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

spinal cord segments to the effector muscle in the eye or orbit (see Figures 16-5 and 16-21).

In addition to signs of denervation of the iris dilator and orbital smooth muscle, peripheral vasodilation occurs and may cause increased warmth, pinkness of the skin best observed in the ear, and congestion of ipsilateral nasal mucosa. These signs may be difficult to detect, especially in small animals.

Preganglionic or postganglionic destruction of sympathetic innervation of the head in horses causes profuse sweating of the ipsilateral half of the face and cranial neck. The same area is hyperthermic, and the nasal and conjunctival mucosae are congested. Hyperthermia is determined by palpating the ears. There is a prominent ptosis of the upper eyelid, but only slight protrusion of the third eyelid and slight miosis. In cattle, sheep, and goats the most constant signs are hyperthermia detected on ear palpation, and ptosis. Miosis and third eyelid protrusion are subtle. In cattle less sweating is visible on the surface of the nose on the denervated side.

Etiology. Most cases of Horner’s syndrome in dogs and cats are idiopathic. Golden retrievers are especially susceptible to idiopathic Horner’s syndrome. Cases of idiopathic Horner’s syndrome are usually postganglionic, and they resolve spontaneously within 6 to 8 weeks. Other causes should not be

Table 16-6 Frequency of Causes of Horner’s Syndrome

Modified from van den Brock AHM (1987): Horner’s syndrome in cats and dogs: a review. J Small Anim Pract 28:929.

dismissed, however. Based on the anatomic location of the lesion, the possible causes of Horner’s syndrome are as follows (the frequency of the various causes in dogs and cats is listed in Table 16-6):

•Injury, infarction, or neoplastic involvement of the cranial thoracic spinal cord causes signs of paresis or paralysis of the pelvic limbs and deficits in the thoracic limbs, in

addition to ipsilateral Horner’s syndrome. Unilateral infarction of the lateral funiculus of the cervical spinal cord from fibrocartilaginous emboli may cause a persistent Horner’s syndrome, along with hemiplegia, in dogs.

•Avulsion of the brachial plexus roots in dogs and cats, with resultant thoracic limb paralysis, occurs after car accidents. Ipsilateral Horner’s syndrome indicates that the injury to the nerves innervating the thoracic limb is in or adjacent to the cranial thoracic spinal cord.

•Thoracic inlet or cranial mediastinal lesions (such as lymphosarcoma) involving the cranial thoracic sympathetic trunk, the caudal cervical sympathetic trunk, or both, may cause Horner’s syndrome without additions CNS signs (unless the tumor invaded the spinal cord).

•Injury to the cervical sympathetic trunk from a dog bite or from surgical exposure causes an ipsilateral Horner’s syndrome that is usually transient (Figure 16-22). Neoplasms involving the cervical sympathetic trunk, such as thyroid adenocarcinoma, are another cause.

•Mycosis of the guttural pouch in horses may involve the cranial cervical ganglion or internal carotid nerve and produce ipsilateral Horner’s syndrome.

•Otitis media may produce Horner’s syndrome often accompanied by signs of peripheral vestibular disturbance,

A

B

FIGURE 16-22. Preganglionic Horner’s syndrome in the left eye of an Alaskan malamute. A, Before instillation of epinephrine. B, Mydriasis 35 minutes after application of 0.1 mL of 0.0001% epinephrine solution. The lesion was preganglionic and associated with trauma resulting from vigorous lunging on a chain. It resolved spontaneously in 5 to 6 weeks.

facial paresis, or both. There is a higher incidence of the syndrome after ear cleaning.

•Retrobulbar injury, neoplasia, and abscess are common causes of Horner’s syndrome. Involvement of additional cranial nerves may cause blindness and strabismus.

Neurologic lesions causing Horner’s syndrome, their location, and associated neurologic deficits are summarized in Table 16-7.

Diagnosis. Denervation hypersensitivity is a phenomenon peculiar to smooth muscle innervated by the general visceral efferent system. Following denervation there is increased sensitivity of the muscle to neurotransmitters. This is evident in smooth muscle innervated by sympathetic neurons when the postganglionic axon is affected. Such denervated muscle shows hypersensitivity to the application of epinephrine or to circulating epinephrine released during excitement. This phenomenon has been studied in dogs with lesions of the sympathetic innervation of the ocular smooth muscles. Great hypersensitivity is present with lesions of postganglionic axons or their cell bodies than with lesions of preganglionic neurons, and this effect is used in localizing the site of the lesion to the sympathetic system.

