4.2 Neuroanatomy Flashcards
What are the 5 different eye movements that are important?
Saccades
Smooth Pursuits
Vergence
Vestibulo-ocular movements
Directions of gaze
What are saccades?
Rapid eye movements that abruptly change the point of fixation in any direction.
Essential to process the visual scene, moving our fovea to different parts of the visual field every 200-350ms to process it in detail.
During saccades vision is suppressed, processed during fixation
Briefly try to explain the anatomy underpinning saccades
Motor movements (preceding electrical impulses) ~ 200ms before eye movement. During this time movement is calculated and signals are sent to the EOMs to move both eyes the correct distance in the appropriate direction in a conjugate movement.
The anatomy underpinning this is complex and is better understood in primates than in humans. It includes a wide range of cortical and subcortical structures which are then mediated through cranial nerves, neuromuscular junction, and muscle
What is a smooth pursuit?
Fixes a target of interest on the area of best vision (fovea).
Slow tracking movements – not without a target to follow.
Testing using optokinetic tests (OKN drum), eyes follow a stripe until full excursion, quick saccade back the other way then once again by smooth pursuit of a stripe.
What is optokinetic nystagmus?
Alternating slow pursuit and fast saccade in the opposite direction in response to such stimuli = normal response to large-scale movements of the visual scene (eg looking out a train window) (vs pathological nystagmus).
What are vergence movements?
Different in that they are disconjugate eye movements, either convergent or divergent, to support BSV.
Slower movements
Fixate the target on the fovea of each eye at a given distance
Convergence for near is accompanied by pupillary constriction and lens accommodation (presbyopia, convergence insufficiency)
What is a vestibulo-ocular movement?
Stabilise the eyes to compensate during fast head movement.
Vestibular system detects changes in head position – corrective eye movements
Directs the eyes to compensate for head movements, maintaining the position of the visual image on the surface of the retina.
How are vestibulo-ocular movements tested?
Head impulse test – abnormal if catch up saccade
Test of VOR suppression – abnormal if nystagmus seen
What are the 5 levels of the nervous system?
Muscle
Neuromuscular junction
Cranial nerve
Brainstem (&cerebellum)
Supratentorial (cerebral hemispheres, basal ganglia, thalamus)
Draw the table for the 6 EOMs and their actions
MR - Adduction
LR - Abduction
SR - Elevation, intorsion, adduction
IR - Depression, extorsion, adduction
SO - Intorsion, depression, abduction
IO - extorsion, elevation, abduction
Explain CN III
Name - oculomotor nerve
Controls - SR, IR, MR, IO
Nucleus in the midbrain
Explain CN IV
Name - trochlear nerve
Controls - SO
Explain CN V
Name - Abducens nerve
Controls - LR
Briefly explain the pathway of CN III
- Nucleus in the midbrain
- Fibres pass through the midbrain
- Through interpeduncular cistern between posterior cerebral & superior cerebellar artery
- Pierces dura, passes superiorly lateral wall cavernous sinus
- Into orbit via superior orbital fissure split into the superior & inferior divisions
What does the superior division of CN III control?
SR and levator palpabrae
What does the inferior division of CN III control?
IR, MR, IO
Briefly explain the pathway of CN IV
- Nucleus in the midbrain between the superior and inferior colliculi
- Longest intracranial course of any cranial nerve and the only one to exit dorsally
- Passes between the superior cerebellar artery and the posterior cerebral artery, runs in the subarachnoid space (vulnerable) before penetrating the dura above the trigeminal nerve
- Travels within the lateral wall of the cavernous sinus above the abducens nerve and V1 (ophthalmic branch of the trigeminal nerve).
- Enters the orbit through the superior orbital fissure to the superior oblique muscle
- Longest intracranial course, vulnerable to shearing forces in trauma
Briefly explain the pathway of CN VI
- Nucleus at junction of pons and medulla
- Enters subarachnoid space, pierces the dura into Dorello’s canal
- Exits Dorello’s canal (along bony notch) into the cavernous sinus
- Enters orbit via the superior orbital fissure into lateral rectus
- Vulnerable along course to raised intracranial pressure (false localising sign), tumour, stroke, trauma as well as microvascular palsies; congenital (Duane’s)
What pathology can be seen in brainstem lesions?
Stroke, tumour, trauma, inflammation & neurodegeneration
Where is a Supranuclear lesion?
Anywhere above the level of the nucleus
Where is a nuclear lesion?
At the nucleus
Where is a infranuclear (CN) lesion?
This can occur due to pathology within the brainstem if it impacts the exiting nerve fibres after they have left the nucleus but before the exit the brainstem (nerve root entry zone, rarer)
Name the 4 focal brainstem syndromes
Internuclear ophthalmoplegia (INO)
One and a half syndrome
8 and a half syndrome
Parinaud syndrome
What is an INO?
Lesion of MLF, failure of adduction (or slowed adducting saccade ipsilateral) contralateral abducting nystagmus. Usually MS affecting the MLF, or stroke, rarer tumour
What is one and a half syndrome?
Lesion involving abducens nucleus, PPRF, and MLF ipsilaterally causing a conjugate horizontal gaze palsy one way & INO the other
What is 8 and a half syndrome?
One and a half plus VIIth NP. The proximity of the PPRF, facial nerve nucleus, and MLF located in the dorsal pons makes this syndrome much more likely. The lesion is most often vascular or demyelinating in the dorsal tegmentum of the caudal pons
How would you assess CNI
Smell: Simply ask ”how is your sense of smell e.g. coffee, mint, freshly peeled orange/mown grass?)
How would you assess CNII?
Acuity, Ishihara, Pupils, Confrontation fields, Lids (incl sustained upgaze, ?Cogan’s lid twitch if ptosis)
How would you assess CNIII & IV and VI?
Pursuit and saccades; ask about diplopia (orthoptic assessment); sustained gaze position if ?myasthenia
How would you assess CNV?
Sensation V1-V3, muscles of mastication
How would you assess CN VII?
Raise eyebrows, show teeth
How would you assess CN VIII?
Hearing, balance (some insight to vestibular function from EM assessment already)
How would you assess CN IX & X?
Open mouth wide, say ‘ah’ – confirm symmetrical palatal elevation and central uvula
How would you assess CN XI?
Shrug both shoulders up against my hands (trapezius), push cheek/chin sideways in to my hand
How would you assess CN XII?
Protrude tongue (confirm stable central tongue protrusion)
Extra investigations needed for suspected neuroanatomy
- Assessment saccadic and pursuit movements
- Examination VOR
- Demonstration Bell’s phenomenon
- Measurement convergence and accommodation amplitudes
- OKN
- Assessment of nystagmus if present
- Pupils
- Eyelid position and movements
- Visual fields
- Full medical examination
- Neuroimaging
- Laboratory investigations
What symptoms can we treat in neuro conditions?
o Head position
o Diplopia
o Oscillopsia
o Cosmetic concerns
What treatment options do we have for neuro conditions?
o Prisms
o Sector occlusion
o Extraocular muscle surgery
o Botulinum toxin
o Medication
