Optical coherence tomography (OCT) is a useful tool for ocular imaging. Recently, the capabilities of this technology have expanded beyond the posterior segment to facilitate the analysis of anterior structures as well. The qualitative, cross-sectional data obtained from anterior segment optical coherence tomography (AS-OCT) allows visualisation of the cornea, limbus and sclera, which is of great value to practitioners who treat corneal pathologies, such as keratoconus and corneal grafts, and to those who fit specialty contact lenses. This article explores the valuable imaging capabilities that AS-OCT provides to a specialty contact lens practice.
AS-OCT offers many advantages to clinical practice, one of the most important being to enhance patient education, which helps achieve treatment compliance. For example, by displaying to a patient with keratoconus their corneal profile on ASOCT, the patient can better visualise the unique corneal shape, providing improved knowledge and understanding of the nature of their corneal condition. Using a profile to demonstrate shape is far easier for a patient to comprehend, as opposed to relying on interpretation of shape from corneal topography maps, which can be more difficult for patients to visualise without an underlying appreciation of visual optics.
the capabilities of this technology have expanded beyond the posterior segment to facilitate the analysis of anterior structures as well
Corneal topography is the tool most optometrists use to understand the shape and elevation of the eye. Unfortunately, there are limitations to corneal topography, including less accurate peripheral data and the inability to measure contours of the limbus and sclera.1 Additionally, corneal topography cannot evaluate posterior corneal elevation, precluding the measuring of corneal thickness. Fortunately, AS-OCT is able to capture this data.
AS-OCT provides a non-contact, noninvasive method of measuring corneal thickness. This is extremely useful in patients with keratoconus who have focal corneal thinning. Monitoring corneal pachymetry measurements is essential for observing any keratoconus progression and for determining eligibility for corneal collagen cross-linking (CXL). Moreover, there is less measurement variability with AS-OCT compared to ultrasonic biometric methods of corneal thickness measurements, because AS-OCT does not rely on indentation of a probe and is not influenced by off-axis probe measurements.2
AS-OCT can also qualitatively capture corneal asymmetry, which is again useful in keratoconic patients. This is advantageous over topographical systems because corneal topography often fails to capture accurate corneal shape in the case of more advanced pathologies. This is due to the system’s difficulty in capturing very advanced corneal irregularity, poor ocular surface and corneal scarring.3 AS-OCT can capture corneal asymmetry with even the most advanced forms of corneal pathology.
Patients who have undergone CXL to treat progressive corneal ectasia often demonstrate a demarcation line, visible on AS-OCT (Figure 1a).4 This represents CXL treatment depth, which separates treated anterior stroma from untreated posterior stroma. In addition to a CXL demarcation line, corneal haze is a common post-CXL complication that can be visualised on AS-OCT.3 Corneal haze (Figure 1b) is often an incidental finding when fitting miniscleral or hybrid contact lenses following CXL surgery. This finding usually fades with time, and patients love being able to visualise the reduction in their corneal haze on subsequent AS-OCT scans.
Some keratoconic patients are treated surgically with intrastromal corneal ring segments, and AS-OCT is helpful in identifying their location within the cornea. Correct placement of these ring segments is imperative. If the ring segments are inserted too posteriorly in the cornea they can perforate the endothelium and migrate into the anterior chamber.3 If the segments are too anteriorly positioned, there is a risk of an epithelial break and extrusion of the segment. Figure 2a demonstrates intrastromal ring segments that have migrated beyond the anterior stroma and through the epithelium. This ring extrusion was extremely painful and required immediate surgical removal (Figure 2b).
The diagnosis of acute corneal hydrops can be confirmed on AS-OCT scans as it allows the visualisation of the Descemet’s membrane break. AS-OCT is then used to monitor the resolving corneal oedema and subsequent scar formation. The scar density and depth can ultimately determine whether a patient undergoes a partial or full-thickness corneal graft, should transplantation be required.3 Figure 3a demonstrates an impending corneal hydrops, with separation of Descemet’s membrane from the stroma. Figure 3b demonstrates the acute phase of corneal hydrops and Figure 3c demonstrates the remaining scar six months later. Despite being refit with a mini-scleral after corneal hydrops resolution, this patient required a full-thickness corneal graft due to unsatisfactory vision from the residual stromal scarring.
