Equine ocular and pre-purchase examinations (Proceedings)
A good ocular examination begins with a thorough medical history. The saying goes that the eyes are the window to the soul – to the ophthalmologist they are often a window to illness somewhere else in the body. Start with the basics; signalment, use, as well as housing, work, and turnout environments. Clinical signs or performance issues not thought to be directly related to the ocular condition may be very important in obtaining an accurate diagnosis. A vaccine and deworming history can also be integral in finding answers to the problem at hand. When assessing the primary ocular complaint, the timing of the onset of signs, the duration of signs of and response or lack of response treatments as well as current medications should be determined and considered.
The globe is contained within the orbit and averages 4.48cm horizontally, 4.76cm vertically and 4.37cm in an anterior/posterior direction. The globe's movement is controlled by four rectus muscles, a dorsal and a ventral oblique muscle, and a set of retractor bulbi muscles. The eyelids are relatively thin with the upper lid being larger and more mobile than the lower lid. The palpebral fissure is normally oval with the lateral canthus being more rounded than then medial canthus. There are eyelashes only on the upper lid, with course vibrissae dorsonasal to the upper lid and ventral to the lower lid. The third eyelid is a fold of conjunctiva enclosing a T-shaped piece of cartilage. It is large in the horse, with only the leading edge normally visible. The leading edge of the third eyelid may or may not be pigmented dependent upon coat color.
The lacrimal gland lies below the dorsolateral orbital rim and is innervated by parasympathic postganglionic fibers of CN VII. The third eyelid gland surrounds the base of the third eyelid and is innervated by parasympathic postganglionic fibers of CN IX. There are two lacrimal puncta, one each in the upper and lower lid approximately 8-9 mm lateral to the medial canthus. The puncta are 2mm in diameter and the nasal lacrimal duct averages 3-4mm in diameter. The nasal lacrimal duct follows a line drawn from the medial canthus to just dorsal and rostral to the infraorbital foramen and then to the skin of the floor of the nostril near the mucocutaneous junction.
Corneal thickness varies being thickest peripherally at 1mm and thinnest centrally at 0.6mm. The iris is normally golden to dark brown, but can have heterochromia or be blue or white. The granular iridicans (corpra nigra), are extensions of the dorsal and ventral pupillary margins. These structures augment pupillary constriction and may have a sun-shade function. The lens in the horse has prominent developmental sutures that can be visualized without magnification using retroillumination. Most horses have a dorsally located tapetum that is green or yellow in color. Coat color can affect the presence or absenc e of a tapetum. In albinotic or subalbinotic coat colors the tapetum is absent or variable respectively. End on choroidal capillaries can be seen as black dots thought out the tapetal fundus and are called the stars of Winslow. The non-tapetal retina is normally dark in color, but again can lack pigment dependent upon the coat color of the animal. If the non-tapetal retina lacks pigment, the choroidal vessels will be visible. The optic nerve head is oval in shape with the long axis oriented horizontally. It is located in the non-tapetal fundus ventrotemporal to the axis of the globe. The vasculature of the equine retina is paurangiotic and normally vessels are only visible extending a few millimeters from the optic disc. The central aspect of the ventral margin of the optic disc has fewer vessels and can appear white in color.