Vestibular System Flashcards
What is the vestibular system?
A special proprioceptive system that primarily influences postural (axial) and proximal limb muscle tone and the oculomotor system
The vestibular system signals changes in what?
The position of the head with respect to gravity (static detection) and motion of the head (kinetic detection)
What is the doll’s eye maneuver?
In the unconscious pt without cervical injury, side to side movement of the head results in horizontal movement of the eyes in the opposite direction
If while testing the doll’s eye maneuver the pt turns their head to the right and the response is right eye adduction and left eye remains neutral what is the indication?
Left VI palsy
While testing the doll’s eye maneuver, the pt turns their head to the right and the response is the right eye remains neutral while the left eye abducts what is the indication?
Right III palsy
In a conscious pt ice water injected into the external auditory meatus creates what?
A convention current in the endolymph of the lateral semicircular duct
In pts with an intact brainstem, ice water caloric testing causes what?
The eyes to turn toward the stimulus and they show horizontal nystagmus to the non-stimulated (opposite side)
Warm water caloric testing normally causes what?
The eyes to turn away from the stimulus with horizontal nystagmus towards the stimulated ear
What is the mnemonic for nystagmus during caloric testing?
COWS
Cold opposite, warm same
Absent vestibuloccular response (VOR) suggests what?
Ipsilateral vestibular impairment of the stimulated semicircular duct
Both the doll’s eye maneuver and oculocaloric testing use which fibers?
MLF
Vestibular receptors are innervated by what?
Dendrites of bipolar neurons that form the vestibular nerve
Where is the vestibular (Scarpa’s) ganglion found?
Near the vestibular portion of the membranous labyrinth and consists of bipolar neurons
The vestibular nuclei projects influences to what 4 principal tracts or areas?
Medial longitudinal fasciculus (MLF), medial vestibulospinal tract (MVST), lateral vestibulospinal tract (LVST) and reticular formation
The medial longitudinal fasciculus (MLF) is the critical link what?
Synchronized eye movements especially horizontal gaze
Describe the MLF
Conveys reciprocal connections b/w the abducens nucleus/PPRF and the contralateral oculomotor nucleus
Fibers from the vestibular nuclei and PPRF decussate and bifurcate into ascending and descending components
Terminates in the oculomotor, trochlear and abducens nuclei (referred to as the vestibulo-ocular pathway)
Horizontal gaze involves the coordinated contractions of which muscles?
Lateral rectus of one eye and the medial rectus of the other
Ex. Looking to the right = lateral rectus of R and medial rectus of the L
Describe the vestibulospinal tract (MVST)
Originates from the medial vestibular nucleus and is primarily an uncrossed descending tract but it does contain some crossed fibers
Terminates in the intermediate gray of the cervical and upper thoracic levels of SC
What is the function of the medial vestibulospinal tract?
Primarily influences the muscles of the neck, upper back and proximal upper limbs
Key link in coordinating the positioning of the head relative to eye movement
The lateral vesitbulospinal tract coordinates what?
The positioning of the body relative to head position and gaze
What is the vestibulo-cerebellum?
Flocculonodular lobe + fastigii
Coordinates the oculomotor system for gaze and has a critical role in coordinating axial and proximal limb musculature and the oculomotor system
The vestibulocerebellar fibers provide direct input from what?
Vestibular nuclei and RF to the cortex of the flocculonodular lobe
Which fibers provide cerbellovestibular efferents?
Vestibulocerebellar and cerebellovestibular fibers
What are cerebellovestibular fibers?
From the cortex and fastigii to vestibular nuclei
Involves the direct fastigiobulbar tract
Describe the direct fastigiobulbar tract (uncinate fasciculus)
Descending limb of the uncinate fasciculus terminates in the vestibular nuclei and RF
Ascending limb exits the cerebellum via superior cerebellar peduncle
Terminates in the midbrain tegmentum and ventral lateral nucleus of thalamus
Important vestibular link to the EPS
Unilateral lesions of the vestibular system cause what?
Ipsilateral signs and sx including nystagmus, vertigo, postural impairment, falling toward the side of the lesion, eyes head and body turn toward side of lesion and visceral disturbances
Nystagmus is described according to what?
The fast component which is directed toward the side of the lesion
Vertigo usually occurs in which direction?
Opposite the lesion
How do you assess postural impairment?
Using the Romberg test
Describe the Romberg test
If a standing pt with his eyes closed sways or falls during this test its positive and a type of sensory ataxia
If the pt is ataxic but the Romberg’s test is normal it suggest what?
Cerebellar ataxia
Many vestibular deficits will attenuate with time due to what?
The compensation of the visual and general proprioceptive system
A unilateral lesion of the MLF superior to the abducens nucleus results in what?
A contralateral disturbance of horizontal gaze (impairment of adduction (MR) of the ipsilateral eye and nystagmus of the abducting eye)
If there is a lesion of the right MLF what is the result?
Horizontal gaze to the right is normal
Horizontal gaze to the left exhibits paresis of the adductor/MR of the R eye and nystagmus of the left (abducting) eye
Referred to as right internuclear opthalamoplegia (INO)
Accommodation does not use which type of fiber?
MLF and is unaffected by INO
The corticotectal fibers directly descend from the frontal eye field to the oculomotor and Edinger-Westphal nucleus
Unilateral lesions of the PPRF result in what?
Paresis or paralysis of horizontal gaze toward the same side of the lesion and gaze preference is away from the side of the lesion
A lesion of the right PPRF would result in what?
Pts inability to move both eyes in conjugate horizontal gaze to the right
