by Noelle La Croix, DVM, Dip. ACVO
Ocular and Oral Disease
The complex anatomy of the eye and orbit is intimately juxtaposed with that of the oral cavity. Oral disease can adversely impact ocular health in several ways. Foreign bodies and/or infectious organisms can directly traverse the oral cavity and enter the orbit. Tooth root infections can result in extra luminal compression of the nasolacrimal duct. Diseases of the zygomatic salivary gland can cause orbital pain and swelling. A thorough understanding of ocular anatomy can help illustrate these relationships.
The orbit of most carnivores is grossly conical in shape, but it is more accurately described as a four-sided rhomboid shape with an apex, base, and four sides. The apex is at the superior orbital fissure where the optic nerve and the majority of extraocular muscles pass through the presphenoid bone to enter the orbit. The base of the orbit opens outwards from the face and is surrounded by an incomplete orbital rim. The lateral wall and floor of this open orbit are formed by soft tissue. Specifically, the orbit is bounded laterally and dorsolaterally by the temporal muscle, zygomatic arch, and orbital ligament. The orbit’s ventral floor consists of the medial pterygoid muscle and zygomatic salivary gland.
The ventral floor of the orbit therefore lacks any boney protection to stop foreign bodies from penetrating through the oral cavity into the orbit. Conversely, orbital masses can sometimes extend through the ventral floor into the oral cavity. For the veterinarian this soft tissue also allows for convenient access to the orbital space through the mouth. Orbital abscesses can be drained and masses can be biopsied though the ventral floor (behind the last maxillary molar of the oral cavity). The orbit’s medial aspect is defined by the frontal lacrimal and presphenoid bone (Figure 1). The orbit’s dorsal aspect is boney as well, being defined by the frontal bone’s zygomatic process and its associated orbital ligament.
Although not technically part of the orbit, the roots of several teeth are within close proximaty to the rostral orbital floor. Specifically, only a thin layer of alveolar bone separates the soft tissue of the orbit from the maxillary fourth pre-, first-, and second-molars. The shallow orbits of brachycephalic dogs are located further rostral to these teeth when compared with those of mesaticephalic or dolichocephalic breeds. Infections of the roots of these teeth can spread to the orbit and/or cause draining abscesses of the face (Figure 2). Pain elicited upon the opening of the mouth is a cardinal sign of an orbital abscess or cellulitis. The ramus of the mandible is embedded within the masseter and temporal muscle immediately caudal to the orbit. When the mouth is opened, the dorsal aspect of the ramus moves rostrally and compresses the orbital contents. If the normal compression is compromised this will result in an acutely painful response.
The zygomatic salivary gland is technically part of the orbit, and diseases affecting it are therefore orbital disease. Disorders associated with an abnormal zygomatic gland include the formation of mucoceles (swellings), sialoliths (stones), and in cats infarctions (necrosis). Sialadenitis (inflammation) and zygomatic salivary gland tumors can also occur. In dogs, of all these disorders, mucoceles are the most commonyly found. Facial trauma can rupture a salivary gland. Resection of the gland can stop the subsequent salivary fluid accumulation within local tissues including those of the orbit. The resection is usually curative but must be accomplished with atraumatic tissue handling through an orbitotomy.
The nasolacrimal ducts normally drain tears away from the eyes. Tears initially drain into dorsal and ventral puncta (oval openings) of lachrymal canaliculi within the inner surface of the upper and lower lid margins. For each eye, these upper and lower canaliculi converge to enter a lacrimal sac within the lacrimal bone. From there a nasolacrimal duct will move the fluid through a lacrimal canal (between the lacrimal bone and maxilla). In dogs, each duct passes by the apex of a canine tooth before ending as an opening within the nasal vestibule. Periapical abscesses of these teeth can lead to nasolacrimal duct obstruction or even dacryocystitis (Figure 3).
Dental and ocular disease are often linked due the anatomical relationship between the oral cavity and orbit. An understanding of that relationship can aid in the diagnostic evaluation of your patients. If you have further questions about oral and ocular disease, please feel free to consult with your veterinary dentist and/or ophthalmologist.
Noelle La Croix, DVM, Dip. ACVO
Veterinary Medical Center of Long Island
75 Sunrise Highway
West Islip, New York 11795
(631) 587-0800; fax (631) 587-2006
Figure 1: Lateral view of a dog skull with the zygomatic arch removed. The figure illustrates the temporal (4), frontal (5), palatine (6), zygomatic (7), maxillary (8), lacrimal (9), and sphenoid (13) bones. Illustration courtesy of Dr. M. S. A. Kumar (Tufts University, unpublished).
Figure 2: A draining fistula, caused by a maxillary tooth root abscess, beneath the left eye of a Yorkshire terrier.
Figure 3: Dacryocystitis, secondary to a periapical tooth root abscess, in a Labrador Retriever.