Ординатура / Офтальмология / Английские материалы / Slatter's Fundemental of Vetrinary Ophthalmology 4th edition_Maggs, Miller, Ofri_2008

result of environmental selective pressures that were altered by domestication. It is unclear whether there are differences in the visual acuity of dog breeds that have been developed to hunt by sight (sight hounds) and breeds that have been developed to hunt by smell (scent hounds), although the finding of a large number of beagles (a scent hound) with a pronounced visual streak suggests that there are insignificant differences between these two groups of dogs, despite their uses.

Estimates of Visual Acuity

The most familiar indicator of visual acuity for the human eye is the Snellen fraction, which relates the ability of a subject to distinguish between letters or objects at a fixed distance (usually 20 feet, or 6 m) with a standard response. Snellen fractions of 20/20, 20/40, and 20/100 mean that the test subject needs to be 20 feet away from a test image to discern the details that the average person with normal vision could resolve from 20, 40, and 100 feet away, respectively. This test actually measures the ability of the area of greatest visual acuity (the fovea) to discriminate between objects of high contrast. Peripheral visual acuity in humans is typically quite poor (i.e., 20/100, 20/200, or worse), presumably because the photoreceptor density is lower and the ratio of photoreceptors to ganglion cells is higher in these regions of the retina than in the fovea. The visual acuity of the normal dog ranges between 20/50 and 20/140, with 20/75 or so being the likely average. Feline visual acuity has been estimated to be between 20/100 and 20/200, whereas the larger eye of the horse (hence a greater numbers of photoreceptors) may result in a visual acuity of 20/30. The visual acuity of cattle, however, is unclear, because the size of their eyes and density of ganglion cells would suggest they possess a visual acuity comparable to that of horses but behavioral studies that depend on the cooperation of the animal have documented a visual acuity of only 20/240 to 20/440.

Most commonly used procedures to determine vision in animals (e.g., determination of menace responses by moving a hand across the visual field or having the animal’s eyes follow

a moving cotton ball) test the motion sensitivity of virtually the entire retina, and positive responses are still present even though visual acuity may be very poor (up to 20/20,000). Visually distinguishing the fine details in objects is less important for most domestic mammals (even working animals) than it is for most people. The trade-off of improved vision in dim light for less acute vision in bright light allows such animals to exploit ecologic niches inaccessible to people and aids in both seeking prey and avoiding predators.

Color Vision

Color vision in domestic mammals has been the subject of numerous studies with conflicting results. More recent, wellcontrolled studies suggest that most domestic mammals possess, and use, color vision.

The presence of cone photoreceptors in domestic mammals suggests the potential for color vision, although the numbers and types of cones are smaller than those in humans. Cones constitute less than 10% of the visual streak in the dog, whereas they occupy almost 100% of the human fovea. Additionally, instead of three types of cones (“red,” “green,” and “blue”) found in humans with normal color vision, dogs have only two functional cone types. One type of canine cone is maximally sensitive to light at 429 to 435 nm (“violet” to normal humans and corresponding to the “blue” cone), and another type has maximal sensitivity to light at 555 nm (“yellow-green” to normal humans) with extension into the red end of the color spectrum (corresponding to the “red” cone). Dogs lack, or do not use, “green” cones and appear to confuse red and green colors (redgreen color blindness, or deuteranopia). This means that dogs are unable to differentiate middle to long wavelengths of light, which appear to people as green, yellow-green, yellow, orange, or red.

Although it is not known whether the dog’s “blue” and “red” cones perceive colors in the same way as those of humans, the canine visible spectrum may be divided into two hues: one in the human violet and blue-violet range (430 to 475 nm), which

is probably seen as blue by dogs, and a second in the human greenish-yellow, yellow, and red range (500 to 620 nm), which is probably seen as yellow by dogs. Dogs also appear to have a narrow region (475 to 485 nm, blue-green to humans) that appears colorless. Light in this spectral neutral point probably appears to be white or a shade of gray to dogs. In people with deuteranopia, however, the neutral point is in a greener (505-nm) region of the spectrum, so dogs are not exactly the same as redgreen color blind humans. Wavelengths at the two ends of the spectrum (blue at one end and yellow at the other) probably provide the most saturated colors. Intermediate wavelengths are less intensely colored, appearing as if they were blends with white or gray.

