GP Lens Case Grand Rounds Troubleshooting Guide – 37

Advanced Keratoconus: Marlane Brown, OD, FAAO

Background

This patient JH presented with advanced keratoconus. Her vision was poor with glasses and she desired improved vision. She was wearing old glasses at this visit and had not been able to tolerate contact lenses in the past. She had tried gas permeable (GP) and soft lenses in the past without success. Her chief complaint was poor vision and poor comfort with the soft and the gas permeable lenses.

Entrance Acuity with Previous Glasses (4 – 5 Years Old):

OD: -1.75 +1.00 x 147 20/50
OS: -1.75 +1.25 x 022 20/60

Test Procedures, Fitting/Refitting, Design & Ordering

Manifest Refraction and Best Corrected Spectacle Acuity (BSCVA):

OD: -2.25 +2.25 x 163 20/30-2
OS: -3.00 +2.50 x 002 20/40

Keratometric Readings:

OD: 46.50 x 49.50 (irregular mires)
OS: 46.87 x 52.00(irregular mires)

Thinnest Pachymetry Reading (Inferior to Central Cornea):

OD: 325 um
OS: 370 um

Corneal Topography:

Initiated the fitting process with Conforma’s ESC contact lenses. These are parabolic aspheres by design. They were fit “on K”, then slightly steeper, then slightly flatter, all without success. There was poor comfort and apical bearing in all cases with best visual acuity of 20/40 OD and OS. Spherical design gas permeable lenses provided her with improved vision but reduced comfort. This design, likewise, caused too much bearing on the cone.

Patient Consultation and Education

My options with this patient were to inform her due to the advanced nature of her cone, this was the best we could do, and simply to try to adapt to the best gas permeable lenses we had attempted to date, or keep trying. She did feel the vision was somewhat better with contact lenses versus her glasses, but was unable to tolerate the lenses for more than 3-4 hours at a time. We decided to continue to fit with a more complex lens, the “Comfort Kone” lens from Metro Optics.

This is a “triaspheric” lens that aims for maximum alignment on an irregular cornea. It has a small spherical central optical zone which fits the peak of the cone and a flatter “A” curve which aligns with the cone’s slope in the periphery. The greater the A value, the greater the change from base curve to peripheral fitting curve. This allows maximum alignment in the mid-cornea where the bulk of the lens will rest. The manufacturer states that one can identify the midperipheral curve by adding the base curve and the “A” value. For example, if the base curve radius is 6.2mm and the A value is 14, then the curvature at midperiphery is 7.6mm. The lens is available in the following parameters:

Comfort Kone Parameters:

Overall Diameter (OAD): 7.5 – 9.5mm
Base Curves: 75.00D to 42.00D
‘A’ Values: A3 to A 20
Powers: Any sphere

To fit the Comfort Kone lens without corneal topography, it is important to determine the starting “A” value. If the flatter keratometry reading is 49.00D or flatter the A5 design should be used. If the flatter keratometry reading is between 49.00D and 52.00D, the A10 design should be used. If the flatter keratometry value is 52.00D or steeper, the A15 design should be used. Next, the base curve radius should be selected and should be similar in value to the steeper “K” reading.

Follow-Up Care/Final Outcome

For patient JH, I chose a Comfort Kone lens base curve slightly flatter than the steepest K reading. In the right eye I fit an A5 “A” curve based on the manufacturer’s fitting plan. I needed an A10 “A” curve for the left eye due to a faster steepening peripheral curve. She had immediately improved comfort and a contact lens visual acuity of OD 20/20-2 and OS 20/25.

Discussion/Alternative Management Options

Keratoconus complications include punctate epithelial keratopathy (PEK), epithelial abrasions, and raised nodular subepithelial scars. All these are more likely to occur over the steepest area of the cone, and all may cause contact lens intolerance. Changing lens designs may change the way the lenses fit or “bear” on the cone, and may improve comfort and vision.

A steep cornea is an indicator of keratoconus. However, the Collaborative Longitudinal Evaluation of Keratoconus (CLEK) study cohort showed that 5% of patients with keratoconus had flatter than 45.00D keratometry readings in the steeper meridian. All clinical signs including slit lamp signs and corneal distortion is probably more indicative of disease than keratoemtry alone. How steep the central corneal curvature is and how fast the curve of the cornea changes will help determine the lens fitting parameters.

Irregular corneas require creativity. It is important to not be fearful of attempting different lenses such as some of the newer keratoconus designs. Utilize the resources of your CLMA member laboratory (i.e., consultants, designs, diagnostic fitting sets, fitting aides). Remember that the goal is to satisfy your patient by improving vision and comfort.

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