The vestibuloocular reflex is one of the brainstem reflexes. When the head turns, the eyes reflexively move in the opposite direction to stabilize the image on the retina. If the reflex cannot be triggered on unconscious or comatose patients, this association suggests that brain death has occurred.
What is the vestibuloocular reflex?
The vestibuloocular reflex is a brainstem reflex designed to provide stable visual perception during head movements. Reflexes are involuntary and automatic. Most of them are protective reflexes intended to protect important body organs or even to ensure survival. The vestibulo-ocular reflex does not belong to the protective reflexes in the strict sense. This reflex is a brainstem reflex that is intended to ensure stable visual perception during head movements. The eyes move in the opposite direction when the head is turned. Fixed objects can thus continue to be viewed without loss of visual acuity. The most important element of the reflex arc is the interconnection of the vestibular arcuate pathways with eye muscle nerve nuclei. This interconnection corresponds to a connection with the nucleus nervi oculomotorii and trochlearis and nucleus motorius nervi abducentis. The vestibulo-ocular reflex is based on a compensatory movement of the eyes, which is facilitated by the aforementioned nerve nuclei. The compensatory movement is also called the doll’s head phenomenon and stabilizes the image on the retina. Other reflexes from the brainstem reflex group are the pupillary reflex, the cough reflex, and the gag reflex.
Function and task
The vestibulocular reflex causes humans to reflexively and slowly compensatory move their eyes to the opposite side during head movements to allow gaze fixation of what was previously viewed. Three different neurons are involved in the circuitry of the vestibulookular reflex. The afferents of the reflex arc similar to the vestibulospinal reflex. At each turn of the head, the circuitry of the reflex eye movement runs via arcuate ganglion afferents of the vestibular organ to postganglionic nerve fibers of the vestibulocochlear nerve. This nerve is located in the vestibular ganglion and corresponds to the first neuron. From here, the stimulus is projected to the vestibular nuclei, which correspond to the second neuron. Switching occurs in these nuclei. The signals thus reach the projection fibers leading to the contralateral abducens nucleus. The nerve fibers located there are wired to the sixth cranial nerve and, via fasciculus longitudinalis medialis, to the contralateral nucleus nervi oculomotorii. By connecting to the third neuron and thus the motor nerve nuclei of the eyes, an eye movement occurs as a motor response to the stimulus. This movement corresponds to an abduction of the eye facing away from the direction of rotation. At the same time, an adduction is initiated that involves the eye in the direction of rotation. In opposing vertical eye movements, the afferents of the maculae utriculi et sacculi play a role, which is initiated via the vestibular nuclei, the vestibulocochlear nerve and the downstream eye muscle nuclei nucleus nervi trochlearis and nucleus nervi oculomotorii. The vestibulocochlear reflex stabilizes the retinal image. From the point of view of evolutionary biology, it thus ensures survival in the broadest sense, because humans are among the visually controlled creatures. Visual perception is the most important type of perception for him: it helps him to recognize danger and to identify food sources. The vestibuloocular reflex ensures that humans can still rely on their visual perception even when their heads are turned.
Diseases and ailments
The vestibuloocular reflex can be tested clinically. The test procedure is usually the same as the Curthoys and Halmagyi head impulse test. To perform the test, the patient sits opposite the physician in an examination chair and is asked to fixate the physician’s nose. The examiner passively and slowly moves the patient’s head to the left and right or down and up. The passive movements take place in one plane each. With a short and jerky movement, he finally transports the patient’s head to the middle position. The jerky middle position initiates the vestibuloocular reflex. The patient’s gaze thus remains unchanged on the doctor’s nose.The presence of the reflex movement speaks for the intactness of all nerve structures involved in the reflex, such as intact arcades. If one of the structures involved is not intact, the reflex is impaired or fails completely. Complete failure, for example, indicates damage to the cranial nerve nuclei involved. If the head impulse rotation test produces pathological results, there is usually an acute peripheral vestibular disorder. This is a disturbance of the vestibular organ that can manifest itself in a variety of symptoms. If there is an acute unilateral failure of the organ, nausea or even vomiting will present in addition to spinning dizziness. Sweating or involuntary oscillating movements of the eyes are also possible symptoms. In individual cases, these complaints are associated with horizontal rotating spontaneous nystagmus. Characteristically, the patient also suffers from a tendency to fall in the sense of trunk ataxia toward the affected side. The sense of hearing is not affected by the disturbance of the organ of equilibrium. By means of the head impulse rotation test, the physician can localize on which side the disturbance of the organ of balance is located. When symptoms such as acute vertigo and nystagmus fail to collect unremarkable results on the turn test, a brainstem infarction or cerebellar infarction is usually present. Even unconscious or comatose patients usually still have the vestibulo-ocular reflex. In connection with unconscious patients, this is also referred to as the doll’s eye phenomenon. The examination is mainly relevant for the diagnosis of brain death. If the reflex can no longer be triggered in a comatose or unconscious patient, there is probably no longer any brain activity. This association may confirm the diagnosis of brain death.