The cat has a limited but detectable capacity for color vision and can distinguish between two stimuli if they differ greatly in spectral content, especially if the stimuli are also large. Cats appear to have the physical capacity (based on the presence of three types of cones) for trichromacy like humans, although behavioral studies have not demonstrated this ability and it is, at best, a pale copy of human trichromacy. Horses appear to have both a short-wavelength-sensitive (blue) cone with a peak sensitivity of approximately 428 nm, and a second cone with a peak sensitivity between the human red and green cones (539 nm, and called a middle–long-wavelength-sensitive cone). Therefore although horses have only two cones like dogs, orange and blue colors appear similar (shades of gray) to horses, whereas red and green appear similar (as shades of gray) to dogs (Figures 1-13 and 1-14). Cattle and swine also appear to have two functional cone pigments. The yellow tint to the equine lens probably filters out blue wavelengths, diminishing certain optical aberrations as well as glare and increasing the contrast of certain objects on select backgrounds.

Restrictions in color vision are probably of limited consequence to domestic mammals, as it is likely that they react only to colors of biologic importance to them. Problems may arise when one is teaching hunting and working dogs to

B C

FIGURE 1-13. A, The color spectrum and corresponding wavelengths of light. B and C, Difference between dichromatic color vision of the horse and normal human color vision. B, Color wheel representing the spectrum of colors perceived by the trichromatic human visual system. C, Reducing the number of types of cone from three to two results in dichromatic color vision and an enormous reduction in the number of different colors seen. (A from Gilger B [2005]: Equine Ophthalmology. Saunders, Philadelphia. B and C from Carrol J, et al. [2001]: Photopigment basis for dichromatic color vision in the horse. J Vis Sci 1:80.)

distinguish red, orange, yellow, and green objects solely on the basis of color. In these cases, other visual clues, such as relative brightness and contrast, or the other senses—smell, sound, taste, and touch—are required to differentiate objects that appear similar in color. Additionally, dogs, and probably most other domestic mammals, are able to differentiate perfectly between closely related shades of gray indistinguishable to the human eye. This ability is far more valuable in exploiting their ecologic niche than color vision, because it increases visual discrimination when insufficient light may be present to effectively stimulate cones.

CENTRAL VISUAL PATHWAYS

The eye is only the first step in “seeing” (Figure 1-15). Vision is not simply a recording of each pixel in a scene, as a camera would make, because that would quickly overwhelm the visual system with massive amounts of information that may not be pertinent to the animal’s survival or lifestyle. The brain does not, and cannot, consciously pay attention to the flood of information it receives from the eyes, but instead categorizes the information into specific “topics” that are channeled to specific areas of the brain for further processing. These “topics” are “internal” features such as texture and contrast, the direction and velocity of the object’s movement, its overall orientation as represented on the retinal surface, its shape, its color, and many other aspects. Additionally, unlike a camera, the brain compares the current image with previous images, images from the other eye, and input from other senses such as hearing, smell, and touch. Once this comparison is completed, only the information that is relevant for the task at hand, or the animal’s survival, rises to the level of conscious attention. Therefore the act of seeing depends not only on the function and health of the eye but also on the cognitive processes in the brain that decide what information merits conscious attention and what is to remain subconscious or ignored.

The central visual pathway begins with the retina, which is in effect an extension of the brain. In this tissue, information is processed in three functional stages. The first stage occurs in the rod and cone photoreceptors. These cells have varying sensitivity to different wavelengths of light and require different numbers of photons to strike them in order to elicit a response. The second stage of retinal processing occurs in the outer plexiform layer. At this level the photoreceptors, bipolar cells, and horizontal cells synaptically interact, and the responses of some photoreceptors are modulated by what is happening to other photoreceptors (so-called “on” and “off” or “center” and “surround” responses). These responses are associated with the static and spatial aspects of an object and serve to better define an object’s brightness and borders by altering its contrast with surrounding objects. The third stage occurs in the synaptic interactions in the inner plexiform layer, which is more concerned with the dynamic and temporal aspects of vision. At this stage, transient responses that may underlie motion and direction sensitivity may occur in the amacrine and retinal ganglion cells. The amount of retinal processing of an image before it gets to the brain varies greatly by species.

