**4.2 Subclinical keratoconus**

Xu et al. reported UHR-OCT epithelial vertical thickness profiles in the diagnosis of subclinical keratoconus. Data showed statistically significant thinning of the central corneal epithelium; 53.48 μm in normal eyes and 51.92 μm in those with subclinical keratoconus. There was no significant inferior epithelial thinning in subclinical keratoconus; 54.94 μm in normal eyes and 54.85 μm in eyes with subclinical keratoconus [51]. However, our unpublished data found that the epithelium in patients with subclinical keratoconus had localized thinning of inferior epithelium quantified with minimum thickness. We also found that the epithelium has relative superior thickening by maximum thickness and that standard deviation of epithelial thickness was increased significantly in all regions.

#### **4.3 Keratoconus**

Corneas with keratoconus show epithelial remodeling, which minimizes local topographic irregularities and improves corneal curvature [52]. Epithelial thinning precedes other corneal changes in keratoconus [53, 54], and the location of the thinnest zone of the epithelium corresponds with the steepest zone seen on Scheimpflug tomography [10]. Xu et al. reported no significant thinning of the inferior cornea in eyes with keratoconus as compared to normal eyes. However, there was significant thinning of the central epithelium; 53.48 μm in normal eyes and 46.10 μm in eyes with keratoconus [51]. Yadav et al. reported that variation in epithelial thickness across the central 3 mm was significantly larger in eyes with keratoconus. This finding was supported by Pircher et al. who wrote that "epithelial thickness, irregularity, and asymmetry seem to be the most promising diagnostic factors in terms of discriminating between keratoconic eyes and healthy eyes" [55].

### **4.4 Ocular surface pathology**

UHR-OCT can be used for the diagnosis of ocular surface squamous neoplasia (OSSN) and detection of sub-clinical disease [16, 56, 57]. OSSN has several classical features on anterior segment OCT, including thickened, hyper-reflective epithelium with an abrupt transition from normal to abnormal epithelium [16, 57, 58]. The gold standard for diagnosis of OSSN is examination of pathology, but non-invasive methods of diagnosis are helpful as topical chemotherapy becomes increasingly utilized [16]. UHR-OCT provides high-resolution imaging with cross-sectional views; dynamic non-contact scanning modality reduces need for technical expertise compared to UBM and confocal microscopy. However, it has poor penetrance with thicker lesions and cannot reliably detect

invasion [16]. UHR-OCT is especially useful as it can non-invasively detect OSSN in the presence of other ocular surface diseases. Co-existing conditions such as mucus membrane pemphigoid or limbal stem cell deficiency make it difficult to diagnosis OSSN based on clinical exam [59].

Corneas with scarring and Salzmann's nodular degeneration have a normalthickness epithelium overlying a dense, hyper-reflective lesion overlying Bowman's layer on UHR-OCT [16]. Epithelial hypo-reflective cysts without basement membrane thickening are seen in Meesman's dystrophy [60]. Eyes with secondary corneal amyloidosis show deposits of amyloid above Bowman's layer, and destruction of Bowman's layer as the disease progresses [61].

#### **4.5 Physiologic changes after contact lens wear**

UHR-OCT can show corneal changes after soft contact lens wear. Epithelial thickness increased by 3.5% and total corneal thickness increased by 10% after 3 hours of patching with soft contact lens wear [62]. Endothelium and Descemet membrane showed no significant change in thickness. Long-term hydrogel lens wearers have been shown to have uniform epithelial thinning [63]. Orthokeratology lenses caused the central epithelium to thin in vertical and horizontal meridians, while the mid-peripheral nasal and temporal epithelium became thicker and the superior mid-peripheral epithelium became thinner. Bowman's layer showed no change from orthokeratology lenses [64].

