**3. Corneal pathology within the tear film**

### **3.1 Dry eye and tear evaluation**

OCT assessment of the tear meniscus has been extensively studied during the last 10 years [32]. The pathophysiology of dry eye disease is characterized by instability of the pre-corneal tear film along with increased osmolarity and ocular surface inflammation and damage [33]. Anterior segment OCT imaging in patients with dry eye disease is clinically useful as it can help directly image the tear film, Meibomian glands and conjunctival folds.

Tear film imaging is inherently limited by the fact that the tear film constantly changes with blinking [1, 23, 24, 34, 35]. OCT meniscometry solves this problem by continuously measuring the tear film, using a tear meniscus to represent the total volume. This method of acquiring images is objective, non-invasive and rapid [32, 34]. However, data analysis is complex, operator-dependent and time-consuming [36]. Tear meniscus height, curvature, and cross-sectional area are widely used in clinical practice and demonstrate good diagnostic performance and correlations with other tests [32, 34].

Cui et al. reported the first visualization of the pre-corneal tear film in dry eye patients with UHR-OCT [12]. The patients were asked to blink normally and then delay each blink as long as possible. The average pre-corneal tear film significantly increased from 4.4 μm during normal blinking to 6.6 μm during delayed blinking. The lipid layer of the tear film can be directly visualized using a contrast mechanism based on sample OCT reflectance, and measures 40–80 nm [37]. The tear film also increases on UHR-OCT after the use of artificial tears and punctal occlusion [38–41].

#### **3.2 Contact lens fitting**

UHR-OCT can perform a dynamic evaluation of contact lens movement with blinking and shifting gaze [42]. Soft contact lenses usually overlap 2mm on the bulbar conjunctiva, but can displace further and overlap 4 to 5 mm onto the bulbar conjunctiva during blinking [43]. UHR-OCT can image the location of the edge of the lens and the tear film underneath the periphery of the lens. This data can help us to understand the normal physiology of the tear film and the dry eye associated with contact lens wear.

The tear film allows contact lens wearers to maintain vision and comfort and health [44]. Lens adherence and ocular surface staining can result from a decreased tear film [43]. Both pre-lens and post-lens tear film contribute to contact lens-associated dry eye [44]. Chen et al. imaged the tear film of 22 subjects before and after contact lens wear with a 15 mm scan width and 3 μm resolution UHR-OCT. Data showed that the pre-lens tear film increased after the instillation of artificial tears, whereas the post-lens tear film remained the same [18]. Cui et al. used UHR-OCT to study soft contact lens conjunctival overlap and the post-lens tear film. They found that increased conjunctival overlap was associated with reduced post-lens tear film underneath the peripheral region of soft contact lens during lens daily wear. Contact lenses with rounded edges also had more conjunctival overlap than the lenses with angled edges [43].

#### **4. Corneal epithelial pathology**

#### **4.1 Dry eye**

Artificial tears eye drops use increased central corneal epithelial and midperipheral corneal thickness in dry eye patients. Epithelial thickness can be a useful

**7**

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

**4.2 Subclinical keratoconus**

**4.3 Keratoconus**

**4.4 Ocular surface pathology**

lial thickness was increased significantly in all regions.

discriminating between keratoconic eyes and healthy eyes" [55].

measurement when evaluating treatment response in dry eye patients, but the pattern of epithelial changes in this disease remains inconclusive [45]. The epithelium has been reported to be thinner [46–47], the same [48], and thicker [49] in dry eye patients in several conflicting studies. Central epithelial thickness in female dry eye patients was thicker than that of normal control patients by 6.5 and 6.2 μm, respectively [49]. Further cell morphology studies may be warranted to differentiate the possible explanations of increased epithelial thickness associated with dry eye. Epithelial hypertrophy or hyperplasia, edema, or increased number of cellular layers may be contributing [49]. Abou Shousha et al. demonstrated that dry eye patients had increased corneal epithelial irregularity compared to controls, quantified by corneal epithelial thickness profile variance and range. Both parameters were significantly correlated with questionnaire scores and improved after dry eye treatment [50].

