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Anterior Segment OCT for Anterior Segment Surgery

Nov 1, 2021
13 min read
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Optical coherence tomography (OCT) is a non-contact imaging technology that produces high-resolution cross-sectional images of ocular tissues.1,2 First introduced in 1994 and commercially available in 2001, anterior segment OCT (AS-OCT) enables precise visualization of the anterior segment structures and can therefore be utilized in various corneal, anterior segment, and ocular surface diseases.1-6

It is important to note that the cross-sectional imaging capability of AS-OCT is similar to ultrasound biomicroscopy (UBM); however, AS-OCT is non-contact and minimizes patient discomfort and image distortion.1-8 OCT also offers the benefit of higher resolution than UBM, for a more detailed analysis of anatomic structures.1-8

There are two OCT platforms available: Time-Domain (TD-OCT) and Spectral-Domain (SD-OCT).1,2

TD-OCT produces cross-sectional images by varying the position of a reference mirror; it allows for deep penetration through the sclera and limbus with 15-20µm resolution.1,2 It also offers a broader range of image capture. Two platforms that utilize TD-OCT are Visante (Carl Zeiss Meditec, Jena, Germany) and Heidelberg (Heidelberg Engineering, GmBH, Smithfield RI).1,2

SD-OCT (also known as Fourier-domain OCT ) uses a spectrometer to detect an image signal by varying the wavelength of light source with time; the interference between the reference reflection and sample is detected as a spectrum. SD-OCT has a more rapid image acquisition but a shorter horizontal scan width compared to TD-OCT.1,2 Examples of platforms that use SD-OCT are Spectralis (Heidelberg Engineering, GmBH, Smithfield RI), RTVue (Optovue, Inc, Fremont CA), and Cirrus (Carl Zeiss Meditec, Jena, Germany).1,2

Additionally, there is ultrahigh-resolution OCT available, which can have an axial resolution of 1-4µm.1,2 This may be more useful in differentiating between various corneal and ocular surface pathologies (e.g., ocular surface neoplasia vs. pterygium vs. ocular surface lymphoma).1-7

Anterior Segment OCT Diagnostics

Diagnosis of corneal and anterior segment disease

AS-OCT can provide cross-sectional imaging of the cornea, conjunctiva, and anterior chamber angle. This proves useful in diagnosing and delineating details in specific ocular surface and anterior segment conditions.

Conjunctivochalasis: AS-OCT can measure a cross-sectional area of the conjunctiva prolapsing into the tear meniscus.1-7 It can also evaluate the ocular surface post-treatment.1-7

Figure 1 demonstrate clinical images of conjunctivochalasis before (left) and after (right) repair.

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Figure 1

Pterygium/Pingueculum: While pterygium and pinguecula are usually easily identified on clinical examination, SD-OCT can highlight subepithelial hyper-reflective wedge-shaped areas within the overlying epithelium.1-7 Ultra-high resolution OCT will also show a dense hyper-reflective subepithelial lesion; this may be more useful in identifying an early recurrence of pterygium post-surgery.1-7

Conjunctival lymphoma: Conjunctival lymphoma constitutes 1.5% of conjunctival tumors.8,9 It is a tumor of the ocular surface that usually presents as a painless, salmon-pink, fleshy area.8 It may appear to have a smooth or nodular surface and is often associated with minimal symptoms, leading to a delay in diagnosis.9 Use of AS-OCT here, which identifies a hyporeflective subepithelial lesion with a thin, slightly hyperreflective layer, may add to the diagnosis.1-9 It is vital to prevent delayed diagnosis (as well as subsequent biopsy and treatment), as systemic workup may be necessary.1-7,9 Treatment may consist of radiation, localized chemotherapy, in conjunction with excisional biopsy.9

Ocular surface squamous neoplasia (OSSN): Here, AS-OCT displays hyperreflective, thickened epithelium with an abrupt transition from normal to abnormal.1-7 Ultra-high resolution OCT can also be used to monitor treatment response and normalization of epithelial appearance.1-7 While most cases of OSSN can be differentiated from benign growths like pterygium, AS-OCT does confirm abnormal anatomy.

A clinical example of ocular surface squamous neoplasia (OSSN) is shown in Figure 2.

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Figure 2

Melanoma/Nevi: Here, SD-OCT displays epithelial hyperreflectivity, most intense within the basal layer and less so throughout the epithelium.1-7 Ultra-high resolution OCT shows normal-to-slightly thickened overlying epithelium with variable hyperreflectivity.1-7 Intralesional cystic spaces may be present in conjunctival nevi, whereas melanoma often demonstrates intense shadowing of sub-lesional tissue on OCT.1-7

Salzmann nodular degeneration: AS-OCT may help in the diagnosis of Salzmann nodular degeneration, as well as differentiation from other ocular diagnoses.1-7 OCT here may show a dense, hyperreflective subepithelial lesion overlying Bowman’s layer with normal surrounding epithelium.1-7

Iris/Ciliary body tumors: It is important to note that UBM may be superior in comparison to AS-OCT in identifying the extent of anterior segment intraocular tumors.1-7 However, SD-OCT may be a more reliable alternative for small/less-densely pigmented anterior iris tumors, especially when cyst-like in nature.1-7

Treatment planning: OCT may provide pachymetry measurements and a physical picture of corneal thickness/irregularities that can help plan for procedures such as phototherapeutic keratectomy (PTK).1-7 This is especially helpful when PTK is being used as a modality to treat corneal haze, scarring, and superficial corneal dystrophies.1-7

Microbial keratitis: In cases of severe keratitis/corneal ulceration, OCT may serve as an adjunct modality to help determine response to treatment and the possible need for more urgent surgical intervention (e.g., severe corneal thinning, impending perforation).1-7 Here, AS-OCT may show keratitis as a hyperreflective area in the stroma.1-7

Keratoconus/ectasia: While topography and tomography are considered the standard for diagnosis in keratoconus and ectasia, AS-OCT can provide helpful information regarding the state of the epithelium (usually thickened over the cone), as well as an anterior hyper-reflective area at Bowman’s layer, which can be associated with increased risk of hydrops.1-6,10 AS-OCT can also help focally measure pachymetry in the presence of corneal scarring.1-6,10

Anterior segment trauma: In the case of anterior segment injury needing surgery, AS-OCT is an optimal imaging method as it is non-contact and high resolution in nature, allowing for evaluating injury depth and extent.1-7 AS-OCT can also be used to follow clinical progress post-treatment and look for signs of scarring, epithelial downgrowth, and retained foreign matter in the anterior segment, which may be missed on slit-lamp examination.1-7

Angle configuration: AS-OCT can be used to determine the anterior chamber angle configuration, which is especially useful before and after performing laser peripheral iridotomy. 1-7

Figure 3 is an example of AS-OCT showing a moderately open anterior chamber angle.