Vasculature

Branches of the external carotid artery are the primary source of blood supply to the eye and its adnexa. Venous blood leaves the orbit through the angular vein of the eye, the deep facial vein, and the ophthalmic veins. The retinal vascular pattern is unique to each animal and does not change with age (Gionfriddo et al., 2006) and may someday serve as a means of identification.

Arteries

The major blood supply to the eye in the dog is from the external carotid artery via the maxillary and external ophthalmic arteries. Rostral to the base of the ear, the external carotid terminates by branching to become the superficial temporal and maxillary arteries. The superficial temporal artery (a. temporalis superficialis) courses dorsally, supplying branches to adjacent structures, and terminates as the superior and inferior lateral palpebral arteries, which supply the lateral aspect of the eyelids and conjunctiva.

The corresponding superior and inferior medial palpebral arteries that supply the conjunctiva and eyelids adjacent to the medial commissure arise from the malar artery (a. malaris), a branch of the infraorbital artery. The malar also sends branches to the third eyelid, to the ventral oblique muscle, and to the nasolacrimal duct.

The maxillary artery (a. maxillaris), gives rise to the inferior alveolar, caudal deep temporal, rostral tympanic, pterygoid, and middle meningeal arteries before entering the caudal alar foramen. The maxillary artery traverses the alar canal and emerges through the rostral alar foramen on the lateral aspect of the maxillary nerve. A few millimeters rostral to the rostral alar foramen, the maxillary artery gives rise to the external ophthalmic artery (a. ophthalmica externa), which passes dorsally to enter the apex of the periorbita. Within the periorbita an anastomotic branch leaves the external ophthalmic and passes caudally through the orbital fissure to unite with the internal carotid artery (ramus anastomoticus cum a. carotis interna) at the level of the sella turcica. A similar branch anastomoses with the middle meningeal artery (ramus anastomoticus cum a. meningea media). These anastomotic branches may arise independently from the external ophthalmic, but more commonly arise from a single trunk that divides within the orbital fissure.

The external ethmoidal artery (a. ethmoidalis externa) arises from the external ophthalmic artery distal to these anastomotic branches and curves dorsomedially over the extraocular muscles to enter an ethmoidal foramen on the medial orbital wall. There are usually two muscular branches of the external ethmoidal artery, although they may arise from a single trunk or from the external ophthalmic directly. A ventral muscular branch runs rostrally toward the eyeball between the ventral and lateral rectus muscles, supplying these muscles as well as the medial rectus, the ventral fasciculi of the retractor bulbi, and the superficial gland of the third eyelid. The blood flow (milliliter per minute per gram) to the extraocular muscles of the dog (0.33 ± 0.06) is significantly higher than that determined for other skeletal muscle. The muscular branches are continued as the anterior ciliary arteries (aa. ciliares anteriores), which follow the tendons of the rectus muscles to their insertions anterior to the equator of the eyeball. Ciliary vessels course anteriorly to the limbal region where they pierce the sclera posterior to the scleral venous plexus and divide into lateral and medial branches. These branches run circumferentially through the sclera just anterior to the scleral venous plexus forming a complete circle. Along the course of this circle, numerous arteriolar branches are given off, which pass anteriorly to supply the capillaries of the limbal region. From this arterial net fine arterioles also pass inward to anastomose with terminal branches of the posterior ciliary arteries supplying the ciliary body and iris. Terminal branches of the muscular branches also contribute to the capillary loops of the bulbar conjunctiva (aa. conjunctivales posteriores) at the limbus and to the deeper episcleral vessels (aa. episclerales).

A dorsal muscular branch crosses over the lateral rectus to run rostrally between the lateral and dorsal recti. It supplies branches to the lateral and dorsal rectus muscles, the dorsal fascicles of the retractor bulbi, the dorsal oblique and the levator palpebrae superioris muscles. At the equator of the eyeball, the dorsal muscular branch terminates as anterior ciliary, episcleral, and posterior conjunctival vessels analogous to those of the ventral branch. A distinct lacrimal artery (a. lacrimalis) usually arises from a muscular branch dorsally but may originate independently from the external ethmoidal or external ophthalmic arteries. It runs rostrally on the lateral edge of the dorsal rectus to supply the lacrimal gland.

