Keratoconus: Diagnosis and Management

Introduction

Keratoconus is characterized by progressive thinning and steepening of the central cornea. It presents a challenge to the optometrist because early diagnosis is difficult, prognosis is unpredictable and progression is variable. Contact lenses can improve vision, but they can also scar the cornea. Keratoconus is a non-inflammatory, self-limiting ectasia of the axial portion of the cornea (1). As the cornea steepens and thins, the patient experiences a decrease in vision which can be mild or severe depending on the amount of corneal tissue affected. Typically, vision loss can be corrected early by spectacles; later, irregular astigmatism requires optical correction with rigid contact lenses (2) Contact lenses provide a uniform refracting surface and therefore improve vision. Patients should be informed upon diagnosis that they will likely require contact lenses eventually, and that the disease does not cause blindness. Although most patients can continue to read and drive, some feel quality of life is adversely affected. Patients need to know that eye examinations will be required annually or more frequently to monitor progression. About 20% of patients will eventually need a corneal transplant (3,4).

Keratoconus occurs in about .15% to .6% of the general population (5-9) Data on prevalence of keratoconus vary greatly. However, best estimates range from 50-230/100,000. Onset of keratoconus occurs during the teenage years--mean age of onset is age 16 years (7,10)-but onset has been reported to occur at ages as young as 6 years (1,11). Keratoconus rarely develops after age 30 years (10). Keratoconus shows no gender predilection and is bilateral in over 90% of cases (5,8,12). In general, the disease develops asymmetrically: diagnosis of the disease in the second eye lags about five years after diagnosis in the first (6). The disease process is active for about five to 10 years (1,6,8,9), then it may be stable for many years. During the active stage, change may be rapid; although unusual contact lenses may have to be refit as often as every three to four months. It has been reported that pregnancy and perhaps even menopause, may cause the disease process to become active (7).

Etiology

The proposed etiology of keratoconus includes biochemical and physical corneal tissue changes, but no one theory fully explains the clinical findings and associated ocular and non-ocular disorders. It is possible that keratoconus is an end result or final common pathway of many different clinical conditions. It has been found in association with hereditary predisposition, atopic disease, certain systemic disorders, and rigid contact lens wear.

Corneal tissue change - whether in the stroma or the epithelium and its basement membrane - has long been proposed as the cause of keratoconus. Teng (13) originally proposed the epithelium as the primary site of abnormal tissue because the earliest histopathologic change occurs there. Now many researchers believe that the primary site is the stroma and that tissue change occurs because of destruction of stromal tissue by collagenase (14-16). If research can identify the ultrastructural or biochemical abnormality causing keratoconus, it may then be possible to intervene in the disease process to halt or even reverse its expression.

Hereditary factors seem to play some role in the development of keratoconus (17,18) which may be inherited as a dominant or recessive trait (6,9). Hammerstein (18) found that in families in which one member had keratoconus, the incidence of keratoconus in other family members was 8%. Rabinowitz et al (17) showed a high prevalence of keratoconic, corneal topographic abnormalities in family members of keratoconus patients.

Atopic disease (eg, hay fever, atopic dermatitis, asthma) has also been suggested as an etiologic component of keratoconus. Rahi et al (19) found atopic disease in 35% of keratoconus patients compared with 12% of normal patients. Raised serum levels of immunoglobulin E, a known immunologic disturbance associated with atopic disease, was present in 47% to 52% of keratoconus patients (19,20). Ridley (10) noted that keratoconus patients are chronic eye rubbers, and he theorized that rubbing indents the cornea, increases intraocular pressure, and may make the cornea yield at its weakest point, the center (9,21,22). On the other hand, patients with atopic disease may rub their eyes because the condition causes itching. (20). Hormonal influence has been addressed as a possible cause, a supposition supported by the initial onset around puberty and tendency to progress during pregnancy or to exacerbate during menopause.

Systemic conditions linked to keratoconus include Down syndrome, Ehlers-Danlos syndrome (a connective tissue disorder in which the skin is loosely attached to the bones), Rieger's syndrome anomaly (posterior embryotoxon), Crouzon's syndrome (craniofacial dysostosis), and Marfan syndrome, according to Bennett.(1).

A final theory is that long-term wear of rigid contact lenses may trigger the onset in patients who are predisposed to the disease (22-26). Studies were retrospective and showed no direct cause and effect of wearing rigid contact lenses and the development of keratoconus. Most research documented PMMA lens wear; as yet no study of which we are aware has indicated any association between keratoconus and gas-permeable rigid, or soft contact lens wear. Macsai and coworkers (27) retrospectively reviewed keratoconus patients at the University of Iowa and found that patients who wore rigid contact lenses before diagnosis of keratoconus had less severe disease (ie, flatter corneas, more centered cone) and were older.

To date, no single, clear-cut cause of keratoconus has been found. The various proposed causes may interact: atopic disease may interact with rigid contact lens wear, ocular trauma may interact with decreased ocular rigidity, or, as Mandell (28) stated, keratoconus might be a part of an as yet unidentified systemic syndrome.

