Keratoconus
Edward S. Bennett
Joseph T. Barr
Loretta Szczotka-Flynn
Keratoconus is a progressive, asymmetric disease of the cornea that is characterized by steepening and distortion, apical thinning, and corneal ectasia.1,2 The cause of keratoconus is unknown, although it is probably a genetic disease. The management of keratoconus is most often in the form of rigid gas permeable (GP) contact lenses; however, more severe cases require penetrating keratoplasty.
The Collaborative Longitudinal Evaluation of Keratoconus (CLEK) Observational Study was the largest multicenter, prospective, observational study designed to describe the course of keratoconus and the associations among its visual and physiologic manifestations. The CLEK study characterized the course of keratoconus and identified factors related to vision, progression, and corneal scarring in keratoconus. A total of 1,209 patients were enrolled between 1995 and 1996, at 15 participating clinics, and patients were re-examined annually for 8 years, through mid-2004. The results of the CLEK study will be referenced throughout this chapter.
CHARACTERISTICSKeratoconus is bilateral in approximately 96% of the cases.3 Typically, one eye is diagnosed earlier and progresses more than the fellow eye. It is often diagnosed in the early teens to early 20s.4,5 There does not appear to be a significant difference in the incidence of keratoconus between left and right eyes nor between men and women.
Epidemiology
The prevalence of keratoconus is stated to be 1 per 2,000 persons. With 270 million Americans, this translates to 135,000 Americans with the diagnosis of keratoconus. However, the only estimate of the incidence and prevalence of keratoconus comes from a study conducted in Olmsted County in Minnesota, which identified 64 incident cases of keratoconus between 1935 and 1982 at Mayo Clinic.6 Case ascertainment was based solely on medical record review with the diagnosis determined by the “examiner’s description of characteristic irregular light reflexes observed during ophthalmoscopy or retinoscopy, or irregular mires detected at keratometry.” Kennedy et al. estimated the overall average annual incidence rate at 2 per 100,000 population with a prevalence of 55 per 100,000. These estimates are most likely low given current diagnostic techniques such as corneal topography. Other groups have reported the prevalence as high as 86 per 100,000 in Denmark7 and incidence as high as 25 per 100,000 in populations that have traditions of consanguineous marriages.8 The higher incidence was suggestive of a genetic factor being significant in the etiology.
ETIOLOGYThe cause of keratoconus is unknown, although metabolic/chemical changes in the corneal tissue have been documented. However, the disease has been associated with atopy,9,10,11 connective tissue disorders,12,13,14,15,16 eye rubbing,17,18 contact lens wear,19,20,21,22 and inheritance.23
Histologic Changes
It is still unclear what causes the corneal changes that occur in keratoconus. A triad of classic histopathologic changes have been observed1:
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Thinning of the corneal stroma
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Breaks in the Bowman layer
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Iron deposition in the basal layers of the corneal epithelium
Apparently, degeneration of the epithelial cells is followed by breaks in the Bowman layer. Specifically, fragmentation of the Bowman layer—possibly caused by degradative enzymes and reduced inhibitors in the epithelium—may represent an early change in keratoconus.24,25 Abnormal enzymes in the corneal epithelium lead to excess collagenase and reduced protease inhibitors in the stroma, most likely resulting in keratocyte death.26 This increased collagenase activation may slowly break down stromal collagen, resulting in stromal thinning.27 Biochemical studies have shown the total amount of protein in the cornea to be decreased.28 The pathology of collagen fibrils has shown abnormally low numbers of collagen lamellae.29 It has been suggested that the lamellae are released from their interlamellar attachments or the Bowman layer and become free to slide, resulting in thinning without collagenolysis.30
Atopic Relationship
There appears to be a relationship between atopic conditions (i.e., eczema, hay fever, asthma) and keratoconus, as approximately 50% of keratoconic patients have some form of atopy.1,3,31,32 As itching is a primary symptom in atopic conditions, it has also been found that most keratoconic patients are vigorous eye rubbers, although a cause-and-effect relationship has not been proven.17,18,33
