You are not logged in.
Etiology, Diagnosis, and Management of Keratoconus: New Thoughts and New Understandings
- Types of Keratoconus
- RGP Fitting Approach
- Surgical Treatments for Corneal Ectasia
- Management of Corneal Problems that can Mimic Keratoconus
In 1931, Professor Alfred Vogt at the University of Zurich described in detail many of the classic biomicroscopic findings in keratoconus. Von der Heydt and Appelbaum classified the corneal changes into seven distinct types of tissue alterations. These changes may appear at varying times throughout the progression of keratoconus but may not be present in all cases.
When an eye with advanced keratoconus is viewed in optic section, the apex of the cornea can be reduced in thickness to one third that of the periphery. (Figure 23.) (Due to the excessive corneal curvature, it may be difficult to keep the entire section in exact focus at one time.)
In the later stages of keratoconus, this can lead to Munson’s sign in which an angular curve is present at the lower lid margin when the patient looks down. (Figure 24.) In the early stages of keratconus, apical thinning is often difficult to detect with the slit lamp. Other positive slit lamp findings often precede apical thinning and therefore may be more helpful in early diagnosis.
Fleischer's ring is a yellow-brown or olive-green pigmented line that partially or completely encircles the base of the cone. (Figure 25.) It is the result of deposition and collection of iron (haemosiderin) anterior to Bowman's layer in the adjacent epithelium. The broken or interrupted ring occurs in approximately 50 percent of keratoconus cases and the ring is often best viewed using blue slit lamp illumination. (Figure 26.)
Ruptures in Bowman’s Layer
These opacities form at or near the apex of the cone and represent structural breaks in Bowman's layer resulting in irregular superficial opacities and scars. (Figure 27.) The opacities begin as grayish dots located at the level of Bowman's layer. Later the spaces between the opacities become opaque and an irregular superficial opacity forms. (Figure 28.) These changes are caused by ruptures in Bowman’s, which are followed by filling in of the spaces with fibrillar connective tissue. In advanced cases, this can account for a considerable loss of visual acuity secondary to the induced irregular astigmatism and loss of corneal clarity. (Figure 29.)
Vertical striae are a series of parallel whitish lines seen in the deep stroma. They are most likely tension lines caused by apical stretching of the corneal lamellae. The striae are most often orientated vertically, but they can sometimes be aligned in the meridian of the greatest corneal curvature. (Figure 30.)
Most often the striae are seen in the region of the corneal apex before it becomes densely scarred. Crossing systems of striae may produce a lattice-like design. (Figure 31.) As a rule, the lines do not cross at the same level. Vertical or Vogt's striae can often be the earliest slit lamp finding noted in keratoconus. Some clinicians believe that the diagnosis of keratoconus cannot be made without the presence of vertical striae.
Increased Visibility of the Corneal Nerve Fibers
The corneal nerve fibers may become more visible in certain cases of keratoconus. The nerves can be seen as a network of grayish lines with corpuscle-like nodes at the point of branching. (Figure 32.)
It is not likely that the nerve fibers are more numerous in keratoconic patients, but only that they are more easily seen due to changes in density. Because a similar clinical picture is often seen in both normal corneas and in keratitis, an increased visibility of the corneal nerve fibers cannot be considered a singular diagnostic sign of keratoconus. (Figure 33.)
Ruptures in Descemet’s Membrane
Spontaneous ruptures of Descemet's membrane occur in approximately 5 percent of patients with keratoconus. The ruptures are characterized by a crescent-shaped tear in Descemet's and the endothelium near the apex of the cone. (Figure 34.)
Aqueous from the anterior chamber passes through the tear resulting in corneal edema and opacification (hydrops). (Figure 35.)
The extent of the opacification varies with rupture size. In most cases, the endothelium recovers and within days begins a slow but steady deturgescence of the corneal opacification. This process can take 3 to 4 weeks. (Figure 36.)
Following resolution of the hydrops, the rolled edges of the tear in Descemet's can be noted. Corneas that do not regain transparency may require keratoplasty surgery. (Figure 37.)
Endothelial Cup Reflex
This brilliant reflex is seen at the apex of the cone and accounts for the characteristic "dew-drop" or crystalline appearance of the cornea. The intensified reflective properties are related to the increased curvature of the posterior corneal surface that can appear as a convex mirror. (Figure 38.)
Throughout the years, the slit lamp has allowed practitioners to examine the multitude of corneal changes occurring in keratoconus. However, it is important to remember that different slit lamp findings may appear at various stages throughout the course of the disease. In addition, not all the classic slit lamp findings may be seen in every patient.
