By Ieuan Rees
A scleral lens fitting for a presbyopic anaesthetist with Salzmann’s nodular degeneration took nearly two years to get to a point where I was happy. It cost me more than I made and required advances in lens design along the way.
Salzmann’s nodular degeneration is a rare, non-inflammatory, slowly progressive, degenerative condition characterised by bluish-white nodules raised above the surface of the cornea.
Corneas may have a history of phlyctenulosis, trachoma, vernal keratoconjunctivitis, measles, scarlet fever and various other viral diseases. The majority of cases are seen without recognised previous keratitis.
Dense irregularly arranged collagen tissue with hyalinisation between epithelium and Bowman’s layer or beyond were seen in the anterior OCT image of the nasal side of the cornea with scleral lens on eye.
When patient GM presented, the right cornea had scars from 3 to 8 o’clock counter-clockwise in patches just in from limbus with an iron line deposit from 3 to 1 o’clock, and a mild pannus from 11 to 9 o’clock with the scarring tending to be raised from 8 to 10 o’clock (Figure 1).
Figure 1. Salzmann’s nodular degeneration
Anterior chambers were open, crystalline lenses were clear, and lid margins, tarsal plates and lower fornices were healthy. The left eye had a 3 mm nasal pterygium with a 4 mm apex, 4 mm base and a flat calm appearance (Figure 2). The ophthalmologist whom GM had been seeing was not recommending surgical intervention for either the pterygium or Salzmann’s nodules, but was not averse to contact lens fitting.
Figure 2. Nasal pterygium
Topography revealed 5.4 dioptres of irregular astigmatism for the right eye and 3.4 dioptres for the left.
Subjective refraction was:
R 6/15= +8.00 / -7.50 x 67 6/9++ L 6/6= +2.75 / -2.50 × 165 6/6
Anisometropia with this correction caused diplopia and flare was a significant presenting complaint. Fine detail at arm’s length was required to perform his duties as an anaesthetist.
Having not developed a fitting protocol as this stage, initial trial lenses were:
R #10 7.1 / 7.1 / 4.0 / 16.50 / -5.00 Sag: 4704
L #12 7.5 / 6.5 / 4.0 / 16.50 / -3.00 Sag: 4809
These resulted in an over-Rx of:
R -0.25 / -1.25 × 75 6/6++ L -0.50 / -0.50 × 95 6/6++
Fenestrated scleral lenses 18.5 mm to 23.5 mm in diameter were tediously slow to fit, adjustments were not repeatable and in-office grinding and polishing was very noisy. Every time the base curve was altered to improve the sagittal depth, the front surface needed changing.
When the elevation differential inside the pupil zone exceeded 100 microns, it was not possible to grind curves that eliminated an overlap of the ‘pump’ bubble inside the pupil zone.
Non-fenestrated smaller scleral lenses (16.5-18.5 mm) had improved repeatability but many resulted in an inadequate landing zone. The older style of scleral lenses tended to push into the conjunctiva whereas the landing zone of the Innovative design scleral landed softly, spreading the weight like a snowshoe.
The Smith technique to determine the anterior chamber depth and subsequently select a suitable trial lens requires extrapolation of a suspect measurement and is subject to variation dependent on operator and slitlamp and in my opinion is not worth the effort.
I determine the height along the steepest axis at a chord of 10 mm and add to this 2,500 microns to determine the sagittal depth of the lens required to vault the cornea by around 200 microns. Always assuming it is better to have more clearance than no clearance, I select a lens with that sagittal depth or greater.
Lens diameter is determined by the horizontal visible iris diameter (HVID). If the HVID is greater than 11.5 mm, I select a 17.5 mm trial lens and if less than 11.5 mm, a 16.5 mm trial lens.
I have found that if the HVID is greater than 11.8 mm, an 18.5 mm lens is best. The best sphere as determined by the elevation map becomes the base curve of my final prescription.
Sometimes we just take an educated guess and fit a lens on eye to see what we get, which is what I did in this case.
Initially the selected lens for the right eye was a front surface ballasted toric lens but it tended to rotate off axis about 20 degrees and was generally unstable.
As the techniques for using the warpage of a scleral lens developed while the fitting of this first lens was occurring, I refitted a rotationally toric periphery with central spherical back optic and a front surface toric, which generally lies along the primary axes of the peripheral toricity.
Once well-fitted, the lenses gave good stability and great vision, and topography shows regular front surface optics.
GM preferred some monovision to improve his functioning at 75 cm and a +0.50 addition for the right lens was applied.