Topical application of 0.1 mL of 0.001% epinephrine causes pupillary dilation in 20 minutes with lesions of postganglionic axons or their cell bodies, and in 30 to 40 minutes with lesions of preganglionic neurons (see Figures 16-19 and 16-22).

Treatment. The most important task is to determine the site of the lesion in an animal with Horner’s syndrome. In general, preganglionic lesions have a less favorable prognosis than postganglionic lesions. With postganglionic Horner’s syndrome, in which an exact cause cannot be determined, symptomatic treatment may be instituted with phenylephrine drops (0.125% or 10%) as necessary to relieve the clinical signs. Most cases of postganglionic Horner’s syndrome resolve spontaneously within 6 weeks. If the lesion is preganglionic, additional diagnostic procedures are undertaken to determine the site and cause, including neurologic examination and imaging (cervical and thoracic radiography; computed tomography or magnetic resonance imaging of the neck) (see Table 16-7). Because of the frequency of lymphosarcoma with cranial mediastinal lesions, thoracic radiographs are routinely taken in cats affected with Horner’s syndrome.

DYSAUTONOMIA (KEY-GASKELL SYNDROME). Synonyms: Dilated pupil syndrome, feline autonomic polyganglionopathy.

Dysautonomia is an idiopathic disturbance of systemic autonomic innervation with a marked reduction in the number of neurons in autonomic ganglia, resulting in complete sympathetic and parasympathetic denervation of the eye (and other organs). The majority of affected animals are younger than 3 years. The disease was initially regarded as a feline syndrome, and it is still most common in the cat, but lately it has also been reported in clusters of dogs living in the American Midwest. The disease is of acute onset, with signs developing within 2 days in cats and 14 days in dogs.

Clinical Signs. Clinical signs of dysautonomia are as follows:

•Dilated unresponsive pupils

•Protrusion of the third eyelids

•Blepharospasm

•Keratoconjunctivitis sicca

•Dry, crusted nose

•Dry oral mucous membranes and oral cavity

•Anorexia and lethargy

•Megaesophagus and difficulty in swallowing

•Vomiting/regurgitation

•Slow gastric emptying

•Fecal and urinary incontinence

•Bradycardia

•Distended bladder

Diagnosis. Dysautonomia is diagnosed from its clinical signs, especially protrusion of the third eyelids with fixed, dilated pupils. Together with the other signs, these two findings differentiate the disorder from Horner’s syndrome, in which the denervation is limited to the sympathetic system, is usually unilateral and presents with miosis. Lack of the normal flare response in intradermal histamine injection has also been used for diagnosis of dysautonomia.

Pharmacologic testing can be used to demonstrate denervation hypersensitivity of both sympathetic and parasympathetic systems (see Figures 16-19 and 16-20). The principles are similar to the testing described for Horner’s syndrome. The procedure consists of the following steps:

Demonstration of Parasympathetic Denervation. Parasympathetic denervation, which is also called Adie’s pupil and pupillatonia, can be demonstrated as follows:

1.Instill a drop of 0.1% pilocarpine and measure pupillary diameter every 5 minutes. If denervation is present, the resulting miosis will occur much faster compared with a normal animal.

2.Instill a drop of 0.06% echothiophate iodide (phospholine iodide). The denervated eye will show no change in pupillary diameter, but miosis will occur in a normal eye.

Demonstration of Sympathetic Denervation. Instill one drop of 1:10000 epinephrine into the eye. The third eyelid will

retract in a denervated eye because of hypersensitivity of the orbital smooth muscle to the epinephrine.

Treatment. The prognosis is poor, and in cats the reported survival rate is only 25% to 50%. Many patients are euthanized owing to systemic complications. Treatment consists of the following approaches:

1.General supportive therapy, including subcutaneous and oral fluids.

2.Routine topical agents for keratoconjunctivitis sicca (KCS), including compound KCS drops and artificial tears.

3.Laxatives and prokinetic gastrointestinal drugs.

VESTIBULAR SYSTEM

The vestibular system maintains the position of the eyes, trunk, and limbs in reference to head position or movement. It consists of receptors and cell bodies in the vestibular ganglion in the petrosal bone (inner ear), axons in CN VIII, neurons in the vestibular nuclei of the cerebellum, and axons in the MLF. The MLF connects vestibular neurons with neurons in the brainstem nuclei that innervate extraocular muscles (CNs III, IV, and VI). Figure 16-23 illustrates prominent anatomic features of the system.

