Essential Principles of Vestibular Physiology
Traditionally there have been five “senses,” namely vision, hearing, smell, touch, and taste. It is entirely reasonable to name balance as the true “sixth sense.” What sets balance apart from the other senses is that it is not used to actively “investigate” our surroundings in the same way the others are. Balance is automatic and subconscious until there is a disruption of the vestibular system and symptoms develop. These symptoms are often substantial.
The vestibular system has two broad functions—the maintenance of balance and the maintenance of stable gaze. The vestibular end organs comprise the otolith organs (the utricle and saccule) and the three semicircular canals (lateral, superior, and posterior). The semicircular canals (SCCs) are activated during rotational movements and the otolith organs during linear movements.
The SCCs are paired structures. While the lateral canals are paired with each other, the superior canal on the left is functionally paired with the posterior canal on the right and vice versa. Eye movements are produced in the plane of the canal being stimulated. Stimulation of the semicircular canal occurs when the cupula is deflected as a result of endolymph within the canal remaining relatively still, as a result of its inertia, as the head is moved.
The vestibulo-ocular reflex (VOR) serves to maintain the visual field in a stable fashion on an area of interest. The area of high visual acuity afforded by the fovea centralis is relatively small when compared to the entire visual field and must be kept accurately directed toward the area of interest even during head and body movements. Systems of smooth pursuit are not sufficiently fast to allow this to be undertaken voluntarily and thus the VOR is used to ensure that eye movements are produced that are equal and in an opposite direction to head movements. Defects in this reflex cause reduced dynamic visual acuity owing to the “retinal slip” caused by an image not being held consistently over the fovea.
Although there is substantial crossover between the function of these systems, the otolith organs play the greatest role in the maintenance of an upright posture through the detection of body or head tilt while the SCCs play the greatest role in the VOR.
Hair cells within the SCCs fire at a baseline rate when at rest with no head movement. When the head is moved in a rotational fashion, one of the pair of canals will increase its firing rate while the other will decrease. This differential will signal a head movement in the plane of that semicircular canal. In the case of the lateral canal, there will be an increased rate of firing of the hair cells on the side to which the head is being rotated and a decrease in the contralateral side. The eye movement produced by the VOR will be the vector of the signals produced by the vestibular end organs, primarily the SCCs.
In most physiological conditions of head movement there is no clinically relevant upper limit on the rate of firing whereas the reduction in the rate of firing in the contralateral ear can reach easily zero, below which there is no further reduction possible. This limit results in the principle that, although the canals are paired, at higher peak velocities of head movement, the contralateral ear cannot provide accurate velocity signals alone. As a result, a unilateral lesion will affect the VOR if sufficient acceleration is applied to the affected ear.
The difference in firing rates of the SCCs is the determinant of the signal received by the central nervous system (CNS) when detecting head movement. In the case of a sudden, pathological loss of vestibular function there will be a sudden difference in firing rates between the paired canals. In the case of the lateral semicircular canal, the affected baseline rate would be zero, whereas the contralateral side would be still firing at its baseline rate. This differential would be interpreted by the CNS as the head being rotated in the direction of the unaffected ear. The VOR then produces a slow-phase eye movement in the direction of the affected ear and a fast phase movement in the direction of the unaffected ear to reset.
The VOR is velocity/acceleration dependent. To maintain an equal and opposite eye movement, the CNS must receive signals from the vestibular system that are capable of delivering information on the velocity of the head movement, not just the direction. This is of importance when considering the results of caloric testing that assign a single value to a loss of vestibular function.
Over time the CNS can compensate for changes in the baseline firing rate of an affected vestibular end organ, but the weakness will remain and, owing to the fact that the contralateral ear cannot compensate at higher velocities for the affected semicircular canal to which it is paired and the VOR will remain pathological.
Testing of vestibular function is by no means reliable, exhaustive, or complete. The SCCs are most easily assessed, but the otolith organs are not easy to test and the tests that are currently available produce a very crude assessment of function. It is worth considering that the amount of neuroepithelium contained in the otolith organs is similar to that contained by the cochlea, yet current tests of otolithic function produce an output that determines whether a response is either “absent” or “present.” Clearly an audiogram that presented hearing in those terms would be unacceptable. No test provides a “gold standard” and no test is indicative of overall vestibular function. Normal vestibular function testing does not exclude vestibular pathology and all abnormal test results should be viewed in a clinical context.
Three paired SCCs and the otolithic (macular) organs within the otic capsule.
SCCs (superior, posterior, and lateral): for angular acceleration perception
Otolithic organs (utricle, saccule): for linear acceleration perception
Cristae = end organs containing hair cells; located within the ampullated portion of the membranous labyrinth
Cupula = a gelatinous matrix that the cilia of hair cells are embedded into; acts as a hinged gate between the vestibule and the canal itself
Otoliths = a blanket of calcium carbonate crystals on a supporting matrix; found only in the otolithic end organs (not in SCCs)
Vestibular nerve = the afferent connection to the brain stem nuclei for the peripheral vestibular system
Superior vestibular nerve: superior, lateral SCCs and utricle
Inferior vestibular nerve: posterior SCC and saccule
Each vestibular nerve consists of approximately 25,000 bipolar neurons whose cell bodies are located in the Scarpa ganglion within the internal auditory canal
The fundamental units for vestibular activity inside the inner ear
Type I hair cells
Surrounded by the afferent nerve terminal at its base in a chalice-like fashion
High amount of both tonic and dynamic electrical activity
Largely stimulatory effect
Type II hair cells
Surrounded by multiple nerve terminals
Predominately inhibitory effect
Each hair cell contains 50 to 100 stereocilia and one long kinocilium that projects into the gelatinous matrix of the cupula or macula.
