Vestibulo-Ocular Reflex - Overview

Introduction

The vestibulo-ocular reflex (VOR) stabilizes eye movement while the head moves. It helps in maintaining vision during activities that involve all types of head movement. When the head moves in one direction, the eyes move in the opposite direction. As a result, the image captured by the eyes is unaffected. Defects in the vestibulo-ocular reflex may occur due to conditions like abnormal nystagmus and oscillopsia.

What Is the Vestibulo-Ocular Reflex?

The vestibulo-ocular reflex (VOR) is an involuntary eye movement that occurs in response to the head movement to stabilize vision captured by the retina of the eye. There are two kinds of vestibulo-ocular reflexes:

• Rotational Vestibulo-ocular Reflex: This reflex is produced in response to horizontal (yaw), vertical (pitch), or torsional (roll) head movements.

• Translational Vestibulo-ocular Reflex: This reflex is produced in response to interaural (side to side), anterior and posterior (fore and aft), or vertical (up and down) head movements.

How Does the Vestibulo-Ocular Reflex Work?

The vestibular system belongs to the inner ear and is also known as the organ of balance. It comprises of semicircular canals and two otoliths or inner ear organs, such as the utricle and saccule. The word "ocular" is related to eyes or vision. The vestibulo-ocular reflex occurs as a result of the interplay between the ear, eyes, and central nervous system. The semicircular canals and other organs in the inner ear are sensitive to motion, and they detect all types of head movements.

The information perceived by the inner ear organs is collected in the form of signals by the motion sensors present within the periphery of the inner ear. These motion sensors in turn carry information to the central nervous system (CNS), which sends the output signals to the eye muscles. This causes the opposite movement of the eyes with the head movement, which stabilizes the vision.

Other reflexes associated with the vestibular system include:

• Vestibulo-Spinal Reflex (VSR): The VSR is a group of reflexes that control muscles of the body to stabilize the posture and balance of the body.

• Vestibulocollic Reflex (VCR): The VCR is a reflex that acts on the muscles of the neck to stabilize the head during head movement.

How Is the Vestibulo-Ocular Reflex Tested?

Some of the methods that are used to test the vestibulo-ocular reflex are:

• Standard Head Impulse Test: In this test, an individual is asked to view a fixed target. The head of the individual is made to move unpredictably in any direction. During each movement, the eye response is noted, and the gaze will remain fixed on the target if the individual has a normal vestibular system. But in individuals without vestibular function, the eyes will move to follow the head movement, and at the end of the movement, the individual has to return to gazing at the target.

• Curthoys and Halmagyi Head Impulse Test: This test was developed by Curthoys and Halmagyi in 1988 to detect the functioning of semicircular canals. All components of the vestibular organs can be assessed with less effort. A video camera with high resolution and high sensitivity is placed in front of the individual's face and connected to the computer, which analyzes the image.

• Rotational Chair Testing: This test is done to check whether the impairment of balance is caused by vestibular or neurological problems. In this test, the patient is made to sit in a chair that is connected to an apparatus without any gaze fixation. When the chair starts rotating, eye movement is observed.

What Causes the Absence or Dysfunction of the Vestibulo-Ocular Reflex?

Vestibulo-ocular reflex is frequently associated with other vestibular problems. The dysfunction or absence of the vestibulo-ocular reflex is associated with many disorders and syndromes. Some of them are:

• Acute Vestibular Syndrome: It starts suddenly with symptoms like nausea, vomiting, and continuous vertigo, which may last for more than 24 hours. Acute vestibular syndrome can occur due to conditions like stroke and neuritis. Patients usually experience signs like an abnormal vestibulo-ocular reflex, nystagmus, abnormal gait, and hearing loss.

• Nystagmus: It is defined as the to-and-fro motion of the eyes rhythmically. The vestibulo-ocular reflex is disrupted in nystagmus. It can be either acquired or congenital. Congenital nystagmus persists throughout life. Even though it cannot be cured completely, it can be managed to a certain extent. Acquired nystagmus occurs due to a head injury, stroke, etc. In most cases, it is temporary.

• Oscillopsia: It is a vision problem that causes the illusion of objects jumping or vibrating when they are not. Oscillopsia can be caused by stroke, meningitis, brain injuries, etc.

• Gaucher’s Disease: It is a metabolic disorder caused by the buildup of certain types of fatty acids in organs like the spleen and liver. Recent research has discovered a link between Gaucher's disease and the vestibulo-ocular reflex.

• Multiple Sclerosis: It is an autoimmune disorder that affects the protective covering of nerves. During progression, multiple sclerosis can affect vision and balance. It can have an impact on the vestibulo-ocular reflex over time.

• Head Injury: It can cause vertigo, blurred vision, nausea, and problems with balance. In some cases of traumatic head injuries, the vestibulo-ocular reflex becomes dysfunctional. In severe traumatic head injuries, the vestibulo-ocular reflex is used to predict cerebral death.

• Herpes Simplex Virus Type 1 (HSV1): This has been found to cause vestibular neuritis. This can affect the vestibular system, which in turn causes the dysfunction or absence of the vestibulo-ocular reflex.

• Severe Alcoholism: Severe alcoholism can affect multiple organs. Using various types of head impulse tests, some researchers discovered that binge drinking alcohol can affect the vestibulo-ocular reflex.

What Is the Treatment for the Dysfunction or Absence of the Vestibulo-Ocular Reflex?

Treatment for vestibulo-ocular reflex varies according to the underlying cause.

• Vestibular rehabilitation therapy (gaze stabilization exercise): It is an exercise-based treatment where patients are made to practice head and eye movements with different activities.

• Anti-inflammatories.

• Physiotherapy.

• Immunosuppressants.

• Corticosteroids.

• Vitamin D supplements.

• Surgical treatment.

Conclusion

The vestibulo-ocular reflex aids in the perception of surroundings and objects during activities such as walking, running, bending, jumping, and so on. This reflex is also used to diagnose many vestibular and neurological disorders. The dysfunction of the vestibulo-ocular reflex due to any cause can affect an individual's life to a certain extent. A lot of diagnostic methods have been developed so far to detect it at a very early stage. With proper therapeutic and rehabilitative techniques, the dysfunction of the vestibulo-ocular reflex can be treated, which allows the affected individuals to lead healthy lives.