Introduction

Cataract surgery is one of the most frequently performed surgeries in ophthalmology. Ocular biometric parameters such as keratometry and axial length are known as very important variables for minimizing postoperative refractive error. Due to advances in biometric devices and intraocular lens (IOL) calculation formulas, we can obtain postoperative refractive error within 0.5 diopter (D) of targeted spherical correction [1, 2].

Toric IOLs are widely used in modern cataract surgeries to reduce astigmatism because they can provide stable and reliable results that are equal or superior to limbal-relaxing incision results [3,4,5]. In the Middle East, toric IOLs are widely used. To reduce astigmatism after cataract surgery, it is very important to measure corneal astigmatism correctly.

Corneal astigmatism can be measured by different keratometry systems. Conventional, automated refractokeratometer is still used widely. This instrument based on partial coherence interferometry (PCI) was introduced and used popularly with advantage of noncontact evaluation and provision of additional data allowing IOL calculations to be performed. The machine, based on optical low-coherence interferometry (OLCI), was introduced to provide additional information regarding white to white distance, pupil size, and pupil and visual axis decentration by image analysis.

Most of previous studies regarding corneal astigmatism and biometric parameters using PCI or OLCI have shown results of Western, American, and Far East countries [6,7,8,9,10]. To the best of our knowledge, few studies have reported corneal astigmatism and biometric parameters for cataract surgery using OLCI from the Middle East. Thus, the purpose of this study was to analyze biometric parameters and the patterns of corneal astigmatism using OLCI in Northern United Arab Emirates (UAE), the Middle East.

Methods

This retrospective study analyzed cataract patient data acquired between December 2015 and December 2018 in Sheikh Khalifa Specialty Hospital, Ras Al Khaimah, United Arab Emirates (UAE). This study protocol was approved by the Ministry of Health and Prevention Research Ethics Committee (MOHAP/DXB/REC-23/2018). It followed the tenets of the Declaration of Helsinki. Inclusion criteria were UAE citizens, Middle Easterner with local last name living in northern part of UAE with comprehensive preoperative evaluation with OLCI (Aladdin software version 1.4.0, VISIA imaging and Topcon, Italy). Exclusion criteria were those who aged under 20 years or over 100 years, expatriates, a history of previous ocular surgery or ocular trauma, corneal opacity or disease such as keratoconus, presence of ocular surface, and intraocular disease that could affect the measurement. Microcornea (white to white, WTW 10 mm or less) and megalocornea (WTW 13 mm or more) were also excluded. Warning sign by OLCI software was indicated in case of bad focus, closed palpebral, tear film insufficiency, high standard deviation on repetition, movement, or measurement not in range. If warning sign was shown in OLCI device, patients’ data were also excluded. All patients underwent the same comprehensive ophthalmic evaluation for both eyes before cataract surgery, including visual acuity, tonometry, refraction, slit-lamp examination, and fundus examination through dilated pupils.

All patients underwent a full ophthalmologic examination including pupil dilation with 1.0% tropicamide (Bausche&Lomb, France). Two experienced optometrists routinely checked vision and intraocular pressure and performed OLCI examination before pupil dilation. Patients were positioned with a chin and forehead rest. They were instructed to fixate on the internal fixation target. Axial length (AL), flat (K1) and steep (K2) keratometry, anterior chamber depth (ACD) and horizontal corneal diameter (WTW) were optically measured with the OLCI. AL was defined as the distance between the cornea and the inner limiting membrane. For acquisition of AL measurements, an interference signal was produced if the measuring light was reflected by the retinal pigment epithelium of the eye. Because the Aladdin system can automatically adjust for the distance difference between the inner limiting membrane and the retinal pigmented epithelium, Aladdin displays adjusted values that can be compared directly to those obtained by immersion ultrasound. Keratometry acquisition was based on the reflection of 24 rings of the Placido disk on the eye at a distance of 80 mm from the patient’s eye. WTW, the value of the horizontal corneal diameter, was obtained automatically with information of corneal diameter and decentralization. ACD was the distance between corneal epithelium and the anterior surface of the crystalline lens. It was measured along the optical axis where the distance was the biggest with slit light projection measuring method. Corneal astigmatism was defined as the difference between K1 and K2. Corneal astigmatism pattern was categorized into three types: with-the-rule astigmatism, if the steepest meridian of the cornea was within 90° ± 30°; against-the-rule astigmatism, if the steepest meridian of the cornea was within 180° ± 30°; and oblique astigmatism, if the steepest meridian of the cornea was between 30° and 60° or between 120° and 150°.

