ECTATIC DISEASES

Staging Ectatic Diseases: A Therapeutic Approach

How to determine the severity of the anomaly so that the proper treatment regimen can be implemented.

By Robert L. Davis, OD, FAAO, & William L. Miller, OD, MS, PhD, FAAO

The classification of corneal ectatic conditions is important because the severity of the anomaly and the stage at which patients are diagnosed can provide guidance for the appropriate treatment regimen. Corneal ectatic disorders are non-inflammatory in nature and characterized by thinning of the central, paracentral, or peripheral cornea, causing structural instability. Normally, the central corneal thickness is 535 microns (Belin et al, 2012). In these anomalies, the reduced thickness can no longer support the corneal structure; the tissue begins to bend, causing the shape of the cornea to distort (Figure 1).

Figure 1. Keratoconus bend.

Theories for the exact biomechanical cause range between collagen lamellae slippage and collagen lamellae splitting (Bergmanson et al, 2014; Meek et al, 2005). Paraxial stromal thinning causes the protrusion of the anterior surface in anterior keratoconus. Posterior keratoconus is caused by thinning of the posterior surface of the cornea, whereas the curvature of the anterior surface remains normal.

When the majority of the stroma thins from limbus to limbus, the cornea takes on a globular shape defined as keratoglobus (Figure 2). The thinning in keratoglobus generally occurs at birth and is greatest at the periphery, but the corneal diameter is normal.

Figure 2. Keratoglobus.

In pellucid marginal degeneration, the stromal thinning is located in a peripheral band in the inferior stroma. Terrien’s marginal degeneration is a progressive thinning of the peripheral corneal stroma. Post-refractive ectasia is a complication of the surgery in which the corneal shape has changed due to the alteration of the corneal structure. According to the “Global Consensus of Keratoconus and Ectatic Disease,” ectatic disease included keratoconus, pellucid marginal degeneration, keratoglobus, and post-refractive disorders (Ambrósio et al, 2014). Terrien’s marginal degeneration, dellen, and inflammatory melts should not be classified as ectatic diseases but rather as thinning disorders. Keratoconus, keratoglobus, and pellucid marginal degeneration are differentiated by thinning location and pattern configuration (Ambrósio et al, 2014).

True corneal ectasia is a progressive degenerative condition characterized by a thinning and steepening of the cornea, resulting in increased corneal sagittal height, myopia, altered asphericity, and irregular astigmatism. The one exception to this definition is post-refractive ectatic conditions in which the corneal shape is irregular due to both variations in corneal sagittal height in multiple zones within the cornea and to irregular astigmatism.

The outcome of the corneal shape change is blurred vision that differs from refractive anomalies such as myopia, hyperopia, and astigmatism. The non-symmetrically shaped cornea causes monocular diplopia, blurred or distorted vision, and possible sensitivity to light and glare.

Due to the irregular shape of the cornea, when light hits the anterior surface, it refracts in a very unpredictable manner, causing visual distortion as the disease progresses. In later stages, vision is uncorrectable with spectacle lenses. A contact lens must neutralize the corneal irregularity by creating a new refractive surface. The regular refracting surface of a contact lens aims to create a distortion-free combination focusing the image onto the retina. This article will discuss the various classifications of corneal ectasia as it progresses through the various stages. It will also discuss the nonsurgical methods of ectasia correction.

The shape and size of the cornea in conical protrusion conditions has been classified into three types. The nipple cone is the smallest size (Figure 3). It is about 5mm and is usually either central or displaced inferior nasal. The second is the oval cone, which is a 5mm to 6mm ellipsoid usually positioned inferior-temporal. The third is the globus cone, which is the largest. It is greater than 6mm and covers more than 75% of the cornea.

Figure 3. Nipple Cone.

The contact lens fit in these cases must respect the diametric size of the protrusion. A successful clinical outcome depends on choosing the appropriate contact lens type; the contact lens design and optic zone will be selected based on the magnitude of the sagittal height (Caroline et al, 1978; Perry et al, 1980).

Staging Criteria

The simplest staging is based on analyzing the topographic image. It can aid in the early diagnosis of keratoconus, but it can also be misleading without the accompanying reduction in corneal thickness. Topographic images are classified into three groups: Group I – symmetric patterns (round, oval, and symmetric bow tie); Group II – all other asymmetric patterns except for asymmetric bow tie with skewed radical axes; and Group III – asymmetric bow tie with misaligned hemi-meridians (Rabinowitz et al, 1996; Li et al, 2004).