CORNEAL GRAFT MANAGEMENT
In patients who have had a corneal graft, AS-OCT can be used to confirm which type of procedure was performed. In deep anterior lamellar keratoplasty (DALK), a white line of higher reflectivity separates the endothelium from the stroma.5 No such line exists with penetrating keratoplasty (PK). If there are issues at the graft-host junction interface, such as interface haze, these issues can be visualised on AS-OCT. Figure 4a demonstrates post-DALK interface haze, which severely limits visual potential in this eye. Figure 4b shows the fellow eye with a failing PK with epithelial bullae.
An often overlooked, yet simple and convenient function of AS-OCT, is the ability to measure cornea oedema. This is important when fitting mini-sclerals (and especially to corneal graft patients), as it is well established that mini-sclerals create low levels of corneal oedema with lens wear.5 As such, it is important to monitor corneal thickness changes pre- and post-contact lens wear. Caution should be taken fitting a miniscleral to a PK graft with a low endothelial cell count, as compromised corneas are already prone to oedema and graft rejection.
While corneal grafts are variable in shape and size,6 grafts often have flatter central curvatures and steeper mid-peripheries, requiring an oblate contact lens design. Oblate mini-sclerals can be fit with minimal central clearance and greater clearance toward the mid-periphery.
CONTACT LENS FITTING
The benefits of using AS-OCT for fitting specialty lenses are well documented.8-14 The thickness of the post-lens tear layer (PLTL) of a mini-scleral must be monitored to ensure complete corneal clearance. The central corneal, limbal and graft-host junction clearance levels, as well as the landing profiles are all easily visualised on AS-OCT scans. Additionally, the scan can be positioned to any given location, which helps problem solve issues such as lens awareness from an unsuitable edge profile. Furthermore, the infrared light source used by AS-OCT does not alter pupil size, enabling comparison of pupil size to the size of the contact lens optic zone, and minimising the inconvenience of haloes and glare at night.
CENTRAL CORNEAL CLEARANCE
Most mini-scleral fitting guides specify an initial central clearance of 200 to 400μm to allow for 100 to 200μm of settling during wear. The optimum central clearance has been debated in the literature, but most are in agreement that as long as there is no insult to the cornea or post-tear layer debris, and the patient is happy, then the vault is acceptable.15
At our specialty contact lens clinic, we notice large variations in mini-scleral settling between lens designs and between patients. To aide in monitoring the amount of lens settling in any given patient, the duration of lens wear prior to the appointment is recorded at all visits, including during the initial fitting process and all subsequent aftercare visits.
AS-OCT can quantitatively measure limbal clearances and allow identification of mechanical compression at the limbus, which would otherwise rely on detecting the presence of limbal staining following lens removal.
AS-OCT displays the location and amount of limbal compression, which guides clinical decision making on how much to adjust the lens parameters. This is particularly useful if you know how much settling occurs throughout the day. Insufficient limbal clearance causes complications such as corneal staining, epithelial breakdown, corneal neovascularisation, and scarring. Conversely, excessive clearance can cause epithelial bogging and conjunctival prolapse.
PERIPHERAL LANDING PROFILE
Mini-sclerals rest on the conjunctival tissue and are designed to ‘seal off ’. Lenses that fail to do so cause discomfort, visual instability, and lens fogging. As excessive or insufficient compression can cause comfort issues, positioning the AS-OCT in the reported location of discomfort can be helpful to examine the lens-sclera relationship. Often, scleral asymmetry is identified, guiding lens design modifications. Incidentally, excessive scleral compression has been linked to increased intraocular pressure.14
Fogging is a common mini-scleral complication and is often attributed to either excessive corneal clearance or scleral asymmetry causing tear exchange. Figure 5 demonstrates post-lens debris in a patient with an asymmetrical sclera. The fogging is resolved upon being refit with a mini-scleral with a toric edge profile.
