EQUINE VETERINARY EDUCATION / AE / MAY 2015
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Case Report
Post mortem magnetic resonance imaging of multiple congenital ocular anomalies in a Comtois mare
M. Oosterlinck*, A. Grebert, I. Gielen†, L. Bosseler‡, R. Ducatelle‡ and F. Gasthuys Department of Surgery and Anaesthesiology of Domestic Animals, Faculty of Veterinary Medicine, Ghent University, Merelbeke, Belgium; †Department of Veterinary Medical Imaging and Small Animal Orthopedics, Faculty of Veterinary Medicine, Ghent University, Merelbeke, Belgium; and ‡Department of Pathology, Bacteriology and Poultry Diseases, Faculty of Veterinary Medicine, Ghent University, Merelbeke, Belgium. *Corresponding author email:
Maarten.Oosterlinck@UGent.be Present address: A. Grebert, 4 rue des Remparts, 51170 Fismes, France.
Keywords: horse; anterior segment dysgenesis; eye; ophthalmology
Summary Multiple congenital ocular anomalies syndrome consists of several abnormalities mainly localised to the anterior segment of the eye. This condition, comprising iridal or ciliary cysts with or without iridal hypoplasia, an excessively protruding cornea (‘cornea globosa’), miotic pupils, retinal dysplasia, cataract, and other lesions, has been reported in several breeds and is strongly related to the silver coat colour, as a result of a dominant mutant allele at the PMEL17 locus. This report describes the macroscopic, ultrasonographic, histological and magnetic resonance imaging findings of the ocular abnormalities in a Comtois mare with multiple congenital ocular anomalies syndrome.
Introduction
Multiple congenital ocular anomalies (MCOA) syndrome consists of several abnormalities mainly localised to the anterior segment of the eye. This condition has been reported in purebred and crossbred Rocky and Kentucky Mountain horses (Ramsey et al. 1999; Grahn et al. 2008; Ségard et al. 2013), in Icelandic horses (Andersson et al. 2011), Comtois horses (Depecker et al. 2013; Ségard et al. 2013), Shetland and Deutsches Classic ponies (Komáromy et al. 2011; Premont et al. 2013) and Miniature Horses (Plummer and Ramsey 2011). There are 2 phenotypes of these anomalies: either iridal or ciliary cysts alone (‘cyst phenotype’), or in association with other anomalies including iridal hypoplasia, an excessively protruding cornea (‘cornea globosa’), miotic pupils, retinal dysplasia, cataract and other lesions (‘MCOA phenotype’) (Grahn et al. 2008). MCOA syndrome is strongly linked to the ‘silver’ coat colour, as a result of a dominant mutant allele at the PMEL 17 (premelanosomal protein 17) locus (Brunberg et al. 2006; Andersson et al. 2008, 2011, 2013). Its mode of inheritance is believed to be autosomal codominance (Ewart et al. 2000), being defined as dominant expression of an allele with phenotypic differences between homozygotes and heterozygotes. Iridal or ciliary cysts are the main expression of this pathology in heterozygotes, generally without clinical symptoms, whereas multiple anterior segment abnormalities and impaired vision are observed in homozygotes (Ewart et al. 2000). However, incomplete penetrance is suggested (Ewart et al. 2000). The gross and histological characteristics of the ocular
anomalies in MCOA-affected cases have been described in detail by Ramsey et al. (1999), Ségard et al. (2013), Premont
et al. (2013) and Depecker et al. (2013). However, to the best of our knowledge, the use of magnetic resonance imaging (MRI) for clinical examination of this disease has not been described. Therefore, this study is the first to document (post mortem) MRI of the ocular abnormalities in a MCOA-affected horse, which are presented in combination with the macroscopic, ultrasonographic and histological findings.
Case history and clinical findings
A 4-year-old bay silver-coated Comtois mare was referred to the equine hospital of the Faculty of Veterinary Medicine of Ghent University with bilateral keratitis of 2 weeks’ duration, unresponsive to topical administration of aureomycine and acetylcysteine and to systemic administration of flunixin meglumine by the referring veterinarian. According to the owner, the horse had not presented ocular pathology before, besides a small white corneal spot in both eyes. On admission to the clinic, the horse presented with
bilateral cornea globosa (‘pop-eye’) and blepharospasm. Decreased vision was suspected based on poor response to the menace reflex in both eyes; the dazzle reflexes were positive, but pupillary reflexes were minimal in both eyes. There was only minimal response to instillation of tropicamide (tropicol 0.50%)1. The horse was sedated with 0.8 μg/kg bwt romifidine
(Sedivet)2 and 0.1 mg/kg bwt morphine (Morphine HCl)3. Subsequently, an auriculopalpebral nerve block was performed with 5 ml mepivacaine (Scandicaine 2%)4, and corneal anaesthesia was achieved by topical administration of 0.2 ml of oxybuprocaine hydrochloride (Unicaine 0.4%)5. Standard direct and indirect ophthalmoscopic examination was performed in a dark room and the eyes were assessed with a slit lamp biomicroscope (SL 15)6. Both eyes presented corneal lesions: in the left eye, multifocal ulcerative corneal lesions were accompanied by superficial peripheral neovascularisation. The right eye presented severe corneal ulceration and corneal oedema, with a mixture of deep and superficial peripheral neovascularisation, miosis and an irregularly outlined pupil. The fundus of the eyes could not be adequately visualised because of the persistent miosis. Culture of corneal swabs of both eyes did not yield bacterial or fungal growth.
Ultrasonography Ultrasonographic examination (B-mode) of the eyes was performed with a 6 MHz macro convex and a 9 MHz linear
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