All collected data were recorded in a spreadsheet and analyzed using SPSS for Windows software version 17.0 (SPSS Inc., Chicago, IL, USA). Distributions of involved eyes and patients were described. An age of patients, AL, ACD, WTW, corneal astigmatism, flat keratometry, and steep keratometry were expressed as mean ± standard deviation (SD) with range. P value of less than 0.05 was considered significant. This study’s results regarding corneal astigmatism were compared with those reported previously in other countries [11,12,13].

Results

This study comprised 238 eyes of 123 UAE citizen patients (Middle Easterner with local last name). Of these 120 patients, 65 (53%) were men. The mean age of patients at the time of surgery was 67.1 ± 9.4 (mean ± SD) years (range 20–89 years). Demographics and biometric parameters of patients are shown in Table 1. The difference in mean AL between men and women was 0.81 mm (23.60 ± 0.94 mm in men and 22.79 ± 0.86 mm in women, p < 0.001). Male eyes had significantly deeper ACD (3.10 ± 0.37 mm in men and 3.00 ± 0.35 mm in women, p = 0.049) and larger corneal astigmatism (1.44 ± 0.98 D in men and 1.18 ± 0.93 D in women, p = 0.035) compared with female eyes. Significant differences were also found in K1 (43.10 ± 1.56 D in men and 44.43 ± 1.80 D in women, p < 0.001), K2 (44.54 ± 1.79 D in men and 45.60 ± 1.43 D in women, p < 0.001), and WTW (11.26 ± 0.48 mm in men and 11.09 ± 0.41 mm in women, p = 0.003).

Table 1 Demographic characteristics of patients and biometric parameters
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The distribution of corneal astigmatism in all eyes is shown in Fig. 1. Mean corneal astigmatism was 1.32 ± 0.97 D (range 0–5.48 D). Corneal astigmatism was 1.5 D or less in 161(67.6%) eyes, 0.5 D or less in 42 (17.6%) eyes, between 0.51 and 1.00 D in 61 (25.6%) eyes, and between 1.01 and 1.50 D in 58 (24.4%) eyes. Corneal astigmatism pattern was with-the-rule in 29.8%, against-the-rule in 57.1%, and oblique in 13.0% of eyes.

Fig. 1
figure 1

Distribution of corneal astigmatism. Corneal astigmatism of more than 1.5 diopters was found in 32.4% of eyes in this study

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Discussion

This study evaluated the distribution of ocular biometric parameters and characteristics of corneal astigmatism measured by OLCI in cataract patients living in northern part of UAE, the Middle East. Some studies regarding corneal astigmatism in cataract patients have been published from several countries but relatively rare in the Middle East [6, 7, 9,10,11,12,13,14,15,16,17]. Our data could be used to represent local cataract patients in Northern UAE because we checked all patients’ nationality, residence area, and ethics.

In the present study, OLCI was used for cataract surgery evaluation. OLCI is quick and easy to use. It can measure the axial length in eyes with central posterior subcapsular cataract for which PCI is not possible [18, 19]. Many studies have reported corneal astigmatism using PCI [6, 9,10,11, 14, 15, 17]. There are two studies showing excellent agreement and repeatability between OLCI and PCI [20, 21]. However, Hoffer et al. [22] have reported that keratometry measured by OLCI is 0.14 D flatter than that measured by PCI. They explained the reason for the difference by suggesting that PCI could analyze smaller corneal diameter than OLCI and most corneas were steeper in the center. This should be considered when comparing keratometry between OLCI and PCI results. According to their previous study, the magnitude of corneal astigmatism measured by OLCI was 0.06 D smaller than or the same as that evaluated by PCI [22]. Hence, it should be considered when comparing corneal astigmatism reports from different countries as shown in Table 2.