Retinal ganglion cell nerve fibers then form the nerve fiber layer of the retina, converge at the optic disc, turn posteriorly, gain a myelin sheath, and pass through the sievelike opening in the sclera, the lamina cribrosa. The fibers pass via the optic

nerve to the optic chiasm (see Figures 1-2, 1-15, and 1-16). Fibers coming from different parts of the retina maintain definite positions within the optic nerve and throughout the path to the visual cortex. Fibers from both optic nerves enter the optic chiasm, where partial decussation, or “crossing over” of fibers from one side to the other, may occur (see Figure 1-15).

The proportion of optic nerve fibers that decussate in the chiasm is related to the relative laterofrontal positioning of the orbit and eye in the skull and degree of binocular overlap. Animals with laterally directed eyes and no overlap between the visual fields of the two eyes exhibit complete decussation at the chiasm, and the information from the right (or left) visual field is processed entirely by the opposite visual cortex. As the eyes become more frontally directed in different species, however, it becomes possible for an object to be seen with both eyes. For example, an object in the animal’s right visual field (on its right side) falls on the nasal area of the right retina and the temporal area of the left retina. In order for the same side of the brain (the left side in this example) to continue to process all the information from the right visual field, some optic nerve fibers must remain ipsilateral and must not decussate at the chiasm. This percentage varies by species, but in horses and cattle, which have relatively laterally directed eyes, 83% to 87% of the optic nerve fibers cross, whereas the percentage decreases—75% for dogs, 67% for cats, and 50% for

humans—as the eyes become more frontally directed. This finding is clinically relevant because species with extensive binocular overlap will not necessarily bump into objects if only one eye is blind or there is a lesion on only one side of the brain.

The optic chiasm receives the optic nerves as they enter the cranial vault via the optic foramen and canals (Figure 1-17). The chiasm lies at the base of the brain adjacent and anterior to the hypophysis, which sits in the pituitary fossa of the postsphenoid bone. This relationship between the pituitary and the chiasm and optic tracts is important in considering the potential effect of space-occupying masses of the pituitary on vision.

From the optic chiasm, fibers enter the left and right optic tracts, which then pass laterally from the chiasm, anterior to the hypophysis, and beneath the ventral surface of the cerebral peduncle. The tracts then curve dorsally and posteriorly, between the cerebral peduncle to which they are attached laterally and the pyriform lobe. The tracts thus pass to the lateral geniculate body.

Before reaching the lateral geniculate body, some 20% to 30% of the fibers leave the tracts and enter the pretectal area. Some of these fibers enter the superior colliculus directly, and others pass via the tracts and lateral geniculate body to the colliculus indirectly. The majority of fibers entering the lateral geniculate body synapse here with the third ascending neuron in the visual

Optic radiation

Left occipital lobe Right occipital lobe

FIGURE 1-15. The visual pathway.

system, which passes without further synapse to the visual cortex. The first two synapses are the photoreceptor-bipolar and bipolar-ganglion cell interfaces.

Those fibers that leave the optic tracts before entering the lateral geniculate body pass to the pretectal area, carrying afferent impulses of the pupillary light reflex. In the pretectal area much decussation occurs, and the fibers pass to the midline

Edinger-Westphal nuclei of the oculomotor nerve (see Figure 1-15). Efferent impulses pass from these nuclei to the pupillary sphincter muscle in each iris.

A positive pupillary light reflex does not mean that the eye can see. Fibers that mediate the reflex arc leave the optic tracts before the tracts enter the lateral geniculate body.

From the lateral geniculate body, fibers pass forward and lateral to the lateral ventricle as the fanlike optic radiation, which enters the occipital or visual cortex, where interpretation of some visual stimuli occurs in domestic animals (Figure 1-18). Increases in intraventricular pressure (hydrocephalus) can affect the visual pathway at this point.

In dogs and cats the visual cortex is not the sole center of interpretation of visual stimuli. If the cortex is removed, light perception and discrimination of light intensity are retained, but familiarity of surroundings is lost. Subcortical integration is believed to occur in the superior colliculus.

VASCULAR ANATOMY AND PERIPHERAL NEUROANATOMY

Further details of orbital anatomy may be found in Chapter 17.