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 epithe-

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

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

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

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

OCT assessment of the tear meniscus has been extensively studied during the last 10 years [32]. The pathophysiology of dry eye disease is characterized by instability of the pre-corneal tear film along with increased osmolarity and ocular surface inflammation and damage [33]. Anterior segment OCT imaging in patients with dry eye disease is clinically useful as it can help directly image the tear film,

Tear film imaging is inherently limited by the fact that the tear film constantly changes with blinking [1, 23, 24, 34, 35]. OCT meniscometry solves this problem by continuously measuring the tear film, using a tear meniscus to represent the total volume. This method of acquiring images is objective, non-invasive and rapid [32, 34]. However, data analysis is complex, operator-dependent and time-consuming [36]. Tear meniscus height, curvature, and cross-sectional area are widely used in clinical practice and demonstrate good diagnostic performance and correlations with other tests [32, 34]. Cui et al. reported the first visualization of the pre-corneal tear film in dry eye patients with UHR-OCT [12]. The patients were asked to blink normally and then delay each blink as long as possible. The average pre-corneal tear film significantly increased from 4.4 μm during normal blinking to 6.6 μm during delayed blinking. The lipid layer of the tear film can be directly visualized using a contrast mechanism based on sample OCT reflectance, and measures 40–80 nm [37]. The tear film also increases

on UHR-OCT after the use of artificial tears and punctal occlusion [38–41].

also had more conjunctival overlap than the lenses with angled edges [43].

Artificial tears eye drops use increased central corneal epithelial and midperipheral corneal thickness in dry eye patients. Epithelial thickness can be a useful

UHR-OCT can perform a dynamic evaluation of contact lens movement with blinking and shifting gaze [42]. Soft contact lenses usually overlap 2mm on the bulbar conjunctiva, but can displace further and overlap 4 to 5 mm onto the bulbar conjunctiva during blinking [43]. UHR-OCT can image the location of the edge of the lens and the tear film underneath the periphery of the lens. This data can help us to understand the normal physiology of the tear film and the dry eye associated

The tear film allows contact lens wearers to maintain vision and comfort and health [44]. Lens adherence and ocular surface staining can result from a decreased tear film [43]. Both pre-lens and post-lens tear film contribute to contact lens-associated dry eye [44]. Chen et al. imaged the tear film of 22 subjects before and after contact lens wear with a 15 mm scan width and 3 μm resolution UHR-OCT. Data showed that the pre-lens tear film increased after the instillation of artificial tears, whereas the post-lens tear film remained the same [18]. Cui et al. used UHR-OCT to study soft contact lens conjunctival overlap and the post-lens tear film. They found that increased conjunctival overlap was associated with reduced post-lens tear film underneath the peripheral region of soft contact lens during lens daily wear. Contact lenses with rounded edges

**3. Corneal pathology within the tear film**

Meibomian glands and conjunctival folds.

**3.1 Dry eye and tear evaluation**

**3.2 Contact lens fitting**

with contact lens wear.

**4. Corneal epithelial pathology**

**6**

**4.1 Dry eye**

measurement when evaluating treatment response in dry eye patients, but the pattern of epithelial changes in this disease remains inconclusive [45]. The epithelium has been reported to be thinner [46–47], the same [48], and thicker [49] in dry eye patients in several conflicting studies. Central epithelial thickness in female dry eye patients was thicker than that of normal control patients by 6.5 and 6.2 μm, respectively [49]. Further cell morphology studies may be warranted to differentiate the possible explanations of increased epithelial thickness associated with dry eye. Epithelial hypertrophy or hyperplasia, edema, or increased number of cellular layers may be contributing [49]. Abou Shousha et al. demonstrated that dry eye patients had increased corneal epithelial irregularity compared to controls, quantified by corneal epithelial thickness profile variance and range. Both parameters were significantly correlated with questionnaire scores and improved after dry eye treatment [50].
