Dustin J. Gardner, O.D.
In cases of large keratoconus or pellucid marginal degeneration, fitting gas permeable lenses can prove to be difficult and uncomfortable for the patient. Often the use of mini-scleral or scleral lenses is beneficial.
I. Case History
A forty two year old Caucasian female presented to the clinic for a contact lens fitting for keratoconus OU. Ocular history showed the patient was diagnosed with keratoconus OU 18 years ago, and long-standing vitreal floaters. The patient previously wore gas permeable contact lenses, but discontinued them due to poor fit, and issues with lenses repeatedly falling out. Patient was currently wearing soft toric contact lenses which provided acceptable comfort but poor vision. No history of ocular trauma, surgeries, or other disease was reported. Patient reported no significant medical history. Family medical history revealed hypertension in her father and diabetes mellitus in her mother. The patient reports taking Zoloft (sertraline) and had no known drug allergies.
II. Pertinent Findings
Patient currently wears soft toric contact lenses of an unknown brand. Distance visual acuity was 20/40-2 OD and 20/30+2 OS. Best corrected visual acuity with manifest refraction was OD -12.75-2.25x082 with 20/50 visual acuity, and OS -10.75-3.25x106 with 20/40+ visual acuity. Slit lamp examination revealed mild inferior central stromal scarring and striae OD, and scarring with associated neovascularization of the inferior cornea adjacent to the limbus OS. Topography results revealed simulated K values of 48.6D/50.4D@7degrees OD and 44.1D/50.8D@164degrees OS. Both corneas had large areas of inferior steepening in a possible “kissing bird” formation OU. Patient was recently seen for a complete eye exam at another practice, and was referred for a contact lens fit only. No dilated fundus exam performed at this visit and no previous exam data was provided.
III. Differential Diagnosis, Differential Treatment
Differential Diagnosis
The differential diagnosis includes keratoconus, pellucid marginal degeneration, and keratoglobus. Keratoconus is the primary diagnosis due to inferior corneal steepening, the physical findings of apical scarring, striae, and clinical finding of high myopia and astigmatism. Pellucid marginal degeneration is a secondary diagnosis. This is supported by topography images revealing a “kissing bird” or “butterfly” like formation of large inferior region of steepening. However it is not a typical classic presentation, and a large inferior cone can sometimes have a similar appearance. Keratoglobus typically presents with diffuse thinning/steepening of the entire cornea and is therefore ruled out by the topography. Differential Treatment
Differential treatment for this patient includes spectacle correction, soft toric contact lenses, spherical gas permeable (GP) contact lenses, specialty GP contact lenses for irregular corneas, corneal collagen cross linking, and corneal transplant/graft. The patient currently wears soft toric contact lenses and is experiencing uncorrectable visual blur. The irregular astigmatism, high minus and high astigmatic refractive error is not ideal for a soft toric contact lens fit. These lenses often have rotation and stability issues when fitted over a steep cornea with keratoconus. Also, soft toric contact lenses are unable to correct irregular astigmatism. Spherical GP lenses can often provide great vision for a patient with an emerging cone. The rigid lens allows for optical correction of the irregular refractive corneal surface. This patient reports a previous unsuccessful history of gas permeable wear. This patient’s presentation is too advanced for correction with traditional spherical GP lenses. Corneal collagen cross-linking is used to strengthen the cornea and results in stopping/slowing the progression of keratoconus.1 This is a relatively new procedure with a high cost. Fairly advanced cases may not receive significant benefit from the procedure and therefore was not explored with this patient. Surgical options include complete or partial transplants of the cornea. These are typically reserved for severely advanced cases in which vision is significantly reduced, and uncorrectable due to scarring, and/or is causing the patient pain from corneal hydrops secondary to keratoconus. This patient is not experiencing hydrops and has stable, relatively clear corneas. Specialty GP lenses are made to be steeper, larger, and more customizable than traditional spherical GP lenses, and allow for a more comfortable and stable fit. This treatment method was determined to be the most beneficial.
