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Basics ⬇

Outline

BASICS

Definition!!navigator!!

  • The equine orbit houses the globe, and is composed of the frontal, lacrimal, zygomatic, and temporal bones, with the medial wall of the orbit formed by the palatine and sphenoid bones. The orbital rim is complete in horses. The orbit is a closed conical cavity with a broad opening anteriorly. There are numerous foramina that open into the orbit, allowing nerves and blood vessels to enter this space. The orbit also contains additional extraocular supportive structures, including muscle, glands, fat, and fascia. The canaliculi of the nasolacrimal duct pass through the lacrimal bone. Pathology of any of these extraocular tissues, including the bony orbit, broadly defines orbital disease
  • As orbital disease progresses, tissue within or adjacent to the orbit loses its ability to function. Discomfort may develop, and vision may be compromised. Compression by a space-occupying lesion, anatomic rearrangement by trauma or disease, or invasion of a systemic illness into the orbit highlights the major mechanisms of orbital disease

Systems Affected!!navigator!!

Ophthalmic, musculoskeletal, vascular, nervous, and upper respiratory systems, including sinuses can be involved.

Signalment!!navigator!!

Older horses tend to develop neoplasia, whereas foals and yearlings, as well as polo ponies, may be more prone to acute trauma.

Signs!!navigator!!

  • Variable according to disease process
  • Exophthalmos, or anterior displacement of the globe, with decreased ability to retropulse the globe. May also be associated with nictitans protrusion, exposure keratitis, and lagophthalmos
  • Enophthalmos, or posterior displacement of the globe, which can occur secondary to atrophy of tissue behind the globe, nerve damage, and following some orbital fractures. Concurrent nictitans protrusion is often seen
  • Strabismus
  • Swollen periorbita
  • Orbital asymmetry from fractures, cellulitis, and orbital emphysema
  • Blepharedema, chemosis, corneal edema
  • Epiphora or other ocular discharge
  • Vision loss (usually unilateral)
  • Nasal discharge or epistaxis, decreased airflow through the ipsilateral nostril
  • Abnormal sinus percussion possible with conditions affecting the upper airways and sinuses
  • Phthisis bulbi (globe atrophy)
  • Fever, pain

Causes!!navigator!!

  • Trauma causing orbital fractures, cellulitis, and/or proptosis
  • Foreign bodies leading to orbital abscesses
  • Orbital neoplasia—meningioma, neuroendocrine tumor, extra-adrenal paraganglioma, lipoma, adenocarcinoma, lymphoma, melanoma, sarcoid, squamous cell carcinoma, hemangiosarcoma, multilobular osteoma, anaplastic sarcoma, medulloepithelioma, schwannoma, rhabdoid neoplasia, angiosarcoma, and neurofibroma have all been found in the equine orbit
  • (Pyo)granulomatous diseases such as Cryptococcus neoformans, Actinomyces spp. or epizootic lymphangitis
  • Retro-orbital cysts, dermoid cysts, dentigerous cysts
  • Guttural pouch disease
  • Sinusitis involving frontal, maxillary, sphenopalatine sinuses
  • Tooth root abscesses
  • Sinus neoplasia
  • Orbital fat prolapse
  • Varices or abnormal distention of venules causing a displacement of normal tissue
  • Parasitism as in hydatid cysts, Halicephalobus gingivalis, and Strongylus edentatus
  • Nutritional myopathy

Risk Factors!!navigator!!

See Causes.

Diagnosis ⬆ ⬇

Outline

DIAGNOSIS

Differential Diagnosis!!navigator!!

  • Any cause of ocular pain
  • Exophthalmos, a sign of orbital disease, can be confused with buphthalmos, which is a marked increase in globe diameter associated with chronic glaucoma
  • Primary extraorbital disease that is close to, but not affecting, the orbit—sinusitis, guttural pouch disorders, dental disease

CBC/Biochemistry/Urinalysis!!navigator!!

CBC may show a leukocytosis and elevated fibrinogen or other nonspecific inflammatory indicators.

Other Laboratory Tests!!navigator!!

Systemic illness should be considered.

Imaging!!navigator!!

  • Very important diagnostic tool in evaluating orbital disease
  • Skull radiographs, orbital ultrasonography, CT (ideal for evaluation of fractures), or MRI where available (foals and small ponies primarily)

Other Diagnostic Procedures!!navigator!!

  • Aspiration of fluid or biopsy of tissue should be performed
  • Cytology, microbial culture and susceptibility, and histopathology are recommended as part of an orbital disease workup
  • Orbitotomy can be diagnostic and therapeutic; however, it is difficult surgery that may require orthopedic instruments
  • Trephination into paranasal sinuses may be indicated for microbial culture, irrigation, and drainage
  • In cases of proptosis, careful ophthalmic examination will dictate viability of the eye. Miosis with severe hypotony and hyphema indicates severe trauma and poor visual prognosis

Pathologic Findings!!navigator!!