Atopy and histologic changes are linked together via the “Cascade Hypothesis” from Kenney et al.34,35,36 According to this theory, young patients have a propensity to produce high levels of reactive oxygen species (ROS, or free radicals) in the cornea. These free radicals are typically cleared by superoxide dismutase, ALDH3, and other enzymes that prevent accumulation of these potentially harmful substances. Some individuals are unable to produce protective healthy enzymes; therefore, free radicals accumulate, causing damage to the structural integrity of the cornea. This results in a cascade of events in which a compromised cornea weakens, thins, and becomes steeper in curvature. As exposure to ultraviolet B light generates free radicals, it is important for the appropriate eyewear protection to be used. Likewise, mechanical stress such as that caused by a poorly fitted contact lens or via eye rubbing can exaggerate ROS formation. If this is the case, these individuals need to avoid eye rubbing with the appropriate treatment of allergies and atopic conditions also advised. Antioxidative therapy, like those currently used in retinal treatments, could be a possible future option.36
Heredity
Several studies have suggested that keratoconus is genetic,37,38 with reports from single families and twin studies also reported in earlier literature. Between 6% and 16% of patients with keratoconus have a history of familial disease.32,39,40,41 Because the severity of the disease varies from asymptomatic forme fruste conditions to disabling scarring requiring corneal transplantation, the true incidence, prevalence, and familial aggregation is difficult to ascertain. Specifically, in asymptomatic conditions, keratoconus may only be detected using sophisticated instrumentation such as videokeratography. Some authors have shown that intermediate traits, as measured by videokeratography, are highly inheritable. Keratoconus has shown association with rare genetic syndromes, such as Woodhouse-Sakati syndrome42 and Down syndrome,43 further supporting the genetic hypothesis.
Contact Lenses
It has been suggested, in many reports, that rigid contact lenses may cause keratoconus because of such factors as mechanical pressure and hypoxia.20,44,45,46 In one study, 27% of the keratoconic patients were contact lens wearers, all of whom were not diagnosed with keratoconus at the time of fitting.46 It was found that these individuals were older at the time of diagnosis, had central (versus decentered) cones, and had a tendency toward flatter corneal curvatures. However, once again, it is difficult to establish a cause-and-effect relationship, as the incidence of keratoconus would be expected to be higher with contact lens wearers.4 This is the age in which keratoconus typically is diagnosed, and these individuals may be corrected for their myopia before being diagnosed with keratoconus. A case report of one of the authors supports a hypoxic etiology. A long-term rigid lens wearer lost one of her gas permeable lenses and went back to wearing an older polymethylmethacrylate (PMMA) spare lens for 8 years. Keratoconus developed in the eye wearing the PMMA lens, whereas the eye wearing the gas permeable lens was unaffected.47
DIAGNOSISThe earlier a patient can be diagnosed as having or possibly having keratoconus, the sooner the practitioner can institute the appropriate management and adequately educate the patient. For the condition to be diagnosed, the practitioner must be aware of the symptoms and clinical signs of keratoconus and encourage frequent follow-up care for monitoring progression.
Early Symptoms and Clinical Signs
The diagnosis begins with a thorough case history. The practitioner must ask the right questions and be a good listener, as the symptoms of incipient keratoconus are varied and frequently confused with psychogenic complaints. Quite often the patient has difficulty explaining the actual problem. Monocular diplopia or “ghost” images and blurring of images are common symptoms, but practitioners may fail to ask questions eliciting a description of image blur.48 Vision may not actually be blurred but distorted; letters may be confused, and parts of letters may be missing or altered. Therefore, the practitioner should ask patients whether they are experiencing this distortion out of one eye only. In addition, if this condition had been previously undiagnosed, the patient may own several pairs of glasses, none of which is satisfactory. Finally, asthenopic complaints, polyopia, photophobia, and halos around lights may be reported.
A gradual decrease in visual acuity is often the first clinical sign of keratoconus. The vision of one eye will be affected before the other, and the blur will be present for both near and far distances. The patient may be able to see better by squinting or by holding printed material very close to the eyes. In addition, because of the conical distortion, an early clinical sign of keratoconus is a “scissors-like” motion observed during retinoscopy. In the early stages, manifest refraction will often result in satisfactory visual acuity; however, an increase in both myopia and regular or irregular astigmatism may be found. An absence of parallelism of keratometry mires and a localized region of corneal steepening is often observed with videokeratography (to be discussed).