The location of the kinocilium relative to the stereocilia gives each hair cell an intrinsic polarity that can be influenced by angular or linear accelerations.
The hair cells of the ampulla within the lateral SCC all have the kinocilia located closest to the utricle.
The hair cells within the superior and posterior SCC all have the kinocilia located away from the utricle or on the crus commune side of the ampulla.
In the otolithic membranes, the hair cells are lined up with the kinocilia facing a line which almost bisects the membrane, called the striola.
Displacement of the stereocilia toward/away from the kinocilium alters calcium influx at the apex of the cell → release/inhibition of neurotransmitters
Four distinct second-order neurons within the vestibular nuclei
Superior (Bechterew nucleus): major relay station for conjugate ocular reflexes mediated by the SCCs
Lateral (Deiters nucleus): control of ipsilateral vestibulospinal (the so-called “righting”) reflexes
Medial (Schwalbe nucleus): coordination of eye, head, and neck movements with connections to the medial longitudinal fasciculus
Descending (spinal vestibular nucleus): integration of signals from the vestibular nuclei, the cerebellum, and reticular formation
Neural integrator = amorphous area in the reticular formation responsible for the final velocity and position command for conjugate eye movements
Vestibulocerebellum = the phylogenetically oldest parts of the cerebellum (the flocculus, nodulus, ventral uvula, and the ventral paraflocculus) into which the vestibular nerve directly projects
Conjugate eye movements, VOR, smooth pursuit
Holding the image of a moving target within a certain velocity range on the fovea of the retina
Cancelling the effects of VOR (eg, figure skater can twirl without getting dizzy)
Compensation process for a unilateral vestibular loss
Vertigo: illusion of rotational, linear, or tilting movement, either of self (subjective) or the environment (objective)
Disequilibrium: sensation of instability of body positions, walking, or standing
Oscillopsia: inability to focus on objects during head movement
Lightheadedness: sense of impending faint, presyncope
Physiologic dizziness: motion sickness, dizziness in heights
Multisensory dizziness: cumulative loss from deterioration/degeneration in the multiple sensory systems responsible for balance (ie, vision, proprioception, vestibular and central integration) often related to age, diabetes, stroke, etc.
A minimum vertigo history should address the following (Figure 16-1):
Duration of individual attack (seconds/minutes/hours/days)
Frequency (daily vs weekly vs monthly)
Effect of head movements (worse, better, or no effect)
Inducing position or posture (eg, rolling onto right side of bed)
Associated aural symptoms such as hearing loss, tinnitus, and aural pressure
Concomitant or prior ear disease and/or ear surgery
Family history (eg, neurofibromatosis, diabetes, or other factors)
Head trauma, medications, comorbidities
See Table 16-1.
Otoscopic and fistula test
Identify middle ear pathology (eg, labyrinthine fistula, cholesteatoma)
Cranial nerves (I-XII)
Cerebellar (midline, hemispheric)
Oculomotor testing (saccade, pursuit, convergence, fixation)
Brain stem lesions or tumors in the cerebellopontine angle
Assesses vestibulospinal pathways and the posterior fossa
CNS pathology often involves oculomotor function; pursuit pathways most often involved
Cardinal sign of a vestibular lesion (except congenital nystagmus)
General balance function
Tandem gait test (eyes open and closed)
General test of proprioception, vestibulospinal/cerebellar tracts
Assessment of balance and corresponding tracts
Confirmation of typical benign paroxysmal positional vertigo (BPPV) or atypical positioning nystagmus
Hyperventilation for 60 s
Reproduction of symptoms suggests underlying anxiety
Head shake test
High-frequency vestibular test (2 Hz; 15 seconds).
Presence of post head shake nystagmus (HSN) correlates well with increasing right/left excitability difference on caloric testing.
Fast-phase of HSN usually directed (but not always) away from the involved ear. The presence of atypical nystagmus (either vertical or rotatory) after horizontal head shaking is called cross coupling and requires exclusion of a CNS disorder.
Halmagyi (horizontal high-velocity/acceleration head thrust) maneuver
High-velocity/acceleration test of angular VOR function.
VOR deficit suspected if refixation saccades (to stabilize eyes on a target following fast head movement) are present.
Deficit can be unilateral or bilateral.
Loss of dynamic visual acuity: loss of visible lines on Snellen or LogMAR chart (more than five lines) with rapid horizontal head shaking (> 2 Hz) suggests a bilateral vestibular loss.