Other stages of keratoconus have been documented. Krumeich and Kezirian’s (2009) classification is divided into four stages. Stage one can include eccentric steepening-induced myopia and/or astigmatism of ≤5.00D, K reading of ≤48.00D, Vogt’s lines, and typical topography. Stage two can include induced myopia and/or astigmatism of >5.00D to ≤8.00D, K reading of ≤53.00D, and pachymetry of ≥400µm. Stage three can include induced myopia and/or astigmatism of >8.00 to ≤10.00D, K reading of >53.00D, and pachymetry of 200µm to 400µm. Stage four can include refraction that is not measurable, K reading of >53.00D, central scars, and pachymetry of ≤200µm.

John’s (2012) classification is based on corneal curvature and corneal thickness alone. Subclinical keratoconus indicates that the cornea is at risk for developing keratoconus over time and is based on topography. Mild keratoconus has a corneal curvature less than 45.00D, and the thinnest part of the cornea is 506µm. Moderate keratoconus has a corneal curvature between 45.00D and 52.00D, and the thinnest part of the cornea is 473µm. Advanced keratoconus has a corneal curvature between 52.00D and 65.00D, and the thinnest corneal thickness is 446µm. Severe keratoconus has a corneal curvature of >62.00D.

Defining the progression of ectatic conditions has been limited in the past by corneal curvature measurements and corneal thickness. With the advent of new diagnostic devices, such as anterior segment optical coherence tomography (AS-OCT) and Scheimpflug tomography, measuring the progression of corneal changes has provided useful information. Newer instruments use resident algorithms that measure specific data to provide a relative indication of the suspicion of keratoconus. Typically, after the diagnosis, practitioners can follow the progression of these anomalies by monitoring the corneal shape, which is performed at six-month intervals for two years and annually thereafter.

Recent information on the biomechanical attributes of the keratoconic cornea have indicated that certain parameters decrease as the severity of the keratoconus increases (Viswanathan et al, 2015). The Contact Lens Evaluation in Keratoconus (CLEK) Study developed the Keratoconus Severity Score (KSS), which included clinical signs such as Vogt’s striae, Fleischer rings (Figure 4), and corneal scarring as well as the mean corneal power and root mean square of the higher-order Zernike polynomials (McMahon et al, 2006).

Figure 4. Fleischer ring.

Subjective criteria such as the National Eye Institute’s Visual Function Questionnaire, comfort, and foreign body sensation are also reliable measures to differentiate between mild and severe cases of keratoconus, but they are less sensitive in distinguishing severity grades in the middle (Wu et al, 2015).

Mean keratometry dioptric powers are often used to distinguish between mild, moderate, and severe keratoconus. But other topographic parameters are also helpful in defining the severity and include flat/steep corneal powers, corneal asphericity quotient, and corneal thickness (Abu Ameerh et al, 2014).

Exact corneal thickness measurements that are useful in aiding the grading of severity include a decrease at the apex, pupil, and thinnest point of the cornea (Sahebjada et al, 2014). Others have also indicated that the thinnest corneal thickness, as well as central corneal thickness, may be helpful in distinguishing grades of keratoconus severity. However, they are less helpful in differentiating between normal and keratoconic corneas (Demir et al, 2013).

Other methods that have been used and correlated with the severity of keratoconus include parameters related to the anterior limiting lamina or Bowman’s layer. High-resolution OCT demonstrates that the average and minimum anterior limiting lamina, as well as an index that compares the inferior anterior limiting lamina minimum thickness to the mean superior anterior limiting lamina thickness, correlate well with the severity of the keratoconus (Abou Shousha et al, 2014). The central and cone apex corneal thickness should be monitored in grading the severity of keratoconus. In addition, the peripheral corneal thickness also should be assessed (Brautaset et al, 2013).

As mentioned previously, parameters specific to certain topographic and tomographic instruments also can be used to monitor the progression of keratoconus and, thus, grade its severity. These might include the indices of surface variance and height decentration (Kanellopoulos et al, 2013). Corneal volume as measured with a Scheimpflug tomographer significantly decreases between moderate and severe keratoconus (Mannion et al, 2011). This decrease is due to the decrease in corneal tissue as the disease progresses.

Staging of keratoconus using a Fourier domain OCT has proved useful in identifying stages of keratoconus (Sandali et al, 2013). However, this staging is more grossly accomplished and may not be as useful as other grading systems. It encompasses thinning in stage 1 to scarring and hydrops in stages 4 and 5. Elevation corneal maps also show changes in anterior and posterior elevation differences that can be correlated to the severity of keratoconus (Ishii et al, 2012). Table 1 provides a platform for grading the severity of ectasia.

Staging the rate of progression of ectatic diseases is important to accurately prescribe the proper treatment options. At the initial stages, soft lenses can adequately provide good vision. But, as the disease progresses, the corneal irregularity cannot be corrected by a standard soft lens, and a more rigid lens will need to be refit. Accompanying the progression of the irregularity, the cornea begins to thin, and corneal cross-linking may be appropriate. These patients should be monitored in six-month intervals to properly chart the progression of the disease.

