Chapter 141 Neuro-ophthalmology
Clinical signs of ocular dysfunction may appear as disorders in the neuroanatomic pathways that allow normal vision. Vision is optimal when sufficiently protected eyes receive the proper amount of light while holding images steady on the retina. Many complex neurologic systems are involved in vision, including the visual sensory system (retina to visual cortex), the autonomic nervous system (pupillary function and lacrimation), the ocular motor system (neural control of eyeball, eyelid, and third eyelid position and movement), and the trigeminal somatic sensory system (pain sensation) of the eye and adnexa.
NEURO-OPHTHALMIC ANATOMY
• The nerves of the petrous temporal bone are of two types: those in transit through the bone and destined for distant locations that subserve the eye or some of its supporting functions, like lacrimation, and those that mediate other functions, like hearing or taste. Those in transit to and destined for the eyeball or to protective and supporting structures for vision (e.g., the adnexa or the vestibular apparatus for head-eye coordination) include cranial nerve (CN) 5, 7, 8; postganglionic neurons of the sympathetic nervous system; and preganglionic and postganglionic fibers of the parasympathetic nervous system.
• The parasympathetic system (general visceral efferent [GVE] fibers) of the glossopharyngeal nerve (CN9) may be indirectly involved in ensuring ocular health and vision. The preganglionic parasympathetic fibers of CN9 form a synapse on a collection of cell bodies known as the otic ganglion, which lies within the ventrorostral petrous temporal bone. This ganglion is the source of postganglionic parasympathetic neurons that innervate the parotid and zygomatic salivary glands.
• These glands may become denervated and dysfunctional (leading to xerostomia) by the same process and at the same time that lacrimation ceases or is diminished from the lacrimal gland (leading to keratoconjunctivitis sicca [KCS]). Be sure the parotid gland is normal before performing parotid duct transposition in cases of medically refractory KCS.
• Two neuroanatomic pathways modulate the pupillary light responses: the pupillary light reflex (PLR) pathway and the efferent sympathetic pathway (Fig. 141-1).
• The afferent arm of the PLR pathway from each eye is a three-neuron pathway. Each consists of a retinal chain neuron (photoreceptors and bipolar cells), optic nerve, and pretectal neuron that distribute information bilaterally (i.e., to both parasympathetic nuclei of the third nerve) as a result of the crossover that occurs at both the optic chiasm and the caudal commissure.
• Note that the crossover of optic nerve fibers is unequal at the level of both the optic chiasm and the caudal commissure.
• The afferent arms of the PLR are part of the central nervous system (CNS). Each efferent arm of the PLR is a two-neuron pathway from the autonomic nervous system (ANS) and consists of preganglionic and postganglionic parasympathetic neurons that are distributed as part of the oculomotor nerve (CN3). The efferent arms of the PLR do not cross to the opposite side en route to the iris sphincter muscles (for pupil constriction) and therefore do not distribute information to both eyes.
• The other pathway that controls the iris is the efferent sympathetic pathway. This pathway, which originates centrally in the posterolateral hypothalamus, remains unilateral throughout its course.
• The central neuron pathway (which may actually consist of several linked pathways leaving the brain stem) proceeds down the lateral cervical spinal cord to synapse with preganglionic sympathetic fibers within the lateral-rostral thoracic spinal cord. After these fibers exit the vertebral column, they return to the head by ascending the neck in the vagosympathetic trunk, where they synapse on the cranial cervical ganglion medial to the tympanic bullae.
CLINICAL SIGNS OF NEURO-OPHTHALMIC IMPORTANCE
Decreased Tear Production:
• A Schirmer tear test of less than 10 mm of wetting per minute may indicate a disorder of either the afferent arm (trigeminal nerve) or the efferent arm (parasympathetic neurons of the facial nerve) of the trigeminolacrimal reflex. Differentiate injury along the trigeminolacrimal reflex pathway from injury to the lacrimal gland proper (see Chapter 139).
Nystagmus:
• Nystagmus is an involuntary, repetitive, to-and-fro movement of one or both eyes that includes smooth sinusoidal oscillations (pendular nystagmus) and alternation of slow drift and corrective quick-phase oscillations (jerk nystagmus). Nystagmus occurs in any plane, can be permanent or transient, and may be either acquired (vestibular disease) or congenital (animals born congenitally blind).
DIAGNOSIS
History
General History
• Obtain information about prior neurologic disturbances, systemic illness, head or neck disease, and drug use.
General Physical Examination
Perform a complete physical examination including a thorough neurologic examination as described in Chapter 125, with particular emphasis on CN2 through CN9.
Visual Sensory System Evaluation
• Visual field testing, strictly speaking, cannot be performed in animals because their eyes will not hold a target (fixate) long enough to assess all visual fields. However, that should not preclude use of helpful clinical tests, albeit crude, to obtain information on vision quality or the integrity of the visual sensory system.
Visual Placing Reaction
• Lift and support the patient under the abdomen and thorax while holding the head in a forward stationary position and advance toward a table edge. Present the table edge frontally (perpendicular to the long axis of the head and body) and then laterally (parallel to the long axis of the head and body). The normal response of a sighted patient is the forward advancement and placement of both feet on the table when the table edge is presented head-on and a lateral reaching movement with the leg nearest the table edge when it is seen by the nasal hemiretina (extreme lateral visual field).
