[Abstract] Cone cornea is a common corneal dilatation disease. With the introduction of corneal refractive surgery, the diagnosis of cone keratoconus has received increasing attention from surgeons. Preoperative screening for cone corneas or subclinical cone corneas can prevent the possibility of medical cone corneas in patients with unrecognized cone corneas from occurring postoperatively. Although conical corneas are a common cause of corneal hyperbolicity, there are other conditions that can lead to corneal hyperbolicity in the clinical setting. The identification of the two has far-reaching implications for carrying out clinical work. Keywords: corneal hyperbolicity; cone cornea; keratoconus Introduction Corneal refractive surgery, especially excimer laser corneal surgery for refractive errors, is becoming more and more accepted by the public. Some patients with high corneal curvature choose to forego excimer laser keratomileusis mainly to reduce the occurrence of postoperative medically induced corneal dilation. Medical corneal dilatation is a serious complication after excimer laser surgery, also known as secondary cone cornea, and its clinical manifestations include progressive, non-inflammatory central corneal thinning, dilatation of the bulge causing refractive regression, increased astigmatism, decreased best-corrected visual acuity, and increased corneal curvature in corneal topography [1]. Although the incidence is low, it can lead to serious consequences and is therefore receiving increasing attention from surgeons. Preoperative occult cone corneas are now considered to be an important factor in their occurrence. High corneal curvature is often seen in cone corneas or early cone corneas, but normal corneas can have high curvature and cone-like corneal topographic manifestations due to mechanical or artifactual factors. On the one hand, it can improve the early diagnosis of conical corneas, especially in the subclinical stage, which can lead to early treatment with rigid gas permeable corneal contact lenses to obtain good visual results and control disease progression, and reduce the incidence of corneal deformation in patients after refractive surgery [2-4]; on the other hand, it can reduce the rate of misdiagnosis in patients with non-conical corneas and can prevent patients from On the other hand: reducing the misdiagnosis rate of non-cornea patients can avoid unnecessary economic and psychological burdens for patients. This paper will summarize the common causes of corneal hyperbolicity and their characteristics, and provide a basis for the identification of conical corneas and other corneal hyperbolic conditions. I. Cone cornea Cone cornea (keratoconus) is a common non-inflammatory, chronic, progressive, locally dilated disease of the cornea, characterized by thinning of the central or paracentral corneal stroma, prominent conical deformation of the central apex, loss of normal curvature of the cornea, irregular astigmatism and scar formation, resulting in a severe reduction in the optical properties of the cornea. Cone corneas usually develop in adolescence and gradually stabilize around the age of 40 years, usually involving both sides, but progression is mostly asymmetric in both eyes [5]. The prevalence of cone corneas in the population is approximately 1 in 2000 [6]. The epidemiological data reported in the literature regarding cone corneas vary widely between data. The exact etiology of the disease is still unclear, and studies suggest that it may be associated with a variety of factors such as genetics, metabolic and developmental disorders, allergic reactions [7], and trauma [8]. Therefore, clinicians should pay attention to the patient’s family history, allergy history and whether they frequently rub their eyes when taking medical history. 1, clinical characteristics of cone cornea The signs and symptoms of the eye are related to the severity of the disease. Early clinical manifestations are mainly myopia and irregular astigmatism, corneal curvature is generally 48D-50D. As the disease progresses, the patient’s visual acuity decreases significantly, frame glasses cannot correct the vision, bilateral asymmetry, refractive aberration is obvious, and corneal curvature further increases. In the early stage of slit lamp examination, there may be no positive signs, but the examining glasses may show “oil-drop” reflection, and the examining glasses may show “scissor shadow” during optometry. One or more of the following signs are observed in the mid- to late-stage conical cornea under the slit lamp: corneal stromal thinning (central or paracentral, mostly inferior or inferior temporal); conical and marked anterior convexity of the cornea; Fleischer’s ring; Munson’s sign; Vogt’s stripe; Rizzuti’s sign. Depending on the severity of the disease, the cornea may develop various scarring patterns. Some patients may develop acute cone corneas: acute rupture of the posterior elastic membrane and entry of atrial fluid into the corneal stroma and epithelium, resulting in edema and clouding, rapid vision loss, ocular discomfort, and tearing. 2, Application of corneal topography detection system in cone cornea diagnosis Corneal topography has an important reference value in the diagnosis of early cone cornea. The corneal topography of early cone cornea is mainly manifested as: steepening of the lower cornea, especially the inferior temporal cornea; greater central corneal curvature (generally >47D); and greater difference in central corneal curvature between the two eyes [6,9,10]. The diagnostic criteria for early cone corneas are not unified at home and abroad.Rabinowitz’s diagnostic criteria are one of the more frequently cited by scholars at home and abroad.Rabinowitz [11] et al. used normal corneas as a control and two standard deviations from their mean values as a reference and suggested that the criteria for screening early cone corneas are (1) central corneal refractive power >46.5 D; (2) corneal The difference between inferior-superior diopter asymmetry (I-S value) > 1.26 D; (3) the difference between the diopters of both eyes of the same patient > 0.92 D. The modified Rabinowitz-McDonnell method ( central K-reading, I-S value) [12]: central corneal curvature > 47.20 D and/or superior and inferior asymmetry (I-S) > 1.40 D. This method has better sensitivity (96%) and poorer specificity (85%) . In addition, foreign scholars have proposed various diagnostic criteria such as automatic expert diagnosis system, neural network-like classification system, and KISA% index, all of which have good sensitivity and specificity [13-15]. In recent years, some new examination means such as Orbscan II and Pentacam have made up for the deficiencies of traditional corneal topography in terms of information on the posterior surface of the cornea, corneal deformation caused by long-term contact lens wear, tear distribution and orbital height resulting in inaccurate measurement [16-17]. 