Day 12 (3): The Efferent Visual System Flashcards
What is the ultimate purpose of the ocular motor system?
Ensure CLEAR, STABLE and BINOCULAR vision
What are the two basic human eye movements?
- Gaze Stabilization
- Vestibulo-ocular reflex
- Optokinetic nystagmus - Gaze shift
- Vergence
- Pursuit
- Saccades
What parts of the brain are involved in the initiation of eye movements?
SUPRANUCLEAR
1. Cortical
- Frontal Eye Fields
- Supplementary Eye Fields
- Posterior Parietal Cortex
- Subcortical
- Basal Ganglia
- Thalamus
- Superior Colliculus
NUCLEAR
- location of the cranial nerve nuclei responsible for eye movements
- pre-motor coordination of conjugate eye movements
- coordination of the vestibulo-ocular system
1. Pons
2. Medulla
INFRANUCLEAR
1. Cranial nerves
2. Neuromuscular junctions
3. EOMs
What is the Vestibulo-Ocular Reflex?
- a gaze stabilization reflex that allows for eye movements in the OPPOSITE direction of head movement to maintain steady gaze and prevent retinal image slip
- only occurs during movements of the head to stabilize gaze on a STATIONARY object
- eye movement FOLLOWS the direction of fluid movement within the semicircular canal (which is OPPOSITE the direction of head movement)
Pathway:
1. Utricle, Saccule, and/or Semicircular canals
2. Utricular, Saccular, and/or Ampullary nerves
3. Vestibular nucleus (pons)
4. Oculomotor, trochlear and abducens nucleus 5. Corresponding nerves and muscles
Note:
Horizontal VOR: involves coordination of the abducens (LR) and oculomotor (MR) nuclei via the medial longitudinal fasciculus.
What is the Medial Longitudinal Fasciculus?
- area of crossed heavily-myelinated axon tracts comprised of ascending and descending fibers
Location:
- both sides of the medial brainstem close to the periaqueductal gray matter in between the pretectum and the spinal cord
Function:
- main central connection for the three nerves that control eye movements: oculomotor nerve, trochlear nerve, and abducens nerve
- relay pathway for horizontal conjugate eye movements
- receives inputs from the superior colliculus, the vestibular nuclei, and the cerebellum
Nuclei:
1. Interstitial Nucleus of Cajal: oculomotor control, head posture, vertical eye movement
2. Rostral Interstitial Nuclei: vertical gaze center
Descending Fibers:
1. From superior colliculus: visual reflexes
2. From accessory oculomotor nuclei: visual tracking
3. From pontine reticular formation: extensor muscle tone
Ascending Fibers:
1. Vestibular nucleus –> oculomotor nucleus, trochlear nucleus and abducens nucleus (VOR)
2. Abducens nucleus –> oculomotor nucleus (for horizontal conjugate gaze movement)
Causes:
1. Multiple sclerosis
2. Tumors
3. CVA
Lesion: Internuclear Ophthalmoplegia
1. ADduction lag in the IPSIlateral eye upon CONTRAlateral gaze
2. Nystagmus in abducting CONTRAlateral eye
Causes:
1. Bilateral: Multiple Sclerosis
2. < 45 yo: Demyelination, Trauma, Tumor
3. > 45 yo: Ischemia
What is the Oculocephalic Reflex?
Doll’s Eye Reflex
- application of the vestibular-ocular reflex used for examination of CN 3, 6, and 8, the reflex arc including brainstem nuclei, and overall gross brainstem function
- demonstrates communication between the vestibular system and the ocular system in the absence of cortical suppressive signals
+ SUPPRESSED in a conscious adult with normal neurologic function
+ ACTIVE in a comatose patient with gross brainstem function
+ ABSENT if there is damage to the reflex arc
Purpose: Determine localization of lesions causing horizontal or vertical gaze palsies
- Supranuclear: POSITIVE Doll’s Eye
- eyes move in the OPPOSITE direction of head movement such that their eyes stay looking forward (like a doll’s eyes)
- lesion is cortical and above the midbrain
- indicate an INTACT brainstem function - Nuclear/Infranuclear: NEGATIVE Doll’s Eye
- eyes moving towards the SAME direction of head movement or remain fixed in the center
- indicate brainstem DYSFUNCTION
Pathway:
1. Head rotation causes endolymph in the horizontal SCC to rotate OPPOSITE to the direction of the head
2. Vestibular nerve IPSILATERAL to the head direction activates the ipsilateral vestibular nucleus and the contralateral vestibular nerve to inhibit the contralateral vestibular nucleus.
