Customized Lens Fitting in Microcornea
Successful fitting of microcornea—or any cornea—is influenced most by a particular corneal measurement.
By Rajni Rajan, MPhil, & Rajeswari Mahadevan, PhD
Ms. Rajan is an optometrist in the Department of Contact Lenses at Medical Research Foundation in Sankara Nethralaya, India.
Dr. Mahadevan is head of the Department of Contact Lenses at Medical Research Foundation, India, and is also an assistant professor at the Elite School of Optometry in India.
Microcornea-cataract syndrome is an autosomal dominant inherited disease characterized by the association of congenital cataract and microcornea without any other systemic anomaly or dysmorphism (Rüfer et al, 2005). Microcornea-cataract syndrome can be associated with other ocular manifestations including myopia, iris coloboma, sclerocornea, and Peter’s anomaly (Stefiniak et al, 1995; Gronemeyer et al, 2002; Rabah et al, 2009; Batra et al, 1967; and others. Full reference list available at www.clspectrum.com/references.asp.).
In the past, microcornea has been used as a marker for microphthalmos because they are often (but not invariably) associated. The normal range for horizontal corneal diameter at birth is 9.8mm ± 0.33mm in boys and 10.1mm ± 0.33mm in girls; the adult corneal diameter of approximately 11.7mm is reached by the age of 7 years (Hornby et al, 2000). Corneal diameters of <11.0mm in males and of <10.7mm in females are regarded as microcorneas (Hazin et al, 2008).
Because microcornea is commonly associated with high refractive errors, visual rehabilitation is essential. Optical forms of visual rehabilitation include spectacles and contact lenses. As the optical advantages of contact lenses outweigh those of spectacles, consider contact lenses first. Contact lenses become the sole choice of visual rehabilitation in conditions such as anisometropia, unilateral aphakia, or unilateral myopia.
Figure 1. The left eye of our patient who presented with microcornea.
Although several rare conditions such as isolated microcornea (Hazin et al, 2008), microcornea-cataract syndrome (Stefiniak et al, 1995; Gronemeyer et al, 2002; Rabah et al, 2009), and Peter’s anomaly (Rabah et al, 2009) have been described in the literature, there are hardly any reports of contact lens fitting of microcornea in the literature. The purpose of this article is to report a case of successful contact lens fitting in unilateral high myopia in a patient afflicted with Microcornea-cataract syndrome. To the best of our knowledge, this is the first case that reports successful contact lens fitting in microcornea.
A 4-year-old male presented to us with bilateral microcornea. Figure 1 shows his left eye. There was no family history of ocular/systemic diseases, the birth history was normal, and the elder sibling was healthy. The patient had no systemic illnesses. The ocular motility and ductions were full. There were no indications of strabismus. Horizontal visible iris diameter (HVID) was 7mm in both eyes.
Anterior segment examination by slit lamp revealed inferiorly decentered pupils and posterior subcapsular cataracts. Cycloplegic retinoscopy (1% cyclopentolate) was –10.00DS –6.00DC @ 180 OD and OS. Posterior segment examination by indirect ophthalmoscopy showed a myopic, tessellated fundus. There were no other significant ocular findings. We prescribed new glasses as he declined the option of contact lenses.
He came for a follow-up visit six months later. Visual acuity with glasses was 20/120 in both eyes. There was no change in the cycloplegic refraction. We assessed the following ocular measurements using Ultrasound A-scan: axial lengths were 26.02mm OD and 26.56mm OS, anterior chamber depths were 2.18mm OD and OS, and lens thicknesses were 4.18mm OD and 4.31mm OS.
The patient returned for follow up three years later. Apart from progression in myopia, the ocular findings were stable (OD and OS –14.00DS –5.50DC @ 180). The posterior segment examination revealed an inferior retinal detachment in the right eye, for which he underwent a scleral buckling procedure. Due to recurrent retinal detachment, he subsequently underwent lensectomy, vitrectomy, endolaser, and silicone oil injection to reattach the retina. Iotim (timolol maleate, FDC Limited, India) eye drops were prescribed because of an increase in intraocular pressure (29mmHg OD). Visual acuity was 20/60 OU with Plano –6.00DC @ 180 OD and –14.00DS –4.50DC @ 180 OS. The central corneal thicknesses were 729 microns OD and OS.
He was referred by his ophthalmologist for a contact lens in the left eye as he experienced double vision binocularly with glasses. Keratometry values were 39.50 @ 170/41.75 @ 80 in the left eye. Soft lenses with a base curve radii (BCR) of 8.9mm/9.1mm and 14.00mm diameter and GP lenses with a 9.0mm BCR and 8.5mm/8.0mm diameters caused a large central bubble due to steep fitting (Figure 2). Soft and GP contact lenses of smaller diameters were then custom designed and manufactured for the patient (Table 1).
Figure 2. Standard lenses caused a large central bubble because they were too steep.
|Custom Soft and GP Lenses for Microcornea|
|LENS||BCR (mm)||BVP (DS)||DIAMETER (mm)|
The soft lens demonstrated adequate coverage, movement, and centration (push up: positive, Figure 3). Visual acuity with an over-refraction of –6.00DC @ 180 was 20/80; N6. The first GP lens (GP1, 60 Dk) was centered and demonstrated adequate coverage, movement, and a diffuse fluorescein pattern (Figure 4). Visual acuity was 20/60; N6 with an over-refraction of –6.00DC @ 180. The second GP lens (GP2) was steeper compared to GP1. It was low-riding with increased central pooling and reduced edge clearance compared to the GP1 lens fit. There was no binocular diplopia, and the patient reported improved visual performance with the GP lenses. We dispensed GP1 and instructed the patient on the care regimen. He wears the contact lens 12 to 14 hours daily with no complaints to date.