Nystagmus (Box 16-1)

Normal Vestibular Nystagmus

Nystagmus is an involuntary, rhythmic ocular movement, the aim of which is to keep the eyes fixated on a visual target as the head moves. The head movement induces impulses in the vestibular component of CN VIII by stimulating the receptors in the semicircular ducts of the inner ear. The afferent neuronal pathway that results in nystagmus continues through the vestibular nuclei in the medulla and via the MLF to the brainstem nuclei of CNs

Box 16-1 Characteristics of nystagmus

Normal

•Occurs with head movement

•Quick phase in same direction as head movement

•Horizontal plane Æ horizontal nystagmus

•Vertical plane Æ vertical nystagmus

Abnormal

•Occurs spontaneously or with head held flexed laterally or extended (positional nystagmus)

•Peripheral receptor disease: The nystagmus is either horizontal or rotatory (direction from 12 o’clock point on globe). Its quick phase is constantly to side opposite from lesion. The direction of the nystagmus does not alter when the head position changes.

•Central vestibular disease (pons, medulla, cerebellum): The nystagmus may be horizontal, rotary, or vertical. The quick phase may be in any direction, opposite from or toward side of lesion, or vertical. The quick phase varies in direction with different positions of the head.

•Quick phase varying in direction with different positions of the head

•Lack of any response to head movements or rapid rotation indicates severe bilateral receptor or severe brainstem disease.

•Lack of any response to cold water irrigation of one of the external auditory canals (caloric test) indicates a severe lesion in the receptor of that side.

III, IV, and VI, whose axons provide efferent innervation to the extraocular muscles (see Figure 16-23). Normal vestibular nystagmus is tested by slowly moving the patient’s head from side to side and observing the limbus to note the resulting eye movement. This form of nystagmus has a rapid phase in one direction and a slow phase in the other direction.

The direction of nystagmus is defined by the direction of the rapid phase.

The rapid phase of the normal (physiologic) nystagmus is in the same direction as the movement of the head: Left movement causes left nystagmus, and ventral movement causes ventral nystagmus. Normal vestibular nystagmus occurs only as the head is being moved. Both eyes are affected and move simultaneously in conjugate fashion. Testing normal vestibular nystagmus evaluates the patient’s ability to abduct and adduct each eye. The result is abnormal if nystagmus persists after the head movement is stopped or if the head is extended or flexed laterally and nystagmus develops in that position.

Lesions that destroy the vestibular system, the MLF, or neurons of CNs III, IV, and VI cause loss of normal vestibular nystagmus.

In neurologic evaluation after intracranial injury it is important to distinguish between signs of diffuse cerebral edema and of brainstem contusion. Loss of vestibular nystagmus indicates a

severe lesion in the brainstem affecting the vestibular nuclei, MLF, or the nuclei of CNs III, IV, and VI.

Observation of ocular movements in normal nystagmus also allows evaluation of specific extraocular muscles. For example, signs of right abducent nerve paralysis are esotropia of the right eye and failure of the eye to abduct fully when the head is moved to the right.

Abnormal Nystagmus

When the head is flexed laterally to either side or extended fully, no nystagmus is normally found. In vestibular disease, nystagmus may be observed as follows:

•Spontaneous nystagmus: Present when the head is at rest

•Positional nystagmus: Present when an abnormal head position is induced

In peripheral vestibular receptor disease the nystagmus is either horizontal or rotary, and always in a direction (quick phase) away from the side of the lesion. The direction of rotary nystagmus is defined by the change in the dorsal limbus during the quick phase. This direction does not change when the position of the head is changed.

With disease of the vestibular nuclei or vestibular pathways in the cerebellum, the nystagmus may be horizontal, rotary, or vertical and may change in direction with position of the head; any of these types of nystagmus suggests central involvement of the vestibular system. However, in most cases it is impossible to further localize of the lesion based only on the nystagmus.

NEUROOPHTHALMOLOGY 341

Disorders of the Vestibular System

Otitis Media and Otitis Interna

Otitis is the most common cause of pathologic nystagmus in animals. Vestibular signs occur in animals when middle ear inflammation indirectly or directly affects the function of the membranous labyrinth. Varying levels of unilateral vestibular disturbance appear, which consist of asymmetric ataxia with strength preservation. Sometimes only a head tilt and positional nystagmus are evident. If the inflammation spreads to the inner ear, these signs may be accompanied by an ipsilateral facial paresis or palsy, Horner’s syndrome, or both. This is because of concurrent involvement of the facial and/or sympathetic nerves, which pass adjacent to or through the inner ear in the dog and cat (see Figures 16-5 and 16-6). Unilateral deafness may occur but is difficult to determine clinically.

