The vestibular system is responsible for maintaining balance and equilibrium. Precise eye movement control is necessary to obtain a fair resolution of the visual field during motion, either self –motion( translational or rotational) or object motion. These are the oculomotor movements controlled at various levels. The gaze is fixed in space during the oculomotor system’s head movement, using vestibule-ocular reflex( by semicircular canals) and ocular counter rolling reflex( by otoliths).
Oculo-Vestibular reflex is an involuntary reflex that is responsible for the stabilizing of the visual field during head motion. Specific reflexes are involved in the maintenance of the stable visual areas, including Extra retinal signals about head motion, retinal signals, neuro control of stabilization reflexes, and motor apparatus-extra ocular muscles. Any disturbance in the oculo-vestibular reflex results in the symptoms such as nausea, head tilt, imbalance during walking and other life activities, dizziness, and blurred vision during motion. The oculo- vestibular reflex is testable through different examinations, including head impulse testing, rotational chair testing, velocity step test, and caloric reflex test.
Clinical Anatomy of Oculo-Vestibular Connections
The vestibular system has its connection with the central nervous system along with the spinal cord. Finally, the archi-cerebellum, the flocculo-nodular portion of cerebellum, is in the overall modulatory control of these multi-segmental vestibular projections. The primary neural connectivity of the oculo- vestibular is provided by the three-neuron arc, namely primary sensory afferent neuron, a vestibular nucleus neuron in the porno-medullary region, and an oculomotor neuron in the III, IV, or VI nuclei in the brain stem.
The vestibular labyrinth consists of three semicircular canals( angular acceleration) and two otolith organs (utricle and sacculus). The primary organization involved in the 3-neuron arc comprises the vestibular nuclei on the same side of the labyrinth sending excitatory projections to the oculomotor nuclei on the opposite side, inhibitory projections to antagonistic oculomotor neurons on the same side of the brain stem.
Physiology Involved in Oculo-Vestibular Reflex System
The oculo-vestibular reflex is an involuntary reflex that stabilizes the visual field and the retinal image during head motion.
Certain types of reflexes are involved in the maintenance of the stable field, and they include,
Extra retinal signals about head motion
The vestibular system consists of semicircular canals, and they are involved in the angular motion as occurs during rotation of the head. The two otolith organs, utricle, and saccules transduce the head’s linear motion during head tilt and roll.
The head’s angular stimulation plays a part in the inspiration of the semicircular canal’s hair cells and results in the eye rotations that are roughly equal and opposite the head’s motion. The head rotations about the horizontal, rotational, and nasal occipital axes produce oculo-vestibular reflex. The rotations are made with the horizontal, vertical, and torsional counter rotations of the eye. These rotations show the slow phase of nystagmus, which is the eyes’ abnormal movement.
The head rotation also produces retinal image motion of the visual field. The head’s process stimulates the reflex eye movements with a slow phase following the moving area interrupted by resetting cascades. This reflex is known as optokinetic nystagmus. This optokinetic nystagmus reflex complements the vestibule-ocular reflex during low velocity sustained head movements such as walking.
Neuro-control of Stabilization Reflexes
The three semicircular canals convert the head motion signals into a neural stimulus driving the vestibular-ocular reflex. When the endolymph moves, the hair cells in the horizontal canal undergo depolarization. Also, the impulses from the left medial vestibular nucleus pass via the right abducens nucleus causing abduction of the right eye, the left eye abduction is caused by the left eye’s medial rectus via the oculomotor nucleus. The left eye abduction leads to conjugate eye movements.
Depending on the stimulation of the semicircular canals and the otolith organs, the four recti muscles and two oblique muscles perform the eyeball movements.
The sensory information from the vestibular system is used to control reflexes responsible for maintaining images’ stability on the fovea (the central area of the retina) during head movements. The information is perceived from the vestibular receptors and is essential for posture and gait. In the case of the normal vestibular function, the reflexes operate with high accuracy.
In the vestibular system, the change in the eye angle is gain. The vestibule-ocular reflex is used to stabilize gaze during head movement. The eye movements are in the direction which is opposite to the head movement. The ocular vestibular system even works in the dark. The ocular-vestibular system is activated by the hot and cold stimulation of the inner ear. In addition to the working and activation of the ocular-vestibular reflex, the fixation reflex is activated for movement. The utricle, the gravity otolith organ, works along with the semicircular canals that cause most reflexes. The ocular-vestibular reflex works in a way to provide both rotational and translational movements. The signals drive the reflex, and the signs arise from the vestibular system of the inner ear. The semicircular canals of the vestibular system are responsible for the detection of head rotation. The otolith organs are responsible for the detection of the translational component. Additional pathways are present, which prevent the eye from rolling back when the head stops moving. For clear vision and proper head movement, the oculo-vestibular reflex should be fast.
Clinical Importance of Oculo-Vestibular Reflex
Some people have reduced ocular-vestibular reflex function. The reduction in the reflex’s process leads to eye movements that are not large enough to compensate with their head movements. The patients also find the vision blurring due to the decreased function of the reflex. This condition is known as oscillopsia.
