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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; ; tumour; neoplasia; cellulitis; infection; trauma

Introduction Supraorbital Frontal bone fossa Exophthalmos refers to the anterior displacement of the 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 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 and the internal 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 will be distorted. Conjunctival inflammatory or invasive process occurs posterior to the globe, hyperaemia and , , distorted 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 . 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 , or the inability to close the 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 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 . 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 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 of exophthalmos have nerve and the integrity of the posterior globe (Boroffka and become more precise and the limitations of conventional van den Belt 1996; Michau 2005) (Fig 5). radiography for interpreting superimposed structures have Radiography is most useful to identify involvement of bone been overcome. These methods with their sequential images in the disease process. Skull films of the exophthalmic patient permit reconstruction of the region of interest, which allows can reveal fractures that may have displaced orbital soft tissues for better lesion localisation and more accurate determination or bone deformation or destruction when invasive neoplasia, of disease extent (Michau 2005). Unfortunately, both such as squamous cell carcinoma, is present (Butler 1993; modalities require general anaesthesia, the risk of which must Michau 2005). However; their usefulness is limited when the be compared to the benefits of the diagnostic information disease process affects mainly the soft tissue structures of the obtained. CT and MRI allow for the identification of lesions orbit. Also, many significant, but smaller bony lesions may be that are not readily apparent with other methods and lesions missed due to superimposition of bony structures. that are nonresectable, eliminating the need for potentially Additionally, precise images are best obtained while the traumatic exploratory surgery (Fig 6). If surgical intervention is animal is under general anaesthesia to prevent motion indicated, CT and MRI can be invaluable to planning of the artifacts. Diagnostic films may be achieved with standing approach and procedure. Due to the large amount of low- sedation alone in some cases only if the temperament of the density fat in the equine orbit, good contrast for animal allows and the sedation is adequate. Skyline views may differentiating orbital soft tissue structures is achievable even reveal lesions against air-filled background and DV views in CT studies (Michau 2005). The primary advantage of CT permit comparison of a diseased structure with contralateral over MR, however, is its ability to detect osseous changes in structures (Butler 1993). Radiography of the sinuses may the tissues of interest (Michau 2005) (Fig 7). MR provides reveal disease that extends into the orbit resulting in exophthalmos. Contrast studies of the orbit may assist in localisation of space-occupying masses or foreign bodies (Butler 1993; Michau 2005).

Fig 7: Computed tomography of orbital and sinus fractures sustained from a traumatic event. Note the multiple bone fragments surrounding the area of the right globe. These Fig 5: Ultrasound appearance of an orbital fracture. fractures were not evident with plain film radiography. EQUINE VETERINARY EDUCATION / AE / DECember 2007 587

better overall resolution of the retrobulbar tissues, but may fail fibrous union will begin within days of the injury. Orbital to detect defects in cortical bone or soft-tissue mineralisation trauma rarely causes avulsion of extraocular muscular (Cutler 2005). Increased soft-tissue densities and lytic bone attachments in the horse, in contrast to the dog (Cutler 2005). changes may be associated with chronic inflammatory If trauma results in a slower or progressive exophthalmos, it is diseases and neoplasia and soft-tissue calcification may occur usually because of progressive swelling of the retrobulbar and with (Brooks 1999). periorbital soft tissues or retrobulbar haemorrhage. Foreign If the exophthalmos is accompanied by nasal discharge, bodies that penetrate the oral or pharyngeal cavities or the epistaxis or stridor, or if the lesion is known to originate within may also inoculate the retrobulbar space with the nasal cavities or sinuses, endoscopy of the nasal cavities or infectious agents resulting in orbital cellulitis or abscess trephination and endoscopy of the sinuses is indicated. These formation, the presentations of which may be delayed. modalities allow for direct visualisation of the disease and access for diagnostic sampling. Inflammatory conditions