Arterial Supply

The major arterial supply of the eye is from the external ophthalmic artery, a branch of the internal maxillary artery,

Infraorbital foramen

Cribriform plate

Lateral part of frontal sinus

Salcus chiasmatis

Rostral clinoid process

Hypophyseal fossa

Caudal clinoid process

Dorsum sellae

Crista petrosa

Canal for transverse sinus

Cerebellar fossa

Hypoglossal canal

F = face

A = arm-forelimb

T = trunk

L = leg-hindlimb

M = mouth and tongue

N = nose and lips

Auditory area

Prefrontal area

Motor area

Somatosensory area

Visual area

which arises from the external carotid artery (Figure 1-19). The contribution from the internal carotid artery is small, unlike the situation in primates, and is via an internal ophthalmic artery, which arises from the circle of Willis. The internal ophthalmic artery enters the orbit through the optic canal with the optic nerve. From the external ophthalmic artery, numerous short posterior ciliary arteries arise (Figure 1-20) and penetrate the sclera around the optic nerve head. These arteries supply the retina and choroid.

There is no central retinal artery in domestic species. Single medial and lateral long posterior ciliary arteries pass around the globe horizontally, within the sclera, to supply the ciliary body. Muscular branches of the orbital artery, which supplies the extraocular muscles, also enter the globe near the insertions of these muscles. These anterior ciliary arteries anastomose with the long posterior ciliary arteries to form the ciliary arterial supply. When the globe is prolapsed, these muscular branches may be destroyed, decreasing the available supply to

the anterior segment of the eye. Branches from the ciliary arterial network form the major arterial circle of the iris. The deep conjunctival arterioles at the limbus anastomose with the anterior ciliary arteries before they enter the sclera, and also with arterioles in the ciliary body. Vascular events of clinical importance (e.g., inflammation in one area of this network of anastomosing vessels) can often be seen in other parts, and their origin must be distinguished clinically if possible (see Figure 1-20).

The eyelids are supplied by the superficial temporal artery, a branch of the external carotid artery, and by the malar artery, a branch of the infraorbital artery.

Venous Drainage

The retina is drained by the retinal veins and venules, which run from the peripheral retina toward the optic nerve head (see

Orbital plexus

Ophthalmic vein

Vortex veins

Angular vein

Internal maxillary vein

Deep facial vein Facial vein

External jugular vein External maxillary vein

FIGURE 1-21. The venous drainage of the eye and orbit of the dog. (Modified from Startup FG [1969]: Diseases of the Canine Eye. Williams & Wilkins, Baltimore.)

Figures 1-20 and 1-21). The venous circle they form at the optic disc may be complete or incomplete in the dog. The venous circle drains posteriorly through the sclera via the posterior ciliary veins to a dilation in the orbital vein, the superior (dorsal) ophthalmic vein.

The choroid is drained by approximately four vortex veins, which leave the globe near the equator and join the superior and inferior ophthalmic veins. The ciliary body is drained by the anterior ciliary veins to the same superior and inferior ophthalmic veins that drain to the orbital venous plexus at the apex of the orbit. This plexus drains to the cavernous venous sinus within the cranial vault. The cavernous sinus drains via

the vertebral sinuses, external jugular vein, and internal maxillary vein. Venous blood thus passes posteriorly from the orbit via this route. It may also pass anteriorly via anastomoses between the ophthalmic veins and the malar, angularis oculi, and facial veins to the external maxillary and external jugular veins.

Considerable species variation exists in the vascular supply and drainage of the eye and orbit.

Nerve Supply of the Eye and Adnexa

The general plan of nerve supply to the eye is shown in Figure 1-22. For further details, see Chapter 16.

Optic Nerve (Cranial Nerve II)

The optic nerve and meninges pass from the globe, through the cone formed by the retractor bulbi muscles, via the optic canal to the optic chiasm. The dura covering the nerve is continuous with the outer layers of the sclera. The optic nerve consists of ganglion cells, whose cell bodies lie in the ganglion cell layer of the retina. It is a tract of the CNS, not a peripheral nerve.

