The advancement of science and technology has yielded a variety of ophthalmic examination equipment, among which UBM and AS-OCT are two important pieces. UBM scans the tissue at a depth of 5 mm 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 enabling the quantitative analysis of many structures in the anterior segment. As a new ophthalmic imaging device, AS-OCT uses 1310 nm infrared light for scanning, which can penetrate the cornea and scleral tissue. It can complete the imaging and measurement of the anterior segment anatomy and pathology for cataracts, 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 a maximum depth of 6 mm. AS-OCT is a form of cross-sectional imaging based on light that does not require contact with the eye. Compared to UBM, AS-OCT is simpler and faster in operation, as well as more acceptable to patients [4]. The imaging principles of the two methods are similar. AS-OCT offers higher-resolution images, while UBM detects biological tissues using ultrasonic waves (at a much higher frequency than light waves) that penetrate more deeply. For a normal transparent cornea, the imaging findings of the two are not significantly different, but for the examination of pathological changes that affect light penetration, such as obvious edema, thickening of the corneal epithelium, and scarring in the stromal layer, UBM has advantages over AS-OCT. Many articles have reported that under the condition of a normal clear cornea, there is no significant difference between the measured values obtained by UBM and AS-OCT in a comparative study of the measurement of the biological parameters of the anterior segment of the eye [5–8]. However, there are few reports on UBM and OCT imaging of the anterior segment of keratoconus in the acute phase, and there are even fewer comparative analyses of these two methods of examination.
Esteban Fuentes et al. applied anterior segment OCT to analyze the anatomical features, such as the thickness of epithelial and stromal layers, the hyperreflexia of Bowman’s layer, Vogt striation, and the opacity of the stromal layer of keratoconus in the acute phase [2]. In their study, Wang et al. found that the resolution of images 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 acute keratoconus revealed by AS-OCT and UBM were corneal Descemet membrane rupture, discontinuous echo, localized obvious edema, and thickening of the stroma layer. The stroma layer was thin, and no definite perforation was seen. AS-OCT images clearly showed corneal epithelial integrity, epithelial edema formation, and epithelial separation from the Bowman layer. To this end, the UBM images showed that the interface of the Descemet membrane was divided into two layers. There were no hemidesmosomes between the Descemet membrane and endothelial cells. The attachment relationship between the two approaches depends on the endothelial cells themselves permanently producing new Descemet membrane fibers. When the Descemet membrane ruptures, there is usually wavy bending and reflecting double lines with a darker place in the middle [10]. Therefore, when the aqueous humor flowed back due to the formation of a fissure in the stroma layer, it may have penetrated through the interface of the Descemet membrane layer and separated from the endothelium.
In general, acute edema of keratoconus often occurs just below the central part of the cornea or under the nose. Thus, it is highly challenging to find any subepithelial effusion when conducting a UBM examination of the same case if separation from the Bowman membrane layer occurs. However, it may also be caused by ultrasonic waves with different angles of incidence. In some instances, corneal edema presented as a 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, thus allowing the posterior cornea and anterior chamber to be observed, which cannot be done with a slit-lamp microscope when the cornea is cloudy. The corneal stromal layer thickens during acute corneal edema, and the thickest edema area in this study was about 2.63 mm. Considering the imaging principles of AS-OCT, the permeability of light waves was relatively weak, which may have caused an unclear image of the posterior part of the cornea, and no interfacial layer of Descemet’s membrane was found. However, AS-OCT offered a higher resolution, and we observed that the Descemet membrane rupture opening in our patient was at least about 0.1 mm, and the basal layer rupture resulted in an increased contact area with the anterior aqueous humor, which also explains the fact that acute corneal edema is less likely to occur when the Descemet membrane layer is simply avulsion-ruptured.
Although the differences between preoperative and postoperative best-corrected visual acuities in all patients with acute keratoconus studied in this group were statistically significant, the deprived postoperative visual acuity was related to various factors, such as the severity of corneal edema, the size and location of the Descemet’s membrane rupture, and the degree of stromal opacity in the patient’s cornea. All patients were treated with deep lamellar keratoplasty under general anesthesia by the same operator. In this study, there was no statistical difference between the preoperative and postoperative best-corrected visual acuity among Groups 1 and 2 patients. Deep lamellar keratoplasty could 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, to achieve better visual quality for patients, we were reminded that for patients with acute keratoconus, partial penetrating keratoplasty can be used. The Descemet’s membrane rupture size was significantly correlated with its height, but it showed no correlation with the stratification of the Descemet membrane. AS-OCT and UBM enabled accurate corneal imaging and measurements of ocular biological parameters, accurate localization, and indications of the size of the Descemet’s membrane layer hiatus, thus providing a precious reference for clinical surgery.
Esteban Fuentes et al. found that the lack of scars in the keratoconus stromal layer might be a factor inducing the acute edema of keratoconus [2]. Due to the small number of cases in this study and the failure to acquire pre-acute corneal edema images of our patients, as well as the lack of research on its influencing factors, we will strengthen the observations of relevant factors in future studies. Notably, the stromal fissure in keratoconus at the acute stage is rarely seen under a slit-lamp microscope. In our case, both AS-OCT and UBM examinations showed stromal fissures of different sizes communicating with the anterior chamber due to different causes. The primary reason for this could be the rupture of the stromal layer itself; the structural changes and appearance of the fissure were caused by corneal deformation. The secondary reason could be the rupture of Descemet’s membrane layer. Notably, corneal edema is aggravated by fissures in the stromal layer because the contact area between the stromal layer and the anterior aqueous humor is enlarged due to the fissures. The fissures in the stromal layer also affect the healing of Descemet’s membrane and the recovery of corneal edema [15–17].