiltrative keratitis, are just as prevalent in silicone hydrogel wearers. Superior epithelial arcuate lesions can occur with silicone hydrogel wearers, and experts believe that a mechanical shearing of the basement membrane in the superior cornea and may be associated with the topographic characteristics of some patients. The higher modulus of the silicone hydrogel lenses may contribute to this problem. Changing the base curve of the lens or switching to a different, more pliable lens material may solve this problem.
Giant papillary conjunctivitis (GPC) can also become an issue with silicone hydrogel lens wear and may be related to the higher modulus of the lens, which may cause mechanical irritation. The symptoms with silicone hydrogel wear are usually less severe than what ordinary hydrogel lens wearers report.
A specific response mainly seen with silicone hydrogel wearers is the formation of mucin balls. Experts believe that this is a mechanical shearing of the tear film's mucin layer into little round balls that become trapped under the lens. After the patient removes the lenses, you may see a dimpling impression that will stain with fluorescein. Fortunately, mucin balls don't seem to have any lasting adverse effects.
Another valuable reason to use this type of lens is to take advantage of its high oxygen transmissibility (Dk/t). One such example takes advantage of the recent approval of the soft silicone hydrogel lens for use as a bandage lens. In this situation, the lens helps reduce the risk of hypoxic-related events as it provides protection from external insult. The unique properties inherent in this material also afford us an alternative for additional systems such as orthokeratology, piggyback lenses and post-surgical considerations. Patients can also wear this lens on a daily wear basis when extra oxygen is particularly beneficial.
Many times we recommend to our daily wear hydrogel wearers that they try the new silicone hydrogel lenses. In doing so, our experience has been that patients are more comfortable in these lenses for longer periods of time and they experience less dryness, especially at the end of the day.
Dr. Boucher has been in a group private optometric practice in Laramie Wyoming since 1966, specializing in cornea and contact lenses. He's a Diplomate in Corneal and Contact Lenses of the American Academy of Optometry. He's also a past chair of the American Optometric Association Contact Lens and Cornea Section and served 18 years on the FDA Ophthalmic Device Panel. He was named the American Optometric Association Optometrist of the Year in 1980.
Dr. Poteet is partners with Dr. Boucher in their Wyoming group practice. Their practice is an investigative site for multiple contact lens companies. Dr. Poteet currently serves as an adjunct assistant professor at the Illinois College of Optometry and the University of Wyoming and as an officer on the board of the Wyoming Optometric Association.