**Abstract**

**Anterior segment optical coherence tomography (AS-OCT)** has become an essential tool in the diagnosis and management of corneal degenerations. AS-OCT optical findings and thickness measurements are useful for the proper evaluation of the ocular surface diseases. AS-OCT imaging provides noninvasive information necessary to decide clinical and surgical management. This device helps to achieve a correct pre-intervention investigation and will allow physicians to compare the corneal status after the surgical process. Thus, it is useful to evaluate the corneal thickness, areas of hyper-reflective material, and corneal fibrosis in certain disorders such as **Salzmann's nodular degeneration (SND)** and **Terrien's marginal degeneration (TMD)**, before and following the surgical process.

**Keywords:** anterior segment optical coherence tomography, corneal degenerations, Salzmann's nodular degeneration, Terrien's marginal degeneration, Dellen, band keratopathy, ocular surface disease, keratoplasty, lamellar keratoplasty

### **1. Introduction**

**Optical coherence tomography (OCT)** was developed to assess the ocular posterior segment. **Anterior segment OCT (AS-OCT)** was not described until 1994, and the first AS-OCT device was commercialized in 2005 [1–3].

The improvement from time domain to spectral domain OCT allowed higher axial resolution images. AS-OCT devices can achieve high-resolution imaging, ranging from less than 5 μm (ultra-high-resolution) to greater than 5 μm (highresolution), providing a noninvasive, in vivo, cross-sectional image of the ocular surface and corneal structure [4, 5].

Spectral domain OCT (SD-OCT) devices include the **Spectralis® HRA + OCT system (Heidelberg Engineering GmbH, Germany).** It can achieve 40,000 A-scans/ second and has a 3.9–7 μm axial resolution, a 14 μm transverse resolution, a 1.9 mm scan depth, and an 870 nm average wavelength [4–6]. This AS-OCT device has been used to capture the corneal OCT images showed along this chapter (**Figure 1**).

AS-OCT is clinically useful for the examination, diagnosis, and management of most of the anterior segment pathologies [4, 7–9] (**Figure 2**). Additionally, it is

#### **Figure 1.**

*Corneal AS-OCT imaging from Heidelberg Spectralis OCT system. The corneal AS-OCT image displays a normal corneal tissue. The layers with the highest hyper-reflectivity are the anterior surface of the cornea and the posterior limit of the cornea with the anterior chamber. The stroma appears as a large band of variable intensity, increasing the signal in the apical zone, due to the perpendicular impact of light reflection on the disposition of collagen fibers.*

#### **Figure 2.**

*(A) Anterior segment (AS) slit-lamp biomicroscopy shows a corneal Dellen due to a previous pterygium surgery: the perilimbar elevation produces an inadequate hydration of the adjacent cornea. (B) Corneal AS-OCT imaging demonstrates a thinning peripheral zone. This thinning area can be measured, and periodic controls can be made by AS-OCT.*

helpful for planning and performing surgery as well as monitoring postoperative cares [1]. AS-OCT imaging requires no contact, which prevents patient discomfort and image distortion. The development of axial resolution, the improvement in scans speeds, and the deeper tissue penetrance allow corneal AS-OCT to recognize structural details in the corneal epithelium, stroma, and conjunctiva, allowing a characterization of corneal disorders [9].

Clinical evaluation and anterior segment (AS) slit-lamp biomicroscopy exam are still the first step in the diagnosis of corneal pathologies and cannot be replaced by corneal AS-OCT images. This technique has to be used as an adjunctive tool, especially in cases in which the diagnosis is clinically equivocal [4, 10]. Corneal AS-OCT imaging has shown to be useful helping to decide the pathology management and to assess the disease resolution. Unfortunately, in some corneal diseases, a histopathologic corneal exam will be necessary to confirm the diagnosis [10].

Corneal AS-OCT imaging can provide optical diagnostic signs for some specific corneal disorders [11]. Thus, corneal hyper-reflectivity can be defined as an increased whiteness compared to corneal tissue of the same location seen in normal subjects, whereas corneal hypo-reflectivity can be defined as an increased darkness compared to corneal tissue of the same location seen in normal individuals [10] (**Figures 2** and **3**). In cases of pathological cornea, a variable increase in reflectivity can be found in case of scars, edema, fibrosis, or material deposits. An attenuated signal is shown in case of fluid accumulation and cystic lesions [10]. In certain cases, AS slit-lamp biomicroscopy evaluation is not able to differentiate an inflammatory disorder from a degenerative disease. In patients with clinical history of inflammation and corneal thinning, corneal AS-OCT imaging shows a hyperreflective band under the corneal epithelium in the area of thinning, which is not

**23**

**Figure 5.**

**Figure 3.**

**Figure 4.**

*anterior stroma and endothelial fibrosis.*

*Clinical Application of Optical Coherence Tomography in the Corneal Degenerations*

*(A and B) Corneal AS-OCT imaging from the Heidelberg Spectralis OCT device. The image demonstrated a thinning central corneal area with a hyper-reflective signal. The hyper-reflective zone is due to a subepithelial,* 

