584 EQUINE VETERINARY EDUCATION / AE / DECember 2007 Satellite Article Exophthalmos in the horse C. E. PLUMMER Departments of Small and Large Animal Clinical Sciences, College of Veterinary Medicine, University of Florida, Gainesville, Florida 32610-0126, USA. Keywords: horse; eye; exophthalmos; orbit; tumour; neoplasia; cellulitis; infection; trauma Introduction Supraorbital Frontal bone fossa Exophthalmos refers to the anterior displacement of the globe within the orbit. In its purest description, Lacrimal bone exophthalmos affects a normal-sized globe, rather than an Supraorbital foramen enlarged globe that appears to protrude from the orbit, as is Ethmoidal foramen Temporal the case when an eye is afflicted with chronic glaucoma and Sphenoid bone subsequently becomes buphthalmic. Exophthalmos is a Optic foramen bone relatively uncommon condition in the horse and when present usually indicates that a significant disease process is Orbital foramen occurring. The most common culprits include trauma, inflammatory conditions and neoplasms that reduce the Rostral alar foramen potential space of the orbit and force the globe outward in Dorsal the direction of least resistance. orbital rim Zygomatic bone The orbits are bony cavities in the skull that function to protect the globe. In the horse they are open anteriorly and closed posteriorly. The horse, as a herbivore, has a complete Fig 1: The bony orbit of the horse. Note the complete bony bony orbital rim, unlike carnivores that have ligamentous orbital rim, the open supraorbital fossa and the open ventral tissue at the lateral aspect of the orbital rim rather than bone aspect. The optic nerve and the internal ophthalmic artery enter the orbit via the optic foramen, while cranial nerves III–VI (Brooks 1999; Cutler 2005). The dorsolateral and ventral enter via the orbital foramen. The external ophthalmic artery aspects of the orbits caudal to the anterior rim consist of and the maxillary nerve enter via the rostral alar foramen. fascial support, muscle and fat. The rest of the orbit is entirely bone (Fig 1). The soft tissue aspects of the orbit represent sites to the periorbital sheath, usually extending into the orbit from through which extension of disease may occur or where beyond its confines (Brooks 1999). sampling of diseased tissue may be facilitated, as is the case Exophthalmos in the horse is usually most easily identified with the dorsal supraorbital fossa. The bony equine orbit is by viewing the animal from the front and comparing the surrounded by sinuses, specifically the frontal, maxillary and relative prominence of the globes on either side of the head sphenopalatine, and disease in any of these potential spaces (Fig 2). Most often exophthalmos will present as a unilateral may intrude upon the orbit through the bony septa separating finding or in the extremely rare bilateral cases, the eyes will be these spaces (Brooks 1999; Cutler 2005). The globe resides in asymmetrically affected, allowing for a difference to be the anterior portion of the orbit and is supported from behind appreciated. Often, the size of the palpebral fissure and the by extraocular musculature and fat. Therefore, when an orientation of the eyelashes will be distorted. Conjunctival inflammatory or invasive process occurs posterior to the globe, hyperaemia and chemosis, epiphora, distorted eyelid contour it is pushed forward and in the opposite direction of the and distention of the supraorbital fossa may be present as well lesion. Lesions of the medial orbit displace the globe laterally, (Fig 3). The nictitating membrane may be prominent and while intraconal lesions, or those that occur within the depending upon the location within the skull or orbit of the extraocular muscle cone, displace the globe directly anteriorly. inciting lesion, the direction of the globe may be altered Orbital disease processes can occur within the extraocular resulting in strabismus. Both the degree and direction of the muscle cone, between the muscle cone and periorbital sheath exophthalmos depend on the size and location of the lesion. (periosteum), which lines the interior orbital walls, or external Lagophthalmos, or the inability to close the eyelids over the EQUINE VETERINARY EDUCATION / AE / DECember 2007 585 globe completely, is sometimes noted secondary to exophthalmos and necessitates therapy to lubricate and protect the cornea from exposure. A thorough physical examination should be performed in any patient presenting with exophthalmos since there may be other clinical signs occurring, such as respiratory stridor, nasal discharge, odour or epistaxis. These abnormalities may direct the investigative diagnostics toward the primary cause or the site of origin. One of the easiest ways to confirm a