3. Vestibular nuclei then activate the CONTRALATERAL abducens nuclei which sends signals to the corresponding LR to contract.
4. The abducens nuclei also sends signals to the contralateral oculomotor nucleus (ipsilateral to the vestibular nucleus) through the MLF to contract the corresponding MR.
Steps:
1. Hold a patient’s eyelids open
2. Briskly rotate the patient’s head from side to side while examiner observes the patient’s eyes
NOTE: only performed in patients with a stable cervical spine
What is the Optokinetic Reflex or Nystagmus?
- gaze stabilization reflex of alternating slow and fast movement of the eyes in response to large-scale movements of the visual scene
- consists of two components:
1. Slow, pursuit phase
2. Fast, refixation phase: saccade in OPPOSITE direction of the pursuit - test: Optokinetic drum/tape with alternating stripes of varying spatial frequencies
- used to assess visual acuity in infants and children
- present in newborns, semi-obtunded patients, and malingering patients
Pathway:
A. Slow Pursuit Phase
1. Afferent: Retina –> Occipital lobe
2. Pontine horizontal gaze center
3. Oculomotor and Abducens nuclei
4. Oculomotor and Abducens nerve and corresponding muscles (MR, LR)
B. Fast Refixation Phase
1. Afferent: Retina –> Occipital lobe
2. Frontal Eye Field
3. Superior Colliculus
4. Pontine horizontal gaze center
5. Oculomotor and Abducens nuclei
6. Oculomotor and Abducens nerve and corresponding muscles (MR, LR)
Clinical Correlates:
1. Parietal lesions: ASYMMETRIC OKN
- efferent pursuit fibers pass close to afferent optic radiations
2. Occipital lesions: SYMMETRIC OKN
3. Subclinical Internuclear Ophthalmoplegia
- slower response by the ipsilateral MR
4. Parinaud’s syndrome
- downward OKN target accentuates convergent retraction movements on attempted upgaze
What are Saccades?
- rapid, conjugate, eye movement that shifts the center of gaze quickly to another part of the visual field towards an object of interest or toward visual, auditory, or tactile stimuli
- voluntary (skimming a text) OR involuntary/ reflexive (fast phase of nystagmus/REM sleep)
- ballistic: movements are predetermined at initiation, and the system cannot respond to subsequent changes in the position of the target after saccade initiation
- conjugate: eyes move at the same time, at the same direction and at the same speed
Pathway:
A. Afferent pathway: retina –> visual cortex
B. Supranuclear/Cortical Initiation Centers
- Initiation center activated is OPPOSITE the saccade direction
- pathology: saccade CONTRAlateral to lesion
1. Frontal Eye Fields: voluntary saccades
2. Superior Colliculus: involuntary saccades
C. Nuclear Gaze Centers
- activation of horizontal and vertical gaze centers together allows for oblique movements
- trajectories are specified by the relative contributions of each gaze center
- final common pathway for horizontal and vertical gaze
Horizontal Gaze Centers:
Paramedian Pontine Reticular Formation &
Abducens Nucleus
- location: Pons
- activated PPRF is CONTRALATERAL
- bilaterally-connected to FEF and SC on BOTH sides for preservation of horizontal gaze even with damage to one FEF or SC
- Abducens nerve (contralateral)
- LR (contralateral)
- Interneuron to decussating Medial Longitudinal Fasciculus (MLF)
- Oculomotor nucleus and nerve (ipsilateral)
- MR (ipsilateral)
Vertical Gaze Center:
Interstitial Nucleus of Cajal & Rostral Interstitial Nucleus of the Medial Longitudinal Fasciculus
- location: Midbrain (Thalamomesencephalic Junction)
Upgaze: CN3 (SR, IO) only
1. UNILATERAL innervation: CROSSED fibers at the Posterior Commissure
2. Oculomotor nuclei (contralateral)
- dorsal midbrain at level of Superior Colliculus
3. Oculomotor nerve (contralateral)
4. SR and IO (contralateral)
Downgaze: CN3 (IR) and CN4 (SO)
1. BILATERAL innervation: UNCROSSED fibers
- harder to get a downgaze palsy with supranuclear or midbrain lesions than an upgaze palsy
- affected only if:
+ late stage disease
+ large SOL
+ bilateral disease
2. Oculomotor and Trochlear nuclei (bilateral)
- IV: dorsal midbrain, level of Inferior Colliculus
3. Oculomotor and Trochlear nerve (bilateral)
4. IR and SO (bilateral)
What are Pursuits?
