Our goal with keratoconus patients is to provide them with the best possible acuity for as long as possible before resorting to a corneal transplant. Myriad contact lens designs allow us to achieve this goal.

Still, keratoconus patients will be among your most challenging contact lens fits. Here, we"ll look at lens designs to consider, discuss practical fitting tips and offer troubleshooting suggestions.

Lens Choice
Lens options for keratoconus patients include the following:

• Soft lenses. While RGPs help mask some corneal irregularity, soft lenses tend to drape over the cornea. So, they are indicated only in the early stages. Soft lenses are acceptable if they do not induce abrasions, which lead to corneal scarring, and if they yield best-corrected visual acuity of 20/40 or better.
• RGPs. When soft lenses no longer work, early keratoconic patients may achieve adequate acuity with large-diameter RGPs. These patients often wear their contacts all day, so use lenses with high oxygen permeability. Some keratoconus patients may need a lower Dk material for more lens stability.
• Hybrid lenses or piggyback fits. If a keratoconic patient is intolerant of rigid lenses, you might need a hybrid lens such as SoftPerm.

A piggyback fit also works well for many patients. With this technique, you first fit the patient with a large diameter soft lens with proper centration and movement. Next, take keratometry or topography readings of the "new" corneal surface and fit an RGP on top of the soft lens. Both materials should be highly oxygen permeable.¹ I use a lens such as PureVision (175 Dk/t) and then fit an RGP over it. Make sure there"s enough movement-preferably at least 1mm in primary gaze-to allow for adequate tear exchange under the lens.

Patients with the nipple form of keratoconus may have limited options. Some RGPs will erode the fibroplastic nodule, leading to abra- sion and decreased vision. These patients often require piggyback lenses, specialty keratoconus designs or surgical excision of the nodule.²

Specialty Designs
More advanced cases may require a lens designed specifically for keratoconus.

I shy away from the few soft lens designs for keratoconus. They do not allow enough oxygen to the cornea, nor do they provide the visual clarity that RGPs do. I consider these soft lenses for a patient who is intolerant of RGPs and unwilling to undergo surgery.

I do, however, use Epicon, which combines the rigidity of an RGP with the comfort and flexibility of a soft lens, and offers a high Dk value.

Three of the more common gas permeable keratoconus fitting sets:

• Soper Cone. This is a bicurve contact lens with a fitting philosophy based on sagittal depth. The vaulting effect of the lens increases as the base curve decreases. This change in the steep central posterior curvature is what is used to fit the cone. The theory behind the Soper design is to avoid apical bearing in keratoconus because it will ultimately lead to corneal scarring.
  The Soper design consists of 10 lenses designated by letters A-J and divided into three groups, each with a different diameter/optic zone relationship:3 A-D for mild keratoconus (K-readings <48.00D in either corneal meridian); E-G for moderate keratoconus (K-readings 48.00-54.00D in either corneal meridian); and H-J for the advanced stage (K-readings >54.00D).¹

• McGuire. This lens system is a modification of the Soper design.³ It consists of three diagnostic lens sets, formulated for the nipple, oval or globus type of keratoconus.
  The fitting philosophy aims to achieve a three-point touch and is predicated upon the size of optic zone in relation to conical size. In this design, the optical zone sizes are 6mm for the nipple cone, 6.5mm for the oval cone and 7mm for the globus. Each lens also incorporates a series of four peripheral curves blended together to create an almost aspheric relationship.¹

• Rose K. This lens design features complex, computer-generated peripheral curves based on statistical data Dr. Paul Rose of New Zealand collected on his keratoconus patients.3 The lenses incorporate standard, flat and steep peripheral systems to achieve the ideal edge lift of 0.8mm. It is available in base curves of 4.75-8mm and diameters of 7.9-10.2mm.
  The Rose K design decreases the optic zone diameter as the base curve steepens. Toric curves are available on the front and rear surfaces as well as peripherally. Rose K lenses are traditionally made in the Boston ES material, but some labs make them in the Boston EO material, which offers increased oxygen permeability. Regional laboratories distribute the Rose K set, making it accessible. Also, the labs use a software program to cut and blend the multiple curves, so replacement lenses are easy to reproduce.³

• Other specialty lenses which I tend to use less frequently include the ComfortKone and Valley Kone.

Fitting Protocol
Once you"ve selected the appropriate material, follow this six-step protocol to achieve an ideal fit:

1. If possible, identify the morphological shape of the cone and the stage of the disease. An easy way to accomplish this is to view the cornea with retro-illumination after dilation. The mean K readings will tell you whether the patient"s disease is early stage (<50.00D), advanced (50.00D-55.00D) or severe (>55.00D). If you have a corneal topographer, you can use that instead to get a more accurate idea of the stage of keratoconus.

2. Instill a drop of anesthetic in each eye before lens insertion. Do this even in previous contact lens wearers to decrease adaptation time. Otherwise, patients tear excessively, causing the lens to sit low and give abnormal fluorescein patterns.

3. Insert the lens, wait at least 20 minutes and then evaluate the fluorescein pattern. As with all contact lens fits, look at the central area, the mid-periphery and the periphery. The lens should locate centrally when you evaluate it. If the lens lags down, use upward pressure on the lower lid to improve centration. Of course, you must realize that the lens may not center on the cornea during the final fitting. If that is the case, then try to achieve the best fit where the lens does center.

