On the front lines of eye care, you must know how to identify and treat this disease.
As primary eye care providers, optometrists are the best care givers for patientssuffering from keratoconus. We can properly prescribe eyeglasses for mild cases, fit specialty contact lenses for other cases, and handle postoperative care for that small percentage of patients requiring surgical intervention. Treating such patients is both challenging and rewarding.
Know your enemy
Keratoconus is a progressive, non-inflammatory conical deformity of the cornea, resulting in corneal distortion and decreased vision. Corneal thinning and a corneal protrusion are characteristic of the condition.
Keratoconus is classified by several means:
* Degree of conicity indicates early or advanced keratoconus
* Morphologically by the shape of the cone
* Corneal topography.
A small island of irregular astigmatism in the inferior paracentral cornea is generally the first manifestation of early keratoconus. Advanced keratoconus, however, falls into three categories: nipple, oval, or globus. A small, near-central ectasia of 5mm in diameter or less is a hallmark of the nipple form. There is sometimes an elevated fibroplastic nodule at the apex of the cone. The oval shape is the most common in advanced keratoconus. It's characterized by displacement of the corneal apex below the midline. This results in an island of inferior midperipheral steepening. Unlike the nipple or oval forms, the globus cone has no surrounding island of normal midperipheral cornea. This form of keratoconus encompasses nearly three-quarters of the corneal surface.
Presenting symptoms of keratoconus include decreased vision, photophobia, diplopia, visual distortion, asthenopia, and glare around lights. Young adults with irregular astigmatism and patients of any age with myopic astigmatism whose eyeglass prescription is changing more frequently than normal are suspect. Be even more suspicious if this patient shows poor repeatability or you can't correct subjective refraction to 20/20 in the phoropter. In many cases, you will be able to rule out all other diagnoses except keratoconus or meridional amblyopia. Slit lamp findings and corneal topography will help you differentiate between the two.
Major slit lamp findings on a patient with keratoconus include:
* Fleischer's ring. This yellowbrown or olive green line marks the peripheral edge of the cone. A cobalt blue filter with diffuse illumination can enhance the appearance of a subtle ring.
* Vogt's striae. These vertical stress lines appear near the apex of the cone. They are seen as a series of sharp, whitish, vertical or oblique lines just anterior to Descemet's membrane.
* Corneal thinning.
* Munson's sign. This bulging of the lower lid in downgaze is typical of keratoconus patients.
In most keratoconus cases, your slit lamp examination will demonstrate displacement of the apex of the cornea below a hypothetical line bisecting the pupillary axis.
Several features on keratornetry will be present in keratoconus patients. In many cases, the mires are oval due to the amount of corneal astigmatism. The irregular corneal surface usually distorts these. In addition, you will often be unable to superimpose the central keratometric rings, suggesting irregular corneal astigmatism, which is a hallmark of keratoconus.
Early keratoconus produces a characteristic pear-shaped elongation of the central mires midperipherally below the corneal midline on topography. The steepening usually spreads nasally to include the inferonasal cornea as the condition progresses. Rotational steepening above the midline along a superotemporal path is characteristic of advanced keratoconus. The superior nasal quadrant of the cornea is usually the last affected.
Types of treatment
Your treatment of keratoconus will depend on its severity. Eyeglasses are initially successful in restoring vision. As the condition progresses, the patient will require contact lenses to provide optimal visual acuity. You have several lens choices: soft (hydrogel) lenses, basic GP lenses and specialty GP lenses.
Soft contact lenses
Soft lenses, unlike GPs, tend to drape over the cornea but don't mask any of the corneal irregularity, so they're used only in the early stages. Choose a toric lens in these cases. Soft lenses are an acceptable option if they do not induce scarring and if best corrected visual acuity is 20/40 or better (usually the legal driving acuity).
GP contact lenses
Early keratoconic patients may do well with large diameter GP lenses when soft lenses are no longer an option. Use lenses with high oxygen permeability to minimize the risk of neovascularization because this group often wears lenses all day. The materials I recommend are the Boston EO, Fluorex 700, or the Fluoroperm 90 or 151. More advanced cases of keratoconus may require a lens designed specifically for keratoconus, such as the Soper, McGuire, or Rose K in order to achieve adequate vision and a proper fit.
* The Soper Cone. The fitting philosophy for this bicurve contact lens is based on sagittal depth. The vaulting effect in the lens is designed to increase as the base curve decreases for a given diameter. This change in the steep central posterior curvature is what is used to fit the cone. The idea that apical bearing will ultimately lead to corneal scarring in keratoconus is the basis for this lens design.