- tracking, conjugate movement of the eyes to maintain fixation on a slow-moving stimulus
- initiation requires a MOVING stimulus
- voluntary ONLY: observer can choose whether or not to track a moving stimulus
- asymmetric:
+ better at horizontal than vertical pursuit
+ better at downward than upward pursuit - conjugate: eyes move at the same time, at the same direction and at the same speed
Pathway:
A. Afferent pathway: retina –> visual cortex
B. Supranuclear/Cortical Initiation Centers
- Initiation center activated is TOWARDS the pursuit direction
- pathology: pursuit IPSIlateral to lesion
- Frontal Pursuit Area
- Parieto-Occipito-Temporal Junction
C. Nuclear Gaze Centers and Infranuclear Pathway
- similar as the saccade pathway
- with some added input of the Cerebellum
What are Vergences?
- align the fovea of each eye with targets located at DIFFERENT distances from observer
- disconjugate eye movements: fixation does NOT move in the same direction
- Convergence
- eyes move towards the center
- see an object that is nearer
- component of the near-reflex synkinesis:
+ accommodation reflex: focusing of image
+ pupillary constriction: increased depth and sharpness - Divergence
- eyes move away from the center
- see an object that is farther away
Control:
SUPRANUCLEAR: Frontal Eye Fields
NUCLEAR: PPRF and EW Nucleus
INFRANUCLEAR: Oculomotor nerve
What is the most common complaint of patients with acquired cranial nerve palsy?
Diplopia
- double vision
- determine whether:
- Monocular vs Binocular
MONOCULAR
- diplopia LINGERS when the unaffected eye is covered
- diplopia RESOLVES when the affected eye is covered
BINOCULAR
- diplopia RESOLVES when one eye is covered
- implies that diplopia happens because of misalignment of the visual axis and non-fusion of retinal images from the two eyes
- causes:
+ Orbital diseases
+ Myasthenia gravis
+ Nerve palsies
- Horizontal vs Vertical vs Torsional
HORIZONTAL: images side by side
VERTICAL: images on top of the other
TORSIONAL: images diagonal
- Worsening in L or R gaze or with near or far
- Constant vs only when tired at the end of day
Where are the locations of the nuclei of Cranial Nerves 3 - 7?
Midbrain: Oculomotor and Trochlear nuclei
Pons: Trigeminal, Abducens and Facial nuclei
Discuss the oculomotor nerve anatomy.
Innervated muscles
1. SR
2. IO
3. IR
4. MR
5. LPS
6. Iris sphincter (parasympathetic)
7. Ciliary muscles (parasympathetic)
Parts:
1. Nucleus complex
- within midbrain tegmentum at the level of the superior colliculus
2. Fascicle
- within midbrain tegmentum from the nucleus to the cerebral peduncles
3. Subarachnoid
- ventral to cerebral peduncles to posterior clinoid process
4. Intracavernous
- within the cavernous sinus from the posterior clinoid process to the superior orbital fissure
5. Intraorbital
- enters the superior orbital fissure inside the Annulus of Zinn
Signs of palsy:
Primary gaze: DOWN-and-OUT
- predominance of SO (depressor) and LR (abductor)
- Limited elevation: SR, IO
- Limited depression: IR
- Limited adduction: MR
- Ptosis: LPS
- Fixed mydriatic pupil: iris sphincter
- poorly-reactive to light
- sympathetic innervation predominates - Blurred vision to near: ciliary muscles
- loss of accommodation
- acute: ABSENT near reflex
- chronic: INTACT near reflex with aberrant regeneration
Types of Palsy:
1. Complete: symptoms 1 - 6 ALL present
2. Incomplete: any combination of the above
What are the most common causes of 3rd Nerve Palsy?