4. Determine the correct base curve. Start with a base curve equivalent to the steeper of the two K-readings. Since the mires are often irregular in keratoconus, the K-reading will only provide a rough gauge of the trial lens to start with.
   Since you are starting with a base curve equivalent to the steeper of the two K readings, this lens should fit too steep. Once you assess the fit of this lens, continue to flatten the base curve until you get the slightest amount of apical touch. If you have difficulty discerning where the cone apex is, then the lens is too steep.
   You want the base curve that distributes its weight-bearing forces evenly on the cornea-the so-called three-point touch. There should be minimal touch at the cone apex, plus an area of bearing between the lens periphery and the intermediate zone of the cornea.
   This fit offers the maximum comfort and vision for your patient. As long as the area of mid-peripheral bearing does not encompass 360, the central cornea will have a sufficient supply of freshly oxygenated tears upon blink. By making sure the bearing is as little as possible at the cone apex, you will probably decrease the chance of scarring the cornea.
   I usually examine the central base curve exclusively until I am satisfied with the fit. Then it is easy to flatten or steepen the peripheral curves.

5. Select the optic zone diameter. It should cover the pupil in average illumination, plus 1 or 2mm. This may not always be possible, but if you can, the extra 1-2mm helps provide coverage when the pupil dilates, and it helps cover the cone.
   If the cone is excessively large and eccentric, you may need to make the optic zone even larger.⁴ If you do not fully cover both the cone and pupil, your patient will suffer from glare and haloes around lights.
   An 8.7mm diameter will work for most patients. The upper edge of the lens should hit the tarsal plate of the upper lid. If it doesn"t, measure from the top of the lens to the bottom of the upper lid, and add that number to the overall lens diameter. You may need a smaller diameter if the lens rides too high, or if you see superior or 3-9 o"clock staining.

6. Over-refract the patient. Start with 1.00D steps initially, and then refine with 0.50D and 0.25D as you get closer to the final prescription. Perform the final over-refraction in normal illumination to approximate normal lighting conditions and pupil size. If you cannot obtain adequate vision with a spherical over-refraction, try a spherocylindrical over-refraction.

Acuity for the keratoconic patient may not be 20/20 even with contact lenses, but that is not a problem if the vision is satisfactory for the patient"s needs.

Troubleshooting
Have the patient return for follow-up at least every six months. Examine corneal surface integrity, evaluate the lens fit and look for changes in corneal topography. Remind him or her that keratoconus is a progressive disorder, so regular monitoring for changes is in order.

Symptoms such as decreased vision, redness, discharge, photophobia, decreased lens comfort, decreased lens wearing time or increased difficulty inserting or removing the contacts might indicate changes in the cornea. Evaluate the patient as soon as possible. Some other common problems you might encounter:

• The base curve is improper. A lens that fits too loosely will tend to lead to repeated corneal abrasions. One fit too tight may result in poor tear exchange. This ultimately leads to a buildup of metabolic debris underneath the optic zone, resulting in poor oxygen transmission to the cornea. This usually manifests as dimple veiling and stipple staining around the base of the cone. To alleviate this problem, adjust the central base curve or peripheral curves, decrease the optic zone diameter, or blend the junctions between the peripheral curves.⁵ Your goal: a good, stable fit that allows for enough movement to provide adequate tear exchange.
• The lens fits too flat. In this situation, the lens may move excessively and feel uncomfortable. You can fix this by steepening either the central base curve or peripheral curves, both of which will decrease lens movement.
• You observe coalesced staining of the apex of the cone. This usually results from a lens fit too flat or from excessive deposits on the posterior lens surface. Staining on the apex of the cornea ultimately leads to corneal scarring. If the fit is too flat, I"ve found it best to steepen it.
  If the fit looks good, observe just the lens in the slit lamp.⁵ If deposits are heavy, start the patient with a new lens and begin aggressive enzyming. Patients may need to enzyme once or twice weekly.
• You observe small bubbles under the lens. These indicate a steep fit. Try, increasing the edge lift, reducing the lens diameter, or flattening the base curve until you get the slightest apical touch. If the bubbles persist, you can order fenestration at the juncture of the optic zone and the secondary curve, but do this only as a last resort.
• The lens rides too low. To improve centration either increase the lens diameter or change the base curve. If you decide to change the diameter, in most cases, the changes must be at least 0.3mm in order to see a clinical difference.

While keratoconus patients are among our most challenging contact lens fits, we can still help them. Although they may not achieve perfect vision, your persistence may provide them with sufficient vision to keep them out of the operating room for some time.

Dr. Gupta practices full-scope optometry with Stamford Ophthalmology in Stamford, Conn.

1. Cannella A. Use of contact lenses in the visual correction of keratoconus. www.opticians.org .
2. Caroline P, Andre M, Norman C. Corneal topography in keratoconus. Contact Lens Spect 1997;12(7):36.
3. Cutler SI. Managing keratoconus with proprietary designs. Contact Lens Spect 1999;14(10):21.
4. Buxton JN, Keates RH, Hoefle F. The contact lens correction of keratoconus. The CLAO Guide to Basic Science and Clinical Practice. Boston: Little Brown and Company, 1984:1-48.
5. Edrington TB. Keratoconus problem solving. Contact Lens Spect 1998;13(3):18.

Vol. No: 138:10Issue: 10/15/01