*(A) AS slit-lamp evaluation showing a silicone bubble in the anterior chamber following a retinal detachment surgery. (B) Corneal AS-OCT analysis demonstrated a corneal posterior hyper-reflective zone and fibrosis. The* 

*corneal fibrosis area is caused by the silicone direct damage in the endothelial cells.*

seen in patients with noninflammatory melts and thinning [4]. This phenomenon can be helpful to achieve a correct management of corneal pathologies (**Figure 4**). Corneal AS-OCT imaging is also useful for the measurements of corneal scar depth

*(A) AS slit-lamp examination showing a corneal superior opacity with iris incarceration due to a previous retinal detachment and a complicated cataract surgery. (B) AS-OCT imaging demonstrated a hyper-reflective and fibrotic area. The opacified zone can be measured by the Heidelberg Spectralis OCT caliper tool.*

[1] (**Figure 5**). It has demonstrated to be able to define the rim of a corneal opacity

*DOI: http://dx.doi.org/10.5772/intechopen.84244*

*Clinical Application of Optical Coherence Tomography in the Corneal Degenerations DOI: http://dx.doi.org/10.5772/intechopen.84244*

#### **Figure 3.**

*A Practical Guide to Clinical Application of OCT in Ophthalmology*

helpful for planning and performing surgery as well as monitoring postoperative cares [1]. AS-OCT imaging requires no contact, which prevents patient discomfort and image distortion. The development of axial resolution, the improvement in scans speeds, and the deeper tissue penetrance allow corneal AS-OCT to recognize structural details in the corneal epithelium, stroma, and conjunctiva, allowing a

*(A) Anterior segment (AS) slit-lamp biomicroscopy shows a corneal Dellen due to a previous pterygium surgery: the perilimbar elevation produces an inadequate hydration of the adjacent cornea. (B) Corneal AS-OCT imaging demonstrates a thinning peripheral zone. This thinning area can be measured, and periodic* 

*Corneal AS-OCT imaging from Heidelberg Spectralis OCT system. The corneal AS-OCT image displays a normal corneal tissue. The layers with the highest hyper-reflectivity are the anterior surface of the cornea and the posterior limit of the cornea with the anterior chamber. The stroma appears as a large band of variable intensity, increasing the signal in the apical zone, due to the perpendicular impact of light reflection on the* 

Clinical evaluation and anterior segment (AS) slit-lamp biomicroscopy exam are still the first step in the diagnosis of corneal pathologies and cannot be replaced by corneal AS-OCT images. This technique has to be used as an adjunctive tool, especially in cases in which the diagnosis is clinically equivocal [4, 10]. Corneal AS-OCT imaging has shown to be useful helping to decide the pathology management and to assess the disease resolution. Unfortunately, in some corneal diseases, a histopatho-

Corneal AS-OCT imaging can provide optical diagnostic signs for some specific corneal disorders [11]. Thus, corneal hyper-reflectivity can be defined as an increased whiteness compared to corneal tissue of the same location seen in normal subjects, whereas corneal hypo-reflectivity can be defined as an increased darkness compared to corneal tissue of the same location seen in normal individuals [10] (**Figures 2** and **3**). In cases of pathological cornea, a variable increase in reflectivity can be found in case of scars, edema, fibrosis, or material deposits. An attenuated signal is shown in case of fluid accumulation and cystic lesions [10]. In certain cases, AS slit-lamp biomicroscopy evaluation is not able to differentiate an inflammatory disorder from a degenerative disease. In patients with clinical history of inflammation and corneal thinning, corneal AS-OCT imaging shows a hyperreflective band under the corneal epithelium in the area of thinning, which is not

logic corneal exam will be necessary to confirm the diagnosis [10].

characterization of corneal disorders [9].

**22**

**Figure 1.**

**Figure 2.**

*disposition of collagen fibers.*

*controls can be made by AS-OCT.*

*(A and B) Corneal AS-OCT imaging from the Heidelberg Spectralis OCT device. The image demonstrated a thinning central corneal area with a hyper-reflective signal. The hyper-reflective zone is due to a subepithelial, anterior stroma and endothelial fibrosis.*

#### **Figure 4.**

*(A) AS slit-lamp evaluation showing a silicone bubble in the anterior chamber following a retinal detachment surgery. (B) Corneal AS-OCT analysis demonstrated a corneal posterior hyper-reflective zone and fibrosis. The corneal fibrosis area is caused by the silicone direct damage in the endothelial cells.*

#### **Figure 5.**

*(A) AS slit-lamp examination showing a corneal superior opacity with iris incarceration due to a previous retinal detachment and a complicated cataract surgery. (B) AS-OCT imaging demonstrated a hyper-reflective and fibrotic area. The opacified zone can be measured by the Heidelberg Spectralis OCT caliper tool.*

seen in patients with noninflammatory melts and thinning [4]. This phenomenon can be helpful to achieve a correct management of corneal pathologies (**Figure 4**).