Diagnosis

Identifying moderate or advanced keratoconus is fairly easy. However, diagnosing keratoconus in its early stages is more difficult, requiring a thorough case history, a search for visual and refractive clues and the use of instrumentation. Often, keratoconus patients have had several spectacle prescriptions in a short period, and none has provided satisfactory vision correction. Refractions are often variable and inconsistent. Keratoconus patients often report monocular diplopia or polyopia and complain of distortion rather than blur at both distance and near vision. Some report halos around lights and photophobia.

Reduced visual acuity in one eye, due to the disease's asymmetry, may be a clue with the early keratoconus patient. This sign is often associated with oblique astigmatism. In early keratoconus, the patient may become less myopic six months later as the astigmatism increases.

Many objective signs are present in keratoconus. Retinoscopy shows a scissoring reflex (6,7,29). Direct ophthalmoscopy may show a shadow (Fig. 1). If the pupil is dilated and a +6.00 D lens is in the ophthalmoscopic system, the cone may appear as an oil or honey droplet when the red reflex is observed (1).

Figure 1

The keratometer also aids diagnosis. The initial keratometric sign of keratoconus is absence of parallelism and inclination of the mires. These can easily be missed in mild or early cases. As the cornea advances, the mires appear smaller. To extend the range of the keratometer, an ancillary lens is placed on the front of the keratometer . If a +1.25 D lens is used, this extends the range to 60 D. To record a reading, 8 D is added to the drum reading (for example, if the drum reads 45 D, adding 8 D yields an actual reading of 53 D). A +2.25 D lens extends the range to 68 D by adding 16 D to the reading. The Soper topogometer is useful especially when patients are directed to look up slightly. This enables an assessment of the inferior steepening that often accompanies this disease. The photokeratoscope or placido disc (Fig. 2) can provide an overview of the cornea and can show the relative steepness of any corneal area. Figure 3 depicts the keratoconic cornea. The even separation of the rings in the spherical and the astigmatic cornea and the uneven spacing of the rings--especially inferiorly--in the keratoconic cornea should be noted. The central rings may show a tear-drop configuration termed "keratokyphosis".

Figure 2

Figure 3

Preliminary studies by Maguire and Bourne (30) indicated that using computed corneal topography to identify subtle, inferior corneal steepening is highly sensitive for detecting early keratoconus. However, these devices are expensive and the reliability of such diagnostic topography has not been systematically evaluated clinically. For example, Mandell and Shie (31) have documented a videokeratographic value of 57.79 D for a keratoconus patient's decentered corneal apex in primary gaze. Upon changing the patient's fixation to center the keratoconic corneal apex, the value increased significantly. This example illustrates the limitations of currently available software for evaluating an irregular corneal surface. Psuedo-keratoconus topographic patterns have been found in normal patients with a displaced apex, contact lens wear, topography when there is poor patient fixation, and in thinning of the cornea in uveitis. Other non-inflammatory thinning disorders which resemble keratoconus are keratoglobus, pellucid marginal degeneration, and posterior keratoconus. These conditions may represent varied clinical presentations of the same underlying abnormality.

The biomicroscope is the only tool which allows a clinician to observe many classical signs of keratoconus: Fleischer's ring, stress lines of Vogt, corneal thinning and scarring, various types of staining with and without lens wear, increased visibility of corneal nerves, and corneal hydrops.

Fleischer's ring

The Fleischer ring is a yellow-brown to olive-green ring of pigment which may or may not completely surround the base of the cone (Fig. 4). Formed when hemosiderin (iron) pigment is deposited deep in the epithelium , Fleischer's ring often becomes thinner and more discrete with progression (6,7). A careful inspection of the keratoconic cornea will reveal a line in approximately 50% of all cases. Locating this ring initially may be made easier by using a cobalt filter and carefully focusing on the superior half of the cornea's epithelium. Once located, the ring should be viewed in white light to assess its extent.

Figure 4

Lines of Vogt

Lines of Vogt are small and brushlike lines, generally vertical but they can be oblique. These lines can be found in the deep layers of the keratoconic stroma (Fig. 5) and form along the meridian of greatest curvature (6); the lines disappear when gentle pressure is exerted on the globe through the lid. Lines of Vogt are more easily viewed when they reappear after this pressure is removed. Rigid lens wear sometimes accentuates the lines. In advanced cases of keratoconus, posterior corneal folds may also be present.

Figure 5

Corneal thinning

significant thinning (up to 1/5th cornea thickness) in the advanced stages of the disease (Fig. 6), and a diagnostic criterion based on comparison of central and peripheral corneal thickness has been proposed (32). Additionally, as the disease progresses, the cone is often displaced inferiorly (30,33). The steepest part of the cornea (apex) is generally the thinnest. Apical thinning described by Stallard is believed to represent an actual reduction in the number of stromal lamellae rather an overall thinning process. Polack believes that collagen lamellae are released from their attachments and slide, giving a thinning of the cornea (7). Harrison-Butler thinning represents a similar change to the peripheral cornea and should be monitored closely for any progression necessitating early surgical intervention. A larger graft may ultimately be needed; thus increasing the antigen load on the recipient bed.

