Anterior Segment-OCT Improves Specialty Contact Lens Fitting Experience

The ability to image both anterior and posterior eye structures with exquisite detail has made optical coherence tomography (OCT) an indispensable tool for eye care clinics. For contact lens practitioners, OCT of the anterior segment (AS-OCT) structures is not only valuable for monitoring corneal disease, it can also be instrumental in enabling clinicians to evaluate lens fitting and design. Since scleral lenses are frequently used as therapeutic devices to treat corneal diseases, which include ectasia and surface disease, these large diameter gas permeable (GP) lenses must be fitted properly to provide optimal vision and comfort and to avoid complications such as corneal edema and conjunctival hyperemia. Although clinicians can achieve a proper lens fitting with a thorough slit lamp evaluation, they can use AS-OCT to expedite the evaluation, easily obtain essential measurements, monitor corneal health, and help troubleshoot potential fitting complications. 

Sagittal Depth 

The scleral lens’s fit over the anterior eye creates the sagittal height (OC-SAG), and a well-fit lens design allows the scleral lens sagittal depth (SCL-SAG) to align with the scleral shape while vaulting over the cornea. The SCL-SAG depends largely on the base and intermediate curves and lens diameter. Measurement calipers on the AS-OCT enable the clinician to estimate OC-SAG.¹ Adding this value to the desired central clearance will produce a SCL-SAG value, and since lens manufacturers frequently define their trial lenses in terms of sagittal depth, optometrists can expeditiously obtain this value with AS-OCT. 

Central Clearance 

AS-OCT caliper tools can also determine central clearance based on measurements of scleral lens and post-lens tear reservoir thicknesses. While there is no definitive requirement as to how much clearance there should be, the generally accepted clearances are between 100 and 500 µm, with most practitioners fitting at an average of 200 to 300 µm.² A potential concern for fitting thicker lenses and lenses with higher central clearance is hypoxia-induced corneal swelling.³ 

When determining SCL-SAG, clinicians should expect the lens to settle into the scleral conjunctival tissue with time. The central clearance decreases as the lens settles and compresses the tissue. A 2014 study suggests that the amount of lens settling is largely dependent on its overall diameter, where lenses with a greater diameter settle to a greater extent.⁴ The same study also suggests that 70% of scleral lens settling occurs within the first 2 hours of wear.⁴ A 2017 study produced  similar results, with 3 study lenses demonstrating a mean [SD] reduction of approximately 34 [48] μm in central clearance after 1 hour of lens wear.⁵ Practitioners must factor lens settling in when determining the desired final clearance.

Other complications can result from either minimal or excessive clearance, both of which can immediately be determined by AS-OCT. When the lens is not adequately clearing or is bearing on the cornea, mechanically induced epithelial erosion and patient discomfort may occur, potentially resulting in decreased visual acuity and ocular surface complications related to epithelial defects. Excessive clearance can produce a larger vertex distance, creating a magnification effect or an accumulation of debris from the tear film, resulting in a visually significant disturbance known as midday fogging.⁶ AS-OCT can allow the clinician to visualize the density of the debris and determine the amount of clearance reduction needed to minimize the disturbance.

Credit: Melanie Frogozo, OD, and Anthony Mac, OD

Figure 1: Mechanically induced epithelial erosion and patient discomfort may result from scleral lenses bearing on the central cornea.

Credit: Melanie Frogozo, OD, and Anthony Mac, OD

Figure 2: Excess scleral vault may produce unwanted magnification or result in an accumulation of tear film debris.

Limbal Clearance 

The limbus is the other key structure that a scleral lens is expected to vault. Within the basal epithelial layer of the limbus are the limbal stem cells, which aid in the renewal of the corneal epithelium.⁷ These cells do not regenerate naturally and must be protected from harm, which includes mechanical irritation from a poorly fitting scleral lens. A lens with a flat vault has the potential to create microcystic edema and limbal stem cell deficiency. A lens with excessive limbal vault, on the other hand, may lead to hypoxia due to increased edema. In instances of excessive limbal vault, a thicker tear layer acts as a barrier to sufficient oxygen diffusion to the delicate limbal stem cells, potentially resulting in stem cell death. It may also induce conjunctival prolapse, whereby conjunctival tissue is sucked into the lens chamber by negative pressure created by the lens, subsequently coming to rest on the limbal area. This prolapse may also cause mechanical irritation and hypoxia of the limbus, resulting in similar complications experienced by wearing lenses with a flat vault. As with central clearance, the limbal clearance can also be measured with AS-OCT calipers. The generally accepted range for final limbal clearance is between 100 and 200 µm.⁸

Landing Zone

A scleral lens must exhibit proper scleral conjunctival alignment for optimal centration, vision, and comfort. Ideally, these lenses should land in a manner that allows for an even distribution of weight. If the haptic area is too steep and bears too much weight, conjunctival vasculature impingement may result. This can lead to granuloma formation and difficulty in removing the lenses. If the haptic portion is too flat and lifts off of the conjunctival surface, the patient may experience discomfort that is more apparent with blinking, or an excessive tear exchange that may lead to midday fogging.⁶ AS-OCT can image the landing zone and detect improper alignments that may lead to adverse events.

Credit: Melanie Frogozo, OD, and Anthony Mac, OD

Figure 3: Adequate corneal vault will optimize patient scleral lens wearing experience.

Monitoring Corneal Health 

Corneal pachymetry evaluation is particularly useful when fitting specialty contact lenses in patients who may be at risk for hypoxia and edema. Post-surgical corneas with low endothelial cell counts are particularly susceptible to edema with specialty lens wear.^6,9 AS-OCT instruments may provide either global pachymetry maps or caliper tools to allow clinicians to measure corneal thickness. This method has demonstrated greater repeatability than ultrasound pachymetry instruments typically used in practice.¹⁰ By measuring and comparing pre and post lens corneal thicknesses, optometrists can continue or alter their management strategy — edematous corneas may require lenses with a higher oxygen permeability (Dk) or lower thickness to thin the post lens tear reservoir. Clinicians may also decrease lens wear time, prescribe topical hypertonics, or use a combination of these treatments.¹¹ AS-OCT can also image other corneal anomalies including corneal bullae, stromal scars, and intrastromal ring segments.

AS-OCT is a valuable tool for specialty lens fitting that can provide scleral lens and corneal imaging that may not be discernible by relying solely on biomicroscopy. It is particularly useful for fitting patients with compromised eyes, including individuals with corneal transplants. By witnessing and understanding the interaction between the lens and the eye on a microscopic level, the practitioner may make more informed clinical decisions and provide a more efficient and seamless fitting experience.