Idiopathic Vestibular Disease (Feline Vestibular

Syndrome, Idiopathic Benign Vestibular Disease, Old

Dog Vestibular Disease)

This is a disease affecting dogs, cats, and horses. Patients present with head tilt and spontaneous nystagmus, with the fast phase opposite to the head tilt. The nystagmus is usually horizontal and occasionally rotatory. At onset a head oscillation may occur simultaneously with nystagmus. The disease is self-limiting, and after 3 or 4 days spontaneous nystagmus disappears, but abnormal positional nystagmus may still be elicited on altering the position of the head. The direction remains opposite to the head tilt.

Central Disorders

Peripheral receptor disease is suggested if the direction of nystagmus does not change when the position of the head is changed. Vertical nystagmus or nystagmus that changes direction when the position of the head is changed suggests a central disorder of the vestibular system.

Eye Position in Vestibular Disease

In vestibular disease, most postural reactions remain intact except for the righting response. Usually the patient experiences difficulty righting itself, with an exaggerated response toward the side of the lesion. When the head is extended in the tonic neck reaction, the eyes should remain in the center of the palpebral fissure in the dog and cat. This often fails to occur on the side of the vestibular disturbance, and resulting in a drooped or ventrally deviated eye.

In ruminants it is normal for the eyes to deviate ventrally with neck extension. In horses there is normally a slight ventral deviation, which is more pronounced in the eye ipsilateral to a vestibular system lesion. Occasionally in vestibular disease, an eye is deviated ventrally (hypotropia) or ventrolaterally without extension of the head and neck. This deviation appears as a lower motor neuron strabismus but can be corrected with movement of the head into a different position or with induction of the patient to move the eyes to gaze in different directions. It is referred to as a vestibular strabismus. The cranial nerves that innervate the extraocular muscles are not paralyzed. The ventrally deviated eye is on the side of the lesion in the vestibular system. Sometimes the other eye may appear to be deviated dorsally (hypertropia).

Signs of vestibular system disturbance referable to disease of the vestibular nuclei or their pathways are similar to those seen in diseases of the peripheral vestibular system.

Vestibular signs usually seen only with diseases of the central pathways are as follows:

•Vertical nystagmus

•Nystagmus that changes direction with different positions of the head

•Disconjugate nystagmus

The lesion is localized to the central pathways mostly because of the presence of signs that accompany the brainstem involvement of other functional systems.

The most common cause of central vestibular system pathology is GME. Infectious diseases affecting the central vestibular system include canine distemper, toxoplasmosis, monocytic ehrlichiosis, Rocky Mountain spotted fever, feline infectious peritonitis, and fungal diseases. In ruminants, Listeria monocytogenes causes inflammation of the brainstem with signs referable to this location, including vestibular disturbance.

Intracranial injury may also affect the central vestibular pathways in addition to other systems in the brainstem. The degree of vestibular disturbance manifested depends on the severity of disturbance to other systems, which may mask the vestibular disturbance. Abnormal nystagmus may be the only sign of vestibular disturbance evident in the tetraplegic, semicomatose patient.

Compression of the central vestibular system by tumors, toxicity (e.g., metronidzole overdose in dogs and cats) and thiamin deficiency may also cause pathologic nystagmus.

Congenital Nystagmus

Congenital nystagmus occurs in humans as an inherited functional abnormality or secondary to congenital lesions in the visual system of the infant. The nystagmus is usually pendular. Similar nystagmus has been described in dogs. In cattle pendular nystagmus occurs in the Holstein-Friesian, Jersey, Guernsey, and Ayrshire. It is significant for diagnostic purposes only and does not affect vision.

Congenital pendular nystagmus has also been reported in Siamese and albino cats. It is due to an abnormal crossover pattern of optic nerve fibers at the optic chiasm. The nystagmus and concurrent medial strabismus are believed to be an attempt by the cat to correct for the resulting abnormal projections to the visual cortex.

Congenital nystagmus occurs in young animals with severe visual deficits during the early postnatal period. These occur in animals born with significant corneal opacities or cataracts, in cases of neonatal retinal detachment or intraocular hemorrhage, or following lid suturing (third eyelid flaps or tarsorrhaphy) in young patients.

