Anterior segment OCT (AS-OCT) has come a long way in recent years. Clinicians are now using it to image everything from the tear film to the conjunctiva and cornea. Not surprisingly, many optometrists now use it to help them assess specialty contact lens fits in particular. As specialty lenses, such as scleral contact lenses and hybrid lenses, continue to gain in popularity, AS-OCT can be increasingly helpful in the lens assessment process. AS-OCT makes what was once a challenging clinical encounter a quick, easy and successful experience.
One study found complications in addition to infectious keratitis associated with specialty contact lens wear could cause hypoxia, inflammation, mechanical influence, deposition and visual blur.1 These complications can be better understood using an AS-OCT map of the contact lens in relation to the cornea. Using AS- OCT to image a specialty contact lens on the cornea can help practitioners decrease any adverse events related to contact lens wear.
|Fig. 1. This AS-OCT, taken with an anterior chamber adapter lens, shows a sagittal depth of 391µm (A). Pupil size is 5.32m (B), and angle Kappa is 2.398m (C).|
Initial Lens Choice
Clinicians can use their AS-OCT to better determine several initial lens parameters, including base curve and the need for back toricity:
Base curve. AS-OCT can aid contact lens practitioners in fine-tuning an initial starting point for pulling a properly fit lens from a fit set based on sagittal height or base curve. Clinicians can use the precise sagittal depth (sag) generated from the tip of the cornea down to the 15mm horizontal chord to get a better idea of the necessary vault height. The sag can be used to match an initial fitting lens from the fitting set. This 15mm chord measurement can also show pupil size and angle kappa, which may be helpful in designing optic zones or multifocal contact lenses (Figure 1).
Fig. 2. The angle of the peripheral cornea, limbus and sclera shows a steeper temporal corneo-scleral insertion and flatter nasal landing zone. The angle caliber tool is measured between chord 10mm and 15mm. The 41° here suggests adding more limbal clearance and a toric back surface lens.
Back toricity. When fitting a scleral contact lens, most practitioners do not know when to choose a back surface toric lens (toric haptic) versus a spherical back surface lens. Back surface toricity can help to stabilize the lens for comfort and best vision, while front surface toricity is only for optical outcome.2 Spherical scleral landing zones on a toric sclera would exhibit a poor-fitting, with-the-rule appearance; the lens will touch the horizontal meridian and lift off in the vertical meridian.3 Landing on both meridians can be achieved only with rotationally asymmetric toric haptic scleral landing zones, which are necessary to ensure lens stabilization and centration.4 AS-OCT can be an excellent tool to use when deciding to use a toric haptic in fitting scleral lenses, as it can help you decide on a starting point. Currently, most scleral fitting sets come with a toric haptic component.
The average peripheral corneal, limbal and scleral angle is approximately 38.4° in normal eyes.5 If a patient’s angle measures landing angle is greater than or equal to 41° at horizontal chords between 10mm and 15mm, a scleral lens is likely to have limbal bearing and scleral toricity (Figure 2). These large insertion angles may warrant a toric haptic lens with additional vault added to the limbal zones.5
|Fig. 3. This anterior segment 5-line raster shows tear clearance below a scleral contact lens. The center thickness of the lens vaulting the cornea is 380µm (A). The tear clearance of 464µm shows midday fogging particles trapped in the tear layer (B). The center corneal thickness is 548µm (C).|
Tear Layer Clearance
One of the most widely used views to evaluate specialty lenses with an OCT is the central clearance of the lens vaulting over the cornea. Research suggests a well-fit scleral lens should have a 100µm to 200µm tear layer in the center and 10µm to 50µm over the limbus.1,6,7
Central clearance is important because of its impact on a patient’s visual outcome and ocular health. One study found excessive tear clearance is associated with increased leukocytes, causing midday fogging and hypoxia.6 Every 50µm increase in clearance has an associated 2.24 times higher odds for midday fogging and possible hypoxia.6 A patient with a tear clearance of 464µm, for example, is 20.79 times more at risk of midday fogging (Figure 3). In contrast, minimal tear layer clearance can put the patient at risk of scarring or discomfort from bearing on the cornea. AS-OCT can help image touch of the contact lens on the cornea, an optimal tear clearance after settling and a tear layer in excess that could result in hypoxia (Figure 4).
|Fig. 4. This anterior segment 5-line raster of the tear clearance below a scleral contact lens shows touch (A), good clearance of 166µm (B) and excessive clearance of 308µm (C).|
Computing the theoretical oxygen transmissibility for contact lens material, contact lens center thickness, plus the amount of tear layer thicknesses is of future clinical interest to determine lens-tear system characteristics that will alleviate corneal hypoxia.7 For now, controlling tear layer surface area is the one way to decrease risks of hypoxia.