ADULTS: Ischemic
1. Vascular disease: HPN, DM, Atherosclerosis, Smoking, Aging
2. ICA aneurysm
CHILDREN:
1. Birth trauma
2. Postnatal trauma
What are the component nuclei of the 3rd Nerve Nucleus Complex?
- location: MIDBRAIN/MESENCEPHALON
- level: SUPERIOR COLLICULUS
- area: TEGMENTUM
Components:
Main Motor Nucleus
1. Central Caudate Nucleus
- midline; unpaired dorsally
- innervates bilateral LPS
2. Subnuclei
- paired
- innervates ipsilateral SR, IR, IO, MR
Accessory Parasympathetic Nucleus/Edinger-Westphal Nucleus
- midline; unpaired superiorly
- preganglionic parasympathetic neurons to the ipsilateral ciliary muscles and iris sphincter
Etiology:
1. Ischemia: most common cause
- embolus or thrombus in the basilar artery at the level of the midbrain/mesencephalon/ cerebral peduncles
- profile: > 40 yo with history of HPN, DM, smoking and OCP use
2. Hemorrhage
3. Mass effect/compression: tumors, hydrocephalus (caudal to 3rd ventricle and ventral to cerebral aqueduct of Sylvius)
4. Inflammation
5. Trauma
Presentation:
- nucleus and fascicles are spread over a large area of the midbrain hence variability in presentation
- upper/middle lesions: pupillary dilatation
- lower lesions: pupillary sparing
- Complete or Partial
- Isolated or Syndrome
Describe the anatomy of the oculomotor nerve fascicle.
- location: TEGMENTAL zone
- level: SUPERIOR COLLICULUS
Pathway: travels ventrally adjacent or within
1. Medial Longitudinal Fasciculus
2. Red Nucleus
3. Substantia Nigra
4. Cerebral Peduncle or Interpeduncular Fossa
- emerges as the Oculomotor Nerve (Trunk)
6. Between the Posterior Cerebral Artery and the Superior Cerebellar Artery
Etiology: similar to Oculomotor nucleus complex
1. Ischemia: most common cause
- embolus or thrombus in the proximal basilar artery
- profile: > 40 yo with history of HPN, DM, smoking and OCP use
2. Hemorrhage
3. Mass effect/compression: tumors
4. Inflammation
5. Trauma
6. Demyelination
Presentation
1. Complete or Incomplete
2. Isolated (if small, unilateral) or Syndromic
(if large due to involvement of adjacent midbrain structures
What syndromes involve the Oculomotor nerve fascicle?
- Weber Syndrome
- Nothnagel Syndrome
- Benedikt Syndrome
- Claude Syndrome
What is Weber Syndrome?
Localization: VENTRO-MEDIAL midbrain
Cause: Infarction
- Paramedian Mesencephalic branches (Basilar)
- Peduncular Deep Penetrating arteries (PCA)
Components:
1. IPSIlateral oculomotor palsy
2. CONTRAlateral hemiparesis/plegia
Affected structures:
1. Oculomotor nerve fascicle (interpeduncular cistern)
- Cerebral peduncles: hemiparesis/plegia
- contain the pyramidal tracts or the main descending motor pathway
- controls VOLUNTARY movement
- decussates at the medulla
+ CorticoSPINAL tract: body
+ CorticoBULBAR tract: lower face and tongue
What is the Nothnagel Syndrome?
Localization: INFERO-MEDIAN midbrain
- in the dorsal tegmentum at the level of the INFERIOR COLLICULUS
- BEFORE the decussation at the Wernekinck commissure
Cause:
1. Tumors: more common
2. Infarction: less common; Peduncular Deep Penetrating arteries (PCA)
Components:
1. IPSIlateral oculomotor palsy
2. IPSIlateral cerebellar hemiataxia
Affected structures:
1. Oculomotor nerve fascicle (tegmentum)
- Superior Cerebellar Peduncle: hemiataxia
- carries the dentatorubrothalamic tract (cerebellar efferent fibers to the thalamus)
- ataxia is ipsilateral to the lesion because of two decussations involved:
+ decussation to contralateral Red Nucleus
+ decussation of the rubrospinal tract from the contralateral Red Nucleus back to side of lesion
What is Benedikt Syndrome or the Paramedian Midbrain Syndrome?
