With the advancement of science and technology, there are a variety of ophthalmic examination equipment, among which UBM and AS-OCT are two important examination items. UBM scans the tissue at the depth of 5mm in the anterior segment of the eye through high-frequency ultrasound (50–100 MHz). In the panoramic mode, the exploration depth can reach about 6–7 mm, obtaining high-resolution and high-definition images of any section, as well as quantitative analysis of many structures in the anterior segment. As a new ophthalmic imaging device, AS-OCT uses 1310nm infrared light for scanning, which can penetrate the cornea and scleral tissue, and complete the imaging and measurement of the anterior segment anatomy and pathology after cataract, refractive surgery, glaucoma, corneal transplantation and ocular trauma in a non-contact manner, providing the basis for anterior segment analysis. The scanning angle is measured from 0 to 360 degrees at a full angle, with the maximum depth of 6 mm. AS-OCT is a form of cross-sectional imaging based on light that does not require eye contact. Compared with UBM, AS-OCT is simpler and faster in operation, and more acceptable to patients [4]. The imaging principles of the two methods are similar. AS-OCT has higher resolution, while UBM detects biological tissues through ultrasonic waves, with stronger penetration, which is much higher than that of light waves. For a normal transparent cornea, the imaging findings of the two are not much different, but for the examination of pathological changes that affect light penetration, such as obvious edema and thickening of corneal epithelium and scar in stroma layer, UBM has advantages over AS-OCT. Many articles have reported that under the condition of normal clear cornea, there is no significant difference between the measured values of UBM and AS-OCT in the comparative study of the measurement of biological parameters in the anterior segment of the eye [5–8]. However, there are few reports on the study of UBM and OCT in the anterior segment of keratoconus in the acute phase, and the comparative analysis of the two examinations is even less.
Esteban Fuentes et al. applied anterior segment OCT to analyze the anatomical features such as the thickness of epithelial and stroma layers, the hyperreflexia of anterior elastic layer, Vogt striation, and the opacity of stroma layer of keratoconus in the acute phase, and they found that the thickening of corneal epithelial layer, thinning of stroma layer, the hyperreflexia of anterior elastic layer, and the lack of stroma scar layer are the risk factors for inducing keratoconus edema in the acute phase [2]. Wang et al. found in their study that the imaging resolution of the central and peripheral cornea by UBM was low [9], and the epithelial boundary was unclear in the case of corneal edema. In this study, the imaging features of AS-OCT and UBM in acute keratoconus were corneal posterior elastic layer rupture and discontinuous echo. Localized obvious edema and thickening of the matrix layer; Vertical fissures with different sizes were formed in the matrix layer of the corresponding parts to communicate with the anterior chamber. The thickness of the matrix layer was thin, and no definite perforation was seen. AS-OCT images clearly showed corneal epithelial integrity, subepithelial effusions, and epithelial separation from the anterior elastic layer. The UBM image shows that the interface of corneal posterior elastic layer is split into two layers. As there is no hemidesmosomes between the corneal posterior elastic layer and endothelial cells, the attachment relationship between the two depends on the endothelial cells themselves permanently producing new posterior elastic layer fibers, and the endothelial cells are easily separated from the posterior elastic layer. When the posterior elastic layer is ruptured, there are usually wavy bending and reflecting double lines with a darker place in the middle [10]. Therefore, when the aqueous humor flows back due to the fissure formation of the matrix layer, it may break through the interface of the posterior elastic layer and separate from the endothelium (no related research report has been found in the literature yet)! )。 Acute edema of keratoconus often occurs just below the central part of the cornea or under the nose, so we may not find any subepithelial effusion in UBM examination of the same case, which is separated from the anterior elastic layer, but it may also be caused by different angles of incidence of ultrasonic waves. Corneal edema presented as uniform and weak reflection on AS-OCT images [11–14], and the corneal boundary was relatively clear. The near-infrared light of AS-OCT can transmit the edematous corneal stroma to the endothelial surface, anterior chamber, iris and lens for cross-sectional imaging, making up for the shortage that the posterior cornea and anterior chamber cannot be observed by slit-lamp microscope when the cornea is cloudy. As the corneal stroma layer was obviously thickened during acute corneal edema, the thickest edema area in this study was about 2.63 mm. Considering the imaging principle of AS-OCT, the permeability of light wave was relatively weak, which may cause unclear display at the posterior part of the cornea, and no interfacial layer of corneal posterior elastic layer was found. On the other hand, AS-OCT had a higher resolution, and we observed that the posterior elastic layer rupture opening in our patient was at least about 0.1mm, and the basal layer rupture resulted in an increased contact area with anterior aqueous humor, which also explains the fact that acute corneal edema is less likely to occur when the posterior elastic layer is simply avulsion-ruptured.
Although the difference between the pre-operative best corrected visual acuity and the post-operative best corrected visual acuity in all patients with acute keratoconus studied in this group is statistically significant, the post-operative visual acuity is often poor, which is related to various factors such as the scope of corneal edema, the size and location of posterior elastic layer hiatus, and the degree of stromal opacity of the patient's cornea. All the patients were treated with deep lamellar keratoplasty under general anesthesia by the same operator. In this study, it was found that there was no statistical difference between the preoperative and postoperative best corrected visual acuity of all the groups 1 and 2. Deep lamellar keratoplasty can reduce postoperative rejection to a great extent, reduce postoperative astigmatism and relieve the pressure of insufficient fresh corneal material. However, based on the above research, in order to achieve a better visual quality for patients, we are reminded that for patients with acute keratoconus, can partial penetrating keratoplasty be used? The size of fracture opening is significantly correlated with the height of fracture opening, while it has no correlation with the stratification of back elastic layer. AS-OCT and UBM can provide us with accurate corneal imaging and measurement of ocular biological parameters, accurate positioning and indication of the size of the posterior elastic layer hiatus, and provide a very valuable reference for clinical surgery.
Esteban Fuentes et al. found that the lack of scar in the keratoconus stromal layer might be an inducing factor for the acute edema of keratoconus [2]. Due to the small number of cases in this study and the failure to observe the pre-acute corneal edema image of our patient, as well as the lack of research on its influencing factors, we will strengthen the observation of relevant factors in future studies. The stroma fissure in keratoconus at the acute stage is rarely seen under slit-lamp microscope, and it has been only rarely reported that in our case AS-OCT and UBM examinations both showed stroma fissure of different sizes communicating with the anterior chamber. There are two causes for it, primary and secondary. The primary one is caused by the cracking of corneal stroma layer itself, and structural change and fissure appearance are caused by corneal deformation. Secondary because of rupture of the back elastic layer. The corneal edema is aggravated by the fissures in the stroma layer, because the contact area between the stroma layer and anterior aqueous humor is enlarged due to the fissures. The fissures in the stroma layer also affect the healing of the posterior elastic layer and the recovery of corneal edema [15–17]. In our case, the matrix fractures were more clearly demonstrated on AS-OCT images than on UBM images, with clearer margins. When corneal perforation occurs due to the thin corneal stroma of patients, UBM, as a contact test, is not suitable. Compared with UBM, AS-OCT is simple and fast to operate, and easy to be accepted by patients.