AS-OCT is a wonderful tool for education and clinical decision making. Being able to better educate patients on their corneal conditions and the complexities of their contact lens fitting creates happier patients who better understand the diagnostic and therapeutic processes.
As technology is rapidly developing, moving us more from an era of qualitative to quantitative assessments, we are excited to imagine future OCT applications.
Jillian Campbell graduated in optometry from Queensland University of Technology, then completed a clinical residency at the Australian College of Optometry, as well as two postgraduate Specialist Certificates through the University of Melbourne in the management of paediatric patients and contact lens patients.
Ms Campbell enjoys the challenges of specialty contact lens fitting and ocular pathology, and practices at Richard Lindsay & Associates and at the Australian College of Optometry. She is a committee member for both ECOVICSA and CCLSA committees.
Andrew Huhtanen graduated as a Doctor of Optometry from the University of Waterloo, Canada. He has a strong interest in anterior segment disease and advanced contact lenses and has presented numerous lectures on specialty lens fitting; with topics including keratoconus, mini-sclerals, orthokeratology, paediatric aphakia, corneal grafts and post-refractive surgery fittings.
Dr Huhtanen practiced at Richard Lindsay & Associates and was involved in clinical education at University of Melbourne as a Clinical Supervisor for Optometry students and as Course Facilitator for the Specialist Certificate in the Management of Contact Lens Patients. He is currently the Clinic Director, Melbourne Eyecare Clinic at the University of Melbourne, Department of Optometry and Vision Sciences.
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- Yip, Harry and Chan, Elsie. Optical coherence tomography imaging in keratoconus. 2018, Clinical and Experimental Optometry , pp. 218-223.
- Lhuillier, L , et al., et al. Visibility and Depth of the Stromal Demarcation Line After Corneal Collagen Cross-Linking Using Anterior Segment optical Coherence Tomography: Comparison Between Isoosmolar and Hypoosmolar Riboflavin. 2018, Cornea, pp. 27(5): 567-573 .
- Zhao, Y , et al., et al. Malapposition of graft-host interface after penetrating keratoplasty (PK) and deep anterior lamellar keratoplasty (DALK): an optical coherence tomography study. 2020 , BMC Ophthalmol , p. 31:20(1):41.
- Mukesh, Kumar, Shetty, Rohit and Vincent, Stephen. Scleral Lens-Induced Corneal Edema after Penetrating Keratoplasty. 2020, American Academy of Optometry , pp. 697-702.
- Szczotka, L B and Lindsay, Richard G. Contact lens fitting following corneal graft surgery. 2003 , Clinical Experimental Optometry, pp. 244-249.
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- Conner, Amy. Fit Specialty Contact Lenses with OCT Precision. 2019, Review of Optometry, pp. 1-5.
- DeNaeyer, GW. Improve your scleral lens fitting success. 2014, Contact Lens Spectrum, p. 29(11).
- Gimenez-Sanchis, Imma, et al., et al. Anterior sgment optical coherence tomography angiography to evaluate the peripheral fitting of scleral contact lenses. 2018, Clinical Optometry , pp. 103-108.
- Kojima, Randy, et al., et al. Benefits of OCT When Fitting Specialty Lenses. 2014, Contact Lens Spectrum , pp. 1-7.
- Vincent, Stephen, Alonso-Caneiro, David and Collins, Michael J. Optical coherence tomography and scleral contact lenses: clinical and research applications. 2019, Clinical and Experimental Optometry , pp. 224-241.
- Michaeud, L, et al., et al. Predicting estimates of oxygen transmissibility for scleral lenses. 2012, Contact Lens Anterior Eye , pp. 266-271.