Ultrasonographic appearance of goat eye
Goat eyes in different age group appeared similar as well defined ovoid structures with mostly anechoic contents (Fig 1). A standoff pad is usually required to visualize the cornea
(Hager et al., 1987). Copious amount of ultrasound gel was used for ocular scanning during the study, which obviated the use of standoff pad. The cornea appeared as a double peaked echo
i.
e. two convex interfaces with a central narrow anechoic space as reported earlier
(Hager et al., 1987; El-Maghraby et al., 1995;
MacKay and Mattoon, 2015). Additionally,
El-Maghraby et al., 1995 observed an area of hyper-reflectivity centrally within the corneal image with an echoic region posterior to it. No such observation was recorded in any of the normal eyes scanned during the study.
The anterior and posterior chambers of the aqueous chamber appeared as a single indistinguishable planoconvex anechoic space similar to earlier findings (
El-Maghraby et al., 1995;
Potter et al., 2008; Toni et al., 2010).
The iris and ciliary body appeared as moderately echoic linear shaped structure. The iris is considered difficult to be imaged but can be seen under ideal conditions in contact with the anterior lens capsule by use of a standoff and a high-frequency transducer
(Dziezyc and Hager, 1988; LeMay, 1978; Smith et al., 1986; Wilkie and Gilger, 1998). The iris was located immediately adjacent to the anterior lens capsule with the thicker, more irregular ciliary body lying peripheral
(Whitcomb, 2002).
The lens appeared as two curvilinear echogenicities representing the anterior and posterior lens capsules. The internal appearance of the lens was anechoic. Similar observations were made earlier
(Potter et al., 2008; Soroori et al., 2009).
The vitreous chamber was imaged as a homogenous, anechoic region between the posterior lens capsule and ciliary body anteriorly and retina posteriorly. The posterior ocular wall had the highest echogenicity. Retinal, choroidal or scleral layers were not individually identifiable on ultrasonography. Several parallel linear echogenicites seen within the retino-choroid-sclera complex were representing one or more of these layers, as recorded earlier
El-Maghraby et al., 1995;
Hager et al., 1987; Potter et al., 2008; Ribeiro et al., 2009; Soroori et al., 2009).
On ultrasonography, the retrobulbar region appeared as an area of moderate echogenicity due to the large amount of fat. The optic nerve could be scanned in only few animals. It appeared as a funnel-shaped hypoechoic-to-anechoic shadow posterior to the optic disc. The extraocular muscles looked as homogeneous, hypoechoic structures running tangentially to the back of the eye. Similar observations have been reported by
Barr (1990),
Simon (1996),
Gonzalez et al., (2001).
Ocular echobiometry in animals
Ocular biometry was one of the early uses of ultrasound in human ophthalmology
(Coleman, 1979). The axial measurements of the globe are important in evaluating conditions such as glaucoma, micro-ophthalmia, macro-ophthalmia, phthisis bulbi or persistent hyperplastic primary vitreous
(Gonzalez et al., 2001). In veterinary medicine, ocular biometry can be used in establishing lens implant size, calculating lens power and estimating prosthetic globe size after enucleation
(MacKay and Mattoon, 2015). The echobiometric values obtained in different age group of Bhakarwali and local non-descript goats is summarized in Table 1.
Central corneal thickness (CCT)
For more accurate measurement of the IOP, CCT is an important factor that has to be taken into consideration and adjustments in the measured IOP are needed for the clinical management of glaucoma patients
(Shih et al., 2004; Patwardhan et al., 2008). In present study, there was steady increase in the overall Mean±SE value of central corneal thickness till 12 months of age which incidentally also was the age at which the difference (P<0.05) became significant from the preceding values.
Ribeiro et al., (2009) also reported increase in the value of CCT in 549 days old goats as compared to 45 day old goat. Higher corneal thickness was recorded in Bhakarwali goat than nondescript goats at all the ages but the significant difference between the two groups of goats was recorded only at the age of 6 months. The central corneal thickness (CCT) values of both the breeds of goats of the present study were higher as compared to CCT values of either Sannen (0.064±0.007 cm) or Iraqi goats (0.059±0.001 cm) at any stage of age
(Ribeiro et al., 2009; Al-Redah, 2016). A thick cornea provides falsely elevated intraocular pressure while as a thin cornea is associated with the falsely low intraocular pressure
(Caster et al., 2007).
Anterior chamber depth
The overall Mean±SE value of anterior chamber depth increased with the advancement of the age. This increase was significant (P<0.05) in Bhakarwali and non-descript-goats at 12 months and at > 36 months of age, respectively. Comparison between the breeds did not reveal any significant difference at any age interval.
Ribeiro et al., (2009) also reported that ACD values of the Sannen goats increased with the advancement of age. However, the ACD values in Sannen breed were higher than values obtained in the present study. ACD has been described as an important preoperative parameter in the anterior segment surgery
(Feng et al., 2011).
Antero posterior depth of lens
The Mean±SE values (cm) of antero posterior depth of lens of goats showed increasing trend with the advancement of age, however, significant increase (P<0.05) in the overall antero-posterior depth of lens was observed from 6 month onwards as compared to value of one month. The increase in the values from 12 to 36 months of age was not significantly different. The APDL value of Bhakarwali group of goats was non-significantly lower than those of non-descript goat at every age level. Similar observations have been made by
Ribeiro et al., (2009).
Latero-medial diameter of lens
Significant (P<0.05) increase in the latero-medial diameter of the lens was observed from 6 month onwards in the overall and in Bhakarwali groups of goats from the corresponding 1 month age values. Comparison within the groups revealed significant (P<0.05) difference in latero-medial diameter of the lens at 36 month of age. Significant increase in LMDL values were observed in non-descript goats form 12 months onward as compared to the corresponding 1 month value.
Vitreous chamber depth
Vitreous chamber depth (VCD) also increased with the age, but the increase was significant in Bhakarwali and non-descript goats at 12 and 6 month of age, respectively. Similar increase in values was recorded by
Ribeiro et al., (2009). The VCD values of Sannen goats at 549 days of age (1.203±0.83 cm;
Ribeiro et al., 2009) were higher than those of Bhakarwali (1.02±0.030 cm) as well as nondescript goats (0.99±0.02 cm) even at 36 month of age, recorded during the study. Comparison between the breeds revealed significant difference (P<0.05) only at 6 month of age.
Axial length
The axial length of globe (cm) was 1.87±0.045 and 1.83±0.064 at 1 month age in the goats of Bhakarwali and non-descript breeds respectively, which continuously and significantly (P<0.05) increased with the advancement of age in both the groups of goats. Comparison revealed no significant difference in the value of globe axial length at any stage of age between the breeds. In consonance with the earlier findings of
Ribeiro et al., (2009) which recorded that globe increased with the advancement of age, thus substantiating the findings of our study. Again the axial globe length in Sannen goats (24.37±1.16) was higher
(Ribeiro et al., 2009) than AGL of either Bhakarwali or non-descript goats of this study.
Retinal rim
The retinal rim thickness (cm) at 1 month of age was 0.183±0.009 and 0.16±0.005 in the goats of Bhakarwali and non-descript breeds respectively. The retinal rim thickness values started increasing significantly (P<0.05) at 6 and 12 months of age in Bhakarwali and non-descript goats, respectively. Comparison between the breeds revealed significant (P<0.05) difference in the values of retinal rim thickness at the corresponding 1 and 6 month of age.