Sampling Probably the most common inflammatory condition of the orbit that results in exophthalmos is orbital cellulitis (Brooks After physical examination and imaging studies have defined 1999; Cutler 2005). This condition may or may not be septic; the location of the lesion, sampling will further the diagnostic however, the most dramatic forms thereof tend to be process and may help form the prognosis. Any samples associated with an infectious aetiology. Penetrating foreign obtained via ultrasound or CT guidance should be submitted for material, trauma and septic emboli have all been implicated in aerobic and anaerobic culture, as well as cytological or its development. Entrapped haemorrhage within the histopathological examination, particularly if the exophthalmos retrobulbar space following trauma may become septic and is painful or if trauma is evident in the patient’s history. result in orbital cellulitis or abscessation with subsequent Advancement of a needle for aspiration or a Trucut biopsy exophthalmos (Brooks 1999). Septic , in punch should be observed via ultrasonography from a different which an infectious agent has been introduced into the globe, window or subsequent CT slicing. Biopsy of orbital contents can result in orbital cellulitis if it is allowed to progress may be performed from the supraorbital fossa after making a unchecked and extend into the orbital tissues surrounding the small skin incision and obtaining a large needle core biopsy from globe (Brooks 1999; Cutler 2005). extending from the behind the globe, from the medial canthus along the orbital frontal or maxillary sinuses, however, is one of the more wall or from 1 cm lateral to the lateral canthus. At the latter site, commonly implicated causes of inflammatory orbital disease a 10 cm, 18 gauge needle is bent into a slight curve then (Brooks 1999; Cutler 2005). Disease extending from the nasal inserted temporal to the lateral canthus and directed posteriorly, cavity, particularly when chronic, may also compromise the in a line parallel to the medial canthus (Brooks 1999). It is critical integrity of the orbit and result in exophthalmos. to avoid major vessels and orbital or ocular structures during the Due to the elongated orientation of the equine skull, sampling procedure, particularly the cranial nerves and dental disease is rarely an initiator of orbital disease unlike in vasculature entering the orbit from the foramina posterior to small animals where the oral cavity is directly adjacent to the the globe. Trephination of the paranasal sinuses may be ventral orbit. Dental disease can affect the orbit of the horse indicated for biopsy, microbial culture, irrigation and drainage if when there is extension into the sinuses and from there into the disease resulting in exophthalmos originates there. the orbit but the disease must be substantial and extensive (Cutler 2005). Guttural pouch disease may rarely affect the Trauma orbit due to its close proximity, although the majority of ocular disease in these cases occurs secondary to damage of the Trauma sufficient to cause immediate exophthalmos may internal carotid artery or cranial cervical ganglion, rather than result also in significant damage to the orbital and periorbital direct extension of infection into the orbit (Cutler 2005). structures. Intraorbital swelling from a traumatic or Parasitic cysts have been described in the horse as space- inflammatory insult will result in marked external periocular occupying lesions of the orbit. An hydatid cyst has been and supraorbital distention initially with exophthalmos reported to cause exophthalmos, as have incidents of aberrant occurring with the more extensive conditions. Due to the migration of other nematodes including Strongylus spp., complete bony orbital rim of the horse, proptosis, or the Halicephalobus spp. and Draschia spp. (Barnett et al. 1988; protrusion of the globe from