The Soper design consists of ten lenses with three groups of lenses for a given diameter/optic zone relationship. They are designated by the letters A through H. Lenses A, B, C and D are intended for mild keratoconus or K-readings of less than 48.00D in either corneal meridian. E, F and G are for use in moderate keratoconus or K-readings of 48.00D to 54.00D in either corneal meridian. Finally, H, I and J are useful for the advanced stage or Kreadings of 54.00D or greater.
* The McGuire keratoconic lens system. This a modification of the Soper design introduced in 1978. Each of the three diagnostic lens sets is designed to correspond with each type of keratoconus. The goal in fitting this lens is to achieve a three-point touch and is predicated upon the size of optic zone in relation to conical size. The optical zone sizes are 6.0mm for the nipple cone, 6.5mm for the oval cone and 7.0mm for the globus.
Four peripheral curves that blend together to create an almost aspheric relationship are also incorporated into the lens. In the McGuire system, the secondary curve is 0.5 mm flatter than the central base curve; the third curve is 1.0mm flatter than the secondary curve; and the fourth and final peripheral curve is 2.0mm flatter than the tertiary curve.
* Rose K. Complex, computer-generated peripheral curves are the basis for this lens design. The lenses incorporate three peripheral systems - standard, flat, and steep - in order to achieve the ideal edge lift of 0.8mm. It is available in base curves from 4.75mm to 8.00mm and diameters of 7.9mm to 10.2mm. In this design, the optic zone diameter decreases as the base curve gets steeper.
The Rose K 2 is the newest version of this lens and incorporates a sophisticated parabolic section on the posterior surface to counteract spherical aberration. These occur because the focal points of light rays far from the principal axis of a spherical lens are different from the focal points of rays that pass through the center of the axis. Spherical aberrations is exaggerated further in the steep radius of curvatures where the rays of light passing through the edge of the lens bend more and focus slightly in front of those passing through the center. Varying amounts of eccentricity across the back optic zone diameter counteract the spherical aberration.
The amount of eccentricity can vary with almost every combination of base curve, diameter and power. The fitting regimen for the refit is exactly the same as Rose K.
If a keratoconic patient cannot tolerate rigid lenses, a "piggyback" fit using a rigid lens fit over a hydrogel lens works well for many patients. First fit the patient with a large diameter soft lens with proper centration and movement. Then, take keratometry readings of the "new" corneal surface and fit a rigid gas permeable lens on top of the soft lens. It's even more important to make sure that both lens materials are highly oxygen permeable because the patient is wearing two lenses at the same time.
Once you have exhausted all contact lens options, surgery may be necessary to obtain adequate vision. Surgical options include simple lamellar keratoplasty, epikeratophakia, thermokeratoplasty, or a penetrating keratoplasty. When referring patients for surgery, the patient's vision should be poor enough that it interferes with his ability to work or drive. Like cataract surgery, there's 110 magic number at which the keratoconic patient needs to undergo a corneal transplant. Often, it's based more on the patient's perception of vision. One patient may find that she cannot do her job with 20/30 acuity while another patient may be very satisfied with 20/60.
If a patient has a very large area of thinning, if a stable contact lens fit is no longer possible, or if the patient has significant corneal neovascularization, surgery may be performed earlier than otherwise indicated by visual performance alone.
Many patients with keratoconus will inquire about LASIK. These patients, unfortunately, are not candidates for this procedure. However, they may be candidates for INTACS. In the case of keratoconus, this procedure is not for refractive purposes, but to help stabilize the cornea so the patient may be fit with GPs. The goal is to prevent or delay the need for a corneal transplant in these patients.
One relatively new experimental procedure that's not yet available in the United States is C3-R. It's collagen cross-linked with riboflavin. Basically, this procedure features the instillation of these eye drops followed by UV light exposure for 30 minutes. The premise is that it halts the progression of keratoconus by helping the cornea increase its thickness. Although initial studies look promising, it is still very experimental.
There was a time that both keratoconus fitting and the lenses themselves were covered by most insurance companies. However, in recent years, this policy has changed and insurance coverage has become the exception instead of the rule. Thus you must set your fees accordingly and tell the patient up front about the anticipated costs before you begin the fitting process. The one part of it that's still covered by insurance is the examination and corneal topography.
References available on request.
BY DEEPAK GUPTA, O.D., Stamford, Conn.
Dr. Gupta is the clinical director for The Center for Keratoconus at Stamford Ophthalmology. He can be reached at email@example.com
Copyright Boucher Communications, Inc. Oct 2005
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