Computed tomographic dimensions of the normal adult equine eye

There are limited data on whether the dimensions of the equine eye and its internal structures change with bodyweight in horses. The aim of this study was to identify whether the size of the equine eye and its individual structures varied in a predictable manner with bodyweight, and to examine the reliability of computed tomography (CT) scans in the measurement of the equine eye. Ninety horses which had undergone CT examination of the head for non‐ocular disease were included. All measurements of the eye were performed twice on all eyes by the same operator to quantify intra‐operator agreement, and in 10 cases were also performed by a second operator to quantify inter‐operator agreement. Agreement was excellent for all but the rostrocaudal measurement of the anterior chamber, which was fair. The following variables correlated significantly with bodyweight: anterior‐posterior size of the globe (correlation coefficient: 0.586) and posterior segment (correlation coefficient: 0.554); the latero‐medial size of the globe (correlation coefficient: 0.452), anterior chamber (correlation coefficient: 0.504) and posterior segment (correlation coefficient: 0.455); and the orbital fossa diameter (correlation coefficient: 0.219). The lens size and anterior‐posterior distance of the anterior chamber did not correlate significantly with bodyweight. These results suggest that overall eye size and the size of the orbital fossa change with the bodyweight of the horse, but lens size remains consistent, and that CT measurements of the internal structures of the eye are repeatable and reproducible. This information aids in interventional procedures for ocular pathology and assessment of normal anatomy.     

Ultrasonography of the eye and orbit

The eye and orbit are excellent subjects for ultrasonic evaluation. Examination and interpretation are relatively simple procedures. The normal ultrasonic anatomy of the eye and orbit is presented. Some examples of ocular and orbital pathology are discussed also.     

Computed Tomography and Magnetic Resonance Anatomy of the Normal Orbit and Eye of the Horse

Traumatic and infectious diseases of the eye and orbit can occur in horses. For diagnosis and monitoring of such diseases, medical imaging is useful including computed tomography (CT) and magnetic resonance imaging (MRI). The aim of the current study was to describe CT and MRI anatomy of the equine orbit and ocular globe. The heads from four adult horses were scanned with a 6‐slice Emotion 6 CT (Siemens, Erlangen), and a 3.0 Tesla Siemens Verio 6 MRI using T1 and T2‐weighted sequences. To validate CT and MR reference images, these were compared with anatomical models and gross anatomical sections. The bony limits of the orbital cavity, the relationship of the orbit with sinuses and foramina of the skull were well identified by CT. MRI was useful to observe soft tissues and was able to identify adnexae of the ocular globe (eyelids, periorbital fat, extraocular muscles, lacrymal and tarsal glands). Although MRI was able to identify all components of the eye (including the posterior chamber), it could not differentiate sclera from choroid and retina. The only nerve identified was the optic nerve. Vessels were not seen in this series of cadaver heads. This study showed that CT and MRI are useful techniques to image the equine orbit and eye that can have clinical applications.     

Normal equine ocular anatomy and eye examination.

When compared with small domestic animals, the horse has unique ocular characteristics (complete bony orbit, well-developed eyelid muscles, a nasal and temporal gray limbal line, granulae iridica, paurangiotic fundus). Knowledge of normal equine ocular anatomy is essential for ocular lesion interpretation. It is important to obtain a full history and general examination before sedation, nerve blocks, or other diagnostic ophthalmic tests are performed. All ocular examinations should include a systematic evaluation of both the anterior and posterior segments. Selection of other diagnostic tests depends on information obtained from the history, general inspection, and ophthalmic examination. The appropriate order of performing various special diagnostic tests must be considered before beginning the examination. If a diagnosis is elusive, referral to an ophthalmologist for further diagnostics should be considered.     

The conjunctival crypt of the equine third eyelid

The purpose of this study is to characterise the gross anatomy of a relatively large, cryptic structure within the bulbar conjunctival surface of the equine third eyelid that has commonly been observed by the authors during equine ophthalmic examinations. In this study, cadaveric eyes and adnexa were obtained from 13 Quarter Horses without history of ocular disease. The gross anatomy of the bulbar surface of the third eyelid was characterised by digital macrophotography and with an image capture system using an operating microscope. A well-defined cryptic structure was readily identified in the majority of eyes (17/26). The location of the crypt varied but was most commonly observed within the central aspect of the bulbar surface of the third eyelid, often in close proximity to prominent blood vessels oriented perpendicular to the leading edge of the third eyelid. A patch of lymphoid follicles was often observed surrounding the crypt. The rectangular shape and upward facing orientation of the crypt was akin to a shirt's breast pocket. Width and depth were variable but ranged between 1 and 8 mm in either dimension. Some crypts were complex, with multiple channels observed within the primary opening. In conclusion, this study confirms the presence of a well-organised mucosal crypt within the bulbar surface of the equine third eyelid capable of harbouring foreign material and ocular irritants. Additional anatomical characterisation of the macro-crypt by light microscopy, immunohistochemistry and electron microscopy is underway. Evaluation of this structure by equine practitioners and veterinary ophthalmologists should be performed during the routine ophthalmic examination to better understand its role in ocular disease.     