#### **4.6 Monitoring corneal epithelial defects**

UHR-OCT is a useful way to monitor corneal epithelial healing as it provides an objective and three-dimensional evaluation [65]. Corneal wound healing was assessed after epithelial-off corneal collagen cross-linking, and it was noted that epithelium surrounding the fluorescein stained abrasion was not fully settled to the underlying basement membrane [10, 65]. UHR-OCT can also help monitor corneal epithelial healing under a bandage contact lens and can determine the appropriate time for lens removal after pterygium excision [11].

#### **4.7 Post-operative monitoring**

UHR-OCT revealed a significant correlation between epithelial thickening and the extent of refractive correction after myopic small incision lenticule extraction. Epithelial thickening of approximately 10% was observed during the first six postoperative months and stabilized after 3 months [66]. Epithelial thickness in eyes treated with photorefractive keratectomy was significantly higher than that of normal eyes at (68.2 vs. 55.8 μm) [24]. This difference was thought to be caused by non-uniformly altered Bowman's layer [24].

Rocha et al. reported a reduction in peripheral epithelial thickness and decreased regional variation in epithelial thickness consistent with increased corneal curvature after corneal collagen crosslinking [52]. UHR-OCT of corneal wound healing after epithelial-off cross-linking correlated well with fluorescein photographs and visualized the stromal demarcation line [65]. Most of the data about the demarcation line seen in cross-linking comes from use of the SD-OCT and further research is needed [67].

Zarei-Ghanavati et al. showed that epithelium covers the Boston type I keratoprosthesis edge and seals the potential space in the interface. They proposed that failure to epithelialize this interface and lack of epithelial sealing around the keratoprosthesis edge might be associated with endophthalmitis [68].

**9**

**Figure 2.**

*Corneal Microlayer Optical Tomography Review DOI: http://dx.doi.org/10.5772/intechopen.84750*

**5. Bowman's membrane pathology**

Vertical thickness of Bowman's layer in subclinical keratoconus was decreased inferiorly compared with normal control eyes [51]. Our group demonstrated that Bowman's layer in patients with subclinical keratoconus was significantly thinner centrally and inferiorly, and could be quantified with mean thickness, minimum

Bowman's layer shows thinning, disintegration and breakage on pathological specimens of eyes with keratoconus (**Figure 2**). Histopathological light and electron microscopy studies of these eyes can be helpful for the diagnosis of keratoconus [70]. Interestingly, these changes happen before stromal changes [71]. Abou Shousha et al. demonstrated that vertical topographic thickness maps of keratoconus patients had characteristic localized relative inferior thinning of Bowman's layer. Inferior average thickness, inferior minimum thickness, Bowman's ectasia index and Bowman's ectasia index-max were all correlated with the severity of keratoconus. The inferior average thickness of Bowman's layer in eyes with keratoconus was 12 μm compared with 15 μm in normal eyes [72]. Light scatter from Bowman's layer in eyes with keratoconus was significantly higher but did not correlate with

UHR-OCT can detect early secondary corneal amyloidosis as a dense spot in Bowman's layer [61]. Corneas with Thiel-Behnke Dystrophy have extensive deposits of hyper-reflective material in a saw-tooth pattern on the surface of Bowman's layer [60]. UHR-OCT images may become adjunct to clinical evaluation and provide an optical biopsy image in other conditions. Eyes with spheroidal degeneration show cystic structures in Bowman's layer and superficial stroma, and Salzmann's nodular degeneration has hyper-reflective material which replaces anterior stroma and Bowman's layer with thin epithelium [60] (**Figure 2**). Limbal stem cell deficiency exhibits a hyper-reflective material which replaces Bowman's layer as well as the

*A prototypical cross-sectional UHR-OCT image of a human cornea with keratoconus; focal disintegration of* 

anterior stroma, with irregular overlying epithelium (**Figure 3**).

*Bowman's layer (white arrow), focal stromal thinning (yellow arrow).*

thickness, Bowman's ectasia index and Bowman's ectasia index-max [69].

**5.1 Subclinical keratoconus**

**5.2 Keratoconus**

disease severity [13].

**5.3 Corneal and ocular surface pathology**