IV. Diagnosis and Discussion
This patient has large inferior keratoconus OD and OS. Keratoconus is a non- inflammatory ectasia of the cornea presenting with stromal thinning, typically of the inferior or central two-thirds of the cornea, resulting in a conical protrusion. The condition is nearly always bilateral, but is often asymmetric.2,3,4 The abnormality of the cornea also induces high myopia and irregular astigmatism.2,3 Clinical signs include oil droplet reflex of the cornea (Charleaux’s sign), iron ring deposition in the corneal epithelium (Fleischer’s ring), fine vertical folds of the stroma and Descemet’s membrane (Vogt’s striae), and enlarged corneal nerves.2,3,4 Topography typically reveals a classic inferior-central cone of steepening.4 In more advanced cases it is possible to note a V-shape to the lower lid in downward gaze (Munson’s sign), and illumination from the temporal side of the cornea will become sharply focused near the nasal limbus (Rizzutti’s phenomenon).2,3
A secondary diagnosis of pellucid marginal degeneration (PMD) was made. These conditions are often mistaken for one another due to similar clinical presentation, and management methods. It can be difficult to differentiate the two from each other. There is no clear answer on whether they are in fact two distinct diseases or simply a variation of the same disorder. 4 PMD is also a bilateral, but often asymmetrical, non-inflammatory condition characterized by a one to two millimeter wide band of inferior corneal thinning. This also results in high myopia and against-the-rule astigmatism.4,5,6,7 In advanced cases Munson’s sign, and Rizzzuti’s phenomenon can be noted, adding to the confusion between the two conditions.4 Often topography can be a useful clinical test to provide some distinction. A classic topographical presentation of PMD shows large areas of inferior steepening stretching from the four o’clock to eight o’clock position in a “kissing bird”, “butterfly”, or “crab claw” appearance.4,5,6 However, some studies have indicated that this classic topography is not diagnostic of PMD, and these patterns can be found in cases of keratoconus as well. Diagnosis must be made based upon a combination of slit-lamp findings, topography, and pachymetry maps.4,6 In this case a pachymetry map has not been obtained, therefore a reliable diagnosis cannot be made.
V. Treatment, management
Clinic Visit 1
Previous attempts made to fit Dyna Z Intralimbal lenses were unsuccessful due to unresolveable bubble formation issues and decreased comfort. Lenses were removed and a Medmont topography was performed. Large areas of inferior corneal steepening in a “kissing bird” formation were found, leaving suspicion of possible pellucid marginal degeneration. It was decided that scleral or mini-scleral lenses would most likely allow for a better fit, comfort, and visual outcome.
Diagnostic fit with Jupiter lenses was performed as described by the fitting guide. Selection of base curve begins with slightly steeper than average keratometry or simulated K power. Then steepen or flatten to achieve a tear reservoir of 200-300 microns, which allows for settling of the lens. Once proper central alignment is achieved, analyze the limbal area for 40-50 microns of vaulting and assure the periphery is aligned with the sclera. If blanching is noted flatten the peripheral curve interior to the blanching. Initial lenses selected had a larger diameter of 18.2 mm to allow for more vaulting of the cornea. Base curve selected by adding a half diopter to the average keratometric value of each eye. After insertion of the lenses fluorescein pattern revealed areas of mid-peripheral touch OU. New lenses selected with a one diopter increase in base curve OU. These lenses showed vaulting over the entire cornea and limbus. The right lens presented with a slight mid-peripheral bubble and trace areas of blanching of conjunctiva in the periphery. The base curve of the lens ordered was flattened by 0.50 diopter to reduce bubble formation, and possibly relieve the conjunctival blanching. Spherical over-refractions allowed for a distance visual acuity of 20/20 OD and 20/25+ OS. Accounting for the over-refractions, lenses ordered with parameters of:
OD: BC 6.985/OAD 18.2/Power -12.25
OS: BC 6.62/OAD 18.2/Power -15.25 Clinic Visit 4
The patient returned to the clinic for dispense of the ordered Jupiter lenses. After insertion visual acuity was measured to be 20/20 OD and 20/20- OS. The patient also reported significantly improved comfort. After allowing a settling period of one half hour, fluorescein evaluation showed full vaulting of the cornea and limbus, no presence of bubble formation, or blanching of the conjunctiva. Estimated apical clearance based on optic section was 200 microns. An optic section can be used to compare thickness of the lens, which is approximately 250 microns, the cornea, which is approximately 500 microns and the lacrimal lens. This allows for estimation of the vault over the cornea. Insertion and removal training performed. Patient was able to demonstrate proper insertion and removal of both lenses. Patient was educated on proper wear and care of lenses, released, and scheduled back for a two week follow up.
