**5.2 Keratoconus**

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

difficult to diagnosis OSSN based on clinical exam [59].

tion of Bowman's layer as the disease progresses [61].

**4.5 Physiologic changes after contact lens wear**

change from orthokeratology lenses [64].

**4.6 Monitoring corneal epithelial defects**

time for lens removal after pterygium excision [11].

non-uniformly altered Bowman's layer [24].

**4.7 Post-operative monitoring**

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

Corneas with scarring and Salzmann's nodular degeneration have a normalthickness epithelium overlying a dense, hyper-reflective lesion overlying Bowman's

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

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

UHR-OCT revealed a significant correlation between epithelial thickening and

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

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].

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

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 destruc-

**8**

is needed [67].

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 disease severity [13].