A blurred image is being presented to the brain. Patients are refrained from reading a book because it requires excellent effort. Nystagmus is experienced by the patients in the vestibule-ocular dysfunction in which there are uncontrolled eye movements. Small head movements can cause distortion. Some of the common symptoms experienced by people with abnormal oculo-vestibular reflexes include being clumsy, difficulty maintaining balance, experiencing nausea, motion sickness, or face sensory issues. As the vestibular system is involved in maintaining balance and equilibrium, any problem related to the system causes the brain to receive conflicting signals. The person may face balance issues, problems with movements, and cause vertigo.
Oculo-Vestibular Reflex Testing
Head Impulse Testing ( HIT)
Halmagyi and Curthroys described the test in 1998. The test is usually performed at the patient’s bedside and used to identify any oculo-vestibular reflex deficit. There may be a deficit in the functioning of the lateral, posterior or superior semicircular canals. Head impulse testing is commonly performed on the patient to diagnose any unilateral or bilateral peripheral lesions. The patient is fixated on an individual target, the examiner rotates the head. After the slight tilting of the head, it is thrust to the opposite side. All the movements should be small. If the patient cannot maintain a steady gaze on the target, this indicates an abnormal result. In case of a lesion’s presence, the patient shows anomalous thrust in one or both directions.
Rotational Chair Testing
During head rotation, this system stimulates both of the canals through which the vestibular system is functioning. Several types of rotational chair tests are clinically performed to assess the functioning of the vestibular system. Further measures are then taken to treat any decline in the functioning of the vestibular system.
Sinusoidal Harmonic Acceleration Testing
The test is performed to assess the eye movements by moving the chair in a sinusoidal fashion. There is an increase in the frequency from 0.01 to 2 Hz. The sinusoidal harmonic acceleration testing can detect any abnormality present in the vestibular system.
Constant Angular Acceleration.
This constant angular acceleration test involves the chair’s rotation up to 360 degrees .the acceleration is kept constant, and the rotational velocity increases. After a steady rise in the velocity, there is a gradual deceleration. Through the continuous angular acceleration test, nystagmus can be measured during acceleration and deceleration.
Impulse Angular Acceleration
Another test for measuring the oculo-vestibular function in which the chair rotates at 360 degrees at a constant rate. The chair rotates until it is stopped abruptly. The slow-phase velocity of nystagmus is measured through this test, along with the gain and time constant velocity. The angular acceleration testing is performed in both directions. And at the end of the trial, the results are plotted over time.
Velocity Step Test
During the velocity step test, there is an increased velocity of 100-200/s through which the chair is rotated. There is a maintenance of speed for about 60 seconds. After a phase of acceleration, rapid deceleration occurs, and the gain and time values are measured.
A temperature gradient is created across the lateral semicircular canal of the stimulated ear. Through this temperature gradient being made, there is a change in the density of endolymph. Caloric testing helps in the initiation of the oculo-vestibular reflex. During this test, for the induction of nystagmus, hot or cold water is poured into the patient’s auditory canal. If cold water is run through the auditory canal, the nystagmus’ fast phase is in the same direction as the water-filled ear. While in the case of hot water, this is the opposite.
Vestibular reflex testing is essential for the proper diagnosis of the reflex. In case of a deficit in one or both vestibular organs, a patient needs proper vestibular rehabilitation.
The vestibular system provides a sense of balance and information about the body position. Precise control of head movements is necessary to obtain a fair resolution of the visual field during motion. The gaze is fixed in space during the head movement by the oculomotor system. The oculo-vestibular reflex is an involuntary reflex, and it is involved in the stabilizing of the visual field . The system is connected to the spinal cord , and to the cortex. The vestibular labyrinth consists of the three semicircular canals , and the two otolith organs, depending on the semicircular canals and the otoliths organs, the four recti muscles and the two oblique muscles perform the eyeball movements. During head movements, the information is perceived from the vestibular receptors and is responsible for the proper posture and gait. Also, a specific type of reflex is involved in the maintenance of the stable field, including Extra retinal and retinal signals, neuro control of the stabilization reflexes, and motor apparatus . The sensory information from the vestibular system is used to control the reflexes which are responsible for the maintaining the stability of the images on the fovea. The eye movements are produced in the direction which is opposite to the direction of the head movement. This system evens works in the dark and is stimulated by the cold or hot. The signals drive the vestibular reflex, and those signals arise from the vestibular system of the inner ear. Additional pathways are present which prevent the eyes from rolling back when the head stops moving. The oculo-vestibular system should be fast for proper head movement and clear vision. In some people, there is a decline in the oculo-vestibular reflex.
The drop leads to oscillopsia and leads to blurring of the picture. Any disturbance in the oculo-vestibular system causes nausea, imbalance, head tilt, blurred vision is experienced during motion. (De, 1992) Some people shared the nystagmus that causes uncontrolled eye movements. Several tests are performed in patients with symptoms that show a decline in the oculo-vestibular reflex (Sriram Simakurthy, 2020)The tests include head impulse testing, impulse angular acceleration, velocity step test, and caloric testing. The test usually involves the rotation of the head in different directions and different angles.The head is rotated with specific velocity, and eye movement is observed. Irregular eye movements or the head’s tilting in the opposite direction may show a decline in the vestibular function. the reduction in the vestibule-ocular reflex may also indicate brain death.
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