Diseases and surgery of the globe and orbit

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1 Chapter 5 Diseases and surgery of the globe and orbit Claudia Hartley and Rachael A. Grundon Orbital imaging See Chapter 2 and online material for detailed information on imaging the orbit of a horse. Orbital disease may be more challenging to diagnose because many of the structures are hidden from direct examination. In addition to appearance, displacement of adjacent structures permits deduction of involved structures and suggests the tissue of interest. Imaging systems provide confirmation of suspected diagnoses and are frequently an essential part of the diagnostic process. Imaging is described in greater detail in Chapter 2. Ultrasonography Ultrasonography is convenient, efficient, performed in real time, inexpensive, and readily available to most practitioners [, 2]. Although a 0 or 7.5 MHz probe is necessary for adequate imaging of the globe, the deeper aspects of the orbit may require a 5 or 7.5 MHz probe (Figure 5.). However, orbital ultrasonography is not very sensitive or specific, and other imaging modes such as computed tomography (CT) may be more diagnostic. In fact, even moderately large orbital lesions may not be apparent on some ultrasound examinations. See Chapter for the technique of performing ocular and orbital ultrasound. Aspiration of a lesion (e.g., mass, fluid) in the equine orbit can be performed for cytology, culture, and histopathologic examination. An 8 G, 4 inch, slightly curved needle is inserted cm lateral to the lateral canthus and then directed posteriorly in a line parallel to the medial canthus [2]. Approaching the retrobulbar space via the supraorbital fossa as described for retrobulbar block is also possible, especially if the lesion is in the dorsal retrobulbar area [3]. Exophthalmos and orbital trauma are the two most common indications for orbital ultrasound examination in the horse. After trauma to the orbit and associated structures, ultrasound may be used to evaluate the retrobulbar space for displaced fractures, hemorrhage, swelling, compression of the optic nerve, and integrity of the posterior wall of the globe [4]. Radiography Radiography is indicated primarily to identify bone fractures in the acutely traumatized individual, bone deformation in invasive neoplasia or sepsis, and to localize cellulitis if gas production is evident. Precise radiographs require general anesthesia to avoid motion, although screening images may be obtained in the standing sedated patient, particularly with the advent of digital radiography. Specific skyline techniques to highlight the orbital bones of interest (Figure 5.2) may identify a lesion against an air filled background (e.g., frontal sinus). Contrast orbitography may be performed by artificially introducing air into the orbit, although routine CT and magnetic resonance imaging (MRI) are much more sensitive diagnostic procedures. Other injectable contrast materials are seldom indicated and risk creating complications. Fluid/air interfaces within the sinus indicate pathology that is directly relevant to the orbit. Although infrequently necessary, radiography is also useful for localizing metallic foreign bodies such as gunshot within the orbit. In general, such foreign bodies are preferably left in position unless direct damage to a blood vessel results in substantial hemorrhage or the object is migrating. Computed tomography and magnetic resonance imaging Advanced imaging such as MRI and CT provide the best images to localize the orbital lesion precisely (Figures 5.3 and 5.4). The use of cross sectional and three dimensional reconstruction of CT equine skull images overcomes the superimposition of the complex anatomic features of the equine skull that make conventional radiographic interpretation difficult [5, 6]. The Equine Ophthalmology, Third Edition. Edited by Brian C. Gilger. 207 John Wiley & Sons, Inc. Published 207 by John Wiley & Sons, Inc. Companion website:

2 2 Equine Ophthalmology Posterior segment of globe EOM ON Figure 5. Transcorneal ocular ultrasound of a normal equine eye using a 7.5 MHz transducer. The deeper aspects of the orbit may require a 5 or 7.5 MHz transducer. C, Cornea; I, iris; L, lens; R, location of retina; ON, location of optic nerve; V, vitreous. Orbital ultrasound. EOM, Extraocular muscles; ON, optic nerve. (Source: Image courtesy of the Animal Health Trust.) Figure 5.2 Radiographs of a normal equine skull: anterior posterior view; lateral view. Skyline radiographic technique may isolate the bone of interest against an air filled background (e.g., frontal sinus). This radiograph is of a frontal sinus fracture from blunt trauma, demonstrating subcutaneous emphysema. The globe ruptured at the limbus in the same injury. (Source for and : Images courtesy of the Animal Health Trust.)