the orbit with entrapment of the Cutler 2005). These animals may have a history of inadequate eyelid margins posterior to the equator of the globe, is very deworming or may have other clinical signs suggestive of rare and when it occurs usually leaves the globe severely parasitic infestation. Treatment ideally involves surgical excision injured, usually necessitating its removal. Often after traumatic of the cysts combined with aggressive anthelmintic therapy. insults, there are extensive fractures of the orbital bones, Nutritional myopathy affecting the masseter muscles especially the dorsal orbital rim. If a fracture results in bony secondary to selenium and vitamin E deficiencies has been fragments that threaten the globe or entrap an extraocular reported to cause profound muscle swelling and necrosis in muscle and limit globe mobility, or causes even a minor the horse resulting in exophthalmos (Step et al. 1991). Clinical displacement of the globe, it must be repaired promptly as signs resolve when the dietary inadequacies are corrected. 588 EQUINE VETERINARY EDUCATION / AE / DECember 2007

Inflammatory orbital disease should be treated infiltrate with eyelid and conjunctival swelling and progresses immediately and aggressively. For orbital cellulitis and any to exophthalmos, which can be bilateral. It can also present disease of suspected infectious aetiology, systemic as a solitary orbital mass that rapidly enlarges resulting in antimicrobial agents should be administered at the highest unilateral exophthalmos. Early identification may make recommended doses tolerable to the patient. Ideally, the therapy possible, but the prognosis for these animals is poor choice of antimicrobial medications will be based upon (Cutler 2005). culture and sensitivity, but it is inadvisable to delay therapy in Exophthalmos may be the presenting complaint for these cases until these data are available. Therapy with a neoplastic processes that arise elsewhere and secondarily broad-spectrum antibiotic should be instituted immediately invade the orbit. Many of these tumours are profoundly and when culture and sensitivity results dictate the therapy advanced by the time exophthalmos occurs due to the may be changed. In cases with a fungal aetiology, antifungal considerable potential space for local expansion both within agents should be added to the treatment regime. the cavity of origin and within the orbit before globe Nonsteroidal anti-inflammatory agents should be given prominence is noted. Aggressive carcinomas originating in the promptly and will help decrease the pressure exerted upon richly vascular ethmoid turbinates at the caudal aspect of the the globe and help with the animal’s discomfort. These horses nasal cavity, medial and inferior to the orbit, have been are usually extremely painful and exhibit general malaise. described as orbital invaders (Cutler 2005). Adenocarcinomas Local therapy for any external wounds or corneal injuries of the nasal cavities, frontal and maxillary sinuses have also should be initiated as well. Steps should be taken to prevent been reported to cause exophthalmos and tend to be very exposure keratitis. Often topical lubrication alone is aggressive and carry a poor prognosis as substantial tissue insufficient to protect the cornea and placement of a destruction is possible prior even to diagnosis (Hill et al. 1989; temporary should be considered. Cold van Maanen et al. 1996). Other tumours within the orbit that compresses may help with local inflammation and be have been reported to cause exophthalmos include soothing to the horse. In the rare cases where a true abscess melanoma, neuroendocrine tumours, osteosarcoma, is identified, surgical exploration, drainage and paranasal sinus , haemangiosarcoma, decompression should be undertaken as soon as possible (van medulloepithelioma, , neuroepithelial tumour of den Top et al. 2007). Most cases, however, occur with diffuse the optic nerve, fibroma, neurofibromas or sarcoids, and tissue inflammation, rather than with a pocket of exudate. lipomas (Dugan et al. 1991; van Maanen et al. 1996; Brooks Prolonged therapy is usually necessary. The longer there is 1999; Colitz et al. 2000; Beard and Wilkie 2002; Scotty et al. retrobulbar pressure upon the globe, particularly if the optic 2004; Cutler 2005). Accompanying the progressive nerve is compressed or the vascular supply to the eye is exophthalmos, some of the reported symptoms in