Localization: SUPERO-MEDIAN midbrain
- in the ventral tegmentum at the level of the SUPERIOR COLLICULUS
Cause: Infarction
- Peduncular Deep Penetrating arteries (PCA)
Components:
1. IPSIlateral oculomotor palsy
2. CONTRAlateral extrapyramidal signs
Affected structures:
1. Oculomotor nerve fascicle (tegmentum)
- Red Nucleus +/- Substantia Nigra: EPS
- movement coordination and modulation
- rubrospinal tract immediately decussate
+ Resting tremors
+ Choreoathetosis: irregular, nonrhythmic, purposeless rapid or slow movements of the fingers or toes
+ Rigidity: difficulty initiating movement
What is the Claude Syndrome?
Nothnagel Syndrome + Benedikt Syndrome
Localization: MEDIAN midbrain
- in the ventral tegmentum near the junction of decussated SCP and contralateral Red Nucleus
- AFTER the decussation at the Wernekinck commissure
Cause: Infarction
- Peduncular Perforating branches (PCA)
Components:
1. IPSIlateral oculomotor palsy
2. CONTRAlateral extrapyramidal signs
3. CONTRAlateral cerebellar hemiataxia
- ataxia becomes contralateral to the lesion because of the remaining decussation of the rubrospinal tract to the contralateral side
Affected structures:
1. Oculomotor nerve fascicle (tegmentum)
- Red Nucleus +/- Substantia Nigra: EPS
- Superior Cerebellar Peduncle: hemiataxia
What is the Sphenocavernous Syndrome?
- lesion: Oculomotor nerve within the cavernous sinus OR the superior orbital fissure
Symptoms:
1. Complete ophthalmoplegia
- involvement of cranial nerves 3, 4, 5A, 5B, 6
- absence of convergence reflex to near
- Poorly reactive pupil
- involvement of BOTH sympathetic and parasympathetic fibers
- maybe small or fixed at mid-position
- absent constriction: NO light & NO near reflex
- absent dilation: NO reaction to dim conditions - Blurring of vision to objects at near
- absent accommodation
- loss of parasympathetic fibers to the ciliary muscles
Causes:
1. ICA aneurysm
2. Tumors
3. Infections
How to distinguish between Oculomotor nerve lesions in the Cavernous Sinus vs Orbital Apex?
A. Orbital Apex/Superior Orbital Fissure
- SOF: connects cavernous sinus with orbit
- branches in close proximity to Optic nerve
- (+) Optic Neuropathy
1. Reverse RAPD
2. Vision loss and visual field cuts
3. Optic disc atrophy and papilledema
- (+) Orbital signs
1. Conjunctival injection
2. Dilated episcleral veins
3. Chemosis
4. Proptosis
5. Periorbital flush, edema or venous dilation
B. Cavernous Sinus
- does NOT present with optic neuropathy or orbital signs
CLUE: differences in the involvement of the individual branches of the cranial nerves
Cavernous Sinus:
- CN3 and CN5A both unbranched: ALL structures innervated by both are affected
- affect structures innervated by CN 5B
- may present with isolated CN6 palsy +/- Horner syndrome (adjacent to the ICA)
Orbital Apex/Superior Orbital Fissure
- both CN3 and CN5A divide into individual branches prior to or upon entering the fissure
- divisional palsies: SOME structures may not be affected
- structures innervated by CN 5B NOT affected as it does not enter the SOF
What are the signs of lesions affecting the intraorbital oculomotor nerve?
location: enters the orbit MEDIAL and INFERIOR to the trochlear nerve INSIDE the Annulus of Zinn
- nerve has branched into 2 divisions:
1. Superior Division: LPS, SR
2. Inferior Division: IO, MR, IR, parasympathetic fibers - presents with orbital signs and optic neuropathy due to the limited volume of the orbit
Causes:
1. Trauma
2. Tumors
3. Infection and inflammation
