Tuesday, December 18, 2007

6.4 (Non-)Contacts

Strictly speaking, a contact lens (CL) is not in direct contact with the corneal epithelium. Instead, it floats on top of the tear film. This is, however, only academic.

There are simple steps for fitting contacts:

1. Corneal evaluation:

Maintaining corneal health remains the most important goal in contact lens fitting. It is now known that the corneal endothelium can change its cell density, size and shape after contact lens wear. The endothelium is responsible for the maintenance of water content in the cornea (too much water causes corneal edema hence haze which occurs in eyes with improperly fitted CLs). Healthy cornea of course is a must for the eye to receive the CL. Eyes with corneal defects such as the dry eye will be incompatible with CL wear. The epithelial defects can be readily seen with fluorescein staining when observed under the cobalt (blue) light. Specular microscopy is needed for the evaluation of the endothelium; although this is not commonly done.

2. Parameters needed for CL fitting:

(1) The most important one is corneal curvature measured with keratometry. The curvature in mm (known as the “K”) is the basis for the selection of CLs. In other words, the base curve of the CL must be based on that of the cornea. For example, in hard lens fitting, the base curve can be on mixed K, on flat K, or slightly flatter than the flattest K, and in soft lens fitting, the base-curve is usually flatter than the flattest K. This allows CL movement that facilitates tear (essentially oxygen and nutrient) exchange. It should always be remembered that CLs are NEVER fitted steeper than K as this will cause “central pooling” (immobile CL with tears trapped underneath) leading very quickly to corneal damage. It also should be noted that the K-readings are those of the central 2-3 mm of the cornea. Trial lens fitting therefore remains the most practical approach in the selection of CL base curve.
(2) The palpebral fissure or the gap between the upper and lower eyelids when the eyes are in the normal open position can be a determining factor in the selection of lens diameter (i.e., size). This is based on the assumption that the portion of the cornea covered by the lids does not receive enough oxygen already, larger-diameter lens therefore will deprive the cornea of even more oxygen.
(3) The corneal diameter also can be a determining factor in the selection of CL size. Again, this is based on the area of corneal coverage by the CL. In general, the smaller the CL diameter the better for cornea health. Furthermore, smaller CL diameter has the same effect as increasing the base curve, i.e., smaller CLs will appear flatter than large CLs. However, it also should be noted that sometimes the patients will experience glare and unstable vision because the CL diameter is too small. And by the same token, eyes with extremely large pupils also will experience peripheral disturbances if the CL is too small.
(4) In addition to CL base curve and diameter, it is also necessary to adjust the lens power because of the difference in the vertex distance (which is the distance between the back of the spectacle lens and the cornea/eye). Notice that refraction either with the trial frame or with the phoropter, is done at a distance away from the eye of around 13 mm. The CLs are fitted directly onto the eye, so the power actually needed will be less minus (or more plus) especially when the spectacle lens power is more than 4 diopters. For example, a spectacle lens power of -5.00D requires a CL power of –4.50D. A conversion chart or software is generally available online.

Notice in the very high myopia (malignant myopia) and hyperopia (pseudophakics), the adjustments may have to be made through over-refraction as even slight change in the vertex distance can cause large differences.

3. CL Selection:

(1) Soft lenses are HEMA-based that contain 30-70% water. There are now daily wear, 3-month extended wear, daily disposable, and 1-2 weekly disposable lenses. Selection is based more on the patient’s life style and sometimes on economic concerns.
(2) Hard lenses are now silicone-based RGP lenses. The classical PMMA lens is no longer or very seldom in use. The fitting principle for RGP lenses is the same as that developed for the PMMA lens, i.e., fitting on K or more frequently, flatter than K to allow CL movements. However, because of the high oxygen transmission, RGPs can be fitted on-K for better stability. (3) Special lenses are those with toric design as well as the bifocals. They are also available in hard- and soft-forms. Because of the necessity to limit lens rotation from blinking motion, a prism ballast or similar must be built-in to anchor the lens in a fixed orientation. Nonetheless, lens rotation sometimes is still too extensive to allow stable vision. There is a momentary blur after each blink that some patients cannot seem to tolerate at all.

4. Situations that require extensive management:

(1) Dry eye – deficient tear production and poor tear quality
(2) Severe allergic conjunctivitis – chronic inflammation and discomfort (“itchy” eyes)
(3) Pregnancy and use of birth control pills – these change tear chemistry that can reduce the wear time
(4) Diabetes mellitus – general intolerance to ischemia
(5) POAG – CL wear may mask glaucoma-related corneal change
(6) Chronic and acute infectious keratitis

5. Complications from CL wear:

(1) Punctate stains – epithelial defects due to desiccation and/or oxygen deprivation often in the 3-9 o’clock position
(2) Corneal edema – from severe oxygen deficit detectable by limbal trans-illumination as central corneal clouding (ccc)
(3) GPC (giant papillary conjunctivitis) – response to allergens embedded in the CL; cobble-stone type
(4) Infectious keratitis – improper wear habit (often over-wear and poor CL hygiene) causing breakdown of corneal resistance to micro-organisms, mostly bacteria (e.g., Pseudomonas aruginosa)

6.Remedies for problems in CL wear:

(1) Proper blinking pattern and adequate blinking rate
(2) Use of wetting solutions and lubricants
(3) Shorten CL wear time
(4) Proper cleaning and disinfecting of contact lenses
(5) Use alternative types of CLs (CAUTION: hard CL re-fitting is quite difficult especially if the patient is a long-term CL wearer. It often necessitates suspension of CL wear for 3 months before the re-fit.)


From the doctors' point of view, Good Vision, Ocular Comfort, plus Adequate Lens Movement are the three hallmarks of a successful contact lens fit. However, it should be noted that thorough dispensing/teaching sessions and periodic follow-ups are also required to ensure maintenance of the patients’ corneal health.

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