3, Application of other examinations in cone corneal diagnosis Laser confocal microscopy reveals that only a large number of dark structures are seen in the posterior stroma of early cone corneas in a bundle-like distribution, while the dark lines in the posterior stroma of mid- and late-stage cone corneas gradually increase and thicken compared with the early stage, with a clear tendency of forward development, and the Descemet’s membrane gradually thins with the aggravation of the disease, and Descemet’s membrane fracture is seen in late-stage patients [18 ]. The ocular response analyzer can measure corneal hysteresis (CH) and resistance factor amount (CRF) to reflect the biomechanical characteristics of the cornea without touching the cornea. Domestic and foreign scholars have shown that there are differences in corneal biomechanics between conical and normal corneas, which can provide information for the diagnosis of cone corneas and subclinical cone corneas [19-20]. However, the above two examination tools are still in the research stage and have not been widely used in clinical work. Second, pseudoconical cornea and simple corneal hypercurvature Normal corneas can appear in some cases with corneal topographic manifestations resembling conical corneas, called pseudoconical corneas. Most commonly, corneal contact lenses (rigid or soft) are worn and the corneal topography appears as steepening of the lower cornea, which is difficult to distinguish from the corneal topography of a cone cornea [21-22]. However, their corneal topography can return to normal after stopping wearing them, so it is recommended that patients wearing corneal contact lenses need to stop wearing them for at least 3 weeks before undergoing examination to exclude interference [23]. Patients with dry eye have poor tear film quantity or quality, which can affect the anterior surface of the cornea and interfere with the corneal topography results, manifesting as high corneal curvature, similar to cone corneal-like changes, but not as a specific increase in lower curvature [24], so it is recommended that patients with dry eye blink before undergoing the examination and perform three consecutive examinations to reduce errors [23]. Improper posture while undergoing corneal topography, such as pressure under the eye and poor gaze, can also interfere with the examination results. The results showed that the corneal thickness and intraocular pressure values of the group with high corneal curvature (≥ 46 D) were lower than those of the normal group (corneal curvature < 46 D), and the maximum meridional refractive power (Simk1), vertical meridional refractive power (Simk2), and minimum meridional refractive power (Mink) of the group with high corneal curvature were higher than those of the normal group, and their corneal astigmatism ( The corneal astigmatism (△k) and corneal surface regularity index (SRI) were greater than those of the normal group. However, there is no report on the specific clinical characteristics of simple corneal hyperbolicity. Third, the identification and clinical application of simple corneal hyperbolicity, pseudoconical cornea and conical cornea The distinction between simple corneal hyperbolicity, pseudoconical cornea and conical cornea has certain clinical significance for the selection of excimer keratomileusis. Table 1 summarizes the characteristics of the three, which can provide some basis for differential diagnosis. To distinguish simple corneal hyperacusis, pseudoconical cornea and cone cornea: First, a comprehensive medical history, such as history of corneal contact lens wear, history of dry eye disease, history of previous eye surgery, etc., needs to be taken. And try to exclude interference to ensure the accuracy of the examination. Then comprehensive history, clinical manifestations and various ophthalmic examinations are performed to screen for cone corneas or subclinical cone corneas. Since the signs and symptoms of subclinical cone cornea are progressive, patients with atypical early stage may also be diagnosed with cone cornea during the follow-up process. Therefore, for some patients with high corneal curvature who are difficult to identify, we should pay attention to the changes in corneal curvature during follow-up, and for patients with gradually increasing K values, we should observe them in detail to exclude the possibility of cone corneas or subclinical cone corneas to reduce and avoid the occurrence of postoperative corneal bulging and medically induced cone corneas [26-27]. In patients with high corneal curvature (>47D) who wish to undergo keratorefractive surgery, after ruling out factors contributing to pseudoconical corneas and the possibility of conical or subclinical conical corneas, a thorough evaluation of other risk factors for postoperative development of corneal expansion is required. Currently recognized risk factors include [28-29]: high myopia, thin preoperative corneal thickness (corneal thickness not exceeding 500 μm), thin residual stromal bed after laser cutting (the currently accepted safe residual stromal bed thickness is above 250 μm), and abnormal corneal topography. Some of the more recognized corneal topographic abnormalities include a stoned cone (forme fruste keratoconus), clear corneal limbal degeneration, asymmetric inferior corneal steepening, and asymmetric bowtie with vertical meridian up and down oblique axes. Some other risk factors include chronic trauma, family history of cone keratoconus, refractive instability, male, and young (<25 years). However, no single factor can definitively predict the likelihood of postoperative corneal dilatation, and some cases have shown that corneal dilatation may occur even in the absence of these risk factors, which may be related to biomechanical factors such as corneal stromal elastin fiber strength. Condon [30] et al. showed that single curvature alone is not a risk factor for postoperative corneal dilatation, so for patients with high corneal curvature who have excluded conical or subclinical conical corneas, corneal refractive surgery may still be an option if the risk of postoperative corneal dilatation is considered to be low.