Advances in the management of keratoconus and related non-inflammatory corneal-thinning disorders allow optometrists to more easily diagnose these ocular conditions and educate patients on their prognoses and treatment options. More eye care practitioners seem to be utilizing corneal topographers and pachymeters, which can significantly reduce the number of patient referrals made for advanced testing and diagnosing.

The ability to confirm the presence of keratoconus (KC), keratoglobus (KGB) or pellucid marginal degeneration (PMD) is only half of the battle. Knowing how to best correct the induced irregular astigmatism and ametropia is equally important for patient retention.

A Clear Treatment Strategy
Assure patients that surgical intervention (penetrating or deep anterior lamellar keratoplasty) is only a “last resort.” Then, let them know that, while spectacles and soft contact lenses may be effective forms of vision correction early on, gas-permeable (GP) lenses potentially provide the best visual acuity as their conditions progress.^1,2 A clear GP lens-fitting strategy for ectatic disease provides patients the best vision possible without doctors’ suffering the expense of excessive chair time and episodic anxiety or self doubt.

Before considering the approach to fitting corneal lenses for KC, KGB or PMD during various disease stages, be clear on your preferred fitting style with respect to the apical fluorescein patterns (steep vs. flat fit, or apical clearance vs. apical touch). Generally, the goal of fitting is simple: good VA, comfort and anterior segment ocular health—however, the type of fit that provides the least deleterious effect on corneal health is still under discussion.

Early in the Collaborative Longitudinal Evaluation of Keratoconus (CLEK) study, researchers learned that practitioners in the U.S. tend to fit GP lenses flat relative to the corneal apex, the degree of which increased as the condition advanced.³ (For patients enrolled in the study, the stage of keratoconus was categorized by the steep simulated keratometry, or sim-k, reading on computerized videokeratography.⁴) Of the 88% of GP lens wearers fit flat, those considered to have mild KC (steep sim-k values < 45.00D) were fit, on average, 1.18D flat. Those with moderate KC (steep sim-k values of 45.00D to 52.00D) were fit on average 2.38D flat, and those with severe KC (steep sim-k values > 52.00D) were fit on average 4.01D flat.^3,5

Though this analysis merely reflected the trend of lens fitters at the time, other reported outcomes seemed more influential in deciding which goal to strive for—a fit that yields apical clearance or apical touch. By the end of the CLEK study, eight years after it began, just 14% of those patients who were fit steep for apical clearance developed corneal scarring, vs. 34% of those fit flat for apical touch.6 Along with lens discomfort, flatter fits “were associated with an increased likelihood of PK.”⁷

The inference that a flat-fitting GP lens triggers the development of corneal scarring in KC was first made in 1982 by Donald R. Korb, O.D. and associates in a study involving seven patients.⁸ Though a causal relationship could not be ascertained from this study’s small sample size or the CLEK study’s observational design, many practitioners believe that a flat-fitting GP lens, in the presence of punctate epitheliopathy, contributes to the development of sub-epithelial scarring in keratoconus.^5,6 Flat-fitting lenses in the absence of corneal staining may provide better vision; however, flat fits are more often associated with staining. So, in an attempt to eliminate this, I fit advanced cones with custom piggyback systems or mini-scleral lenses designed for apical clearance. This method allows me to prolong patients’ ability to wear lenses without surgical intervention.

Now that a specific technique of fitting according to fluorescein pattern assessment has been established, the type of condition, location of the ectasia and stage of presentation in a particular patient must be identified.

Also, external ocular measurements need be taken to further refine the process of initial lens selection. In dealing with KC and other conditions associated with irregular astigmatism, many specialty GP lens manufacturers offer diagnostic lenses for fitting, so that doctors do not need to relay sim-k values and a manifest refraction for empirical lens design. But, when using diagnostic fitting sets, the key is to understand which lens design will best suit a particular condition, the location of the apex and other factors. Manufacturers offer a wealth of information on specialty lens designs, including fitting guides, photos of lenses on eyes and troubleshooting techniques.