Corneal Topography Change
Keratometry
Keratometry is beneficial in the diagnosis and monitoring of keratoconus. However, the limitations of keratometry, including the measurement of only a few paracentral points on the cornea, can make early diagnosis difficult, especially if the patient has a decentered apex of the cone.
What is often observed in early keratoconus is a lack of parallelism of the keratometric mires. The mires are somewhat distorted, and one may not completely overlap the other. The corneal astigmatism may increase and shift toward an oblique axis. A steepening of the corneal curvature may be observed. If the condition is incipient, the keratometry readings may be in the 43 to 48 D range. Once the condition advances beyond the curvature values of the keratometer, a +1.25 D ophthalmic lens can be mounted over the objective (patient end) to add approximately 8 D to the keratometer reading (Appendix 1).
Videokeratography
The use of videokeratography (VKG) has been an outstanding tool for the diagnosis and monitoring of patients with keratoconus. The location of the apex and the progression of the condition can be observed via evaluation of the color map. The localized area of steepening can easily be observed with a color map (Fig. 18.1).
Although definitive keratoconus cannot be diagnosed in the absence of several slit-lamp or retinoscopic findings, a keratoconus suspect can be determined quite easily via the appearance of the color map. These individuals can then be monitored to determine if further progression and diagnosis of the condition occurs. In addition, a compression of the mires will be present in the affected region via the reflection of the photokeratoscopic rings. Also, as the unaffected corneal region (typically superior) will not change, an inferior-superior dioptric asymmetry will be present.1
The impacted region or “cone” has traditionally been categorized as “nipple,” oval (i.e., sagging), or globus.49,50,51 The nipple cone is smaller and more centralized than the other two types. The oval cone is more inferior, whereas the globus cone is quite large in diameter. McMahon52 reported on results from the CLEK study in which the impacted areas were divided into nipple (Fig. 18.2A), oval (Fig. 18.2B), globoid (Fig. 18.2C), and marginal (Fig. 18.1), with the latter group pertaining to a nonround or nonoval cone located in the periphery of the cornea (often inferior).
As shown in Table 18.1, most of the cones can be described as nipple or oval. It was also reported that 12.2% have an apex above horizontal with an average location at 262 degrees (inferior-temporal).53
Several VKG systems have developed applications for the screening and diagnosis of keratoconus.48,54,55,56,57 This is particularly important, as the presence of keratoconus is a contraindication for refractive surgery and it has been found that as many as 5% to 7% of refractive surgery candidates have subclinical keratoconus.58,59
FIGURE 18.1 Representative color map of a keratoconic patient. |
FIGURE 18.2 A nipple cone is shown in (A), an oval or sagging cone in (B), and a globus or globoid cone in (C). |
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TABLE 18.1 CONE TYPES AND PREVALENCE | ||||||||||||||||
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In research clinics, keratoconus has been defined using topographic indices only. VKG readings and differences in corneal shape have been used to diagnose keratoconus. Mandell et al.,4,60 with the cone apex aligned with the optical system of a VKG, determined that a true apex power reading can be obtained and therefore compared to the normal range in the detection of keratoconus. It was concluded that if the cone apex power is 48 to 49 D, the patient should be considered a keratoconus suspect. For powers of 49 to 50 D, there is a very high likelihood of keratoconus, and for powers above 50 D, the diagnosis is almost certain. The modified Rabinowitz-McDonnell method1,54,61 uses the following guidelines: if the central corneal power is >47.2 D or if the difference between the inferior and superior paracentral corneal regions (i.e., I-S value) is >1.4 D, then the cornea is considered keratoconus suspect. If the central corneal power is >48.7 D or the I-S value is >1.4 D, then the cornea is classified as keratoconus. Rabinowitz and Rasheed have also developed an index, the KISA%, to grade the presence or absence of keratoconus.62 Although this index has the potential to define disease severity, the developers have only described its role in defining normals, keratoconus suspects, and those with the disease. The KISA index has been used to monitor changes in normal eyes of unilateral keratoconus patients63 and genetic screening where KISA was used to distinguish keratoconus from normal individuals.38
The difficulty in using topographic-only methods to define keratoconus is that early forme fruste conditions—which may never progress—can be labeled as keratoconus with unnecessary worry imposed on the patient. Although topographic evidence of keratoconus should be a strong reason to avoid ablative refractive surgery, otherwise asymptomatic patients may not need any special therapeutic treatment other than monitoring. For that reason, a definitive diagnosis of keratoconus is usually made clinically by requiring either a slit-lamp sign of the disease (as discussed below), refractive and visual acuity changes suggestive of keratoconus, or distortion of the anterior cornea as measured by the red reflex.