CENTRAL VISUAL PATHWAYS

Anatomy of the central visual pathways is discussed in Chapter 1 and was summarized at the beginning of this chapter (see Vision and the Menace Reponse and Figure 16-3)

Clinical Signs of Diseases of the Central Visual

Pathways

The effects of diseases of the central visual pathways on the menace response and PLR have been discussed in detail in the beginning of the chapter (see Figures 16-8 to 16-12 and Table 16-2). Briefly, lesions that destroy the retina or optic nerve in one eye cause blindness and a partially dilated pupil in that eye (see Figures 16-8 and 16-13). The pupil is not fully dilated due to consensual input from the unaffected eye. The menace response and direct PLR are lost in the affected eye but are retained in the contralateral eye. The consensual PLR to the affected eye is normal, but the consensual PLR from the affected eye is absent. There are no other CNS signs. Total bilateral retinal, optic nerve, optic chiasm, or proximal optic tract destruction causes complete blindness, with both pupils widely dilated and unresponsive to light directed into either eye (see Figure 16-9). Retinal degeneration is the most common cause of this deficit.

Unilateral lesions in the distal optic tract, LGN, optic radiation, or visual cortex cause a visual deficit in the contralateral visual field (see Figure 16-10). As noted at the beginning of the chapter, in humans, where 50% of the optic nerve fibers cross over at the chiasm, the visual deficit affects 50% of the visual field in each eye. In domestic animals, where a larger percentage of fibers cross over at the chiasm, a unilateral lesion will cause a significant visual deficit in the eye contralateral to the lesion and a lesser deficit in the ipsilateral eye (see Figure 16-3). The deficit is pronounced from the temporal side (visual field) when there are contralateral lesions in the central visual pathway. In all postchiasmal unilateral lesions the PLR responses are usually normal because of decussation at the chiasm, pretectal area, and Edinger-Westphal nuclei.

Complete bilateral lesions in the optic tracts (after the afferent fibers of the PLR have diverged), LGN, optic radiations, or visual cortex produce total blindness with normal PLR. More often, however, the lesions are partial and clinical signs are difficult to

determine. For example, canine distemper encephalitis often produces extensive lesions in the optic tracts without obvious clinical deficit of vision or pupillary functions.

The magnitude of visual deficit depends on location and extent of the lesion in the visual pathway.

Histopathologic Reactions of Visual Pathways to Disease

Nervous tissue is composed of three elements: neurons, neuroglia (astrocytes, oligodendrocytes, microglia), and vascular connective tissue (Figure 16-24). Neurons—for example, retinal ganglion cells (RGCs)—have large cell bodies with huge nuclei and prominent nucleoli. Dendrites conduct impulses toward the cell body—for example, RGC dendrites receive synaptic input from the midretina in the inner plexiform layer. Axons transmit impulses away from the cell body—for example, RGC axons constitute the fibers of the optic nerve, relaying the visual signal to the LGN. RGC axons in the nerve fiber layer of the retina have no myelin sheath as they converge on the optic disc. However, they are myelinated in the optic nerve by oligodendrocytes. Myelin in the optic nerve differs from that in peripheral nerves, in which Schwann cells (lemmocytes) form the myelin. Diseases affecting Schwann cells do not influence the optic nerve, and diseases that affect oligodendrocytes of the optic nerve do not affect peripheral nerve myelin.

Axoplasmic flow of metabolites and cell organelles occurs in both directions in the axon, to and from the cell body. Interruption of this flow in the optic nerve is significant in the pathogenesis of papilledema (see later).

Acute Neuronal Degeneration

Because neurons are specialized cells that have little ability to regenerate or proliferate, alterations seen are the result of degene-

FIGURE 16-24. A histologic micrograph of the normal canine optic nerve in the lamina cribrosa region. The optic nerve is composed of axons of retinal ganglion cells and microglial and macroglial cells. (Hematoxylin & eosin stain.) (Courtesy Dr. Emmanuel Loeb.)