Modern video keratoscopy techniques have also furthered our knowledge of the many topographical changes occurring across the corneal surface but corneal mapping cannot replace the slit lamp in making the definitive diagnosis of keratoconus, especially in the condition's incipient stages. The topographical changes of inferior steepening and superior flattening are common findings in the normal non-keratoconic population and are simply related to corneal apex position. Therefore, we can state that keratoconus may be suggested by corneal mapping techniques, but the actual diagnosis can only be confirmed through positive slit lamp findings. These findings include vertical striae, ruptures in Bowman’s layer, Fleischer's ring, corneal thinning, ruptures in Descemet's membrane, and increased visibility of the corneal nerve fibers.
Spectacle Correction in Keratoconus
In the early stages of keratoconus, the patient's refractive error can often be successfully managed with spectacle lenses. It is important to communicate to the patient that there is no evidence to support the theory that early contact lens intervention is of therapeutic benefit in preventing or lessening the progression of the disease. However, wearing contact lenses typically provides the patient with better visual acuity than can be obtained with glasses by neutralizing the regular and irregular refractive errors induced by the condition.
As keratoconus progresses, spectacle lenses often fail to provide adequate visual acuity, especially at night. This can be further complicated by the fact that the patient's glasses prescription may change frequently and can be limited by the degree of myopia and astigmatism that must be corrected. Also, keratoconus is often asymmetric therefore full spectacle correction may be intolerable because of anisometropia and aniseikonia. However, despite these limitations, spectacles can often provide surprisingly good visual results in the early stages of the condition.
Contact Lens Designs for Early Keratoconus
The successful fitting of contact lenses for keratoconus requires that three objectives be met:
- The lens-to-cornea fitting relationship should create the least possible physical trauma to the cornea.
- The lens should provide stable visual acuity throughout the patients entire wearing schedule.
- The lens should provide all day wearing comfort.
Although it may be impossible to meet all of these objectives for every patient, we should remain focused on utilizing all of the modern lens designs and fitting techniques at our disposal to achieve the best possible outcomes.
Spherical Lens Designs
Traditional 3 to 4 curve spherical lens designs are generally used only in the early stages of keratoconus when there is minimal topographic difference between the central and mid-peripheral corneal topographies. These lenses can be designed to provide a superior alignment fitting relationship or a more intra-palpebral, three point touch relationship.
Superior Alignment Fitting Technique for Early Keratoconus
Superior alignment fitting lenses for keratoconus commonly consist of aspherical designs that incorporate a larger 9.5 mm overall diameter with an 8.3 mm posterior optical zone. These lenses are designed to provide a superior alignment fitting relationship across the more “normal” portion of the keratoconic cornea. (Figure 39.)
When fitting these lenses, the central keratometry (“K”) readings are of little value. Instead, knowledge of the more normal nasal temporal and superior mid-peripheral cornea (which lies beyond the area measured by the keratometer) becomes the most important fitting consideration.
For example, a patient with early keratoconus presents with topographical findings that demonstrate the characteristic inferior steepening and superior flattening. The central keratometric readings are 46.25 D/ 49.75 D. Inferiorly, the cornea steepens to 51.25 D and superiorly it flattens, to 42.00 D. (Figure 40.)
If a standard spherical contact lens is fitted on flat “K” (46.25 D) or steeper than “K,” the lens will be forced downward due to impingement of the steep curve on the more normal superior cornea. (Figure 41.) This would prevent the lens from freely traversing along the vertical meridian with each blink. It would also prevent the lens from achieving the desired superior resting position, which will inevitably result in superior compression of the epithelium and an inferiorly-fixed lens position. This compression is best noted by the broken lines of the keratoscopy mires, as well as by the broadening and flattening of the reflected rings. (Figure 42.)
Initially, patients may be comfortable with the steeply fitted, spherical lenses. However, with time they often experience increased lens awareness and a decrease in wearing time. Therefore, adequate superior clearance of a standard spherical lens design can only be accomplished by fitting the lens with a base curve radius flatter than that of the central cornea.
Clinical experience has shown that the initial diagnostic lens should have a base curve radius equal to the radius of curvature 4.0 mm to the temporal side of the cornea. This lens is placed on the cornea and evaluated with fluorescein. The ideal lens-to-cornea fitting relationship should be one in which the following are present:
- The base curve is flat enough to provide lens alignment across the flatter superior cornea.