Figure 6

Corneal scarring

Sub-epithelial corneal scarring, not generally seen early, may occur as keratoconus progresses because of ruptures in Bowman's membrane which is then filled with connective tissue (Fig. 7) (28). Deep opacity of the cornea are not uncommon in keratoconus. It has also been reported that flat-fitting contact lenses may produce or accelerate corneal scarring (34). A raised "callous" is possible but is easily treated by simple debridement or laser ablation. In addition apical scarring with an overlying epithelial defect and surrounding edema can be confused for ulcerative keratitis in this disease process.

Figure 7

Corneal nerves

Thickening of the corneal nerves makes them more visible in keratoconus. Prominent nerve fibers forming a network of gray lines with fine white dots are also sometimes seen.

Swirl staining

Swirl staining may occur in patients who have never worn contact lenses because basal epithelial cells drop out and the epithelium slides from the periphery as the cornea regenerates (35-37). Thus, a hurricane, vortex, or swirl stain may occur (Fig. 8). Swirl staining may be due to rubbing of the eye or can also result from flat- fitting contact lenses. When this is the case, the lens is generally too flat. A steeper lens often diminishes staining.

Figure 8

Hydrops

Corneal hydrops occurs, generally in advanced cases, when Descemet's membrane ruptures, aqueous flows into the cornea, and reseals (Fig. 9). Keratoconus patients who are having an acute episode of corneal hydrops report a sudden loss of vision and a visible white spot on the cornea. Corneal hydrops causes edema and opacification. As Descemet's regenerates, edema and opacification diminish (28,38). Occasionally, hydrops can benefit keratoconus patients who have extremely steep corneas. If the cornea scars, a flatter cornea often results, making it easier to fit with a contact lens. An increased incidence of hydrops has also been reported in keratoconus patients with Down's syndrome (39). Excessive rubbing should be discouraged in this population. Anecdotally, hydrops seemed to be more prevalent when scleral lenses were employed as a treatment option.

Figure 9

Munson's sign

Munson's sign is readily observable without using the slit lamp. This sign occurs in advanced keratoconus when the cornea protrudes enough to angulate the lower lid during inferior gaze (7). This sign may be missed,as they look down, if the patient has a markedly inferior cone.

Ruzutti's light reflex

A light reflex projected from the temporal side will be displaced beyond the nasal limbal sulcus when high astigmatism and steep curvatures are present. Although not a pathognomonic sign, Ruzutti's reflex may aid in a diagnosis especially when a biomicroscope or other tools to aid in diagnosis are not available.

Reduced intraocular pressure

A low intraocular pressure is generally found. This is a result of a thinner cornea and/ or reduced scleral rigidity. Due to possible artifact and since the reliability of readings are in question caution must be taken in carefully observing nerve fiber layers and the overall health of the optic nerve.

Classification

Keratoconus can be classified by cone shape (12,40), central keratometric reading (12), or progression (7,8,12). The simplest classification systems are based on keratometric reading or shape:

Based on severity of curvature

Mild <45 D in both meridians

Moderate 45-52 D in both meridians

Advanced >52 D in both meridians

Severe >62 D in both meridians

 

Based on shape of cone

Nipple small diameter (5 mm.); round shape; easiest to fit with contact lenses

Oval large diameter(>5 mm.); often displaced inferiorly; more difficult to fit with lenses, most common by topography

Globus largest diameter (>6 mm.); 75% of cornea affected; most difficult to fit with lenses

By using these two classification systems, most cones can be described by severity of curvature and shape (for example, an advanced, oval cone). Note: It might be helpful to view the cone either laterally or against a dilated red reflex noting also the shape and location of any Fleischer's ring.

Contact lens design

The goal of any contact lens is to provide adequate vision with maximum comfort over a prolonged period of time. With rare exception a rigid gas permeable material is the lens material of choice. Currently, there is some balance needed in choosing a material with high oxygen flux versus a material with greater durability and deposit resistance. A number of contact lens designs for fitting keratoconus have been suggested. Historically, how a rigid lens addressed the apex serves as a method in which to classify a particular fitting relationship or philosophy. The advantages and disadvantages of the major lens designs are discussed in the succeeding sections.

3-point touch design

The 3-point touch design is the most popular and most frequently advocated design used to fit contact lenses for keratoconus (41-42). The diameter of the 3- point touch lens is generally 7.8 to 8.5 mm. An optic zone size is usually at least 1.5-2.0 mm. smaller than the overall lens diameter. Multiple peripheral curves are needed and ultimately determined by fluorescein evaluation. The 3-point touch refers to the support provided for the lens by an area of central bearing and 2 other areas of bearing at the corneal midperiphery, usually in the horizontal meridian (1). The area of central bearing is about 2 to 3 mm in diameter (Fig. 10) (29,40-42). Generally, this design provides a reasonably balance between comfort and vision.