The practitioner must learn to wade through the overwhelming amount of information available on the Web or from sales representatives to determine what techniques, information, lens materials and styles will be most beneficial to various patients. Once the doctor decides upon a basic strategy for fitting, this information can be used to develop a comprehensive management plan.

Corneal Lens Options by Ectasia
	Small GP	Large GP	Piggyback System	Hybrid
KC—Central	Yes; advanced	Yes; mild to moderate KC	Yes; small GP	Yes; SynergEyes KC, A, or ClearKone
KC—Inferior or Sagging	No	Yes	Yes; large GP	Yes; KC, ClearKone
KGB—Large	No	Yes	Yes; large GP	No
PMD—Inferior	No	Yes	Yes; large GP	Yes; A series

Your Fitting Toolbox
Corneal topography using the tangential map is an essential tool in differentiating the type of ectasia present, in highlighting the location of corneal apex and in staging the condition once a comprehensive slit lamp evaluation has been completed and a tentative diagnosis made. In this situation, the value of the sim-k is in staging. It provides a more scientific approach to lens selection in contact lens fitting. Four fitting categories make selecting a lens design much easier for common corneal thinning disorders:

• Keratoconus—central/nipple cone.
• Keratoconus—inferior or sagging cone.
• Keratoglobus—large/globus cone.
• Pellucid marginal degeneration—inferior.

These classifications help the practitioner decide between the fit of a “small” corneal GP lens (overall diameter, OAD < 9.9mm), a large corneal GP lens (OAD = 10.0mm to 12.0mm), a piggyback lens system, a hybrid lens, or a scleral lens—semi-scleral, mini-scleral and full scleral lenses are beyond the scope of this article. (See “Corneal Lens Options by Ectasia,” above.)

When only GP corneal lens fittings are being considered, the process may be further simplified by grouping the conditions into just two categories of interest: central or nipple cone vs. the other three categories combined. Why? While a large corneal lens performs well on many ectatic corneas, it does not perform as well on a central or nipple cone in advanced stages of keratoconus due to the large optic zone over the highly irregular surface—there tends to be excessive paracentral pooling. So, when dealing with a small central cone, it is best to consider the stage of the condition based on the steep sim-K readings prior to selecting a lens brand.

In mild KC (<45.00D), a standard aspheric lens with a 9.3mm to 9.9mm OAD is often successfully fit over the central cone, though a large-diameter keratoconic design with a large optic zone diameter (OZD) could be used as well.

In moderate KC (45.00D to 52.00D), large diameter keratoconic designs are recommended for comfort, though a smaller-diameter keratoconic design, such as the tricurve design used in the CLEK study, may yield a better looking fluorescein pattern, especially in cases where the steep sim-k is at the upper end of the range.⁵ 

In severe KC (>52.00D) small-diameter lenses are recommended for fit, but may be most comfortable and center best over a soft lens carrier with or without a center cut-out, or with a custom silicone hydrogel lens available in steeper-than-average base curves and variable overall diameters. (See “Corneal Lens Options for Central Cones,” below.)

Corneal Lens Options for Central Cones
KC—Central	Small GP	Large GP	Piggyback System	Hybrid
Mild	9.3mm - 9.9mm aspheric or KC design	10mm - 12mm aspheric or  KC design	Small/large GP over silicone hydrogel or daily disposable	SynergEyes A or KC Series
Moderate	8.6mm - 9.2mm KC design	10mm - 11.2mm KC design	Small/large GP over silicone hydrogel or daily disposable	SynergEyes KC series, ClearKone
Severe	< 8.0mm - 8.5mm KC design	--------------	Small GP over custom O2Optix or Flexlens with central cut-out.	SynergEyes KC Series, ClearKone

Fitting an Irregular Cornea
With a normal cornea especially, external ocular measurements are taken to help establish the OAD and OZD in the initial GP lens selection; these include horizontal visual iris diameter (HVID), palpebral fissure depth (PFD) or inter-palpebral aperture (IPA) and pupil diameter in dim lighting.

These dimensions are likewise important when you’re fitting a large-diameter GP lens for a mild central cone in KC or the other three categories previously defined.