Table 2 Comparison of corneal astigmatism in cataract patients between this study and other published studies
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The mean AL using OLCI in our study was 23.22 mm. It was shorter than that (23.43 mm) in Germany measured by PCI [9]. Because a previous study has shown excellent agreement and correlation of AL between OLCI and PCI [22], data could be compared without adjustment. It has been suggested that the reason for AL difference is that AL is highly associated with genetic and environmental factors [23, 24]. Usually, people with smaller body height and weight have shorter axial length. Considering that the average body height of our study subjects was 161.9 cm for men and 149.9 cm for women, genetic factor is thought to be an important factor. Hoffer et al. [22] have reported that mean ACDs measured by OLCI are 0.16 mm and 0.05 mm deeper than those measured by PCI in China and US groups, respectively. ACD values measured by OLCI in our study were smaller than those measured by PCI in other countries [6, 11, 22]. Similar to findings in other studies [6, 9, 12, 25], our study also revealed that men had a longer AL, flatter K1 and K2, deeper ACD, and larger WTW than women.

In our study, the mean corneal astigmatism (1.32 D) was much higher than that reported by others. It has reported to be between 0.90 and 1.03 D in Europe [9, 10, 13]. In Asia, various studies have reported corneal astigmatism from 0.79 to 1.15 D [6, 11, 12, 14, 15, 17]. Isyaku et al. [16] have reported a mean corneal astigmatism of 1.16 D in Africa. The magnitude of corneal astigmatism can be affected by ethnic and environmental factors. High incidence and severity of keratoconus in Arabic peninsula have been reported [26]. This provides a clue for the high corneal astigmatism found in our study, suggesting that ethnic factor and environmental factor such as chronic corneal trauma might play a role.

It is widely proven that corneal astigmatism 0.5 D or less can be controlled by placing a clear cornea incision on the steep axis of the cornea [27]. In the present study, the percentage of the corneal astigmatism of 0.5 D or less was 17.6%, the lowest among all other countries as shown in Table 2. Corneal astigmatism of between 0.5 and 1.5 D before intervention of cataract surgery can be corrected by single on-axis clear corneal incision or with an opposite clear corneal incision [27, 28]. In our study, 67.6% of eyes had corneal astigmatism of 1.5 D or less. This percentage was much smaller than those from other countries [6, 9,10,11,12,13, 15, 17]. It has been widely accepted that moderate-to-severe corneal astigmatism in cataract patients can be corrected by toric IOL implantation. In our study, 32.4% of eyes had corneal astigmatism of more than 1.5 D. This percentage was larger than those from other countries [6, 9,10,11, 13, 15, 17].

This study has some limitations. First, the number of cataract surgery patients in our hospital was not large enough to represent the whole population in Northern UAE. The cataract surgery in our hospital was started in December 2015, and this area has a small UAE citizen population. Many patients were excluded from our study because of the high presence of ocular surface and intraocular disease. Some patients’ data were excluded due to warning sign of OLCI. There is also a possibility of selection bias regarding corneal astigmatism from a referral hospital. Second, we could not provide characteristics of biometric parameters by dividing several groups on the basis of age because some patients did not know exactly their year of birth. Record of birth was unavailable several decades ago. Third, we could not compare our data with those from other countries directly because of different measurement instruments.

In conclusion, this study revealed the variation of ocular biometric parameters of cataract surgery patients using OLCI in Northern UAE, the Middle East. Corneal astigmatism of more than 1.5 D was found in 32.4% of eyes. This percentage was much higher than those reported in other countries. Despite the small number of study samples, results of this study provide valuable and practical information to physicians and government regarding service plan in Northern UAE, the Middle East.