Figure 10

These three areas of bearing distribute the weight of the lens across the cornea and prevent lens rocking because of an excessively flat fit. This type of lens works well for a cornea with a centrally located cone. Care must be taken to avoid peripheral seal-off and lack of movement. Fluorescein must break midperipherally with each blink. Otherwise, a smaller optic zone should be chosen. In addition, apical bearing should not exceed 2 to 3 mm because increased bearing may cause punctate staining, corneal erosion and maybe apical scarring (34). In fitting cones of large diameter or cones which are greatly displaced inferiorly, the 3-point touch design using small lenses cannot be used because poor lens centration usually occurs.

Apical clearance

This design should result in a lens which vaults the cornea and clears the apex (Fig. 11). Korb et al (34) proposed that apical bearing may increase the rate of corneal scarring or abrasion and suggested that apical clearance designs traumatize the cornea less. Apical clearance lenses are small in diameter (8.0 mm) and have small optic zones (5.8 mm). The apical clearance method works well on cones which have central apexes or on displaced apexes which are only slightly inferior to the visual axis. This method, best for smaller cones, is impractical for large cones, such as a sagging oval cone or globus cone. A possible disadvantage is reduced visual acuity accompanying apical clearance and marginal comfort due to the primary support of the lens resting on the peripheral and midperipheral cornea. (7,43).

Figure 11

Large, flat lens

This apical bearing fittting philosophy is useful for displaced apexes (7,12). As keratoconus develops, the apex of the cornea is generally displaced inferiorly. If a small lens is placed on an inferiorly displaced apex, the lens is generally positioned low, and the lid often dislocates the lens with each blink. In such cases, a lens of larger diameter (9.0 to 9.8 mm) is preferable. The fitting method positions the upper edge of the lens under the upper lid to prevent lens dislocation (Fig. 12). The peripheral system must be flat enough--typically 1 to 2 mm flatter than standard lens designs--to permit lens movement. Often, a large lens is too steep peripherally and binds the peripheral cornea. In contrast, a very flat lens without secondary bearing often rocks and is uncomfortable. Large, flat lenses result in a larger area of bearing than that of lenses fitted by using the 3- point touch design and thus may be more prone to erosion or scarring (1,7,11,34). Several lens designs employing large diameter lenses have been described in the literature. One of the most often mentioned is the Bronstein philosophy which uses an alignment approach to the superior cornea.

Figure 12

Keratoconus lens systems

Numerous lens systems exist for fitting keratoconus patients. Often, these systems provide a cookbook approach and suggest that lens diameter be increased as the cone develops. However, we prefer lenses with apical clearance or minimal apical bearing in advanced keratoconus unless the apex is significantly decentered or large.

Aspheric

Aspheric lenses have been recommended for fitting keratoconus patients. Spherical lenses have a constant radius of curvature in the optic zone and different curvatures cut into the lens in the peripheral areas. However, aspheric lenses gradually flatten in curvature from the center to the periphery. The eccentricity, or "e value," determines the rate of flattening and is independent of the base curve. The "e value" of an average cornea is about .65. Decreasing the lens "e value" decreases the rate of flattening, and increasing the "e value" increases the rate of flattening. When fitting an aspheric lens, good centration is desirable because poorly centered lenses may induce astigmatism and reduce visual acuity. The fluorescein pattern should show central alignment or slight central bearing. The peripheral system should show clearance, and lens movement should be apparent. In theory, this approach is ideal; in practice, it may be less attractive. Problems occur in finding the correct "e value" for the cornea as well as in reproducing the lens. Examples of this type of lens are the VFL or Ellip-see-con lens.

Soper lens system

The objective of the Soper lens system, popularized by Soper and Jarrett (44), is based on sagittal depth. The principle is that a constant base curve with an increased diameter results in increased sagittal depth and a steeper lens. The lenses included in the fitting set are categorized as mild (7.5-mm diameter, 6.0-mm optic zone diameter), moderate (8.5-mm diameter, 7.0-mm optic zone diameter), and advanced (9.5-mm diameter, 8.0-mm optic zone diameter). The initial trial lens is selected on the basis of degree of advancement of the cone (45). The more advanced the cone, the larger the diameter of the recommended lens; the smaller and more centrally located the apex, the smaller the diameter of the lens. Soper (46) stated that, when properly manufactured, his lens is not a bicurve lens but incorporates the following curvatures: a 26-D curve (0.2 mm wide), initially generated by using a diamond tool; a 37-D curve (0.2 mm wide), generated by using a velveteen tool; and a 40-D curve (0.1 mm wide), generated by using a velveteen tool. When carefully manufactured, these curves all blend together. However, many laboratories inappropriately manufacture this lens as a bicurve lens with a peripheral system which may be too steep. The secondary curve, 7.5 mm (45 D), remains the same regardless of whether the base curve is 52 or 62 D. When central alignment is achieved, the periphery is often too tight, resulting in inadequate lens movement and tear exchange and the need to modify (flatten) the lens periphery and/or reduce the optic zone size with other lens compensatory changes.