Slit-Lamp Biomicroscopic Signs
Biomicroscopy is essential for the diagnosis of keratoconus. Only with the biomicroscope can the observer detect the subtle changes occurring within the cornea. The hallmark clinical signs of keratoconus, which are present in the majority of clinically confirmed cases, include Vogt striae, Fleischer ring, and scarring. Vogt striae are a series of vertical or oblique lines located in the posterior stroma or Descemet membrane (Fig. 18.3). They are most likely the result of the stretching of the corneal lamellae. They temporarily disappear when transient pressure is applied to the globe through the upper lid.64 The Fleischer ring can be observed in approximately 50% of all diagnosed cases of keratoconus.65 It is a yellow-brown to olive-green discoloration appearing in a broken or interrupted ring encircling the base of the cone (Fig. 18.4). It appears to outline the base of the cone and represents hemosiderin deposits in the deep epithelium near the Bowman membrane. Irregular superficial scars can form at the apex of the cone as the condition progresses. They begin as discrete dots in the Bowman membrane; fibrillar connective tissue invades the space between the opacities, and they proceed to increase and become opaque66,67 (Fig. 18.5).
FIGURE 18.3 Vogt striae. |
Thinning of the cornea can be observed at the region of the cone via an optic section. Increased visibility of the nerve fibers can be observed at the corneo-scleral junction as a result of a change in their density. In severe cases, corneal hydrops can occur secondary to a rupture in the Descemet membrane, allowing aqueous humor from the anterior chamber to flow through the damaged endothelium, causing corneal edema and eventually scarring.
External findings include the Munson sign and Rizzuti phenomenon. In advanced cases, the shape of the lid will be altered because of the protrusion of the cone, and the Munson sign will be present. This can be confirmed by having the patient look down until the lower lid is at the equator of the cone. According to Rizzuti,68 when illuminating the cornea with a penlight from the temporal side of the cornea, focused anterior to the iris, light is sharply focused on the temporal side of the nasal limbus.
Ophthalmoscopy
On ophthalmoscopy, a circular, oblong, or dumbbell-shaped shadow may appear that looks like a large indefinite cataract separating the central from the peripheral reflex.48 On closer evaluation, this phenomenon is observed to be corneal in location. Fundus details are difficult to observe. A technique called photodiagnosis has been useful in diagnosing and monitoring the size, shape, and location of advanced or severe cones. In this technique, the image of the cone is viewed against the red fundus reflex. With the dilated pupil, the examiner views the cornea through the direct ophthalmoscope at a distance of 2 feet. In advanced cases (>50 D), the cone can be easily observed against the red fundus reflex (Fig. 18.6). The image can also be recorded using a fundus camera with a high plus condensing lens.
FIGURE 18.4 Fleischer ring. |
FIGURE 18.5 Corneal scarring. |
The clinical signs of keratoconus are summarized in Table 18.2.
DIFFERENTIAL DIAGNOSISWhen diagnosing keratoconus, it is important to rule out other mimicking conditions such as corneal warpage syndrome, corneal molding from a high-riding GP lens, keratoglobus, and pellucid marginal degeneration.