- However, the base curve of the lens should be steep enough to provide slight touch mid peripherally at 3 and 9 o’clock along the horizontal meridian. This will prevent the lens from rocking and pivoting on the cornea. (Figure 43.)
- The lens will most likely exhibit slight bearing at the apex of the cornea and slight edge lift inferiorly across the steeper portion of the cornea.
It is clear that use of the superior alignment fitting technique is possible only in the early stages of keratoconus because as the central ectasia progresses, the superior alignment fit can result in greater localized apical bearing. This can lead to excessive lens rocking and instability, resulting in symptoms of lens awareness.
The Intra-Palpebral Three-Point Touch Fitting Technique for Early Keratoconus
Using the intra-palpebral or three-point touch fitting technique, a spherical lens design is selected that has an overall lens diameter of 8.0 to 8.5 mm with a posterior optical zone diameter of 6.4 to 6.9 mm. The initial diagnostic lens is selected with a base curve radius equal to the flat keratometric reading. This lens is placed on the cornea and evaluated with fluorescein. The ideal lens-to-cornea fitting relationship should be one in which the following are present:
- The base curve should be steep enough to provide a slight central touch, shown by thinning of fluorescein, at the corneal apex and slight touch mid-peripherally at 3 and 9 o’clock along the horizontal meridian. This creates three points of lens touch along the horizontal meridian. (Figure 44.)
- The lens will most likely position centrally or slight low on the cornea due to slight impingement of the lens across the flatter superior cornea.
Aspheric Lens Designs for Early Keratoconus
Often in keratoconus, the steepness of the corneal apex and the radical flattening of the mid-peripheral and peripheral cornea limit the effective use of spherical lens designs. Therefore, today many practitioners advocate fitting designs that incorporate aspheric radii. These designs accomplish three important fitting objectives:
- They allow the lens to be fitted with apical clearance without the periphery impinging on the flatter mid-peripheral cornea. (Figure 45.)
- The amount of posterior lens asphericity can be independently varied to allow the lens to clear the flatter mid-peripheral cornea.
- The highly aspheric posterior designs provide better alignment and weight distribution over a larger area of the cornea. This often provides improved lens centration and comfort.
Aspheric lens designs for keratoconus are available in three different configurations:
- Lenses with small spherical optical zones and aspheric peripheries.
- Lenses that incorporate two or more aspheric radii (bi-aspheric designs) with a low eccentricity in the center and a higher asphericity in the periphery.
- Lenses that incorporate a specific asphericity (total aspheric) throughout the posterior surface of the lens.
RGP Fitting Approach
The following is our step-by-step approach to fitting aspheric RGP lenses for early keratoconus.
The diagnostic lens fitting begins by performing corneal mapping and viewing the topography as an axial map display. The topography is quantitatively viewed to identify the size, shape, and location of the red zone (steepest area of the cornea) as well as the blue zone (flattest area of the cornea). The dioptric curvature of the corneal apex is also identified for reference. (Figure 46.)
A diagnostic lens is then selected with a base curve radius equal to the dioptric curvature at the corneal apex. The lens is placed on the eye and its position and relationship to the cornea evaluated with fluorescein.
An optimum lens-to-cornea fitting relationship is accomplish when three fitting criteria are present:
- The base curve radius should provide slight clearance across the corneal apex with no fixed, mid-peripheral bubbles.
- The mid-periphery of the lens should “land” (touch) the mid-peripheral cornea at 3 and 9 o’clock to prevent nasal/temporal lens decentration.
The lens should create minimal impingement across the flatter superior cornea. (Figure 47.)
The lower edge of the lens will often lift slightly away from the cornea due to inferior corneal steepening. Additionally, intermittent bubbles or frothing may be noted inferiorly. This is not only acceptable but may be essential to ensure both adequate superior alignment and vertical lens movement with the blink. Any attempt to decrease the inferior edge lift (i.e., steepening the base or peripheral lens design), may result in a tight superior lens-to-cornea fitting relationship. Furthermore, with a steep fitting relationship, the lens edge may cause further mechanical distortion of the cornea that may exacerbate the condition.
Having achieved the desired fit, a sphero-cylinder over-refraction is performed to determine the final contact lens power. A compensating anterior surface aspheric surface is often incorporated to neutralize the radial astigmatism and coma induced by the highly aspheric posterior lens surface.
The final lens is then ordered in a moderate to high Dk rigid gas permeable material. Each lens manufacturing laboratory will incorporate a slightly different aspheric central and/or peripheral lens design, so it is imperative that the diagnostic lens design match the final lens ordered.
|< < previous|