McGuire lens system

The McGuire keratoconic system is a modification of the Soper lens design. In the McGuire system, fitting sets are categorized as nipple (8.1 mm diameter, 5.5 mm optic zone), oval (8.6 mm diameter, 6 mm optic zone), or globus (9.1 mm diameter, 6.5 mm optic zone) (47). The McGuire system has four peripheral curves; the three inner curves are each .3 mm wide, and the peripheral curve is .4 mm wide. From most central to most peripheral, the curves are 3 D (.5 mm), 9 D (1.5 mm), 17 D (3 mm), and 27 D (5 mm) flatter than the base curve. This lens system usually allows adequate edge clearance and movement.

NiCone lens

The NiCone lens system (available from Lancaster Contact Lens Co, Lancaster, PA) is promoted as having three base curves and one constant peripheral curve of 12.25 mm. NiCone fitting sets are designated by the numbers 1 to 3. The Number 1 cone set is for patients with keratometry readings between 40 and 52 D, the Number 2 set covers from 53 to 65 D, and the Number 3 set is for readings >65 D. The preferred lens alignment is feather touch (48). The second base curve is a .3-mm "transition zone" between the central base curve and the "third base curve," which rests on the normal peripheral cornea. Peripheral curves are added to improve lens performance. Large diameter lenses are often successful when employing this fitting design. A disadvantage of this system is that the values the manufacturer uses for the peripheral curvatures are unknown (due to patented design); not knowing these curvatures limits the practitioner's ability to modify the lens, making the practitioner dependent on the laboratory for fitting design changes.

Rose K design

The Rose K design is a unique keratoconus lens design. The system (26-lens set) allows the practitioner to choose lens options based on a systematic fitting approach. The design starts with a standard 8.7 mm. diameter that incorporates a decreasing optic zone as the base curve steepens coupled with an intrinsic, computer designed peripheral curve system. The lens is provided through Paragon Optics and is manufactured on a DAC lathe. Similar concerns raised with the NiCone lens in not knowing the peripheral curve system (proprietary) hold here as well. However, the Rose K system provides the practitioner some latitude in designing the peripheral lens system with the many options that are available. The standard lift lenses should work approximately 70% of the time. Additional lens diameters are available when needed (8.3, 9.0). Peripheral curves and even base curves can be configured in a toric design. Front surface cylinder using truncation for stability is an additional option.

Soft lens and hybrid combination lens alternatives

Soft lenses as well as combination lens alternatives have been advocated in fitting keratoconus patients.

Hydrogel lenses

Soft contact lenses are used to fit keratoconus patients who cannot tolerate rigid lenses or who have early disease (49-50). Soft lenses provide greater comfort. Lenses may partly correct irregular astigmatism and thus permit spectacle overcorrection. The disadvantages of soft lenses are reduced visual acuity and the need for spectacle overcorrection for best visual acuity.

Specialized soft lenses are another option. Because these lenses are extremely thick (.3-.5 mm) and low in water content, they sometimes slightly neutralize the irregular astigmatism of keratoconus almost as effectively as rigid lenses. This design is most often indicated for patients for whom a rigid lens cannot be fitted because of severe apical displacement, an extremely steep cone, or a cone of particularly large diameter. In such cases, the large diameter of the hydrogel lens (14-15 mm) allows it to bear on the sclera and the corneal apex, enabling the lens to be centered, which may not be possible with smaller, rigid lenses. One custom-made soft lens for keratoconus, manufactured by Flexlens, Inc. (Englewood, CO), is made of material with 45% water content and can be made in any power or curvature. When high-molecular-weight fluorescein is instilled in the eye of a patient wearing this lens, the fluorescein pattern under the lens is similar to that of a rigid lens. Lens movement is vital for a successful fit. Movement of this lens can be maximized by varying the secondary curve.

Another soft-cone lens is the Fre-Flex Cone Lens (Optech, Inc., Englewood, CO). This lens design is based on sagittal depth. The base curve (8.4 mm) and diameter (14 mm) of the lens are held constant, and the optic zone diameter (OZD) is varied to produce different fitting relations. The larger the OZD, the more tightly fitting the lens. These lenses, which are made of a 55% water content material, are manufactured in three sizes: Cone A (5.9 mm OZD), Cone B (6.9 mm OZD), or Cone C (7.9 mm OZD). The Cone A lens, although the flattest, is often successful in correcting even steep cones and should be tried before the steeper Cone B and C lenses. Disadvantages of both the Flexlens and Fre-Flex Cone lenses are corneal edema and neovascularization.

Piggyback lens designs

A piggyback lens system consists of a rigid lens fitted on top of a soft lens (51). Indications for this technique are rigid lens intolerance or mechanical problems such as recurrent corneal erosion. A respite is often essential in managing patients with recurrent epithelial breakdown. A combination of lenses often results in visual acuity equal to that achieved by using a rigid lens alone (52). Because the soft lens decreases the corneal curvature difference between the central and the peripheral cornea, the base curve of the rigid lens used with the soft lens is often flatter than the base curve of a rigid lens alone. Steeper rigid lens peripheral curves can be used to assure proper lens system movement. Only materials with high oxygen transmissibility should be used for both components.

Piggyback systems are usually used in difficult cases but are not the preferred lens system for keratoconus. Long-term care for two types of lenses is often difficult. Wearing time should be carefully monitored in these instances.