horses with compromised, the more likely there is to be permanent orbital tumours include orbital swelling, blindness, strabismus, damage to these structures resulting in loss of vision. and behavioural abnormalities. Prognosis will Prognosis for vision is guarded in most cases. depend upon the size of the mass lesion, the extent of its invasiveness, and whether the tumour is amenable to surgical Neoplasia resection or radiation therapy. Many cases are so advanced at the time of diagnosis that palliation becomes the only Many differing neoplasms have been identified within the treatment avenue possible. equine orbit presenting with or progressing to exophthalmos. There are, however, few studies detailing the Other conditions that may result in prevalence and frequency of each type. Overall, the most exophthalmos common periocular tumour in horses is squamous cell carcinoma (SCC). SCC of the orbit is usually an extension There are certain very rare congenital conditions that may from another periocular structure, such as the globe or cause protrusion of the globe from the orbit. Animals that eyelids (Dugan et al. 1991; Beard and Wilkie 2002; Cutler present with these conditions tend to be young. Orbital 2005). Most often the orbital presentation begins as a third dermoids are foci of skin that are located within the orbit that eyelid lesion that extends deep into the orbit and fills the may form cystic, fluid-filled masses that expand and result in retrobulbar space. Usually exophthalmos is not the initial progressive exophthalmos (Cutler 2005). Surgical excision is finding, but may occur if the third eyelid or any other visible the preferred therapy. Prognosis will depend upon the size of tumour has been previously removed and a recurrence in the the mass, the likelihood of complete removal and any posterior orbit pushes the globe anteriorly. SCC is a locally exposure damage to the globe that may have already aggressive tumour that frequently requires extensive surgical occurred. Certain vascular anomalies including venous sinus resection, often accompanied by enucleation or exenteration dilations, arteriovenous anastomoses or varices or other to affect a cure (Beard and Wilkie 2002). Radiation therapy prominent or redundant vessels have been described in the has a high cure rate, but its availability is limited (Theon and orbit and implicated in the development of exophthalmos. Pascoe 1995). Lymphoma is the most common secondary Some orbital varices have been attributed to traumatic tumour of the orbit and is often less obvious as a neoplastic induction. Venous lesions will cause a relatively stable process (Cutler 2005). It frequently begins as a diffuse exophthalmos, while arterial abnormalities may be pulsatile EQUINE VETERINARY EDUCATION / AE / DECember 2007 589

with a palpable or auscultable bruit. Treatment and prognosis or drainage may decrease the likelihood of permanent are highly dependent upon whether access to the lesion can damage to the optic nerve and loss of vision. Any case of be achieved. Coil thrombosis and surgical reduction have been exophthalmos in the horse should be promptly and seriously rarely attempted (Cutler 2005). addressed for the wellbeing of not only the eye, but of the Caudal maxillary sinus cysts have been described that have patient as well. expansive behaviour similar to neoplastic masses which have resulted in exophthalmia (Auer 1992). Dentigerous cysts occur References uncommonly and have occasionally been identified as a cause of exophthalmos. These have a classic radiographic appearance Auer, J. (1992) Equine Surgery, W.B. Saunders, Philadelphia. pp 497- and are usually amenable to surgical removal (Butler 1993). 507. Barnett, K.C., Cottrell, B.D. and Rest, J.R. (1988) Retrobulbar hydatid Treatment cyst in the horse. Equine vet. J. 20, 136-138. Beard, W.L. and Wilkie, D.A. (2002) Partial orbital rim resection, mesh The first consideration for the exophthalmic equine if any sort skin expansion, and second intention healing combined with enucleation or exenteration for extensive periocular tumors in of globe-sparing approach to therapy is to be made is horses. Vet. Ophthalmol. 