The external ophthalmic artery continues rostrally and medially to the center of the periorbital cone, where it comes to lie on the external sheath of the optic nerve. The course of the artery along the optic nerve is sinuous. Approximately midway between the optic canal and the posterior pole of the eyeball, there is a large anastomosis between the external and internal ophthalmic arteries (ramus anastomoticus cum a. ophthalmica interna). The internal ophthalmic artery (a. ophthalmica interna) is a small artery that arises from the rostral cerebral artery at the level of the optic chiasm. The internal ophthalmic artery passes through the optic canal on the dorsal surface of the optic nerve and runs rostrally on the nerve to anastomose with the external ophthalmic. The internal ophthalmic artery is smaller than the external ophthalmic and is distributed almost exclusively to the eyeball itself.

From the anastomosis between the internal and external ophthalmic arteries, two long posterior ciliary arteries (aa. ciliares posteriores longae) arise, which supply most of the blood to the anterior segment of the dog eye. Initially, the long posterior ciliary arteries run rostrally, closely applied to the optic nerve. At the posterior aspect of the eyeball, the long posterior ciliary arteries give rise to a variable number of short posterior ciliary arteries (aa. ciliares posteriores breves). The short posterior ciliary arteries form a ring of 6 to 10 pillars around the optic nerve in the region of the lamina cribrosa. The laminar region of the optic nerve is also supplied by cilioretinal arteries and longitudinal pial vessels. Pathologic findings associated with the vessels of this region have been reported in dogs with glaucoma. The short posterior ciliary arteries pass through the sclera adjacent to the optic nerve and ramify in the choroid. These choroidal arterioles follow primarily in a meridional course to the ciliary body and ciliary margin of the iris. Here they form variable anastomoses with branches of the anterior and long posterior ciliary arteries. In dogs with poorly pigmented ocular fundi, the course of the choroidal vessels may be visible with the ophthalmoscope. The choroidal blood flow of the canine eye has been estimated to be approximately 250 mL per min per 100 g (of tissue). This flow has been shown to decrease with an elevation in intraocular pressure. A defect in the choroidal vasculature in the lateral quadrant of the fundus is the most common manifestation of Collie eye syndrome. The resistance to blood flow within the posterior ciliary artery expressed as the mean resistive index and the pulsatility index are 0.63 (±0.06) and 1.15 (±0.21), respectively. Although these values provide an indirect measure of arterial resistance, they are not correlated with systolic blood pressure or pulse rate.

The retinal arteries are derived from the short posterior ciliary arteries as they pass through the sclera at the periphery of the optic nerve. The retinal arteries continue anteriorly through the choroid and retina and emerge in the periphery of the optic disc, where they are visible ophthalmoscopically. The number of retinal arterioles is variable, but there are usually approximately 15 where they first become visible at the periphery of the optic disc. They divide repeatedly toward the periphery; secondary and tertiary branches are visible in the normal eye. The retinal arterioles are distinguished from the retinal veins by their greater tortuosity, finer caliber, and brighter red color. The arterioles have been reported to undergo a degree of sclerosis associated with aging.

Dogs possess a holangiotic retina with the presence of a circulus arteriosus around the optic nerve forming several choroidoretinal arteries. From these vessels, up to six branches enter the optic nerve head at the level of the sclera and run laterally to the optic nerve head toward the retina. There is no single central retinal artery but retinal veins drain blood toward the center of the optic nerve head, forming a single central retinal vein that leaves the eye through the area of the lamina cribrosa. Because dogs possess several branches derived from a plexus of cilioretinal arteries instead of a single central retinal artery, the pial vessels are in direct contact with the cilioretinal vascular plexus from choroidal vessels of the same source. The central retinal arteries are not involved in the supply of the optic nerve head.

The long posterior ciliary arteries continue anteriorly in the episcleral tissues along the medial and lateral meridians of the eyeball to its equator. Here they disappear from view, passing deep to the sclera into the suprachoroidea where they continue anteriorly to the ciliary margin of the iris. A few branches may be given off that anastomose with choroidal arterioles from the short posterior ciliary arteries. In the periphery of the iris, each long posterior ciliary artery bifurcates into dorsal and ventral branches that run circumferentially, forming the major arterial circle of the iris. The circle is incomplete dorsally and ventrally; however, in most specimens a smaller vessel completes the circle near the base of the iris. Small corkscrew-shaped branches leave the circle to supply the pupillary region or to anastomose with arteries in the ciliary region (see Vascular Tunic).

Angiostatin is a natural component of the canine cornea and retina where this serves to inhibit unwanted angiogenesis.

Vasculature: Veins