When fitting piggyback systems, the soft lens must move adequately. The rigid, gas-permeable lens should have minimal center and edge thickness to promote centration and comfort and should have a rounded edge to prevent tearing of the soft lens. Oxygen bubbles under the rigid lens help evaluate fit. Central bubbles indicate a steep lens; bubbles near the lens edge indicate a flat fit. Patients should use soft contact lens solutions so the soft lenses are not at risk of absorbing preservatives found in many rigid lens solutions, which could be toxic to the cornea in high concentrations.

Another type of piggyback system is the countersunk lens, available from Flexlens, Inc. (Englewood, CO) (Fig. 13) (53). It's a thick, soft lens, 14.5 to 15 mm in diameter, with a rigid lens fitted into a groove cut into the soft lens. The thickness of the rigid lens plus the tear reservoir should be equal to the depth of the groove. If the rigid lens is too thick, it protrudes from the groove and may be blinked out of the groove; if the rigid lens is too thin, the whole system may dislocate if the lid catches the groove during blinking. To permit tear exchange, the rigid lens should be made .1 to .2 mm smaller than the groove diameter. The rigid lens fit is evaluated by observing the location of any oxygen bubbles. The main objective is to center a rigid lens on a displaced corneal apex. Major disadvantages are cost; inconvenience of a two-lens system; corneal edema; neovascularization; and tearing of the soft lens at the groove junction.

Figure 13

SoftPerm lens

The SoftPerm lens (from WJ/PBH, San Diego, CA) is a hybrid lens with a rigid, gas-permeable center surrounded by a soft, hydrophilic skirt. This lens may be indicated for patients with displaced corneal apexes or for patients who cannot tolerate rigid lenses. Its advantages include its one-piece design, better centration on displaced apexes, and improved comfort compared to rigid, gas-permeable lenses. However, the SoftPerm lens has limitations. Most early keratoconus patients can be successfully fitted with rigid lenses. For patients with advanced keratoconus, in which a lens of larger diameter is useful, the lack of steep base curves in the SoftPerm lens (its steepest base curve is 6.5 mm) limits performance. In addition, the lens material has a low DK value (rigid lens, 14 DK; soft portion, 5.5 DK). The major problems are breakage (cost), difficult handling, lack of lens movement, induced corneal edema and neovascularization.

Fitting procedure

Information on the diagnosis of and fitting methods for keratoconus has been presented. A specific procedure to facilitate fitting or refitting the keratoconus patient follows.

Keratometry

Although keratometric readings may be of limited use in fitting keratoconus patients, they can assist in initial trial lens selection and in documenting disease progression. The keratometric range should be extended as necessary.

Refraction

Careful refraction is mandatory. Patients with keratoconus often have unpredictably good best-corrected visual acuity. High amounts of cylinder, frequently at an increasingly oblique axis, are sometimes found. Retinoscopic and keratometric results are an excellent starting point for determining subjective refraction. However, there is often poor correlation between spectacle and keratometric cylinder axes. This is actually a helpful differential between keratometric distortion due to rigid lens wear (where there is a much closer correlation) and keratoconus. Careful refinement of cylinder axis and avoidance of overminusing are necessary. Accurate refraction provides a baseline best visual acuity without contact lenses, which serves as a reference point for expected acuity after correction by contact lenses. The corrected acuity should be at least as good as that achieved with spectacles. Prescriptions are often based on subjective refraction results to provide either backup spectacles for patients who wear contact lenses or as the primary correction for patients who cannot wear contact lenses.

Trial lens fitting

Trial lens fitting is important for patients with keratoconus. No formula exists for predicting proper lens fit. The initial, trial lens should have a base curve which either splits the two keratometric readings or is slightly flatter than the mean corneal curvature. Choosing the steepest K as a lens starting point may help in establishing minimal clearance after lens equilibration. If the patient has never worn contact lenses, instilling a topical corneal anesthetic agent will reduce reflex tearing and help the practitioner to obtain more accurate retroactive endpoint and visual acuity measurement.

Fluorescein pattern analysis

The ideal keratoconic fluorescein pattern depends on the lens design chosen. The authors prefer to use a minimum apical clearance design whenever possible. The pattern should show slight fluorescein pooling in the center and a narrow band of touch in the intermediate curve area. In addition, the peripheral curve should be flat enough to allow a reservoir of tears to collect. This combination aids in tear interchange and movement. The 3-point touch fitting philosophy produces a similar pattern except for the central 2 to 3 mm of bearing.

To analyze the fit, the fluorescein pattern should be divided into two areas: the central portion (Fig. 14), including the entire area under the optic zone, and the peripheral zone (Fig. 15). Each area should be analyzed separately. The location and amount of bearing should be observed. If central bearing is found, the lens should be steepened if an apical clearance pattern is desired. If pooling in the periphery is absent, the peripheral system should be flatter. Insufficient lens movement and excessive bearing in the area of the secondary curves indicate the secondary curve is too steep and is preventing lens movement. Absence of fluorescein in the secondary or peripheral area can result when the lens seals off. A central air bubble may indicate that the lens is too steep. In contrast, paracentral air bubbles may indicate that the optic zone is too large.