Managing Ectasia

The management of ectasia is predicated on the progression of the disease. Essentially, ectasia is managed by refractive and therapeutic options. Refractive options are initially attempted with spectacles until they no longer can provide adequate vision. Contact lenses are the preferred method of correction for ectatic conditions to improve the visual outcome that spectacles cannot provide.

A variety of contact lenses—such as soft lenses, GP lenses (including both corneal and scleral lenses), hybrids, and piggyback lens systems—have been used to correct various stages of ectatic diseases. As ectatic diseases progress, ametropia and irregular astigmatism increase, which necessitates using contact lenses as the only viable refractive option.

Although optically corrective, contact lenses do not slow down the progression of keratoconus. Other refractive options, such as corneal collagen cross-linking (CXL), intracorneal ring segments (ICRSs), and intrastromal rings, strengthen the corneal structure and may arrest disease progression. The hallmark of therapy is to stabilize the shape of the eye and provide visual rehabilitation (Krachmer et al, 1984).

With many contact lens alternatives, what is the best approach for visual correction? The goals are no different than when prescribing contact lenses for patients who have normal corneas, which are to meet patients’ expectations for comfort, acuity, and maintenance.

Soft lenses and soft toric lenses have been used successfully in early keratoconus. With multiple lens choices, they are convenient, affordable, and comfortable when patients are in the emergent stage. However, soft lenses eventually will not provide an adequate visual outcome as the disease progresses. Instead, corneal and scleral GP lenses and hybrid lenses are necessary in later stages to neutralize the increasingly irregular corneal surface.

Recent advances in soft lens designs using lenticularization to increase lens thickness, combined with improved oxygen permeability in materials, have allowed practitioners to achieve the goal of neutralizing corneal irregularities in physiologically compatible materials. These lenses can provide the desired visual outcome in some cases, but they are not as comfortable and are more costly compared to disposable lenses.

On the other hand, GP lenses have been the gold standard and can provide patients with the desired visual outcome, but they can become uncomfortable especially as the tear film breaks down from the frictional force of the lens moving up and down a compromised cornea.

GP corneal lens designs are selected based on sagittal height and by taking into consideration the size and location of the protrusion. This is accomplished by sagittal height measurements, base curve radii, and corneal lens diameter. Fitting strategies employed for the management of corneal ectasia with traditional GP contact lenses have included apical clearance, three point touch, and apical bearing lens designs. Currently, the most common strategies are apical touch and three point touch; apical clearance is less commonly used, mostly as a result of poorer visual performance.

However, a modified fitting strategy involves the use of the first definite apical clearance lens (FDACL), which has provided an alternative approach for reaching a reliable and repeatable endpoint (Edrington et al, 1998; Romero-Jiménez, 2013). Many keratoconus GP lenses are specifically designed based on cone location and severity. For instance, one design is helpful in small-diameter ectasias that are displaced inferiorly, while another modifies the overall diameter and optic zone diameter based on whether the ectasia is a nipple, oval, or globus cone.

As the disease progresses, lens stability on the cornea becomes more difficult to control; intralimbal GP lenses may help improve both lens centration and visual outcome. When conventional lenses cannot be prescribed or cannot provide adequate vision, hybrid lenses and piggyback lenses can be used. These lenses consist of a combination of both rigid and soft materials. The advantage of these lens designs is that they provide comfort from the hydrophilic portion of the lens with the visual characteristics of a GP lens.

A carrier lens with a central groove can help improve centration of an unstable corneal GP lens. The GP lens rests inside the depression, which limits the movement and stabilizes centration.

As the cornea further thins in more advanced corneal ectatic conditions, the cornea no longer can support its structure; the tissue bends, requiring a contact lens design that will vault over the corneal surface to bypass the irregular shape. At this stage, it may become a technically difficult task to design contact lenses that provide acceptable visual acuity. Scleral and semi-scleral GP lenses vault over the distorted ectatic and fragile cornea and land on normal scleral (conjunctival) tissue.

As medically necessary contact lens have become an important option for these patients, it is prudent to document that a series of contact lens fitting attempts have been made, either in the past or present, demonstrating that simpler approaches have not satisfied patient expectations or that the corneal/contact lens relationship is not optimally achieved.

Contact Lens Complications

Complications when prescribing contact lenses for ectatic conditions are similar to those with normal contact lens wear, although these complications are exaggerated due to the compromised cornea and the stress of the contact lens upon the cornea.

An adequate post-lens tear film is important for all contact lens wearers. Abnormal corneal surface topography develops poor tear circulation. Tear film disturbances may be related to the progression of the disease or to the stress from wearing contact lenses for many hours. A thinned post-lens tear film layer resulting from that friction from long hours of wear often occurs in these patients. This ocular surface disease in ectatic diseases is characterized by poor tear quality, squamous metaplasia, and goblet cell loss (Shneor et al, 2013).