Figure 3. Soft custom lens on our patient’s left eye.
Figure 4. The fit of the GP1 lens on our patient’s left eye.
Different Concepts in Contact Lens Fitting
Corneal Diameter As a “rule of thumb,” lens diameter is generally chosen as 2mm less than the HVID for GP lenses and as 2mm more than the HVID for soft contact lens. Total diameters outside of the common range are necessary for individuals who have uncommon interpalpebral apertures and HVIDs. It is sometimes necessary to fit lenses of smaller or larger diameter to achieve adequate centration depending on factors such as lid tonus and scleral or peripheral corneal topography (Davis et al, 2005; Caroline and André, 2010). Similarly in our case, customized smaller-diameter lenses centered on the microcornea with adequate limbal coverage.
Sagittal Depth Sagittal depth (sag) is predicated upon five anatomical features: central corneal radius, overall corneal diameter, corneal asphericity, and radius and asphericity of the paralimbal sclera (Caroline and André, 2010). Jackson (2012) proposed that a larger corneal diameter means a deeper than normal sagittal depth, which requires a lens with deeper sag. A customized soft lens (with a steeper base curve and larger diameter) can provide a good fitting relationship, leading to better comfort and lens fit (Jackson, 2012). The scleral radius and shape contribute little to the overall sag (Caroline and André, 2010). In our case, customized lenses of smaller diameter and flatter base curve fitted optimally on the eye, possibly with reduced sag.
Caroline and André (April 2012) theorized that all soft lens fitting is based on matching the sagittal height of the lens to that of anterior eye. Central radius of curvature plays only a minor role in sagittal height of the eye. Corneal diameter dominates as the major contributor to the height of the anterior segment. Mass-produced soft lenses failed to provide a good lens-to-cornea fitting relationship in our case. Soft lens fitting guides can be used when the HVID is between the common ranges of 11.6mm to 12mm, but when the HVID is outside of these parameters, effective K readings must be assessed. Effective K incorporates both central K and corneal diameter to assist in appropriate selection of BCR, thereby improving our initial base curve selection. Caroline and André (May 2012) summarize the three rules of soft contact lens fitting as follows: 1) the larger the corneal diameter, the greater the sagittal depth; 2) the lower the corneal eccentricity, the greater the sag; and 3) the steeper the central radius of curvature, the greater the sag.
Available soft lenses with flatter base curves were too steep for the microcornea because the smaller-diameter cornea had reduced sagittal depth.
Corneal Asphericity Douthwaite et al (2002) have proven that normal variations in corneal asphericity have the least influence while normal variations in HVID have the greatest influence in changing corneal sagittal depth. The most appropriate measurement to take in selecting optimum soft contact lens parameters may be the HVID. While the variation in corneal asphericity influences the sagittal height of the anterior eye more than the variation in corneal curvature, the corneal diameter is the most influential of all. The influence of asphericity and corneal diameter on anterior eye sagittal height and, consequently, on lens fit illustrates why keratometry alone is not always the best indicator of which soft lens will provide an optimal fit (Douthwaite et al, 2002). Though eccentricity was not calculated in our study, lenses were custom made based on corneal diameter.
Predicting Sagittal Depth and Corneal Eccentricity Numerous methods can help us predict sagittal depth and corneal eccentricity. Becherer et al (2007) have described four unique fitting philosophies to derive the final lens design: 1) Nomogram, 2) Arc length designing, 3) Rule of 3, and 4) Sagittal depth method. André et al (2001) used the “2.50 Diopter rule” to mathematically describe how the diameter is significantly more influential than curvature is in determining sagittal depth. Cross section photos of anterior segments can help us to understand sagittal depth. Accurate measurements of corneal diameters can be achieved using topography technology or with a special reticule in the slit lamp eye piece. Topography systems can provide us with measures of corneal asphericity. Lack of topographical reports in our study led us to design lenses on the basis of corneal diameter and central corneal curvature.
As topography and imaging technologies evolve—such as simulating GP fitting patterns virtually—accurate calculation of anterior segment dimensions (including sagittal depth) may be possible (Eiden et al, 2012). But even practitioners without access to modern technologies can custom design lenses based on a clear understanding of the role of corneal diameter in determining the ideal lens design. Thus, patients who have been unsuccessful or marginally successful wearing mass-produced contact lenses can achieve consistent, comfortable vision with lenses designed for their eyes (Jackson, 2012; Becherer et al, 2007). Custom lenses can be successfully fitted in corneas that vary from the norm to enable optimal visual performance and to maintain the ocular health of patients. Corneal diameter is often a neglected parameter in the current environment of readily available empirical designs. Using corneal diameter and central keratometric readings to predict the base curve and diameter of initial diagnostic lenses would help reduce the number of trials required for finalizing the best lens fits.
Finding the Best Lens Fit
The fit of a contact lens is influenced by anterior segment features such as lid characteristics, palpebral aperture and corneal dimensions, scleral toricity, corneal asphericity, sagittal depth, tear volume, and lens features such as lens design and material, water content, edge profile, and thickness. But the lens fitting goal of matching the sagittal height of a contact lens to that of the anterior segment can be achieved only if variations in corneal diameter are corroborated. This holds true for both GP and soft custom lenses. CLS
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