When selecting a small-diameter GP lens for a moderate-to-severe central cone, however, the stage of the condition has a greater bearing on the OAD/OZD than the HVID, IPA and pupil diameter. Consider the lens diameter before choosing the initial diagnostic lens base curve for an irregular cornea, unless a fitting nomogram for a particular brand indicates otherwise. For a small GP lens, the starting base curve should match the steep sim-k radius of curvature reading or be 0.2mm steeper than the mean sim-k reading; for a large GP lens, the starting base curve should be 0.2mm flatter that the mean sim-k reading. (See “OAD and Base Curves by Ectasia,” below.)

OAD and Base Curves by Ectasia
Small GP: KC (central or nipple)	Large GP: KC (inferior or sagging); KGB; inferior PMD
OAD: Based on the stage of KC	OAD: 11.2mm when HVID = 12.0mm          10.8mm when HVID = 11.5mm          10.4mm when HVID = 11.0mm
Base Curve (mild to moderate KC): 0.2mm steeper than mean-K Base Curve (severe): Match steep sim-K reading in millimeters	Base Curve: 0.2mm flatter than mean-K

Once the first lens is trialed and the back surface fluorescein pattern assessed, the base curve should be further adjusted as needed. Sodium fluorescein strips wet with a non-viscous agent may be applied to the superior bulbar conjunctiva or inferior palpebral conjunctiva at posterior lid margin (Solutions with lubricating agents may dampen the fluorescence and should be avoided). Wait 30 to 60 seconds, then analyze the posterior apical, para-central, mid-peripheral and peripheral curve patterns. When the apical pattern appears relatively close to your fitting goal, 0.1mm-step changes in base curve are recommended until the best pattern is observed. When large changes are indicated, as in a case of extreme apical bearing or clearance, 0.3mm-step changes in base curve are suggested. Once the ideal apical pattern is achieved, if there is excessive paracentral pooling observed at the base of the cone, a lens with a slightly flatter base curve or smaller OZD should be considered for trial. Mid-peripheral bearing helps stabilize a lens on the eye, but when the area of bearing is very wide, a smaller OZD with a flatter peripheral curve system may reduce the amount of bearing and increase lens movement.

To increase the amount of fluorescein observed at the edge of the lens, a higher edge lift is needed. Generally, the request for flatter edges should be made by increasing the lift (in 0.2mm steps) or peripheral curve radii (in 0.5mm steps), unless otherwise indicated by the manufacturer. The reverse is done when the edge lift appears excessive, indicating the need for a steeper peripheral curve system. While acceptable movement of a hydrogel or large GP lens is typically 0.5mm in primary gaze, a small GP lens should have between 1.0mm to 2.0mm of movement to reduce the incidence of lens binding and poor tear exchange. Common methods for increasing lens movement include flattening the base or peripheral curve, widening the peripheral curve, reducing the OAD/OZD or a combination of the two.

You may observe an excellent fit and VA with a particular keratoconic GP lens, but if it is uncomfortable or does not allow for a reasonable wearing schedule, the fit is not successful. But, this situation may be easily rectified; add a soft lens to be worn under the GP lens, changing the modality to a piggyback system. The well-fitting GP lens may be reordered with a 0.2mm flatter base curve to rest over the flatter pre-corneal lens surface—Ks taken over a soft lens may be flatter, justifying the need for a flatter GP lens. The power of the new lens must take into consideration two factors: 

• The tear lens induced by the change in the GP base curve. (See “Tear Lens Adjustments,” below.)
• The power of the soft lens, unless it is available in plano.

Tear Lens Adjustments
Piggyback System	Starting GP Base Curve/Power	Over-Rx	Soft Lens	Final GP Base Curve/Power
Trial	6.5mm BC / -6.00DS back vertex power	-8.00DS	-0.50DS	6.3mm BC / -14.37DS
Adjustment	Assess apical touch pattern; need 0.2 steeper BC (1.62 tear lens: SAM, add -1.62DS). No change in PC or OAD needed.	Vertexed to cornea: -7.25DS	If minus power CL, add to Rx (+0.50DS)	Add -1.62DS tear lens to -6.00DS original trial lens power + vertex over-reaction less soft contact lens power
Final: -6.00DS trial lens + -1.62DS (tear lens compensation) + -7.25DS (vertexed over-refraction) + 0.50DS (soft lens power compensation) = -14.37DS back vertex power of GP lens
When ordering GP lenses in a base curve steeper than the trial lens, add minus (SAM); when ordering a GP lens in a base curve flatter than the trial lens, add plus (FAP). Remember to vertex the over-refraction when greater than +/- 4.00DS.