Aqueous humor and plasma concentrations of a compounded 0.2% solution of terbinafine following topical ocular administration to normal equine eyes

Objective To determine the transcorneal penetration and systemic absorption of a compounded 0.2% terbinafine solution following repeated topical administration to normal equine eyes. Sample population Six healthy adult horses with normal ocular examinations. Procedures One eye of each horse received 0.2 mL of a compounded 0.2% terbinafine solution every 4 h for seven doses. During the 1 h following administration of the final dose, multiple peripheral blood samples were obtained, and a single aqueous humor (AH) sample was collected at the end of the hour. AH and plasma concentrations of terbinafine were determined using high pressure liquid chromatography (HPLC). Stability of the formulation was assessed with HPLC analysis over a 14‐day time period. Results Terbinafine was not detected in the AH or plasma of any horse at any time point. No signs of ocular irritation or systemic toxicity were noted in any horse at any time point. The solution was stable over 14 days. Conclusion Topical ocular administration of compounded 0.2% terbinafine solution does not result in detectable AH or plasma levels following administration to normal equine eyes, suggesting its use for deep corneal or intraocular fungal infections in equine ophthalmology may be limited.     

Ophthalmic disease of fish.

Ocular disorders in fish are common and can occur as primary or secondary manifestations of systemic disease. Because most fish are sight feeders, visual impairments usually have fatal consequences for the individual fish and can lead to substantial economic losses for the client. This article encourages practitioners to perform a complete ocular examination as part of routine physical examinations in fish by providing a review of the functional anatomy of the teleostean eye, clinical work-up, ocular examination, ocular diseases, and treatment.     

The eye and orbit

Computed tomography (CT) and magnetic resonance imaging (MRI) provide excellent morphological detail of the eye and its associated structures. In veterinary medicine, these imaging modalities are most often used to diagnose and determine the extent of ocular or periocular tumors. They may also be used to diagnose inflammatory conditions of the orbital region and to determine the severity and extent of ocular trauma. This article reviews the applications of CT and MRI for ocular and orbital diseases, discusses normal findings, and presents representative case examples.     

Radiographic examination of the equine foot

A complete radiographic examination of the equine foot consists of properly exposed, processed, and positioned radiographs. For radiographic interpretation, in addition to knowing radiographic signs of disease, a knowledge of normal radiographic anatomy and possible insignificant anatomic variations is necessary.     

Equine ophthalmic examination: Routine diagnostic techniques

A complete, thorough ophthalmic examination is essential in the clinical approach to equine ocular diseases. In the majority of cases and more than in most systems, an ophthalmic diagnosis can be achieved at the time of examination because most ocular structures can be visualised either directly or indirectly. This article describes examination protocol and the techniques, instruments and diagnostic procedures currently available to equine general practitioners.     

Congenital ocular dermoid cyst in a river buffalo (Bubalus bubalis) calf

Clinical and histopathological findings of a congenital ocular dermoid cyst, located at the lower eyelid of a river female buffalo (Bubalus bubalis) calf were presented. A soft, fluctuant, non-tender, hyperaemic cystic mass was detected overlaying the left eye. Fine needle aspirate revealed filamentous debris with no malignant cells. The cyst was treated surgically by orbital exenteration and subsequently subjected to histopathological examination. The histopathological study disclosed a conjunctival dermoid cyst. This report is novel, in that; such ocular cyst has not previously been described in river buffalo calves.     

Cavernous sinus syndrome in a holstein bull

A 13‐month‐old Holstein bull was presented for right‐sided exophthalmos. Ophthalmologic examination noted that the animal was visual in both eyes, but that the right pupil was persistently dilated and very sluggish to constrict when stimulated with a bright light and that normal ocular motility was absent. Fundic examination of the right eye was normal as was a complete ophthalmologic examination of the left eye. Radiographs at presentation did not reveal the presence of sinusitis or other skull abnormalities. Initial treatment comprised intravenous antibiotics and anti‐inflammatories for orbital inflammation over a 14‐day period. There was no perceptible change in the appearance or neuro‐ophthalmologic examination of the right eye during hospitalization. The animal was discharged to the owner's care, but 3 weeks later was found recumbent with unilateral strabismus of the left eye and a fixed right pupil. Due to the inability to rise and rapid deterioration, humane euthanasia was performed, and a full postmortem examination, preceded by a MRI, was performed that identified abscesses extending bilaterally through the round foramina obliterating the cavernous sinus region, as well as abscessation of the right mandible, right trigeminal neuritis, right‐sided sinusitis, and right‐sided otitis media. Cavernous sinus syndrome should be considered in cattle with a combination of exophthalmos and neuro‐ophthalmologic abnormalities involving cranial nerves III, IV, V, and VI, whose branches are located within the cavernous sinus.     