FIGURE 16-25. Histopathology micrograph showing inner retinal atrophy secondary to glaucoma in a dog. Note that except for a few degenerative nuclei, no ganglion cells can be observed in the inner retina (right side). A marked, diffuse edema is also noted. (Hematoxylin & eosin stain stain.) (Courtesy Dr. Emmanuel Loeb.)

ration or necrosis (Figure 16-25). Severe, acute insults cause immediate damage and destroy the cell. The duration of the insult is more important than the cause. The sequence is as follows:

Acute insult ˜ Swelling, fragmentation, and dissolution of cell bodies ˜ Necrosis

Tissues become edematous or cystic and later collapse and shrink. There is minimal reactive proliferation, and remnants of disintegrating RGCs are phagocytized by microglia. Neuroglial cells may fill in the defect (“gliosis”) (see Figure 15-38 in Chapter 15).

Chronic Neuronal Degeneration

More stages are identifiable in this process than in acute degeneration. Neurons may swell and accumulate cytoplasmic lipoidal vacuoles. Others may shrink, losing cell bodies and processes, to leave only a pyknotic nucleus.

Axonal Degeneration

Lesions and degenerative processes may occur in axons distant from the cell body. Indeed, numerous degenerative conditions affect the optic nerve, which is a collection of axons, before the RGCs are affected. Segments distal to the injury shrink rapidly, but proximal segments attached to the cell body survive longer, and bulblike swellings develop at the site of injury. However, the bubbles do not persist, and eventually both proximal and distal portions of the neuron, including the cell body, degenerate due to interruption of the axoplasmic flow. Therefore chronic atrophy of the optic nerve ultimately leads to retrograde atrophy of the nerve fiber and RGC layers of the retina. Similarly, loss of RGCs causes anterograde atrophy of the corresponding axons in toptic nerve.

16-26. Myelin degeneration of the optic nerve of a horse with equine recurrent uveitis.

FIGURE 16-27. Severe acute toxic optic neuropathy in a sheep poisoned with Stypandra imbricata (“blindgrass”). Note the loss of normal architecture, loss and disorganization of axons, and presence of gitter (fatladen macrophages).

and fill in defects caused by disappearance of other neuronal tissues, forming “glial scars” or areas of “gliosis.” Histiocytes or fixed macrophages of the CNS are termed microglia. They phagocytize fatty materials released during degeneration of nervous tissue and become large and rounded with a vacuolated cytoplasm (gitter cells).

Diseases of the Central Visual Pathways

Diseases of the Optic Nerve

PAPILLEDEMA. Papilledema (“choked disc”) is not a disease but edema of the optic nerve head caused by elevation in intracranial pressure. As the subarachnoid space of the brain is continuous with the optic nerve sheath, elevation in cerebrospinal fluid (CSF) pressure is transmitted to the optic nerve. The result is disruption of the axoplasmic flow between the RGC body and the axonal terminal. The most common cause of papilledema is brain tumors, with one study reporting a 48% incidence of papilledema in 21 dogs with brain tumor (Table 16-8). Of the 21 dogs, 11 were boxers. In addition to intracranial neoplasia, papilledema occurs in orbital inflammations and neoplasms (including optic nerve neoplasms), in vitamin A deficiency in cattle, and in some forms of toxic optic neuropathy (e.g., male fern and lead poisonings in cattle).

Clinical Signs. The clinical signs of papilledema are as follows:

•The disc is swollen and elevated above the surrounding retina.

•Disc margins are indistinct and fluffy.

•Retinal arterioles and veins show a distinct kink as they pass down over the edge of the disc into the retina.

•The disc has a “watery pink” appearance.

•Retinal veins are congested, dilated, and tortuous, and many more fine veins are visible.

•Small flame-shaped hemorrhages may be present on or near the disc margin.

The main differential diagnosis for papilledema is optic neuritis, which presents with similar clinical signs (see following section). The two are distinguished by the fact that the former causes no functional deficits, whereas the latter causes loss of vision and PLR.

Papilledema itself does not cause visual deficit.

Table 16-8 Clinical Signs in 21 Cases of Canine

Brain Tumor

Modified from Palmer AC, et al. (1974): Clinical signs including papilledema associated with brain tumors in twenty-one dogs. J Small Anim Pract 15:359.

Although papilledema does not cause primary loss of vision, chronic papilledema may lead to progressive visual deficits due to optic nerve atrophy. Furthermore, in cases where papilledema is caused by brain tumors, cortical lesions may cause visual deficits, as well as other neurologic deficits (see Table 16-8).

Locomotor deficiency and change in temperament in a dog with visual dysfunction and papilledema is highly suggestive of an intracranial space-occupying lesion.