Corneal AS-OCT imaging is also useful for the measurements of corneal scar depth

#### **Figure 6.**

*(A) AS slit-lamp biomicroscopy demonstrates a central and deep corneal opacification due to a traumatic perforation in a child. (B) AS-OCT evaluation shows a strange foreign body in the corneal endothelium (red arrow). The iris is incarcerated in the posterior corneal layers because of the perforating traumatism (blue arrow). (C) AS-OCT imaging displays an irregular stroma with hyper-reflective zones in the fibrosis areas. (D) The caliper tool can measure the respective corneal areas helping the physician to achieve a correct surgical management.*

#### **Figure 7.**

*(A) AS slit-lamp evaluation shows a corneal Dellen due to a previous pterygium surgery in the peripheral nasal area. (A, B) AS-OCT analysis demonstrating a thinning area in corneal periphery, allowing direct comparison between scans. (B) An evaluation from November 2013 and (C) an evaluation from January 2016. Even though there are some minimal changes in the thinning morphology, the length is almost equal, and the corneal Dellen can be considered stable during the last years.*

(**Figures 5** and **6**) and measure the corneal scar depth before choosing a surgical procedure [12]. A noninvasive surgical technique, such as lamellar keratoplasty (LK) or phototherapeutic keratectomy (PTK), can be chosen when only the anterior corneal layers have been affected, while in other cases, penetrating keratoplasty (PKP) will be the unique option to restore the normal corneal structure. Corneal AS-OCT device also allows direct measurements and comparison with prior scans (**Figure 5**).

Corneal AS-OCT device allows direct measurements and comparison with prior scans (**Figure 7**). The device is essential to evaluate cases with high risk of corneal perforation [13].

Degeneration can be defined as a gradual disruption of the normal condition of a tissue with a subsequent loss of functionality [14]. Corneal degeneration can be related with systemic diseases, local inflammation, or direct toxic action. In this chapter three corneal degenerations will be described: **Salzmann's nodular degeneration (SND), Terrien's marginal degeneration (TMD), and band keratopathy (BK). Arcus senilis** is not present in the developing due to benignant nature of the disease.

**25**

**Figure 8.**

*Clinical Application of Optical Coherence Tomography in the Corneal Degenerations*

**SND** is a noninflammatory, slowly progressive, degenerative corneal disease. It is characterized by the presence of elevated, bluish white to gray subepithelial nodules located in the anterior cornea**.** The size of the nodules oscillates from 1 to 2 mm. Nevertheless, larger nodules have been described as a result of the fusion of

SND was identified by Maximilian Salzmann in 1925. He described the corneal nodules, usually related to phlyctenular or atheromatous keratitis [17]. SND is often associated with chronic corneal inflammation and irritation. Multiple risk factors have been reported. Interstitial keratitis, vernal keratoconjunctivitis, dry eye disease, meibomian gland dysfunction, pterygium, soft contact lens wearers, and previous trauma or surgical procedures are some disorders that predispose to suffer

SND usually occurs in female patients, ranging from 50 to 60 years old. Patients can present unilateral o bilateral disease (**Figure 8**). The number of nodules oscillates from one to eight. These nodules generally adopt a round shape. However, in some cases, they can be conical, prismatic, or wedge-like. Most nodules are avascular although some can be associated with blood vessels. They are normally located in the superior and inferior cornea. In cases of previous pterygium surgery, they will be often located in the edge of the blood vessels; in patients with history of contact lens wearing, in the interpalpebral

Corneal AS-OCT SND images display prominent, hyper-reflective, subepithelial deposits overlying Bowman's membrane [1]. The corneal opacities are located under a normally reflective, thin epithelium [10, 16]. The intraepithelial fibrosis overgrowth can result in a corneal surface elevation above Bowman's layer. The central part of nodule has heterogeneous signal intensities, and the nodule margin can be differentiated by subepithelial triangle spike. An irregular stromal scarring can be seen below the nodules, limited to the stromal superficial layers. An epithelial hypertrophy may also be observed around the nodules in an attempt to regularize the corneal surface. The structure of the posterior stroma, Descemet's membrane, and endothelium is not affected by the fibrosis, but the AS-OCT imaging shows a modification of the posterior corneal curvature (**Figure 9**). Both modifications of anterior and posterior

corneal curvature induce astigmatic changes and visual loss in these patients.

The destruction of Bowman's layer is considered the most important property in the pathophysiology of the disease [18]. Bowman's layer is replaced by a granular periodic acid Schiff-positive (PAS-positive) eosinophilic material that resembles

*(A and B) AS slit-lamp biomicroscopy image shows the bluish to white nodules localized in the mid-peripheral* 

*inferior cornea. The corneal opacity is present in both eyes of a 56-year-old female SND patient.*

*DOI: http://dx.doi.org/10.5772/intechopen.84244*

**2. Salzmann's nodular degeneration**

several smaller nodules [15, 16] (**Figure 8**).

portion; and in keratoconus patients, in the apex of the cornea.

the pathology.

*Clinical Application of Optical Coherence Tomography in the Corneal Degenerations DOI: http://dx.doi.org/10.5772/intechopen.84244*