When fitting a contact lens on these corneas, care must be taken to either fortify or protect the tear film layer. With mostly corneal GP contact lenses, corneal molding due to contact lens compression—especially with the blink—creates additional stress upon the cornea, resulting in punctate staining from the corneal epithelium sloughing. In extreme cases, corneal abrasions will develop, which introduces the possibility of other complications including infection, corneal scarring, hydrops, and uveitic anomalies.

Eye rubbing has become one of the characteristic symptoms of corneal ectasia. This mechanical trauma is a direct insult to the corneal tissue, increasing the disease progression. Eye rubbing has been investigated as a risk factor for progressing corneal ectatic diseases ranging from 40% to 73% (McMonnies and Boneham, 2003; Karseras and Ruben, 1976; Gordon-Shaag et al, 2012; Georgiou et al, 2004). Although, the CLEK study (Zadnik et al, 1998) cited that 46% did not report any eye rubbing, which shows that the cause of the disease is multifactorial. Typically, antihistamine therapy is prescribed for these cases, limiting the histaminic effect of eye rubbing (Hawkes and Nanavaty, 2015).

Contact Lens Choices

In a retrospective study of 244 keratoconus patients, Shneor et al (2013) discovered that 78.3% were wearing contact lenses. This was confirmed by the CLEK study (74%) (Zadnik et al, 1998) and Owens and Gamble (2003) (80%). Patients wearing soft lenses were reported by Shneor et al at 13%, CLEK at 3.5%, and Owens and Gamble at 7.1%. GP lenses were reported by Shneor et al at 67.7%, CLEK at 92%, and Owens and Gamble at 83%. The use of piggyback lenses was 7.3%, and the use of hybrids was 4.8% (Zadnik et al, 1998). The use of scleral lenses in Shneor et al’s survey was 4.2%.

It is important to remember that these numbers reflect the lens designs available when these studies were conducted. We suspect that the percentage of scleral lenses fitted for ectasia has likely increased since these studies were published based on the number of designs available at the time when the surveys were conducted (Shneor et al, 2013).

Patients who have progressive corneal ectatic diseases can only function adequately while wearing contact lenses. Without their contact lenses, they lose mobility and are unable to perform the simplest of everyday tasks. It is the responsibility of eyecare professionals to supply visual correction devices to allow patients to perform normal, routine tasks.

One of the most underutilized devices is the piggyback alternative. This can be used with GP, hybrid, and even scleral lenses. The soft lens acts as a barrier, protecting the fragile cornea from additional insult created by friction. It is not necessary for this type of piggyback system to be worn everyday, but it can be employed when discomfort and corneal insult prevent patients from wearing normal corrective lenses; this allows patients to continue with normal routines. One potential drawback is the increased number of lens care solutions necessary to clean, disinfect, and store the lenses.

Table 2 presents a suggested list of management options. This is only a suggested list of choices based on the severity of the ectatic condition. Other factors that may play into the management option choice can include patient age, how rapidly the ectasia is progressing, symptoms with previous modality, ocular surface condition, resultant visual acuity, and cost.

Managing these patients who have ectatic disease requires more than just the ability to prescribe and fit these different lens designs. We must ensure that the visual outcomes allow patients to perform adequately in their everyday existence. While the expense of these therapeutic devices can become a financial burden, without them patients can no longer see.

Becoming an advocate with your patients’ third-party benefit plans is an important ingredient to patient management. Documenting the reason for the lens selection is a recent requirement to assure that our patients can receive treatments that are needed and available.

In summary, you should take a pragmatic approach to prescribing contact lens designs for a changing corneal structure in corneal ectatic conditions that follow the various stages in this disease process (Alio et al, 2012). CLS

For references, please visit www.clspectrum.com/references and click on document #241.

Dr. Davis is a co-founder of EyeVis Eye and Vision Research Institute and practices in Oak Lawn, IL. He is an adjunct faculty member at Southern California College of Optometry, Illinois College of Optometry, Pennsylvania College of Optometry at Salus University, and University of Alabama at Birmingham. He is a Diplomate in the Cornea, Contact Lenses and Refractive Technologies section of the American Academy of Optometry as well as an inductee in the National Academies of Practice in Optometry. He has received research funds from SynergEyes, CooperVision, and B+L and has a proprietary interest in SpecialEyes, Alternative Vision Solutions, and in the Recess Pillow Lens System.

Dr. Miller is an associate dean for academic affairs and professor at the Rosenberg School of Optometry, University of the Incarnate Word. He is a consultant or advisor to Alcon and Oasis Medical and has received research funding from CooperVision, Contamac, and SynergEyes and lecture or authorship honoraria from Alcon. You can reach him at wlmiller@uiwtx.edu.