Corneal Warpage Syndrome
Keratoconus and corneal warpage syndrome (CWS) can often be differentiated via a combination of the case history and a comprehensive clinical examination, including videokeratoscopy. Corneal warpage syndrome patients typically have a long-term history of contact lens wear. This condition occurs as a combination of corneal hypoxia and the mechanical effects induced by the contact lens. It has been most often reported with PMMA lens wear, although soft and GP lenses can induce CWS, the latter via a poor lens-to-cornea fitting relationship. Although corneal distortion and often a scissors-like retinoscopy reflex are present in both conditions, CWS patients typically manifest signs of corneal hypoxia while rarely exhibiting corneal steepening beyond 50 D.69 In addition, the degree of mire irregularity or misalignment is typically less with CWS than in keratoconus. Videokeratoscopy will show a localized area of corneal steepening in keratoconus but not necessarily in CWS. Keratoconus can often be differentiated with a comprehensive biomicroscopy examination. Clinical signs such as corneal thinning, Fleischer ring, Vogt striae, and, in moderate to severe cases, corneal scarring are present in keratoconus and not associated with CWS, which is limited to changes in corneal contour.70 The affected corneal region is limited in keratoconus, often of variable size, encompassing some of (but not limited to) the central and inferior regions. This can be verified via photodiagnosis. Keratoconus, unlike CWS, is a progressive condition, typically over a 5- to 10-year period, often resulting in a protruding conical region, easily observable via Munson sign in advanced stages. Corneal warpage syndrome, conversely, is remediated and often results in a healthy, regular cornea via discontinuation of contact lens wear and/or refitting into a lens material with higher oxygen permeability (Dk) exhibiting a well-centered lens-to-cornea fitting relationship. The differences in keratoconus and corneal warpage syndrome are summarized in Table 18.3.
FIGURE 18.6 Photodiagnosis outlining the cone. |
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TABLE 18.2 KERATOCONUS CLINICAL SIGNS | ||||||||||||||||||||||||||||||||||||||||||||||||
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High-Riding Gas Permeable Lens
A pseudokeratoconic corneal topography can result from wearing a highly positioned GP lens. This fitting relationship induces superior corneal flattening accompanied by inferior steepening, thus simulating a keratoconus color map. As with corneal warpage syndrome, none of the classical slit-lamp signs of keratoconus will be present. In addition, once the rigid lens is removed, the inferior cornea will flatten back toward baseline over the course of approximately 7 to 14 days.
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TABLE 18.3 CORNEAL WARPAGE SYNDROME VERSUS KERATOCONUS: DIFFERENTIAL DIAGNOSIS | ||||||||||||||||||||
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Pellucid Marginal Degeneration
As with keratoconus, pellucid marginal degeneration is a progressive disorder affecting both eyes. However, it is typically characterized by a peripheral band of thinning of the inferior cornea from 4 to 8 o’clock with a 1- to 2-mm unaffected area between the region of thinning and the limbus.1 Slit-lamp evaluation should be beneficial in differentiating the regions of thinning between keratoconus and pellucid marginal degeneration. In addition, in pellucid marginal degeneration, the videokeratograph has a classical “butterfly” appearance demonstrating a large amount of against-the-rule astigmatism.71 Many believe the two conditions are similar and perhaps share the same etiology and pathophysiology, yet different areas of the cornea are affected.
CLASSIFICATION AND PROGRESSIONThere are various classification schemes for keratoconus, although no single classification scheme has been universally accepted. One comprehensive classification scheme is presented in Table 18.4, which includes visual symptoms, classical slit-lamp signs, and corneal curvature values as a method to distinguish among different degrees of severity. A recently introduced classification scheme (Table 18.5) utilizes slit-lamp findings, corneal topography map characteristics, and two easily determined topographic indices, average corneal power (ACP) and higher-order first corneal surface wavefront root-mean-square (RMS) error (HORMSE).72
Apical scarring is included to better delineate moderate to severe disease, since corneal scarring is associated with more advanced disease.73 With the inclusion of higher-order RMS errors, normals from suspect cases are better delineated, as coma has been shown to be an excellent differentiator of suspected keratoconus and normal corneas.74
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TABLE 18.4 KERATOCONUS CLASSIFICATION | ||||||||||||||||||||||||||||||||||||||||||||
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Patients first diagnosed with keratoconus want to know the prognosis for progression and loss of vision. Clinicians would like to be able to predict the rate of progression and to identify those patients who will advance to severe keratoconus. However, the rate of progression for a particular patient is impossible to predict. Some patients advance rapidly for 6 months to a year and then stop progressing, with no further change. Often, there are periods of several months with significant changes followed by months or years of no change; this may then be followed by another period of rapid change. However, it typically progresses over a time period of 3 to 8 years.3 In the CLEK study, the slope of the change of flat K was approximately 0.20 D per year; over 7 years, this translated into an expected steepening of 1.44 D. Steepening of 3 D or more in either eye had an incidence of 23%.75
PATIENT CONSULTATIONAs with almost all medical conditions, patients should be informed of the diagnosis (or possible diagnosis) as soon as possible. The progression of the disease can be described and, although most patients will not need a corneal transplant, the possibility of this option should be mentioned. These individuals are often very curious about their condition and desire more information. The internet is always a good source for information, but patients can also impose undue worry upon themselves if they retrieve faulty information or information not pertaining to them or their specific disease state. One excellent source is the National Keratoconus Foundation site available online at http://www.nkcf.org. Keratoconus patients can contact a large number of sources of information here and find answers to commonly asked questions. Likewise, practitioners can utilize this information as well. Additionally, several cities have keratoconus support groups in an effort to allow patients the opportunity to share their experiences with others.