subtle exophthalmia is through retropulsion of the globes. Normally, both globes should move easily and equally posteriorly when external pressure is manually applied through closed eyelids. Both globes should be repelled in the same direction and position and the extent and direction of posterior movement or the resistance thereof should be compared. Retropulsion should be performed in every case exhibiting exophthalmos because it may allow for a gross estimation of the location and extent of the lesion and Fig 3: Exophthalmos in a young horse secondary to trauma. potential cause of the exophthalmos. When pain is associated with retropulsion of the exophthalmic globe, most often the damage to the globe may significantly alter the prognosis or primary cause will be an inflammatory disease or cellulitis. course of treatment. Nonpainful exophthalmos is more commonly associated with slowly expanding neoplastic or cystic mass lesions. Diagnostics In any case of orbital disease in the horse, the condition of the globe should be assessed as thoroughly as possible. The There is a device called an exophthalmometer used to measure status of the menace response and the pupillary light reflexes relative globe prominence; however, its clinical utility is limited may give insight into the extent of the disease process. Eye (Cutler 2005). Similar results, although subjective, are position and movement may help localise an orbital lesion. obtained by simply palpating the globe and its position relative When globe movement is restricted, forced duction testing to the orbital rim. Orbital disease, such as that which results in may be indicated. The health of the cornea is of particular exophthalmos, is a diagnostic challenge because the concern since exophthalmos can result in exposure keratitis. structures and tissues of interest are hidden from view and Fluorescein stain will detect corneal ulcerations and Rose direct examination. Therefore, the most important diagnostic Bengal stain can detect even earlier signs of exposure, and prognostic information when investigating an equine case including tear film quality abnormalities, which frequently of exophthalmos, aside from culture and histopathological precede epithelial defects. The complete ophthalmic examination of the specific tissues involved, is obtained examination will also include determination of intraocular through advanced imaging of the patient’s skull and orbit. pressure and visualisation, if possible, of the posterior segment. The condition of the optic nerve head should be Imaging noted since it is susceptible to injury by retrobulbar lesions. Some aspects of the ophthalmic examination may not be able Ultrasonography is a safe, practical and relatively inexpensive to be performed depending upon the amount of periorbital method for imaging the eye and orbit (Hallowell and Bowen swelling present or the condition of the globe itself. Severe 2007). It is very useful for examining the globe when the eyelids are swollen or when the anterior structures of the eye are opaque limiting visualisation of the more posterior ocular structures (Michau 2005) (Fig 4). Most practitioners will find a 7.5 or 10 MHz probe useful for examining the globe itself. For examination of the orbit, however, a 7.5 or 5 MHz probe may be necessary to reach the deeper structures behind the eye (Cutler 2005). This modality can differentiate solid from cystic fluid or air-filled lesions and their locations within the orbit. Aspirates or biopsy samples may be acquired with ultrasound guidance, which allows observation of needle placement and gives this technique a distinct advantage, reducing the risk of iatrogenic trauma. However, it is possible to miss small lesions with ultrasonography. Even moderately sized lesions may not be apparent; in such cases further imaging should be pursued Fig 2: Exophthalmos in the horse. This particular animal had an (Cutler 2005). Exophthalmos and orbital trauma are the most orbital neoplasm. common indications for orbital ultrasonography. After 586 EQUINE VETERINARY EDUCATION / AE / DECember 2007 Fig 4: Ultrasound image of a horse eye and orbit. A retinal detachment is present; however, the retrobulbar space posterior to the globe (at bottom of image) is normal. Note the homogenous echogenicity and the normal optic nerve and Fig 6: Computed tomography image of a neoplasm which has muscle cone. secondarily invaded the orbit. Note the position of the globe. traumatic insults to the orbit, ultrasound may be used to With the advent of more advanced imaging techniques evaluate the retrobulbar space for haemorrhage, swelling, such as computed tomography (CT) and magnetic resonance foreign bodies, displaced fractures, compression of the optic imaging (MRI), the investigations