CONGENITAL ANOMALIES

Aplasia and Hypoplasia. Aplasia of the optic nerve is complete absence of the nerve, an extremely rare condition. Hypoplasia of the nerve is defined as significant reduction in the number of optic nerve axons. The primary developmental abnormality is thought to be in the number or differentiation of the RGCs whose axons form the optic nerve. Therefore in optic nerve hypoplasia the number of RGCs is usually also decreased, and the nerve fiber layer is thin. Hypoplasia of the optic nerve occur infrequently in dogs, cats, horses, and cattle, and may be unilateral or bilateral. The condition is believed to be hereditary in a number of dog breeds, although it may also be acquired (see the section on vitamin A deficiency in cattle and pigs). Prenatal infection with bovine viral diarrhea– mucosal disease may cause congenital optic nerve atrophy.

In aplasia the optic disc is entirely lacking. If retinal vessels are present, hypoplasia is more likely, with a small remnant of the optic disc present. In horses few if any retinal vessels are present in either hypoplasia or aplasia. In hypoplasia the disc is gray and may be heavily pigmented (Figure 16-28). Secondary retinal degeneration may be present.

In optic nerve aplasia, the eyes are congenitally blind; pupils are dilated and unresponsive to light (Figure 16-29). In optic nerve hypoplasia there are significant visual deficits and PLR abnormalities. Their extent, however, varies with the number of functional RGCs and optic nerve axons.

Aplasia-hypoplasia should be differentiated from atrophy, which is usually not present in young animals. Histologically, the presence of retinal gliosis, inflammatory cells, or degenerative changes in retinal ganglion cells indicates atrophy rather than aplasia-hypoplasia. Another differential diagnosis (for hypoplasia) is micropapilla, a normal variation in which an animal has a smaller-than-usual optic nerve, but no visual or PLR deficits.

Colobomas. Colobomas are pits or excavations in the optic disc and peripapillary area, caused by incomplete closure of the embryonic fissure (see Chapter 2). They are typical if seen in the inferior medial portion of the disc, and atypical if located elsewhere. In dogs, colobomas occur most commonly in the collie eye anomaly in collies and Shetland sheepdogs (Figure 16-30), although they may also be inherited as separate distinct entities (e.g., in basenjis). Colobomas are also inherited (as an autosomal dominant trait with incomplete penetrance) in Charolais cattle but may occur sporadically in any species. The lesions are congenital and nonprogressive, varying in size from small pits to excavations several times the size of the normal optic disc. If a coloboma is large enough, vision and PLR are affected because the nerve fiber layer is disrupted as it enters the optic nerve head. Small colobomas have minimal effect on vision and PLR.

The clinical appearance of a coloboma is a white-gray indentation in the optic nerve. Blood vessels reaching the margin of the coloboma disappear from view as they “dive” into the excavation. The depth of the coloboma may be estimated by focusing on it using a direct ophthalmoscope, and

A

B

FIGURE 16-28. Hypoplastic (A) and normal (B) optic nerves of the left and right eyes, respectively, of a 4-year-old golden retriever. Note obvious differences in color, size, and shape of the optic discs. This was an incidental finding, and the owner was unaware that the left eye was blind.

FIGURE 16-29. Mydriasis in a cat with bilateral aplasia of the optic discs. The two eyes are similar, are normal in size, and have dilated pupils. (From Barnett KC, Grimes TD [1974]: Bilateral aplasia of the optic nerve in a cat. Br J Ophthalmol 57:663.)

comparing the refractive power to that required to focus on the adjacent disc or surrounding retina. Colobomas must be distinguished from glaucomatous cupping, which is an indentation in the optic nerve head (centered in the center of the optic disc) caused by elevation in intraocular pressure.

INFLAMMATORY DISORDERS

Optic Neuritis. Optic neuritis is an inflammation of the optic nerve. The inflammation may be unilateral, though it is usually bilateral; it may affect the entire nerve or parts of it.

FIGURE 16-30. Optic nerve (and scleral) coloboma in a collie with collie eye anomaly. Note the “disappearance” of the blood vessels at the edge of the optic disc as they “dive” into the large coloboma.

Etiology. Causes of optic neuritis are as follows:

•Infectious diseases affecting other nervous tissues (e.g., canine distemper, cryptococcosis, hog cholera, toxoplasmosis, feline infectious peritonitis) (Figure 16-31)

•Inflammatory diseases, most commonly GME or meningitis

•Trauma, especially after proptosis of the globe

•Orbital diseases (e.g., orbital cellulitis and orbital abscess)

•Neoplastic disorders. These may be primary optic nerve tumors (e.g., meningioma) or orbital tumors affecting the nerve.