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TABLE 18.5 KERATOCONUS SEVERITY SCORE | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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When monitoring these patients, it is important to empathize with the possible changes in their quality of life. Typical keratoconus has an onset early in life, often said to be at puberty.76 The CLEK study reported a median age of 39 years at enrollment. Thus, keratoconus is a chronic disease with a long duration affecting people during their prime earning and childrearing years, which may relate to the reported adverse psychological effects reported by some authors.77,78,79,80 In the CLEK study, visual acuity worse than 20/40 was associated with lower quality-of-life scores.81 A steep keratometric reading >52 D was associated with lower scores on subscales representing Mental Health, Role Difficulty, Driving, Dependency, and Ocular Pain. Scores for keratoconus patients were between patients with category 3 and 4 age-related macular degeneration (AMD) except General Health, which was better than for AMD patients, and Ocular Pain, which was worse than for AMD patients.81 This significantly impaired vision-related quality of life continues to decline over time.82 Additionally, in those patients with worsening visual acuity and increasing corneal curvature, a significant 10-point decline over 7 years in National Eye Institute Visual Function Questionnaire (NEI-VFQ) scale scores is found.82 Therefore, although keratoconus rarely results in blindness, its impact on patients is comparable to that of a person with advanced macular degeneration and, because it affects young adults, the magnitude of its public health impact is disproportionate to its reported prevalence and clinical severity.
Additionally, impact on the quality of life was confirmed by a study in which keratoconus patients were compared to normal controls and to age-matched patients with chronic, nonkeratoconic eye disease via use of a personality questionnaire.41 Abnormal results were found on the same psychological scales (passive-aggressive, paranoid, hypomanic, disorganized thinking patterns, and substance abuse) in both the keratoconic and chronic eye disease patients.
SPECTACLE MANAGEMENTSpectacle correction is an uncommon form of correction in keratoconus. The CLEK study found that 16% of patients diagnosed with keratoconus wore spectacles as their primary form of distance correction.32 As a result of the gradual progression of corneal irregularity, spectacles appear to have their best application early in the disease, before contact lens use. Spectacles do not impact the irregular nature of the corneal curvature. In addition, the refractive error can change quite rapidly and, as the condition progresses, anisometropia may exist because one eye is often more affected than the other eye.65
CONTACT LENS MANAGEMENTRigid Lens Fitting
Keratoconus is best corrected with rigid gas permeable contact lenses. Even in the early stages of the disease when spectacle correction may still be an option, rigid contact lenses do the best job of correcting the irregular astigmatism and secondary aberrations that are induced from the irregular cornea. In the process of rigid lens fitting, the practitioner may be presented with a difficult fit as a result of an extremely decentered apex of the cone. This is particularly true in cases in which the apex is decentered several millimeters inferiorly, as the lens tends to position at the steepest region of the cornea.
In the CLEK study, 65% of patients were best corrected bilaterally with rigid lenses, and 8% unilaterally with a rigid lens.32 This value is similar to the 75% of keratoconic patients wearing rigid lenses in the CLEK study83 and is similar to other reports.46,84
A rigid contact lens will neutralize much of the distortion/optical aberrations of the anterior corneal surface, and the subsequent visual acuity can be increased several lines on the acuity chart.85 However, even if visual acuity chart vision is “normal” with rigid contact lens correction, most likely there is some decrement in vision performance. Contrast threshold measurements have shown a vision loss at low spatial frequencies (0.25 c/deg) that is not improved by contact lens fitting.86 Therefore, although rigid contact lenses can provide a significant improvement in visual acuity for keratoconic patients, there may still be residual loss of visual function.
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