Evaluation and Troubleshooting
When evaluating a piggyback system, a high molecular-weight fluorescein must be used so as not to stain the soft lens in most cases. I recommend the liquid form rather than the strips. A daily disposable soft lens would be most convenient for a patient using both a GP and soft lens in each eye.

In the interim, for maximum oxygen potential until a high DK daily disposable lens is available, many eye care professionals prefer a two-week or monthly replacement silicone hydrogel carrier.^9,10 When mild to moderate inferior decentration occurs, as does with some inferior cones, I have found that a -0.50DS Proclear monthly replacement lens (CooperVision) may be more helpful than other brands in maintaining a better GP lens position.

If a traditional soft lens in a piggyback system is problematic because a steeper base curve or non-standard OAD is needed, O2Optix custom quarterly replacement silicone hydrogel lenses (CIBA Vision) may be ordered.

In patients with advanced inferior cones, when a small GP lens decenters in the eye and you don’t wish to proceed with a scleral lens, centration may be sustained by using a custom soft lens carrier with a central cut-out (e.g., Flexlens, X-Cel). The low Dk of the lens is a concern, but it may be the only option in some cases barring a scleral lens. Though the manufacturer recommends ordering a cut-out diameter that is 1.0mm larger than the GP diameter, I have seen a 0.7mm larger cut-out work best for some patients.

When a patient complains of significant GP lens awareness after a reasonable adaptation period, identifying the source of the sensitivity is essential for troubleshooting. When the lids are retracted during lens wear and symptomatic relief is experienced, there are four possibilities to improve patient comfort:

• Polish the edges of the lens.
• Steepen the peripheral curve system.
• Increase the lens diameter.
• Refit to a hybrid (the edges of the GP component are bonded to the hydrogel component and are usually less irritating).

When the lids are retracted and the symptoms remain until topical anesthetic is instilled, try a piggyback system; the soft carrier lens should act as a protective bandage lens.

When lens awareness persists in both situations, a mini-scleral lens designed to vault the cornea (e.g., msd mini-scleral lens, Blanchard Contact Lens) may be the most comfortable option for the patient.

The final step in the process after achieving a good fit with a diagnostic GP lens is calculating the back vertex lens power for best acuity. With a trial lens of known parameters, the most accurate and quickest method of determining which lens power to order is by performing retinoscopy and a subjective over-refraction while the patient is wearing the best-fitting trial lens. When the over-refraction is greater than +/-4.00DS, it should be vertexed to the corneal plane and added to the power of the trial lens before placing the order.

If the best-fitting trial lens is going to be ordered in a slightly steeper or flatter base curve, the tear lens compensation must be factored into the final lens power (see “Tear Lens Adjustments,” above). Fortunately, laboratory consultants are happy to help with these calculations, but the clinical data must be accurately collected and relayed to the technical support person.

Whether one is a novice contact lens fitter employed in a primary care practice or an experienced contact lens practitioner, there is no doubt that working with GP lenses for keratoconus is challenging. Once the diagnosis of a specific corneal ectasia has been made, having a strategic plan formulated, a diagnostic lens set to fit from, an informative website to refer to and an industry rep to consult with can make the overall process much less daunting.

Few rewards in optometry compare to returning the gift of sight to someone who has been robbed of it simply by the lack of proper vision correction, which is certainly within the scope of this profession to provide.

Dr. Janoff is an assistant professor of optometry and Chief, Cornea & Contact Lens at Nova Southeastern University College of Optometry in Fort Lauderdale, Fla. She was a co-investigator on the CLEK study.

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10. Jones L, Woods C. Compromises’ End? The introduction of a silicone hydrogel daily disposable lens. Available at: www.siliconehydrogels.org/editorials/jun_08.asp (Accessed December 2009).