Treatment/Management Discussion
Treatment and management of corneal ectasia is often based upon how advanced the condition is. A common scale of severity is based on keratometry readings. Mild cases are 45 diopters or less, moderate cases are in the 45-52 diopter range, advanced cases are greater than 52 diopters, and severe cases greater than 62 diopters.2 In the early stages spectacles, and soft contact lenses may be sufficient.2,3 In moderate cases, as the cornea becomes more irregular, soft contact lenses are unable to mask high corneal astigmatism and become unstable. Spectacles are also ineffective at correcting the resulting irregular astigmatism. At this stage a gas permeable lens is required. These rigid lenses are able to mask corneal irregularity with a tear reservoir and allows for better refractive correction.2 However as the condition further advances, or if the affected area is large enough, as in cases of suspected PMD, these lenses can become difficult to fit, making them uncomfortable for the patient, unstable, and unable to provide optimal visual acuity. This patient falls into this category. In these cases it is beneficial to fit larger gas permeable lenses such as mini-scleral or even scleral lenses. These lenses are fit aligned with the sclera and provide total vaulting of the cornea. The larger lenses are more stabilized, and provide improved comfort by reducing limbal interaction and lens edge interaction with the lids.2,4,7,8 An ideal fit provides 250 microns of corneal clearance and 50-100 microns of limbal clearance. This amount of vaulting is required because the scleral lens will eventually settle into the conjunctiva and avoids creating suction and touch of the cornea. In a study done by Visser et al. significant increase in both monocular and binocular visual acuity was found with scleral lenses as opposed to best-corrected visual acuity without a scleral lens.8
Conclusion
In general when managing a patient with keratoconus, PMD, keratoglobus, or any irregular cornea, gas permeable lenses will provide the best visual correction. This is due to the inherent nature of the rigid lens to mask corneal irregularity by formation of a lacrimal lens. With increasing severity, smaller lenses will begin to fit improperly, causing both discomfort and reduction of visual acuity. When this occurs, fitting with a larger diameter lens can provide better alignment, comfort, and vision. In the more advanced cases a sclera or mini-scleral lens provides these benefits by vaulting the entire cornea, and reducing lens edge to eyelid interaction.
Bibliography
1. Spoerl, E. Induction of Cross-Links in Corneal Tissue, Exp. Eye Res., 1998; Vol. 66: p 97-103
2. McAlinden, C. Keratoconus I, Optometry Today, 6/13/2008; Vol.48 Issue 12: p.43-47
3. Rabinowitz, Y. Keratoconus SURVEY OF OPHTHALMOLOGY, 1998; Vol.42 Number 4: 297-319
4. Jinabhai, A. Pellucid corneal marginal degeneration: A review, Contact Lens & Anterior Eye, 2011; Vol. 34: p. 56-63
5. Biswas, B. Management of pellucid marginal corneal degeneration, Eye, 2000; Vol. 14: p. 629-634
6. Belin, M. What’s in a Name: Keratoconus, Pellucid Marginal Degeneration, and Related Thinning Disorders, American Journal of Ophthalmology, 2011; Vol. 152, No. 2: p. 157-162
7. Stalboerger, G. Scleral Lenses in the Management of Pellucid Marginal Corneal Degeneration, Optometry, 2008; Vol. 79, No.6: p. 302
8. Visser, E. Modern Scleral Lenses Part I: Clinical Features, Eye and Contact Lens, 2007; Vol. 33(1): p. 13-20