GP Lens Case Grand Rounds Troubleshooting Guide – 34

Intermediate Keratoconus: Heidi Wagner OD, MPH, FAAO

Background

MM was a long-term hydrogel lens wearer complaining of decreased vision in the right eye that was not correctable with spectacles or hydrogel toric contact lenses. He had been referred to the Cornea and Contact Service for consultation regarding a tentative diagnosis of keratoconus and to evaluate the potential improvement in vision with gas-permeable (GP) contact lenses. The referring provider reported that the posterior segment and other ocular health findings from the patient’s recent examination were unremarkable.

Test Procedures, Fitting/Refitting, Design & Ordering

Visual Acuities (with Hydrogel Toric Contact Lenses):

OD: 20/100 Over-refraction: No improvement
OS: 20/40 Over-refraction: No improvement

Slit Lamp Examination:

Good centration, coverage, and movement were observed with current hydrogel lenses. The hydrogel lenses exhibited minimal deposits. Lens rotation was 2˚ nasal OD, 4˚ nasal OS, and stable OU. Corneas were positive for Fleischer’s ring OU and negative for other corneal anomalies including apical scarring characteristic of keratoconus.

Corneal Topography:

Corneal topography revealed inferior steepening in each eye consistent with keratoconus; simulated keratometry (sim-k) readings revealed asymmetric measures, steeper in the right eye than the left (see photo):

Ancillary Testing:

A gas-permeable lens was utilized for diagnostic purposes, revealing best-corrected acuities of 20/30 OD and 20/25 OS. The initial base curve radius was selected based upon the steep sim-k readings.

Patient Consultation and Education

We advised the patient that our findings were consistent with those of the referring doctor and consistent with the diagnosis of keratoconus. We provided the patient with educational materials about the condition. Furthermore, we advised the patient that initial diagnostic lens fitting revealed a significant improvement in best-corrected visual acuity in his right eye and a modest improvement in his left eye. Our recommendation was, therefore, that we refit the patient with GP lenses, thus addressing his chief complaint.

The lens fit was refined by bracketing between apical clearance and touch. The final lens parameters are illustrated in Table 1. The peripheral lens system design was based upon the parameters of diagnostic lenses with a small modification in the peripheral curve system to compensate for excessive edge clearance observed during the lens fitting. A spherical over-refraction was performed to determine the final lens power.

Table 1: Lens Parameters:

Power(D)BCR(mm)OAD/OZD(mm)SCR/W(mm)ICR/W(mm)PCR/W(mm)

OD: -7.75 +lenticular 7.10 8.6/6.0 9.50/.65 10.50/.40 11.50/.25
OS: -6.75 +lenticular 7.30 8.6/6.0 9.50/.65 10.50/.40 11.50/.25

Heavy blend, Boston ES, Blue, Dot OD, 0.10 CT OU

Follow-Up Care/Final Outcome

Lenses were dispensed at a subsequent visit with visual acuities of 20/25 OD, 20/20 OS, and 20/20 OU. Lens evaluation revealed slight apical touch, paracentral clearance, mid-peripheral bearing, and peripheral clearance OU. Although the lenses decentered slightly inferiorly, the patient did not report any adverse visual symptoms associated with lens wear.

The lens care system and wearing schedule were reviewed with the patient prior to dispensing. Although the patient missed his initial follow-up appointment, he reported significant improvement in visual acuity as well as good comfort with lenses after a three-week adaptation period.

Discussion/Alternative Management Options

The patient was advised that alternative treatment plans include continuing with spectacle wear or hydrogel toric contact lens wear, which would not correct for the irregular astigmatic refractive error. A hybrid lens design would provide an alternative option, although until recently, was available only in low Dk lens materials.

The patient questioned whether he was a good candidate for refractive surgery. He was advised that the outcomes of elective procedures such as LASIK (laser-in-situ-keratomileusis) are somewhat unpredictable in patients with his condition and that therapeutic procedures such as penetrating keratoplasty are more commonly utilized for patients experiencing contact lens intolerance and/or decreased visual acuity that is not correctable with contact lenses.

A gas-permeable lens may be utilized as a diagnostic test with multiple applications. First, it may be used to confirm that the underlying cause of decreased visual acuity is corneal irregularity. Second, an experienced clinician can infer the corneal topographical map though assessment of the fluorescein pattern. Third, the test demonstrates the patient’s potential visual acuity with GP lenses.

Although numerous lens fitting sets for keratoconus exist, certain characteristics predominate. Smaller optical zone and overall lens diameters, steeper base curve radii, and flatter peripheral curve systems than cosmetic lenses of equivalent base curve radii characterize lenses designed for this condition. Fitting nomograms generally begin with a base curve radius equivalent to the steep keratometry reading or mid-way between the steep and flat keratometry readings. Regardless of the nomogram selected, it should be noted that this is a starting point that should be refined with diagnostic lens fitting.

End points for base curve radii vary with the clinician but generally avoid excessive apical clearance or touch. The final lens power is better determined through over-refraction than empirical methods. The parameters of the peripheral curve system of the diagnostic lenses may be duplicated or modified appropriately based upon the fluorescein patterns observed during diagnostic lens fitting. Lens care and follow-up for patients with moderate presentations of keratoconus are similar to those of cosmetic lens fits.

Supplemental Readings

  1. Bennett, ES & Weissman BA Clinical contact lens practice. Lippincott Williams & Wilkins 2005
  2. Krachmer JH, Feder RS, Belin MW. Keratoconus and related noninflammatory corneal thinning disorders. Surv Ophthalmol 1984; 28:293-322
  3. Szczotka LB, Barr JT, Zadnik K, the CLEK Study Group. (2001) A summary of the findings from the Collaborative Longitudinal Evaluation of Keratoconus (CLEK) Study. Optometry. Journal of the American Optometric Association 72:574-587
  4. Dowey, SW. What is keratoconus? A reference guide for patients and their families. National Keratoconus Foundation. 1999

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