Oculomotor Nerve (Cranial Nerve III)

The nucleus of the oculomotor nerve lies in the brainstem and has several components serving different extraocular muscles, the ventral rectus, dorsal rectus, medial rectus, inferior oblique, and levator palpebrae superioris muscles. The nerve also carries parasympathetic fibers originating from the Edinger-Westphal nucleus, which lies near the other nuclei of the oculomotor nerve, and serves the sphincter pupillae and ciliary muscles. The oculomotor nerve thus contains efferent

n

Levator palpebra and dorsal rectus muscle Medial ventral rectus muscle

Ventral oblique muscle Dorsal oblique muscle

Skin of upper lid and forehead Medial canthus and third eyelid Lacrimal gland

Nasal mucosa via ethmoid foramen

Cornea, iris, and ciliary body

Extraocular muscles

Lateral rectus muscle

Retractor bulbi muscle

Upper eyelid

Lower eyelid

Facial area

Lacrimal gland

Orbicularis oculi muscle

motor fibers to the striated extraocular muscles, of mesodermal origin, and parasympathetic fibers to the smooth muscles of the iris and ciliary body, of neuroectodermal origin.

The oculomotor nerve leaves the brainstem on its ventromedial surface (see Figure 1-16), and passes ventral to the optic tracts, through the cavernous sinus, and enters the orbit via the orbital fissure (foramen orbitorotundum in cattle, sheep, and pigs). In the orbit, the nerve divides into dorsal and ventral rami. A branch from the ventral ramus passes to the ciliary ganglion, where the preganglionic parasympathetic fibers synapse. For more details on the fibers leaving the ciliary ganglion, see Figures 1-23 and 1-24.

Trochlear Nerve (Cranial Nerve IV)

The trochlear nerve leaves the brainstem on the dorsal surface and runs lateral to the tentorium cerebelli to the orbital fissure. It passes through the fissure with the oculomotor nerve and the ophthalmic branch of the trigeminal nerve. The trochlear nerve innervates the dorsal oblique muscle only.

Trigeminal Nerve (Cranial Nerve V)

The sensory branches of the trigeminal nerve receive the majority of the input from the orbit and periocular area. The nerve has both motor and sensory roots (see Figure 1-16), which pass in a common sheath through the petrous temporal bone to the trigeminal ganglion. The three branches of the nerve—ophthalmic, maxillary, and mandibular—arise from the trigeminal ganglion. The ophthalmic nerve leaves the cranial cavity via the orbital fissure, and the maxillary nerve via the round foramen (see Figure 1-22).

Once in the orbit, the ophthalmic nerve divides into the supraorbital (frontal), lacrimal, and nasociliary nerves. The supraorbital nerve is sensory to the middle of the upper eyelid

and adjacent skin (Figure 1-25). In horses, cattle, sheep, and pigs, the nerve reaches the upper lid via the supraorbital foramen, but in dogs and cats, it passes beneath the orbital ligament.

The lacrimal nerve supplies the lacrimal gland. The nasociliary nerve, the major continuation of the ophthalmic nerve in the orbit, gives rise to the ethmoidal and infratrochlear nerves. The ethmoidal nerve passes through the ethmoidal foramen to supply the mucous membranes of the nasal cavity. The infratrochlear nerve passes beneath the trochlear, penetrates the septum orbitale, and innervates the medial canthus, third eyelid, and adjacent lacrimal system (see Figure 1-25). Within the orbit, the nasociliary nerve gives off the long ciliary nerve, which enters the globe near the optic nerve to provide sensory innervation to the globe itself.

The maxillary nerve passes through the round foramen and via the alar canal to the pterygopalatine fossa. It gives rise to the zygomatic nerve, which divides into zygomaticotemporal and zygomaticofacial branches within the orbit. The zygomaticotemporal branch supplies sensory innervation to the lateral upper lid and rostral temporal area. The zygomaticofacial branch emerges from the periorbita ventral to the lateral canthus and supplies the lateral lower lid and surrounding skin. Postganglionic sympathetic fibers from the cranial cervical ganglion may also be distributed to the orbit via the branches of the maxillary nerve, which has no other branches of ophthalmic significance.

Abducent Nerve (Cranial Nerve VI)

The abducent nerve leaves the ventral surface of the medulla oblongata (see Figure 1-16) and passes through the wall of the cavernous sinus, forward via the orbital fissure (see Figure 1-22), to enter the orbit and supply the retractor bulbi and lateral rectus muscles.