3 Chapter 5: Diseases and surgery of the globe and orbit 3 a dorsal nasal meatus b middle nasal meatus c ventral nasal meatus 2 dorsal conchal sinus 3 rostral maxillary sinus 4 ventral conchal sinus 2 dorsal conchal sinus 3 caudal maxillary sinus 4 ventral conchal sinus a b c 4 3 Red arrow optic chiasm White arrow sphenopalatine sinus Green arrow guttaral pouch 2 frontomaxillary aditus 3 caudal maxillary sinus (D) Figure 5.3 Transverse plane CT images (bony window) of a normal equine skull. At the level of ethmoturbinates. At the level of most caudal maxillary teeth. At level with globes; frontal sinus, 2 frontomaxillary aditus, 3 caudal maxillary sinus. (D) At the level of the optic chiasm (red arrow); sphenopalatine sinus (white arrow); guttural pouch (green arrow). (Source: Images courtesy of the Animal Health Trust.) large amount of low density fat in the equine orbit provides good natural contrast for differentiating various soft tissue structures by CT, although the primary advantage of CT over MRI is its ability to detect osseous changes [5]. Disadvantages of CT over skull radiography in the horse include increased cost and limited accessibility [6]. Pioneering standing sedated equine CT systems make use of an air skates trolley system, allowing near frictionless movement of the horse, developed by the late Alastair Nelson MRCVS. This allows the operator to move the entire patient anteriorly or posteriorly, whilst minimizing extraneous head movements, within the CT gantry [7]. This development has greatly advanced the use of this imaging modality by removing the costs and risks associated with g eneral anesthesia in the horse. There have been several reports of CT

4 4 Equine Ophthalmology Figure 5.4 Transverse and saggital views of a CT image of a horse with left sided exophthalmos caused by a retrobulbar neoplasm. Note the increased soft tissue density in the left orbit compared to the right orbit. Three dimensional CT image reconstructed from transverse plane CT images of the horse in. examination of the equine orbit, including evaluation of the periorbital sinuses, orbital masses, and skull fractures [4 9]. MRI of the normal orbit and eye of the horse has been described. MRI has advantages over CT for delineating soft tissue lesions. Cranial nerves within the orbit were not easily identified in this study, with the exception of the optic nerve (CN II) [9]. MRI has also been used to investigate paranasal sinus disease in the horse [9, 0]. MRI investigations require general anesthesia in the horse and longer capture times than CT. See Chapter 2 for more information on MRI and CT imaging. References. Freestone JF, Glaze MB, Pechman R, McClure JR. Ultrasonic identifi cation of an orbital tumor in a horse. Equine Vet J 989;2: Scotty NC, Cutler TJ, Brooks DE, Ferrell E. Diagnostic ultrasonog raphy of equine lens and posterior segment abnormalities. Vet Ophthalmol 2004;7: Brooks D. Orbit. In: Auer J (ed). Equine Surgery. Philadelphia: WB Saunders; 992: Williams J, Wilkie D. Ultrasonography of the eye. Comp Contin Educ Pract Vet 996;8:

5 Chapter 5: Diseases and surgery of the globe and orbit 5 5. Barbee D. Computed tomography in horses. Vet Radiol Ultrasound 987;28: Tucker RL, Farrell E. Computed tomography and magnetic resonance imaging of the equine head. Vet Clin North Am Equine Pract 200;7:3 44, vii. 7. Barakzai SZ, Barnett TP. Computed tomography and scinitigraphy for evaluation of dental disease in the horse. Eq Vet Educ 205;6: Miesner T, Wilkie D, Gemensky Metzler A, Weisbrode S, Colitz C. Extra adrenal paraganglioma of the equine orbit: six cases. Vet Ophthalmol 2009;2: D Aout C, Nisolle JF, Navez M, et al. Computed tomography and magnetic resonance anatomy of the normal orbit and eye of the horse. Anat Histol Embr J Vet Med 205;44(5): Tessier C, Bruhschwein A, Lang J, et al. Magnetic resonance imaging features of sinonasal disorders in horses. Vet Rad Ultrasound 203;54():54 60.