A Comparison of Two Methods for Corneal Thickness Measurement: Ultrasonic Pachymetry and High-Performance B-Mode Ultrasonography in Beagle Dogs
M.H. Tramontin; L. Lima; D.A.A. Garcia; T.R. Froes; F. Montiani-Ferreira
It is well established that ultrasonic corneal pachymetry is the most accurate and reliable in vivo method currently available to measure corneal thickness in animals and human beings (Gilber et al. 1991, Korah et al. 2000). This technique permits measurement of the cornea in a normal physiologic state (nonanesthetized). Nevertheless, the recently available high performance B-mode ultrasound systems, which are primarily used for abdominal and echocardiographic diagnoses, can be used to acquire corneal images of good quality. The purpose of this investigation is to evaluate the ability of high performance B-mode ultrasound systems to measure corneal thickness compared to ultrasonic pachymetry, which is considered the gold standard.
Materials and Methods
All procedures were conducted in accordance with the ARVO Statement for the Use of Animals in Ophthalmic and Vision Research as well as in accordance with the principles of guidance for the care and use of animals at UFPR. Sixteen Beagle dogs were studied, including 8 males and 8 females, which ranged in age from 3 to 4 months. Central corneal thickness measurement was acquired using two different methods. The first method utilized an ultrasonic pachymeter (Micropach® Sonomed, Model 200P+, Lake Success, NY) with the speed of sound in the cornea pre-set at 1640 m/s. The second method utilized high-performance B-mode ultrasonography using a 14 MHz linear probe (Sonix SP, Ultrasonix, Richmond, BC, Canada). Both procedures were performed in all dogs the same day. Following application of a topical anesthetic (proparacaine hydrochloride 0.5% ophthalmic solution USP, Alcon Laboratories, Forth Worth, TX, USA), the pachymeter probe was placed on the center of the cornea and 3 readings taken and averaged. The same procedure was repeated placing the B-mode ultrasonic probe on the center of the cornea using ultrasonic transmission gel (Aquasonic-100, Parkr Laboratories Inc., Fairfield, NJ, USA). Care was taken during probe placement to avoid corneal indentation for both methods. Measurements were performed at the same time of day (mid-morning) to avoid any effect of circadian rhythm. The left eye was measured first followed by the right. For corneal pachymetry, the measurement of corneal thickness was automatically displayed on the LCD screen of the pachymeter upon probe contact with the center of the cornea and subsequently recorded. For B-mode ultrasonography, 3 optimal images of the central cornea were frozen on the screen. Optimal positioning was considered when the posterior wall of the globe could be clearly visualized on the B-scan ultrasonogram, the globe appeared symmetrical and the reflections from the four principle landmarks (cornea, anterior lens surface, posterior lens surface and retinal surface) along the globe axis were perpendicular. On each image two electronic cursors were placed on the B-scan image: one at the anterior hyperechoic line of the corneal surface (corneal epithelium) and the other at the posterior hyperechoic line of the cornea (Descemet's membrane and endothelium). The data obtained in this study were paired. Statistical analysis was applied using a paired t-test provided by the software JMP, version 5.0.1 (SAS Institute Inc., Cary, NC). Data were deemed significant when p < 0.05.
Mean central corneal thickness obtained by ultrasonic pachymetry was 0.557 ± 0.027 mm and 0.617 ± 0.067 mm by B-mode ultrasonography. The numeric difference of data obtained by these 2 different methods of data acquisition was statistically significant (p < 0.0001). In both cases corneal thickness tends to increase with age p < 0.0001 and p < 0.0024, respectively. Additionally, for both methods of data acquisition no difference was observed between males and females for corneal thickness (p < 0.05).
Discussion and Conclusions
It is possible to acquire corneal thickness data using a high-performance B-mode ultrasound system. Mean corneal thickness values acquired by B-mode ultrasonography and pachymetry could be considered comparable for dogs (Gilger et al. 1991, Gwin et al. 1982, Ekesten & Torrang 1995). However, high-performance B-mode ultrasonography, using a 14 MHz probe, tends to overestimate corneal thickness by about 10% when compared to pachymetry. Additionally, the variability of the data was higher with high-performance B-mode ultrasonography, therefore precision tends to be lower with this method. Nevertheless, we believe that in the absence of a corneal pachymeter, high performance B-mode ultrasonography can be used as a tool to estimate gross changes of corneal thickness. Despite the method used to estimate corneal thickness in dogs, the characteristic and well described (Gwin et al. 1982; Ekesten & Torrang 1995; Montiani-Ferreira et al. 2003) increase of thickness with age was observed in this investigation.
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