Figure 14

 

Figure 15

Corneal apex position relative to the trial lens should be determined. In general, the lower the apex, the larger the lens diameter required for centration. A low-riding lens may be too flat or too small, requiring a larger lens. In attempting to fit a larger lens, a lid attachment fit is desirable; however, the peripheral system must be flat enough to prevent paracentral seal-off, resulting in decreased movement and tear exchange. If adequate centration cannot be achieved, a soft cone lens should be considered. Fitting such specialty lenses requires a trial set, and this lens should be regarded as a last resort.

Each trial lens should be allowed to settle on the eye for about 10-20 minutes before evaluation. Keratoconic corneas are very pliable, and if the patient previously wore flat lenses, the degree of corneal molding can be marked. Figures 16 and 17 depict the corneal change which occurs when a steeper lens is placed on the eye and allowed adequate time to settle. A flat pattern upon initially inserting a lens will only look flatter after equilibration. So, if a clearance fit is desired the flat lens can be removed even before equilibration takes place.

Figure 16

 

Figure 17

Trial lens overrefractions should be compared, and the total power of each system should be about equal. This comparison is important for evaluating the consistency of the diagnostic lenses.

Example:

Base Curve Power Overrefraction Net Power
50.00 -10.00 -3.00 -13.00
52.00 -11.00 -4.00 -15.00

 

Steepening the base curve by 2.00 D creates a fluid lens of +2.00 D. To compensate for this effect, an equal amount of minus power, that is, -2.00 D, must be added. Therefore, a lens with the parameters 50.00/-13.00 is theoretically equivalent to a 52.00/-15.00 lens. However, a direct 1:1 relationship between power and base curve change may not exist with exceedingly steep base curves.

Once adequate fit is achieved, a manufacturing laboratory must make the lens according to specifications. Some laboratories have a standard keratoconic lens design, but these lenses are often too steep peripherally. If the ordered lens looks different from the diagnostic trial lens, modification in the office or laboratory is necessary. Manufacturing a trial lens set according to specifications is important to insure a standard factor for comparison both during fitting and in checking ordered lenses. A gas-permeable material with medium oxygen transmissibility (range, 12-39) is optimal. This choice allows ample oxygen and is less prone to warping and dryness. Adequate center thickness should be ordered to minimize flexure.

Modification

In fitting keratoconus patients, lens modification is crucial. Returning the contact lens to the manufacturer for modification is often not practical. A patient who requires a lens with parameters of 50.00/-16.00 cannot see without the lens. Many modification units are available, but one with multiple spindles for polishing and cutting is the most efficient (Fig. 18). A more extensive selection of tools, especially for steep curvature, is necessary for modifying keratoconus lenses. For major modification, diamond-impregnated brass tools are recommended. The range in tool size provided should be 7.5 mm to 11.0 mm (45.00 D-30.75 D), progressing in 0.5-mm steps. To polish the rough cut, polishing laps with radii of 6.60 mm to 12.00 mm (51.00 D-27.00 D) are suggested. The difference in laps should be .15 mm at the steep end (6.60-7.50 mm), .5 mm to 1 mm at the flat end (9.0-12.0 mm), and 0.2 mm to 0.5 mm in the normal corneal curvature range (7.8-9.0 mm). These size ranges for diamond tools and polishing laps should accomplish most secondary and peripheral curve modifications. A polishing lap which is steeper than the diamond tool is often necessary to completely remove the tool marks.

Figure 18

Referral criteria

In 15% to 20% of the keratoconic population, a corneal transplant is eventually required (2,3). The patient should be referred for transplant if any of the following generally accepted referral criteria are met: 1) contact lens intolerance especially with recurrent abrasions; 2) inability to fit the patient with a contact lens (including frequent lens loss); 3) decreased vision (generally from scarring) which prevents the patient from doing necessary visual tasks - for some patients, this may occur at an acuity level of 20/60; for others, it may occur at 20/100; 4) a large cone with progressive thinning in the periphery (the larger the cone, the more difficult the surgery since because the donor button is sutured to the peripheral cornea); and 5) the danger of perforation. The latter is is extremely rare in keratoconus.

All patients should be informed that after a corneal transplant, the normal healing time required for visual rehabilitation is about 9 to 10 months, although visual correction can be prescribed as early as 3 months postoperatively in some cases when there is a running suture with buried knots.

Surgical alternatives

Various types of surgery are available for the patient with keratoconus. Penetrating keratoplasty is the most common. In this procedure, the keratoconic cornea is prepared by removing the central area of the cornea, and a full-thickness corneal button is sutured in its place (Fig. 19). Usually trephines between 8.0-8.5 mm are used. Fleischer's ring can be used as the limit of the conical cornea. Generally, the second eye is not grafted until the first eye is successfully rehabilitated. Depending on the criteria used to assess the success rate, this surgery is 90% to 95% successful (3,54-59). Most of these patients who are grafted for keratoconus are younger than the majority who are grafted for other reasons. Running sutures, using Merseline, anchored by cardinal sutures provide excellent results (clear, compact grafts). Older patients with a slower healing response and altered tear film generally do better with nylon and interrupted sutures for selective removal. Contact lenses are often required after this procedure for best visual correction.