Fit vs. Corneal Shape
Using an overall anterior chamber view can benefit a specialty contact fit as it helps the practitioner get a better sense of the relationship between the shape of the eye and how the contact lens settles (Figure 5). This view allows practitioners to see where adjustments need to be made to mimic the overall globe shape.
|Fig. 5. This AS-OCT shows a poor-fitting hybrid lens with decentration, which is causing touch.|
Clinicians can use this anterior segment tool to rule out any corneal edema induced by contact lens wear. Corneal thickness should be measured before specialty contact lens wear and within six months after wear to rule out any edema caused by the contact lens. If the central corneal thickness number increases over time, then the corneal edema is induced by contact lens wear.
Current research suggests 5% is an acceptable amount of edema for daily wear contact lenses.8 Beyond 5% edema, striae appear in the posterior stroma.9 Edema greater than 15% is considered pathological, and visual acuity becomes cloudy. Pathological edema can lead to infiltrates, epithelial microcysts, pain, neovascularization and destruction of endothelial pumps.9
|Fig. 6. Anterior segment 5-line raster images show a scleral lens edge that lands too flat and lifts away from the conjunctiva (A); good, stable landing aligned with conjunctiva (B); and impingement into the conjunctiva with a steep landing zone edge (C).|
Peripheral Scleral Lens Landing
Scleral lenses need to distribute their weight along the conjunctiva. Lenses that fit loosely or flat at the scleral landing zone can cause excessive movement, lid irritation and vision fluctuation (Figure 6A). Lenses fit too tight or steep in the scleral landing zone can cause pain and lack of tear exchange (Figure 6C). Landing zones should land parallel to the conjunctiva for best comfort and fit (Figure 6B).
|Fig. 7. Anterior segment 5-line raster shows limbal clearance of a scleral lens vaulting the limbus of the cornea to scleral junction.|
The human eye can only see 50µm to 60µm under evaluation with a slit lamp.10 Using an OCT can precisely show by how many microns the scleral lens clears the limbus, with an indeal clearance of 10µm to 50µm (Figure 7). Excessive limbal clearance can cause corneal bogging and conjunctival prolapse. Conversely, hypoxia, low tear exchange and discomfort can occur when the limbus is not cleared properly.
In fitting and evaluating specialty contact lenses, AS-OCT may be your new best friend. With a little practice, these new imaging techniques can show you the exact relationship between the lens and the eye—giving you the information you need to get the fit right.
Dr. Conner works as a therapeutic glaucoma optometrist at Dallas Eye and Ear in Dallas.
1. Postnikoff CK, Pucker AD, Laurent J, et al. Identification of leukocytes associated with midday fogging in the post-lens tear film of scleral contact lens wearers. IOVS. 2018;60(1):226-233.
2. Johns L. Using toxicity with scleral lenses. Optometry Times. www.optometrytimes.com/article/using-toricity-scleral-lenses. Published July 17, 2017. Accessed May 5, 2019.
3. Barnett M, Fadel D. Scleral lenses: the benefits of toric landing zones. CL Spectrum. 2017;32 (11).
4. Ritzman M, Caroline PJ, Borret R, Korszen E. An analysis of anterior scleral shape and its role in the design and fitting of scleral contact lenses. Cont Lens Anterior Eye. 2017;41(2):205-13.
5. Kojima R, Caroline P, Walker M, et al. Benefits of OCT when fitting specialty lenses. CL Spectrum. 2014;29(10).
6. Michaud L, Worp EVD, Brazeau D, et al. Predicting estimates of oxygen transmissibility for scleral lenses. Cont Lens Anterior Eye. 2012;35(6):266-71.
7. Schornack MM. Scleral lenses in the management of keratoconus. Eye Contact Lens. 2010;36(1):39-44.
8. Efron N, Holden BA. A review of some common contact lens complications; Part I: The corneal epithelium and stroma. Optician. 1986;192(5057):21-6.
9. National Research Council. Contact lens use under adverse conditions. Washington, DC: National Academies Press; 1990:2.
10. Filtration. Nilfisk Industrial Vacuums. www.nilfiskcfm.com/filtration. Accessed May 5, 2019.