What are the guidelines for scleral lens diagnostic fitting?

Scleral Lens Diagnostic Fitting Guidelines

Initial Lens Selection Strategy

Begin scleral lens fitting by selecting the initial diagnostic lens based on sagittal depth rather than base curve alone, as this approach is preferred by experienced practitioners and correlates better with successful fitting outcomes. 1

Pre-Fitting Assessment

Before initiating scleral lens fitting, obtain comprehensive baseline measurements:

• Corneal topography/tomography to assess corneal surface irregularities, keratometry values (steep and flat K), and reference sphere measurements 2, 3
• Slit-lamp biomicroscopic examination evaluating tear film, conjunctiva, sclera, cornea, and anterior chamber depth 2
• Visual acuity testing at distance and near with current correction, documenting baseline corrected distance visual acuity (CDVA) 4
• Tear function assessment including tear breakup time, Schirmer testing, and ocular surface vital dye staining 2, 4
• Corneal pachymetry and endothelial cell analysis when indicated 2

Diagnostic Lens Selection Approach

Experienced practitioners prioritize sagittal depth (63% of experienced fitters) over base curve (preferred by only 60% of new fitters) when selecting the initial diagnostic lens. 1 However, the predictive relationship between topographic parameters and final lens specifications remains weak (r ≈0.50-0.73), making diagnostic fitting the most efficient method. 3

The correlation between corneal topographic indices and scleral lens base curve shows:

• Steep keratometric power correlates with base curve (r = 0.70-0.72) 3
• Flat keratometric power correlates with base curve (r = 0.70-0.71) 3
• Reference sphere correlates with base curve (r = 0.68-0.73) 3

Despite these correlations, diagnostic fitting remains superior to calculation-based approaches, as imaging technology does not eliminate the need for skilled clinical observation. 5

Fitting Process and Assessment

Number of Trials Required

Expect an average of 3.7 ± 1.9 diagnostic lens trials (range: 1-8 trials) to achieve optimal fit. 6 More experienced practitioners typically require 2.1 lenses ordered on average with 2.9 follow-up visits to complete fitting. 5

Critical Assessment Parameters at Each Visit

All experienced prescribers (100%) assess the following parameters at every follow-up visit:

Central Clearance Evaluation

• Estimate central clearance by comparing the thickness of the post-lens tear reservoir to scleral lens thickness using a slit lamp beam - this technique is used by 100% of experienced prescribers versus only 62% of new prescribers 1
• Evaluate vault using both slit-lamp examination and anterior segment optical coherence tomography when available 6

Post-Removal Assessment

All experienced practitioners (100%) schedule follow-ups at specific times and assess: 1

• Conjunctival compression (100% of experienced fitters)
• Conjunctival staining (100% of experienced fitters)
• Corneal staining (100% of experienced fitters)

Lens Characteristics and Advantages

Scleral lenses completely vault over the corneal surface, providing centration, minimal apical touch, stability, and improved central visual acuity. 2 This design is particularly advantageous for:

• Advanced keratoconus when other lens modalities fail due to lens decentration 2
• Failed rigid gas-permeable (RGP) contact lens wear - studies show all patients referred for scleral lens fitting after RGP failure can be successfully fitted with either conventional or custom-designed scleral lenses, avoiding keratoplasty 2
• Scleral irregularities including pterygia and glaucoma blebs, using custom-designed lenses 2

Common Pitfalls and Considerations

Disadvantages to Anticipate

• Decreased tear exchange beneath the lens 2
• Difficulty with insertion and removal requiring patient education and practice 2
• Significantly higher cost compared to larger-diameter or mini lenses, which may be cost-prohibitive for patients with inadequate insurance coverage 2
• Patient refusal rate of approximately 15% (5 of 34 patients) due to cost concerns 6

Clinical Outcomes

When successfully fitted, scleral lenses provide:

• Mean daily wear time of 9.3 ± 4.5 hours (range: 2-16 hours) 6
• Significant visual improvement: mean uncorrected logMAR visual acuity improves from 1.09 ± 0.47 and spectacle-corrected logMAR of 0.67 ± 0.50 to 0.13 ± 0.20 after scleral lens fitting 6
• 93% of patients achieve CDVA of 20/30 or better with appropriate lens wear 2

Important Limitations

Scleral lenses do not arrest disease progression - ectasia can continue despite good corrected visual acuity, making early consideration of corneal cross-linking important for progressive keratoconus rather than waiting until visual acuity declines or corneal thinning precludes treatment. 2

Fitting Algorithm Summary

1. Obtain comprehensive topography and baseline measurements including sagittal depth parameters 2, 3
2. Select initial diagnostic lens prioritizing sagittal depth over base curve alone 1
3. Assess central clearance using slit-lamp beam comparison to lens thickness 1
4. Schedule timed follow-up visits (average 2.9 visits required) 5
5. Evaluate post-removal findings including conjunctival compression and staining patterns 1
6. Expect 2-4 lens trials before achieving optimal fit 5, 6
7. Use anterior segment OCT when available to confirm appropriate vault 6