Evaluating Eye Movement

Neurology of Eye Movement

Michael Davidson, D.V.M. Professor, Ophthalmology Diplomate, American College of Veterinary Ophthalmologists

Department of Clinical Sciences College of Veterinary Medicine North Carolina State University Raleigh, North Carolina, USA Neurology of Eye Movement Lecture Summary

 Extraocular muscles  Cranial nerves III, IV, VI  Types of eye movement  Eye movement reflexes  Paresis of eye movement (ophthalmoplegia)  Strabismus Extrinsic Muscles of the Eye

 Four recti, two oblique and retractor bulbi  Origin at orbital apex, insert anterior to equator of globe (except ventral oblique)  Originate medioventrally to exact orbital apex = visual axis and axis of orbital cone do not coincide at primary gaze: – results in secondary, tertiary muscle actions – complex actions if globe is rotated to other positions besides primary gaze

www.studyblue.com Extrinsic Muscles of the Eye

 Lateral rectus (CN VI) – Abduction  Medial rectus (CN III) – Adduction  Dorsal rectus (CNIII) – Elevation, medial rotation, adduction  Ventral rectus (CNIII) – Depression, lateral rotation, adduction  Ventral oblique (CNIII) – Elevation, lateral rotation, adduction  Dorsal oblique (CNIV) – Depression, medial rotation, abduction  Retractor bulbi (CN VI) – Retraction of globe www.pinterest.com Extrinsic Muscles of the Eye  Coordinated action between muscle groups of same eye is required…none of the EOM act singly, tonus is increased or decreased in opposing muscle at the same time. www.droualb.faculty.mjc.edu

 Coordinated action is also required between EOM groups between the two eyes (e.g. lateral and medial recti when abducting or adducting). www.medicalgeek.com www.123rf.com Cranial Nerves Controlling Ocular Motion  CNIII (Oculomotor): – Motor nuclei in tegmentum of midbrain, near rostral colliculus, exits brainstem as part of CNIII on ventral aspect of midbrain – With preganglionic parasympathetic fibers (Edinger- Westphal nuclei) – Some fibers cross over to opposite CNIII (for coordination of eye movements between the two eyes) – Through orbital fissure, then dorsal rami to dorsal rectus and levator palpebral mm; ventral rami to medial rectus, ventral rectus, ventral oblique ww.bmc.med.utoronto.ca Cranial Nerves Controlling Ocular Motion  CN IV (Trochlear): – mesencephalic tegmentum at level of caudal colliculus – crosses over and courses on dorsal aspect of brainstem – enters orbit through orbital fissure with CN III, VI – innervates contralateral dorsal oblique – unique aspects among all cranial nerves: • fewest axons • longest intracranial course • only CN to exit on the dorsal aspect of the brainstem • only CN to completely decussate **dorsal oblique mm. is the only EOM that arises from a myotome originating from the second somite of the head www.droualb.faculty.mjc.edu Cranial Nerves Controlling Ocular Motion  CN VI (Abducens): – Rostral part of medulla oblongota (floor of fourth ventricle) at the internal genu of the facial nerve – Courses rostrally through middle fossa beside pituitary gland, passes adjacent to cavernous sinus – Enters orbit through orbital fissure, innervates lateral rectus and retractor bulbi – Controls conjugate horizontal eye movements: • 70% of fibers to ipsilateral lateral rectus m. • through medial longitudinal fasciculus (MLF), 30% of fibers contacts contralateral oculomotor (CNIII) medial rectus motor neurons • coordinates abducting of ipsilateral lateral rectus and adduction of contralateral medial rectus www.bmc.med.utoronto.ca Motor Centers Controlling Eye Movement  Vestibular nuclei = fibers project to nuclei of CNIII, IV, and VI through medial longitudinal fasciculus (MLF)  Gaze centers = collection of 4 nuclei in pontine reticular formation which coordinates horizontal and vertical eye movements  Rostral colliculus = eye movements towards auditory or visual stimuli Vestibular System

 Peripheral – Inner ear • Membranous labyrinth with endolymph, stereocilia – Vestibular nerve (part of CNVIII)  Central – Vestibular nuclei (medulla) • Input to CNIII, IV, VI, cerebral cortex, cerebellum, vestibulospinal tract, reticular formation – Flocculonodular lobe of cerebellum Principles of Eye Movements

 Clear vision requires congruence of retinal images on two eyes  Tracking moving object requires coordinated eye movements  As head/orbit is unstable platform, computation of eye movements must account for head movement (through vestibular system)  Coordination of eye movements involves continuous vestibular system input to CN III, IV, and VI Types of Eye Movement

Smooth pursuit Saccade Types of Eye Movement

 Smooth pursuit (eye tracking, involuntary eye movements): – employed when eye following moving object – once visual target fixated, smooth pursuit motor system causes eye to move reflexively to tract target – fixation can be initiated voluntarily (when an object of interest is seen) or involuntarily (cars of moving train) – variable velocity depending on target – but the smooth pursuit cannot be accomplished voluntarily (i. e. requires target) Types of Eye Movement  Saccades: – fastest of eye movements, directs gaze from one target to another, fixed velocity – intentional (voluntary) saccades: • voluntary changes in fixation • reading involves series of microsaccades – reflexive (involuntary) saccades: • quick phase of vestibular nystagmus • reflexive saccades in response to auditory or peripheral visual field stimuli (rostral and caudal colliculi)

Illustrative Exercise: 1)try shift your gaze slowly between two objects in room (impossible as saccades are taking place) 2)focus on your finger and slowly move it about (possible, smooth pursuit) Nystagmus