5, 23-28. protection of the cornea from exposure. Multiple and very Boroffka, S.A.E.B. and van den Belt, A.J.M. (1996) CT/ultrasound frequent administration of a lubricant, such as an artificial diagnosis - retrobulbar hematoma in a horse. Vet. Radiol. tear ointment, should be instituted even if the eyelids are still Ultrasound 37, 441-443. covering the globe. Depending on the extent of the globe Brooks, D.E. (1999) Equine . In: Veterinary protrusion, the animal may be able to actively perform a Ophthalmology, 3rd edn., Ed: K.N. Gelatt, Lippincott, Williams & complete blink when stimulated to do so by an examiner Wilkins, Baltimore. pp 1053-1116. initiating the palpebral reflex, but may not do so on its own. Butler, J. (1993) Clinical Radiology of the Horse, Blackwell Scientific, Of course, if the animal is unable to blink, additional steps are Ames, Iowa. pp 285-354. necessary to protect the globe, especially if artificial tears Colitz, C.M.H., Gilger, B.C., Davidson, C.P. and Ross, M.G. (2000) cannot be applied liberally and frequently. A temporary Orbital fibroma in a horse. Vet. Ophthalmol. 3, 213-216. tarsorrhaphy may be placed to keep the eye moist while the Cutler, T.J. (2005) Diseases and surgery of the globe and orbit. In: primary disease process is being addressed. Several horizontal Equine Ophthalmology, Ed: B.C. Gilger, Elsevier Saunders, St. Louis. pp 63-106. mattress sutures of 3-0 to 4-0 nylon or silk placed split thickness through the eyelid margins with stents may be Dugan, S.J., Roberts, S.M., Curtis, C.R. and Severin, G.A. (1991) Prognostic factors and survival of horses with ocular/adnexal utilised to this end. Corneal ulcerations that occur as the squamous cell carcinoma; 147 Cases (1978-1988). J. Am. vet. result of exposure are painful, difficult to heal and often med. Ass. 198, 298-303. progress to compromise the integrity of the globe. Of course, Hallowell, G.D. and Bowen, I.M. (2007) Practical ultrasonography of any corneal ulcers present must be treated aggressively to the equine eye. Equine vet. Educ. 19, 600-605. prevent further progression that could be sight or globe- Hill, F.W., Moulton, J.E. and Schif, P.H. (1989) Exophthalmos in a horse threatening. resulting from an adenocarcinoma of the frontal sinus. J. S. Afr. Treatment of the lesion initiating the exophthalmos must vet. Ass. 60, 104-105. be tailored to the specific lesion and its aetiology. If it is not a Michau, T.M. (2005) Equine ocular examination: basic and advanced primary orbital lesion initiating the exophthalmos, the primary diagnostic techniques. In: Equine Ophthalmology, Ed: B.C. Gilger, disease process must be identified and treated appropriately Elsevier Saunders, St Louis. pp 1-62. and combined with symptomatic therapy for the orbit and Scott, E.A., Duncan, J.R. and McCormack, J.E. (1974) Cryptococcus the globe. involving the postorbital area and frontal sinus in a horse. J. Am. vet. med. Ass. 165, 626-627. Scotty, N.C., Ford, M., Williams, F. 3rd, Loiacono, C., Johnson, P.J., Conclusions Messer, N.T. 4th, Turnquist, S.E. and Essman, S. (2004) Exophthalmia associated with paranasal osteoma in a Although orbital disease is not a common complaint for Quarterhorse mare. J. vet. diag. Invest. 16, 155-160. horses and does not generally have a significant impact on the Step, D.L., Divers, T.J., Cooper, B., Kallfelz, F.A., Karcher, L.F. and finances of the equine industry, it can have a major effect on Rebhun, W.C. (1991) Severe masseter myonecrosis in a horse. J. the individual animal, its quality of life, potential utility and Am. vet. med. Ass. 198, 117-119. lifespan. Many cases of exophthalmos in the horse are Theon, A.P. and Pascoe, J.R. (1995) Iridium-192 interstitial indicative of the presence of significant disease and it is brachytherapy for equine periocular tumours: treatment results frequently the case that the disease process has been severe or and prognostic factors in 115 horses. Equine vet. J. 27, 117-121. long-standing and that no cure or palliation may be possible. van den Top, J.G.B., Schaafsma, I.A., Boswinkel, M. and Klein, W.R. (2007) A retrobulbar abscess as an uncommon cause of This is particularly true in older animals where neoplasia tops exophthalmos in a horse. Equine vet. Educ. 19, 579-583. the list of differentials. Early imaging and intervention may van Maanen, C., Klein, W.R., Dik, K.J. and van den Ingh, T.S.G.A.M. improve the prognosis for these individuals. In the case of a (1996) Three cases of carcinoid in the equine nasal cavity and traumatic or inflammatory process, early decompression of maxillary sinuses: Histologic and immunohistochemical features. increased orbital tissue pressure via anti-inflammatory therapy Vet. Pathol. 33, 92-95.