Figure 19

An alternative is lamellar keratoplasty, a partial corneal transplant. The cornea is removed to the depth of posterior stroma, and the donor button is sutured in place. This technique is technically difficult, and visual acuity is inferior to that obtained after penetrating keratoplasty (59). As a result, use of lamellar keratoplasty is largely confined to the treatment of large cones or keratoglobus when tectonic support is needed(59). This technique requires less recovery time, and poses less chance for corneal graft rejection or failure(60). Its disadvantages include vascularization and haziness of the graft.

A rarely performed procedure in the United States, thermokeratoplasty involves placing a hot ring along the base of the cone to heat and traumatize the cornea, resulting in a corneal scar which reduces the corneal curvature, and allows a flatter contact lens to be fitted (61,62).

Epikeratoplasty is primarily suited for contact-lens-intolerant patients in whom scarring has not yet occurred (55,63). This is a rarely performed procedure today. Prospective epikeratoplasty patients must have visual acuity of at least 20/40 when wearing contact lenses (55,63-64). In this procedure, the central host epithelium is debrided, and the donor cornea is sewn over the keratoconic cornea. The donor button is regular in shape and provides the uniform surface necessary for good visual correction. Advantages of this procedure are its nonpenetrating nature; retention of mechanical integrity of the globe; and retention of normal host endothelium to avoid risking graft rejection (54,60). Occasionally, contact lenses may be required postoperatively for best visual correction (62,63).

Excimer procedures may have some potential merit, having been used recently with some success in removal of nodular "callous" plaques of the central cornea. New developments in excimer corneal modeling may allow lamellar onlay or penetrating grafts to be lathed, thereby eliminating refractive ametropia following various surgical procedures

Lenses following penetrating keratoplasty

A lens can be prescribed following restorative surgery like full-thickness grafting after adequate healing (generally at least six months). (64-66) Even rigid lenses can be placed on the eye for considerable wearing periods with running sutures in place. Remember to monitor closely for graft rejection or failure.

Intraocular pressures are to be monitored since many of these patients will be on topical steroids for some time and an increased intraocular pressure can be a sign of early inflammation suggestive of graft rejection. Grafts typically heal from the center outward and tend to be flatter in the central than the midperiphery, not unlike some refractive surgery corneas. Usually high oxygen flux rigid lenses predominate since they often provide stable acuity and an excellent physiologic response to wear. The contact lens practitioner's bane is significant astigmatism especially when found in an against-the-rule fashion. Rigid lenses will seek the steepest part of the cornea and hydroplane in a pattern of least resistance. Therefore, there is often significant rocking and nasal or temporal displacement with many grafted eyes fit with rigid lenses. Surgical reduction of astigmatism rather than fitting back surface/bitoric lenses is preferred since rarely is the astigmatism anything other than irregular and non-orthogonal. The topographic map provides the information needed to perform this in-office, slit lamp procedure (66). Such procedures are often successful on reducing high levels of astigmatism and may allow the patient to wear a contact lens or have some functional acuity with spectacles.

Graft topographies may include: nipple-like protrusions, proud grafts, sunken grafts, tilted grafts and eccentric grafts (65,66). All pose significant challenges to the contact lens practitioner and post-graft fitting is best left to the practitioner who wishes to specialize in this area. Lack of lens movement and bubbles around the graft margins are two problems that can be encountered (66). Smaller diameter lenses with flatter peripheral curves may aid in movement of the lens. Frequent lubrication is recommended. Soft lenses or hybrid systems are not contraindicated but must provide adequate oxygen to the front surface of the eye and move adequately to flush retro-lens debris. Otherwise, compromise may result in edema, neovascularization, opacity and scarring and even infection of the cornea. Significant amounts of edema have been found in patients wearing hybrid lenses. Reduced wearing times are helpful in insuring safe and effective lens wear following surgery.

Complications in fitting contact lenses following penetrating keratoplasty include: graft rejection, inadequate vision, corneal edema and scarring, lens intolerance and infection (66). With the advances in surgical procedures, namely adjustable suture procedures and mersilene suture material resulting in less corneal astigmatism, spectacle correction may be adequate in obviating the need for contact lens correction. Bandage lenses can be used for protection when there is impaired healing with ocular surface abnormalities, the surgeon is unwilling to resuture or remove sutures causing irritation, or when a small wound leak occurs in the post-operative phase (64, 65).

Summary

Keratoconus presents a great challenge to the optometrist. The treatment of this disorder ranges from spectacle correction to full-thickness grafting. This disease is generally best managed by appropriately fitted contact lenses. Contact lens management is often a compromise between the quest for an ideal fit and the patient's requirements for comfort and best vision. Information has been provided to allow the practitioner to diagnose keratoconus, to counsel the patient about this condition, to choose the right fitting philosophy, and to decide when surgical consultation is necessary.

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