KEY CONCEPTS
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Retinoscopy can play an important role in keratoconus screening, because of its portability, affordability, sensitivity, and easy accessibility.
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With progression in keratoconus, contact lenses are recommended for vision restoration and delayed surgical intervention.
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Corneal gas permeable contact lenses play a large role in keratoconus management and continue to be the initial lens of choice among practitioners when fitting the irregular cornea.
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Hybrid lenses are a good lens option for those who are intolerant of corneal gas permeable contact lenses.
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Scleral lenses are large diameter gas permeable contact lenses that vault the cornea and land on the scleral conjunctiva.
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SLs offer visual improvement and neutralize irregularities of the corneal surface. Compared to corneal lenses, SLs have improved centration and stability.
Introduction
Early detection and management of keratoconus (KC) is critical and may impact quality of life. Glasses and contact lenses (CLs) may be utilized to aid function early in the course of the disease. However, CLs do not slow or stop the advancement of KC. Spectacle correction may be used in mild cases of KC, or used at home in addition to CLs. Vision may be limited, since glasses do not correct irregular astigmatism. Soft contact lenses (SCLs) are an option in early cases. In moderate to advanced cases, rigid corneal gas permeable (GP), piggyback, hybrid, and scleral lenses (SL) improve vision by correcting irregular astigmatism induced by an irregular cornea. In this chapter we will review various types of spectacles and CLs in the optical management of KC.
RETINOSCOPY
The retinoscope, a common tool utilized by optometrists and ophthalmologists for objective refraction, has been available since the early 1900s and may play an important role in KC screening, as it is portable, affordable, sensitive, and easily accessible. During retinoscopy, a streak of light is directed into the patient’s eye and reflects off the retina, producing a light reflex that can be neutralized by correcting the patient’s refractive error. High myopia, irregular astigmatism, and a scissoring reflex are commonly observed during retinoscopy in a KC patient. , The scissoring reflex is more prominently noted in those with advanced disease and is a strong indicator of KC. When compared with the Belin and Ambrósio Enhanced Ectasia display on the Pentacam (OCULUS GmbH, Wetzlar, Germany), the retinoscope had a 98% sensitivity and 78% specificity in KC diagnosis.
Pinhole visual acuity may help to demonstrate an approximate improvement in vision; however, the amount of improvement in pinhole acuity may not be a true gauge of best-corrected acuity. In patients with KC, spectacle or CL acuity may not match the pinhole acuity because of posterior corneal irregularities and/or corneal scarring.
REFRACTION
A baseline refraction is beneficial in determining best-corrected visual acuity (BCVA). In patients with KC and irregular astigmatism, BCVA may be reduced and may not be improved with manifest refraction. Reduced vision with manifest refraction and spectacle correction may be a strong motivator for the use of GP CLs. Conversely, if vision is relatively good with manifest refraction, the patient may not be as motivated to wear GP CLs. Improvement of visual acuity with CLs compared with manifest refraction and documentation of visual improvement may be important for legal and insurance purposes. Even if vision is poor, glasses to wear at home after CL removal may be beneficial.
A manifest refraction can aid the type of CL selected and in deciding whether CLs or glasses should be recommended. If refractive error is relatively symmetrical, glasses are tolerable, visual acuity meets driving standards, and glasses are comfortable to wear, spectacle correction may be an acceptable option. However, glasses may not optimally correct visual acuity, reduce glare and halos, nor correct higher-order aberrations. In these instances, CLs are a better option. If a prescription for glasses is given, one should consider postponing the refraction until after the CL fitting is finalized, to minimize spectacle blur.
A study of 90 eyes in 61 patients with KC compared corrected distance visual acuity with autorefraction and manifest refraction. Superior vision was achieved with manifest refraction compared with autorefraction. Additionally, the difference between the autorefraction and manifest refraction increases as the cornea steepens. This study concluded that autorefraction is not reliable in patients with KC. However, evaluation of the mires when performing autorefraction may be diagnostic of KC. Irregular mires may be indicative of high and/or irregular astigmatism.
SOFT CONTACT LENSES
As KC progresses and spectacles no longer provide adequate visual acuity, patients and practitioners turn to CLs for vision restoration and delayed surgical intervention. , Viable options are abundant and include commercially available SCLs, custom SCLs, specialty KC SCLs, corneal GP lenses, hybrid lenses, piggyback lens systems, and scleral lenses (SLs). Ideally, the CL should feel comfortable, not compromise ocular health, and provide functional vision. It is important to note that the ability of CLs to treat or prevent KC progression is a common misconception, especially among patients, and has been disproven.
