A Clinical and Morphological Study of the Neonatal Thoroughbred Foal's Eye

Munroe, Graham A (1993) A Clinical and Morphological Study of the Neonatal Thoroughbred Foal's Eye. PhD thesis, University of Glasgow.

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Despite the huge increase in scientific knowledge and understanding of the equine neonate which has taken place in the last 10-15 years, little attention has been paid to the eye. This morphological survey of neonatal Thoroughbred foals was undertaken to define the following: the normal appearance of the eye in the neonate; to highlight the differences between the neonatal and adult equine eye; document the route and time-scale of the development from one to the other; record the incidence and type of anomalies that occur and to distinguish these from true abnormalities; and finally, to study the incidence, morphology, reabsorption and long term effects of neonatal retinal and/or scleral haemorrhages and thereby, suggest an aetiology. Detailed ophthalmoscopic examinations were carried out on 169 randomly selected neonatal Thoroughbred foals born in two separate regions of the U.K. and one in New Zealand. A standardised examination was carried out and recorded using words, illustrations and photographs. Each foal also received a detailed clinical examination. Details of the mother's reproductive, gestational, parturitional and post-partum history were also recorded. Examinations were carried out within 96 hours of birth and repeated as necessary. Recorded data was subjected to an extensive statistical analysis. The recording and analysis of the data from the mare and foal allowed the generation of a database for each foal so that it could be identified apH traced, if required, in later life. An assessment was made of the gestational and parturient factors that might affect the development of the neonatal foal and particularly its eyes. Foal disease processes that may directly or indirectly contribute to variations in the morphology of the neonatal eye were also defined. All the foals in the survey had eyes open at birth and 98% had normal vision at the time of first examination. The most effective way of testing vision in the neonate was close observation of the foal in a box or nursery paddock. The menace reflex was absent in 96.5% of foals and, therefore, of little use in neuropathological testing, whereas 96% of foals had a positive pupillary light reflex, usually of a slow or sluggish nature. The normal appearance of the eyelids of the foals in this survey was similar to previous descriptions in the foal and adult, with long and dense upper cilia, and short, rather sparse lower cilia. The normal pattern of vibrissae was long and between 1 and 5 in the upper eyelid, and long and between 6 and 10 in the lower eyelid. 5% of foals had eyelid abnormalities, the majority of which were secondary entropion. The appearance of the conjunctiva in the neonatal foal differs little from the adult, and conjunctival differences between the foals in this survey partially correlate with coat and nictitans colour. The incidence of subconjunctival (scleral) haemorrhages in this survey was 8.3% and did not change with the period after birth when the first examination was carried out. There were no sex or eye distribution affects and the haemorrhages were not associated with any neonatal disease or abnormality. All haemorrhages were fresh, red and resolved within 4-10 days of birth. Most haemorrhages were present dorsal or dorso-nasally, and extended up to the limbus. The main factors involved in the pathogenesis of the haemorrhages seem to be increased peripheral venous pressure and direct compressive blunt trauma to the orbit during birth canal passage. The cornea of all normal foals was clear, oval and broader nasally. 67% of foals had a scleral shelf, usually dorsal and ventral. 62% also showed the presence of the pectinate ligament corneal insertions on the temporal and nasal aspects of the eye, and these are considered normal in the foal. Although the incidence of congenital comeal abnormalities was low, a case of congenital comeal vascularisation was recorded, this being only the second in the literature. The vast majority of foals had a dark brown or grey-brown iris colour with tan mottling towards the periphery and grey iridescent bands, especially towards the pupillary margin. The resting pupil was approximately 75% of maximal dilation, usually of a circular or broadened oval shape. The size and shape of the pupil were related to the examination time. Post pupillary light reflex pupil sizes were 5-15% of dilation smaller and of an oval shape, with the degree of change depending on the age of the foal. Anisocoria was present in 4 foals, including one with NMS, and return to clinical normality mirrored the resolution of the anisocoria. All foals in the survey had dorsal corpora nigra and 74% ventral structures. Dorsal corpora nigra were usually prominent and of normal size, whereas ventral structures were small and difficult to detect. Collarette tags of the anterior pupillary membrane were common and considered of no pathological significance. (Abstract shortened by ProQuest.).

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Additional Information: Adviser: Neil Gorman
Keywords: Veterinary science
Date of Award: 1993
Depositing User: Enlighten Team
Unique ID: glathesis:1993-75336
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 19 Nov 2019 20:49
Last Modified: 19 Nov 2019 20:49
URI: http://theses.gla.ac.uk/id/eprint/75336

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