• In lateral placing, have the nose extend beyond the corner of the table to ensure that only the most extreme portion of the nasal hemiretina (i.e., the most rostral portion of the nasal hemiretina referred to as the monocular crescent) is stimulated. When testing the visual fields, present the table edge from the front and side (laterally) under monocular and binocular test conditions.
Visual Cliff
• These are crudely created by using multiple variable height platforms or simply a table with variable height.
• A homogeneous texture and color has to cover all testing surfaces. Use a black-and-white checkered tablecloth.
• Normal dogs and cats will jump onto the lower platform (e.g., floor), whereas animals with poor depth perception will be unwilling to do so, will lower their bodies to the tabletop and slide off the table, or will crash their chins into the floor as they miscalculate the depth of the jump.
Visual Obstacle Course
• Use an obstacle course with a standardized layout based on the examiner’s preference for most effective geometric design. This will result in more reliable deductions about the performance differences between sighted and non-sighted animals.
• Use five or six gray polyurethane foam cylinders, 2 feet tall and 4 inches in diameter, rather than the unprofessional and possibly injurious furniture objects customarily used, to ensure a safe, effective course.
• Run the course under monocular and binocular conditions using photopic (ambient room light) and scotopic (an incandescent red light bulb on a dimmer switch) illumination.
• Visually deficient animals will be reluctant to move in a strange environment under ambient light and/or scotopic light conditions or will move slowly through the course with their noses to the floor (i.e., “pick” their way). A few of the visually impaired will perform in a buoyantly bold manner yet run into many cylinders during each trial.
Dazzle Reflex
• This is a subcortical reflex, manifested as a bilateral partial eyelid blink in response to a bright light shined in one eye at a time.
• Use a powerful light source (halogen power with a Finoff transilluminator or, preferably, a fiberoptic light).
• It is present when the optic nerve is intact to the level of the midbrain. It also requires an intact facial nerve to elicit the blink.
Menace Response
• The menace response is a cortically mediated eyelid closure (i.e., cortical blink) and occasional head withdrawal originating in the cerebral cortex in response to a sudden threatening gesture in the near visual field. This response is complex and not totally understood.
• It requires intact peripheral and central visual pathways, including the cerebellum and facial nerves.
• Evaluation of the visual sensory system (CN2 and higher vision centers) requires isolating its function and integrity from the corneal blink reflex pathway (CN5 to CN7). If this is not achieved, a blink following a visual threat may elicit the corneal blink reflex afferently mediated by CN5, rather than the desired CN2-mediated menace response.
• Use a windshield (plastic shield or helmet visor) to block airwaves while still allowing the patient a view of the menacing threat.
• Menace the visual field of each eye (nasal and temporal hemiretina) after patching the other eye, which eliminates its participation in the response. Then evaluate the left and right binocular visual fields (i.e., the nasal hemiretina of one eye and the temporal hemiretina of fellow eye) by eliciting the menace response without patching.
Autonomic Nervous System Evaluation
Pupillary Responses
Pupillary Light Reflexes
• This exam is best conducted in darkened conditions using a powerful light source (see Fig. 141-1). A halogen-powered light is minimally required to adequately evaluate the PLR, but a fiberoptic light source is best.
• The pupil under direct light stimulation (direct PLR) will constrict to a greater degree than the other eye. The constriction of the other eye is called the indirect or consensual light reflex. Thus, this pupil is said to react indirectly or consensually.
• Because the pupil receiving the direct light constricts to a greater degree than the other pupil (indirect pupil), an anisocoria exists during this active stimulation that is referred to as a dynamic contraction anisocoria. This is a normal physiologic response.
• Anisocoria is absent in ambient light unless there is heterochromia of the irides, iris disease, PLR pathway, or efferent sympathetic pupillary pathway pathology.
Measurement of Pupil Size and Symmetry
• Pupillary symmetry is best obtained clinically using the direct ophthalmoscope as a crude pupillometer.
• Direct the light source at the interpupillary space on the bridge of the nose to observe the tapetal reflection of both eyes simultaneously.
Swinging Flashlight Test
• Immediately following direct light stimulation into one eye, swing the light source into the other eye, and then back again to the first eye directly stimulated. Repeat this procedure back and forth (thus the swinging flashlight) as often as necessary to be sure that both pupils constrict directly and indirectly.
• If a pupil dilates rather than constricts during direct stimulation immediately following its return from direct stimulation of the other eye, a unilateral retinal or prechiasmal optic nerve lesion exists. If dilatation of a pupil instead of pupil constriction occurs during the swinging flashlight test, this is referred to as positive for the dilating eye.
Pharmacologic Testing
• Use topical drugs in cases of anisocoria to differentiate anatomic problems in the iris (e.g., synechia or iris atrophy) from neurologic problems. Also use topical drugs to localize the lesion to preganglionic or postganglionic fibers within each of the two efferent pathways (the efferent sympathetic pathway and the efferent parasympathetic pathway of the PLR).
• In an injured nerve whose terminals are allowed to degenerate, the structure supplied by it becomes supersensitive to the transmitter substance released by the terminals.
Lacrimation
Schirmer Tear Tests (Schirmer 1 and 2) (see Chapter 139)
• Use strips of Whatman paper to measure the autonomic innervation to the lacrimal gland (reflex tearing as measured by Schirmer 1 in the unanesthetized eye) and basal secretion from the lacrimal gland (by Schirmer 2 in the anesthetized eye). The contribution of the gland of the third eyelid is also figured into these measurements.
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