•Exogenous toxins (e.g., optic neuropathy in cattle from the ingestion of male fern and in sheep from the ingestion of Stypandra imbricata [“blindgrass”]) (see Figure 16-27). In humans, numerous drugs (e.g., chloramphenicol, alcohol, nicotine) cause optic neuropathy, and drugs are often suspected but unproven causes in sporadic cases in animals.

•Vitamin A deficiency causing abnormal bone growth that constricts the optic canal

•Many cases, especially in dogs, are of unknown etiology, and in fact most cases are classified as idiopathic.

Clinical Signs. The clinical signs of optic neuritis are as follows (in retrobulbar neuritis, the ophthalmoscopic signs marked with an asterisk [*] may be absent, because the more distal part of the nerve is affected; in these cases, the fundus may look normal):

•Acute loss of vision

•The pupil is dilated and unresponsive. In unilateral cases, there is a consensual PLR from the unaffected eye, but not from the affected eye.

•The optic disc is swollen and raised. It appears to be congested, and its margins are blurry (Figure 16-32).*

•Hemorrhages on or around the optic disc*

•The retina around the disc may be edematous or detached. With time, peripapillary retinochoroidal degeneration may appear.*

•Exudation and haze in the adjacent vitreous*

•Concurrent signs of CNS disease may be present, depending on the primary cause.

•Optic neuritis, if untreated or uncontrolled, frequently leads to optic atrophy, with a pale, grayish, shrunken optic disc and attenuation of blood vessels (see Optic Neuropathy later).

50 Mm

A

50 Mm

B

FIGURE 16-31. Histopathologic micrograph showing a case of canine optic neuritis secondary to viral meningoencephalitis. A, Note the extensive, diffuse infiltration of inflammatory cells around the nerve. There is also multifocal infiltration of the optic nerve by inflammatory cells. B, A higher-magnification view of the same case, showing the nonsuppurative nature of the perivascular infiltrate. (Hematoxylin & eosin stain.) (Courtesy Dr. Emmanuel Loeb.)

Differential Diagnosis. Acute blindness with fixed, dilated pupils may also be caused by the following conditions:

•Glaucoma: Other clinical signs are usually present (see Chapter 12).

•Retinal detachment: The detached retina is usually visible behind the lens or can be demonstrated ultrasonographically (see Chapter 15).

•Sudden acquired retinal degeneration (SARD): The electroretinogram response is extinguished in SARD but normal in optic neuritis (see Chapter 15).

The appearance of an inflamed disc should be distinguished from papilledema and from myelination of the nerve fiber layer of the retina surrounding the optic disc (Figure 16-33; Table 16-9).

Treatment. Comprehensive ophthalmic, neurologic, and physical examinations should be performed in order to identify the primary cause (if present), and appropriate therapy should be instituted. The inflammation itself is treated symptomatically with high doses of systemic steroids. However, the prognosis for return of vision is poor, and indeed some of the primary causes (e.g., distemper) may even be life threatening.

Equine Optic Neuritis/Neuropathy. These diseases are usually seen in elderly horses. Equine exudative optic neuritis (Figure 16-34), is characterized by the following signs:

•Sudden, bilateral onset of blindness, accompanied by signs of optic neuritis

•Multiple round or oval yellowish bodies protruding from the borders of the optic disc and extending into the vitreous

•Pupillary dilation and loss or depression of PLRs

•Hemorrhages on or around the optic disc that may precede or accompany the appearance of the yellow bodies

Because the condition is uncommon, cumulative experience in its treatment is lacking, and optic atrophy is the usual sequel.

An important differential diagnosis is equine proliferative optic neuropathy, a disease of elderly horses that is also characterized by papillary or peripapillary white masses (Table 16-10, Figure 16-35). However, this disease causes no visual disturbance and the condition is usually found incidentally

FIGURE 16-32. Optic neuritis in a dog. Note the blurry disc margins and the loss of detail on the disc surface, caused by edema of the nerve head. (Courtesy University of Wisconsin–Madison Veterinary Ophthalmology Service Collection.)

FIGURE 16-33. Myelination of the nerve fiber layer of the retina in a dog, seen as white streaks extending from the disc. This must be distinguished from papilledema and optic neuritis. Clinical signs are lacking, and vision is normal.