Early or mild KC patients may do well in commercially available SCLs as toric and extended range parameters are readily manufactured ( Table 21.1 ). If decreased acuity or lens decentration is noted with commercially available SCLs, numerous laboratories also offer custom SCLs with greater power ranges and parameter availability. These made-to-order lenses may provide improved vision, a better fit, and greater comfort as they are tailored specifically for the patient ( Table 21.2 ). These SCLs are limited when fitting the more advanced KC patient, as they tend to contour to and assume the irregular keratoconic cornea.
| Contact Lens Name and Manufacturer | Sphere Power Range | Cylinder Power Range | Axis | Replacement Schedule | ||
|---|---|---|---|---|---|---|
| Biofinity XR (CooperVision) | −12.50 D to −20.00 D +8.50 D to +15.00 D (0.50-D steps) | N/A | N/A | 1 month | ||
| Biofinity Toric XR (CooperVision) | ±10.00 D (0.50-D steps after ±6.00) | −0.75 to −5.75 (0.50-D steps) | 5–180 degrees (5-degree steps) | 1 month | ||
| Proclear Toric XR (CooperVision) | ±10.00 D (0.50-D steps after −6.50 D and +6.00 D) | −0.75 to −5.75 (0.50-D steps) | 5–180 degrees (5-degree steps) | 1 month |
| Diameter | Base Curve | Sphere Power Range | Cylinder Power Range | Axis | Replacement Schedule | ||
|---|---|---|---|---|---|---|---|
| 10.0 mm–16.0 mm | 6.9–9.5 mm (0.1-mm steps) | +/-30.00 D (0.10-D steps) | −0.25 to −8.00 (0.1-D steps) | 0–360 degrees (1-degree steps) | Practitioner discretion |
SPECIALTY SOFT CONTACT LENSES FOR KERATOCONUS
Masking front surface corneal irregularity requires a neutralizing tear lens under a smooth refractive plane and may be achieved with a GP lens. Specialty SCLs designed specifically for the KC patient exhibit characteristics similar to a GP lens and mask low amounts of corneal irregularity as they are lathed in higher modulus (stiffer) materials or are designed with greater lens center thicknesses. Many KC SCLs are available in silicone hydrogel materials or even utilize fenestrations to prevent corneal hypoxia. Diagnostic fitting and following the manufacturer fit guide are recommended as numerous designs are available and vary greatly in fitting philosophy ( Table 21.3 ). If a patient is adamantly against wearing GP lenses or is intolerant of them, overlay spectacles may be prescribed in addition to SCL wear for possible further vision rehabilitation.
| Manufacturer | Contact Lens Name | ||
|---|---|---|---|
| ABB Optical Group | Concise K KeraSoft IC and KeraSoft Thin | ||
| Acculens | Soft K | ||
| Advanced Vision Technologies | Soft K and Soft K Definitive NaturaSOFT IC and ICR | ||
| Bausch+Lomb | NovaKone and NovaKone Toric | ||
| Art Optical | KeraSoft Thin | ||
| Continental | Continental Kone | ||
| GP Specialists | YamaKone IC | ||
| Gelflex USA | Keratoconus Lens | ||
| Marietta | Soflex | ||
| Metro Optics | Revitaleyes and Revitaleyes Definitive KeraSoft Thin | ||
| Ocu-Ease, Optech | Ocu-Flex K | ||
| TruForm Optics | KeraSoft IC and KeraSoft Thin | ||
| United Contact Lens | UCL K-Lens | ||
| Visionary Optics | HydroKone and HydroKone Toric | ||
| X-Cel Contacts | Flexlens ARC and Flexlens Tri-Curve |
CORNEAL GAS PERMEABLE CONTACT LENSES
Corneal GP lenses play a large role in KC management and continue to be the initial lens of choice among practitioners when fitting the irregular cornea. Fitting goals for a corneal GP lens are that they vault minimally over the corneal apex to prevent epithelial disruption, and provide midperipheral bearing and moderate peripheral clearance. In early KC, an ideal fit is easier to obtain. In moderate to advanced KC, it is more difficult to obtain an optimal fit because of corneal irregularities.
KC patients with paracentral “nipple” cones often do well with smaller diameter corneal GP lenses, whereas patients with larger “oval” cones or decentered cones fare better with medium or large diameter corneal GP lenses ( Table 21.4 ). A poor-fitting GP lens may be uncomfortable and compromise ocular health. A known risk factor for corneal scarring is a flat-fitting GP lens with associated corneal staining. Patients in flat-fitting GP lenses also experienced more lens discomfort and a greater propensity for corneal transplantation. It is imperative that a harsh apical bearing relationship be avoided when fitting the KC patient, regardless of the GP lens diameter utilized. Two common fitting philosophies strive for either very light apical bearing (three-point touch) or mild apical clearance first definite apical clearance lens (FDACL) fluorescein pattern. Adequate peripheral edge lift and movement on blink is also necessary to facilitate tear exchange and debris removal. Lens design software available in some corneal topographers may simplify the fitting process by relaying information directly to GP lens laboratories. Advanced lathing technologies can also be used to fabricate asymmetric peripheral curves, creating a more uniform edge during GP lens wear. Numerous corneal GP lens designs of various diameters are available from every GP lens laboratory.