Varies greatly depending on the particular disease.

Treatment ⬆ ⬇

TREATMENT

  • Highly dependent on underlying cause
  • Minor orbital trauma may be treated medically; however, surgical intervention may be indicated in some cases (e.g. fractures, orbital neoplasia) with short hospital stays recommended until the owner or trainer can monitor and treat the patient at home
  • Activity is based on degree of visual impairment and comfort of the horse. Some of these diseases are very painful especially after invasive surgery. Stall rest may be indicated for the short term
  • No change in diet is necessary unless malnutrition is a cause of the atrophy of orbital contents
  • For primary orbital disease, prognosis for vision is guarded initially. In severe, painful orbital disease with irreversible blindness, and in some cases of orbital neoplasia, the best management may be orbital exenteration
  • In a sighted eye, orbitotomy is best for discrete, solitary retrobulbar cysts or masses that do not invade the optic nerve
  • Enucleation may be recommended to remove a painful, blind eye and its associated conjunctiva and nictitans. An intraorbital prosthesis may be placed in the orbit to replace the globe if risk of infection or tumor recurrence is low. In the absence of infection or neoplasia, intrascleral prostheses are an acceptable alternative to enucleation and can be placed in an eviscerated scleral shell as long as the cornea is not severely diseased and there is no residual lagophthalmos or exophthalmos. Orbital bleeding should be minimized by careful hemostasis
  • Exenteration is indicated if a neoplastic condition extends beyond the confines of the globe or if disease affects both the globe and other orbital tissues
  • Periorbital fractures should be repaired quickly as fibrous union can occur as soon as 1 week post trauma. Tarsorrhaphies are beneficial to proptosed eyes and should not be removed until most of the periorbital swelling has subsided, usually 1–2 weeks
  • Temporary or permanent tarsorrhaphy is indicated in cases of facial nerve trauma, neurogenic keratoconjunctivitis sicca/keratitis, and lagophthalmos

Medications ⬆ ⬇

Outline

MEDICATIONS

Drug(s) of Choice!!navigator!!

  • Systemic antibiotics should be administered in cases of trauma or suspected orbital infection/cellulitis
  • Anthelmintics in cases of parasitic orbital disease
  • The globe itself may benefit from topical ophthalmic lubricants or antibiotics
  • Periorbital swelling can be alleviated by judicious use of anti-inflammatories
  • Occasionally uveitis is seen with orbital trauma and should be treated with topical or systemic anti-inflammatories
  • Flunixin meglumine at a dose of 1 mg/kg IV, IM, or PO BID or phenylbutazone at a dose of 2.2–4.4 mg/kg PO twice daily can be given for pain associated with the orbital disease

Contraindications!!navigator!!

Topical steroids are contraindicated when ulcerative keratitis is present.

Precautions!!navigator!!

Long-term flunixin meglumine or phenylbutazone use may lead to systemic complications such as renal dysfunction or protein loss from the gut.

Possible Interactions!!navigator!!

N/A

Alternative Drugs!!navigator!!

Intralesional iridium implants or cisplatin chemotherapy into the orbital tumor may be beneficial in some types of neoplasia. Chemotherapy and corticosteroids may be beneficial in the treatment of orbital lymphosarcoma.

Follow-up ⬆ ⬇

FOLLOW-UP

  • Recheck visits are indicated for more extensive diseases, especially if orbital surgery is performed as for an aggressive tumor
  • Long-term damage may be sustained in orbital trauma, including eyelid paralysis, chronic keratitis, keratoconjunctivitis sicca, or intermittent nasal or ocular discharge, necessitating chronic therapy and monitoring
  • Providing a safe environment with good training may decrease the opportunity for trauma
  • Recurrence of tumor, reinfection of orbit, and persistent pain and swelling can all occur during and after the treatment period
  • Blindness and loss of the eye are possible sequelae of severe orbital disease
  • Highly variable outcomes are based on correct diagnosis and appropriate treatment. Some orbital diseases such as trauma are one-time events with possible long-term side effects. Other diseases such as tumors may never be cured, only treated palliatively

Miscellaneous ⬆ ⬇

Outline

MISCELLANEOUS

Abbreviations!!navigator!!

  • CT = computed tomography
  • MRI = magnetic resonance imaging

Suggested Reading ⬆ ⬇

Brooks DE. Ophthalmology for the Equine Practitioner. Jackson, WY: Teton NewMedia, 2002.

Gilger BC. Equine ophthalmology. In: Gelatt KN, Gilger BC, Kern TJ, eds. Veterinary Ophthalmology, 5e. Ames, IA: Wiley Blackwell, 2013:1560–1609.

Gilger BC, ed. Equine Ophthalmology, 3e. Ames, IA: Wiley Blackwell, 2017.

Author(s) ⬆

Author: Shari M. Greenberg

Consulting Editor: Caryn E. Plummer

Acknowledgment: The author and editor acknowledge the prior contribution of Dennis E. Brooks.