 Involuntary, rhythmic globe oscillation  Pendular nystagmus = equal movements  Jerk nystagmus = slow phase (smooth pursuit) and quick phase (saccade) – Physiologic (vestibular) nystagmus – Spontaneous (pathologic) nystagmus Eye Movement Reflexes: Vestibulo-Ocular Reflex  Reflex movement of eyes in response to vestibular system  During visual tracking or scanning of object, both static position of head and head movements detected by vestibular system and signal to extraocular muscles adjusted accordingly: – receptors in inner ear ⇒ vestibular neurons/nuclei ⇒ medial longitudinal fasciculus (MLF) ⇒ ipsi- and contralateral ocular motor nuclei ⇒ extraocular muscles

Illustrative Exercise: 1)read this slide while tilting head….instead of simple adduction and abduction of eyes, now eye must move in all axes 2)repeat exercise by tilting head and nodding head up and down…..information about head position and head movement detected by vestibular system, used to adjust action of all nuclei controlling eye movement Vestibulo-Ocular Reflex

 Vestibular system can compensate not only for static head position (e.g. tilted head), but for dynamic head movements  “Physiologic” or vestibular nystagmus: – Horizontal head movement to right creates smooth pursuit movement to left to steady image on retina, then a saccade to the right when the limits of EOM action reached and to return the eyes to primary position to preview upcoming scene – Dorsal flexion of neck causes smooth pursuit movement down,then a saccade up VOR VOR

Vestibulo-Ocular Reflex

 Evaluates: – vestibular system – CNIII, IV, VI – interconnecting pathways  Important tool for evaluating brainstem injury as often affects one or more of these three structures

***Note VOR does not require vision, but absent if congenitally blind Video Post Rotary Nystagmus

 Spin 6-8 times, then halt  Inertia causes endolymph to continue to flow through horizontal duct  As head is stationery, cristae stimulated in the opposite direction….so jerk nystagmus with quick phase or saccadic movement in direction opposite rotation after coming to a halt  Postrotational jerk nystagmus decreased when rotated opposite side of peripheral vestibular disease  Therefore, if difference in response is noted between two rotations = vestibular disease video Eye Movement Reflexes

 Physiologic or Vestibular Nystagmus – quick phase toward the direction of rotation (i.e. head movement to right, saccades to right)

 Post Rotatory Nystagmus – quick phase opposite direction than rotation (i.e. spin to right, saccades to left) Optokinetic Reflex (Optokinetic Nystagmus)  Brainstem and cortical visuomotor centers can also initiate (involuntary) oculomotor reflexes  Repetitive objects moving across visual space, a smooth pursuit movement is initiated to track the object, then the visuomotor centers initiate a saccade to bring upcoming (next) image on retina and track  “Railroad” nystagmus – Person on moving train fixes eyes on passing utility poles – Eyes track until pole passes from view – Eyes saccade back to forward gaze position to next pole  OKN reflex elicited by any repetitive visual stimulus (striped drum) Optokinetic Reflex (Optokinetic Nystagmus)

Video 1 Video 2 Ocular Reflexes

 Vestibulo-ocular reflex-does not require vision, but absent if congenitally blind

 Optokinetic reflex-requires a vision (used to test for malingering or feigning vision loss) Spontaneous Nystagmus

 almost always pathologic  lesions in inner ear, vestibular nerve, brainstem, or cerebellum  unilateral vestibular disease = jerk nystagmus – decreased activity in left vestibular system= right vestibular system drive the eyes in a slow phase (smooth pursuit) to the left, quick phase (saccade) directed to right, away from lesion (i.e. lesion contralateral to quick phase of nystagmus)  central vestibular disease may cause positional nystagmus

Vestibular Syndrome

 Nystagmus  Ataxia and abnormalities of ambulation  Head tilt Evaluating Eye Movement and Position  Static (eye position and alignment): – eye position relative to normal eye – corneal (light) “reflex” technique Evaluating Eye Movement and Position

 Dynamic (eye movement): – Vestibulocular reflex – Voluntary change in gaze – Forced duction: • Normal = slight resistance to duction by functional muscle • Paresis = no resistance to duction in direction of gaze limitation • Restrictive lesion = unable to perform duction…”positive forced duction test” Paresis of Eye Movements: Ophthalmoplegia

 Nuclear (CN nuclei or cranial nerve), internuclear (MLF), or supranuclear (cortex, descending coritomesencephalic fibers, or brainstem gaze centers)

 Evaluate vestibulo-ocular reflex…if intact, paresis is most likely supranuclear in location CN III Lesions  Affects PLR, eyelid movement, ocular motility  Total = ptosis, inability to rotate eye up, down or in, dilated and nonresponsive pupil,  divergent strabismus (when normal eye directed straight) CN IV Lesions

 Exotorsion of contralateral eye – vertical diplopia – compression, hemorrhage, trauma – common in humans after head trauma in humans d.t. long intracranial course of nerve

 Isolated CN IV paresis rarely recognized in dogs/cats CN VI Lesions  lateral rectus, retractor bulbi  lack of ipsilateral abduction  lack of ipsilateral globe retraction (corneal reflex)  slight ipsilateral esotropia with frontal gaze – lack of antagonist tone from lateral rectus  diplopia (humans) Cavernous Sinus and Orbital Fissure Syndromes

 Venous sinus in anterio- ventral cranial vault  CN III, CN IV, CN V (ophthalmic and maxillary branches), CN VI adjacent to sinus  total ophthalmoplegia, mydriasis, abnormal corneal blink reflex, absent globe retraction  generally neoplasia along anterior floor of cranial vault Cavernous sinus syndrome OD Strabismus

 Lesion in vestibular pathway (often positional)  Lesion in CN III, IV, VI, interconnections  Congenital, misrouting of optic nerve axons or visual cortex architecture  Physical displacement of globe  Proptosis (medial rectus)  EOM lesion (fibrosing myositis) Strabismus