A promising surgical approach to equine glaucoma

The preceding case report by Wilson et al. (2015) describes the surgical placement of a Baerveldt glaucoma shunt in a glaucomatous equine eye that had been refractory to aggressive medical and repeated surgical therapies yet maintained functional vision. Shunt placement surgery successfully maintained vision and intraocular pressure in the normal range in the affected eye. This is the first report of long‐term successful glaucoma filtration surgery in a horse. Glaucoma shunt placement shows promise for maintaining vision and comfort in equine eyes affected with glaucoma. There is much to be learned in terms of the most appropriate implant type for the equine eye, the risk of complications such as ocular hypotony or tube fibrosis, and the utility of combination therapy with transcleral cyclophotocoagulation and implant placement, but with the report of one case in which gonioimplant placement has been associated with intraocular pressure control for over one year, further investigations appear warranted. This case provides a glimmer of hope for long‐term treatment of a painful, blinding disease that has traditionally carried a poor prognosis in horses.     

The amphibian eye.

Knowledge of the anatomy of the amphibian eye is important to differentiate normal from abnormal. Special features of the amphibian eye mark the transition from aquatic to terrestrial life. The most often reported spontaneous ocular disease in the anurans (frogs and toads) is lipid keratopathy. An experimental study showed that high dietary cholesterol played a significant role in the development of corneal lipid infiltration. Other diseases, like panophthalmitis, occur with bacterial septicemia.     

Ocular parameters related to drug delivery in the canine and equine eye: aqueous and vitreous humor volume and scleral surface area and thickness

OBJECTIVE: To measure the ocular surface area, scleral thickness, and the aqueous and vitreous humor volumes in the canine and equine eye. Animals studied Fourteen canine and 16 equine cadaver eyes. PROCEDURE: Eyes were measured either fresh or following fixation in formalin. Ocular surface area was calculated using the fluid displacement method. Globes were hemisected and aqueous and vitreous humor were collected and quantitated. Scleral thickness was measured by digital caliper, by image projection, and by ultrasonic biomicroscopy (UBM). RESULTS: Mean +/- standard deviation (SD) scleral surface areas in canine and equine eyes were 12.87 +/- 2.24 and 57.23 +/- 5.63 cm2, respectively. Mean +/- SD aqueous humor volume was 0.77 +/- 0.24 in the dog and 3.04 +/- 1.27 mL in the horse. Mean vitreous humor volume was 1.7 +/- 0.86 mL for the canine eye and 26.15 +/- 4.87 mL for the equine eye. In canine and equine eyes, the sclera was thinnest at the ocular equator: 0.34 +/- 0.13 mm (canine) and 0.53 +/- 0.1 (equine). There were no significant differences between the direct caliper and projected thickness measurements or differences between measurements in the superior, inferior, nasal, and temporal quadrants of the eye. Scleral thickness in fresh tissue was greater than in fixed tissue at most sites. CONCLUSIONS: The UBM measurement method appeared to be most consistent and reproducible when compared to direct measurement techniques. Formalin fixation of the eyes was associated with significantly thinner scleral tissue than with fresh ocular tissue.     

RADIOGRAPHY OF THE EQUINE THORAX: ANATOMY AND TECHNIC

Radiographic evaluation of the entire adult equine thorax in the standing position requires a minimum of four overlapping lateral radiographs. Small horses could often be studied with fewer radiographs. This basic examination provided the optimal radiographic opportunity for detecting lung diseases in the horse. An understanding of normal gross and radiographic anatomy is an absolute necessity if one is to avoid the potential diagnostic pitfalls inherent in radiographic interpretation of the equine thorax. Of equal importance is an appreciation for the technic required to produce thoracic radiographs. The advent of rare-earth intensifying screen and highspeed film systems has enhanced the opportunity for obtaining diagnostic standing lateral equine thoracic radiographs.     

Congenital orbital cysts of neural tissue in two dogs

A 3-month-old English Cocker Spaniel and a 6-month-old miniature poodle presented with clinical signs related to an abnormal right eye since birth. In both dogs, the right globe could not be identified and was replaced by a fluctuant intraorbital mass covered by a vascularized mucous membrane. Ultrasonography demonstrated that both masses were cystic structures markedly larger in size than the normal contralateral globes. In both cases, surgical excision revealed a multilobular cyst filling the whole orbital cavity. Histopathologic examination and immunostaining for glial fibrillary acid protein and S100 protein supported a diagnosis of neural cysts associated with ocular dysplasia. The definitive diagnosis was congenital cystic eye and microphthalmos with cyst for the Cocker Spaniel and miniature poodle, respectively. Karyotype was normal in both dogs. Congenital cystic eye and microphthalmos with cyst result from defects in early embryonic life that arise following formation of the optic vesicle and prior to closure of the optic fissure. To the authors' knowledge neither has been reported in the canine species. They should be considered in the differential diagnosis of orbital cysts in dogs.     

Biology and pathology of the invertebrate eye.

Although this article is not a comprehensive review of ocular biology and disease across the millions of invertebrate species, it is hoped that it highlights the surprising gene homologies that influence ocular development despite the dramatic differences in resulting ocular development and final morphology. In the same way that ocular anatomy and physiology is undergirded by similarities in the genetic foundation of the eye, it is to be expected that eye disease in invertebrates, as it is studied in more depth, will show substantial homologies with the eye disease of the vertebrates that veterinary ophthalmologists are more used to treating.