VOR suppression test
Inability to visually suppress nystagmus during head rotation suggests a defect at the level of the vestibulocerebellum.
Head heave test
High-velocity/acceleration test of translational VOR function while the whole head is moved with a quick pulse linearly along the interaural (horizontal) axis
Assesses utricular function
Shown to be useful in detecting an acute unilateral vestibular injury and in prognosticating recovery in acute vestibular neuronitis
Visual vestibulo-ocular reflex (VVOR) or visually enhanced VOR
Is the addition of VOR and vision (smooth pursuit) activity to investigate visual-vestibular interaction.
An abnormality of the VVOR reflects a compound deficit of cerebellar, vestibular, and oculomotor control (ie, in smooth pursuit, the VOR and the optokinetic reflex).
The clinical utility of the VVOR sign is its unique ability to simultaneously test for the coexistence of pathology involving the VOR and the vestibulocerebellum.
Cardinal sign for a vestibular disorder. The slow phase of the nystagmus = direction of the flow of the endolymph and is vestibular in origin; the quick phase (centrally generated) = compensatory mechanism. Types are as follows:
Physiologic: endpoint nystagmus noted on lateral gaze more than 30 degrees
Spontaneous: nystagmus present without positional or other labyrinthine stimulation
Induced: nystagmus elicited by stimulation, that is, caloric, rotation, etc.
Positional: nystagmus elicited by assuming a specific position
Eye and head movements occur in the plane of the canal being stimulated and in the direction of the endolymph flow.
Ampullopetal flow causes a greater response than ampullofugal flow in the lateral canal.
The reverse is true in the posterior and superior canals.
Alexander’s law: The slow-phase velocity of the nystagmus increases when the eyes look in the direction of the fast phase (commonly observed in peripheral lesions).
First-degree: present only when gazing in the direction of the fast component
Second-degree: present when gazing in the direction of the fast component and on straight gaze
Third-degree: present in all three directions
Formal balance function testing indicated when:
Site/side of lesion not identified through history or physical examination
To ascertain who is likely to benefit from vestibular rehabilitation
To assess recovery of vestibular function
To assess contralateral function if destructive procedure is contemplated
To determine if intervention (ie, gentamicin ablation, vestibular neurectomy, etc.) has been successful
Horizontal and vertical eye movements are recorded indirectly using electrodes measuring changes in the corneoretinal potential (dipole).
Electrodes are typically placed at each lateral canthus and above and below at least one eye with a common electrode on the forehead.
Eye movements are recorded directly using infrared video cameras and digital video image technology.
Eye movements can be observed in real time and/or recorded.
Bithermal caloric tests
Pursuit system evaluation
Optokinetic system evaluation
Fixation system evaluation
Findings suggestive of central pathology
Spontaneous/positional nystagmus with normal calorics
Direction-changing nystagmus; failure of fixation suppression
Bilateral reduced or absent caloric responses without a history of labyrinthine, middle ear disease, or ototoxicity
Abnormal saccades or saccadic pursuit results, especially with normal caloric results
Hyperactive caloric responses (ie, loss of cerebellum-generated inhibition, in absence of tympanic membrane (TM) defect or mastoid cavity)
Findings suggestive of peripheral pathology
Unilateral caloric weakness
Bilateral caloric weakness with history of labyrinthine disease or ototoxicity
Fatiguing positional nystagmus
Intact fixation suppression response
Bithermal caloric test
A bithermal stimulus (eg, water or air) is used to irrigate ears to evaluate function of lateral SCCs in caloric test position (CTP)
Water = body temperature ±7°C (30°C and 44°C) for 30 seconds
Air = body temperature ±13°C (24°C and 50°C) for 60 seconds
Nystagmus response: “COWS”—“Cold-Opposite,” “Warm-Same” responses
Cool water/air → endolymphatic fluid drops → ampullofugal flow in the lateral SCC → deflection of hair cells away from kinocilium → inhibition of involved side → slow drift of eyes toward involved side and compensatory (fast) saccades in the opposite direction
Warm water/air → endolymphatic fluid rises → ampullopetal flow in the lateral SCC → deflection of hair cells toward kinocilium → excitation on the involved side → slow drift of eyes toward opposite side and compensatory saccades in same direction as stimulus
Unilateral weakness (UW)
Directional preponderance (DP)
RW = the peak slow-phase eye velocity of the response following right ear-warm temperature irrigation
RC = right ear-cool
LW = left ear-warm
LC = left ear-cool
UW more than 15% to 30% (laboratory dependent) difference = abnormal
Bilateral weakness = when the total added maximum slow-phase velocity for the left and for the right are less than 12 degree/s (ie, LW + LC < 12 degree/s and RW + RC < 12 degree/s; laboratory dependent)
DP more than 30% difference = significant but its clinical value remains questionable (some believe that DP is directed toward the side of a central lesion and away from side of a peripheral lesion)
Cursory otoscopic inspection should be completed before ordering caloric test.
Use of water as stimulus is contraindicated when TM perforation is present; heightened response expected on perforated side (using air as stimulus).
Excess cerumen can impede accuracy of test results and should be removed; many laboratories are not equipped to do cerumen management on site.