Facial Nerve (Cranial Nerve VII)

The mixed facial nerve contains somatic motor and parasympathetic fibers, innervating the orbicularis oculi and retractor anguli muscles and the lacrimal gland. Cell bodies of the motor fibers are found in the facial nucleus in the medulla oblongata. The parasympathetic cell bodies are located in the rostral salivatory nucleus in the medulla. The nerve leaves the brainstem lateral to the origin of the abducent nerve (see Figure 1-16) and, with the vestibulocochlear nerve, enters the petrous temporal bone near the acoustic meatus (see Figure 1-23), a point of clinical significance to be discussed later with hemifacial spasm (see Chapter 16). The facial nerve enters the facial canal of the temporal bone, where the geniculate ganglion is situated.

From the ganglion arises the major petrosal nerve. Joined by the deep petrosal (sympathetic) nerve, the nerve of the pterygoid canal is formed and passes via the pterygoid canal to the pterygopalatine fossa, ending as the pterygopalatine ganglion. The parasympathetic fibers synapse here, and some pass to the lacrimal gland.

The facial nerve passes from the geniculate ganglion and emerges from the stylomastoid foramen to give numerous branches. The facial trunk terminates as the auriculopalpebral nerve, which crosses the temporal region and zygomatic arch (see Figure 1-25). The palpebral branch supplies the orbicularis oculi and retractor anguli oculi muscles.

Autonomic Innervation

Action of the Ocular Muscles with Autonomic Innervation (Table 1-1)

The pupillary dilator and sphincter muscles are antagonistic to each other and control the size of the pupil. As one muscle contracts, the other relaxes. If either muscle fails to function, the effects of the remaining muscle predominate; for example, paralysis of the dilator muscle alone results in a small pupil (miosis) because of the unbalanced action of the sphincter muscle.

The ciliary muscle varies in size and orientation among species but in general is composed of one set of smooth muscle fibers arranged so as control tension on the lens zonules and the refractive power of the lens and another set that controls the relative width of the iridocorneal angle, through which aqueous humor exits the eye. Müller’s muscle elevates the upper eyelid together with the levator palpebrae superioris muscle. If either muscle is defective or denervated, the upper eyelid droops

(ptosis).

Table 1-1 Actions of the Ocular Muscles with

Autonomic Innervation

Parasympathetic Supply

Parasympathetic fibers arise from the Edinger-Westphal nucleus of the oculomotor nerve and pass via the nerve to synapse in the ciliary ganglion (see Figures 1-24 and 1-26).

Other fibers that pass into the ciliary ganglion but do not synapse in it are as follows:

•Postganglionic sympathetic fibers from the cranial cervical ganglion

•Sensory fibers from the ophthalmic branch of the trigeminal nerve (V)

Sympathetic Supply

Sympathetic fibers from the brain pass down the cervical spinal cord and leave via spinal nerves of segments T1 and T2 (see Figures 1-24 and 1-26). The fibers leave the nerves, pass to the sympathetic trunk, and pass cranially with it, in its common sheath with the vagus nerve. The sympathetic trunk terminates cranially at the cranial cervical ganglion, where many of the fibers synapse. Postganglionic fibers pass via a variety of pathways to the pupillary dilator muscle and to Müller’s muscle.

OCULAR REFLEXES

All ocular reflexes and responses may be thought of as having five parts: (1) a receptor, (2) an afferent neuron, (3) interneuronal connections, (4) an efferent neuron, and (5) an effector. Knowledge of the pathways involved in each of the reflexes and responses is invaluable for the localization of neuroophthalmic lesions. Table 1-2 summarizes these reflexes and responses.

Pupillary Light Reflex (See Figures 1-24 and 1-27)

Constriction of the pupil on the same (ipsilateral) side as the stimulus is termed the direct pupillary reflex, whereas pupillary constriction on the opposite (contralateral) side is called the consensual light reflex. In order to obtain reliable reflexes, a bright penlight or transilluminator must be used. Both direct and consensual responses are present in normal animals, although considerable differences exist in the speed of the reflexes between small and large animal species.

Palpebral/Corneal Reflex

The palpebral/corneal reflex is elicited by touching either the periocular skin (palpebral) or the cornea (corneal). This reflex