What is Unilateral and Bilateral Vestibular Weakness?
The typical person has a right and a left vestibular system housed within their right and left inner ears. Unilateral vestibular weakness is used to describe dysfunction of just one vestibular system in the pair. Bilateral vestibular weakness is used to describe dysfunction of both vestibular systems in the pair.
Possible causes of unilateral/bilateral vestibular weakness may include but is not limited to the following:
Toxicity of certain medications
Weaknesses related to advanced age
Unilateral and Bilateral Vestibular Weakness Symptomology:
Symptoms can vary based on cause of weakness but may include the following:
Vertigo and dizziness
Imbalance and gait abnormalities
Oscillopsia, or blurred vision while moving
These symptoms may have varying severity and duration dependent on cause.
Unilateral and Bilateral Vestibular Weakness Testing:
Unilateral and bilateral vestibular weakness may be revealed during caloric testing, an integral part of videonsytagmography (VNG). During caloric testing, the vestibular system is stimulated by “irrigating” both ears with warm and cool air. When warm air is used to irrigate the ear, endolymph within the horizontal canal becomes lighter and flows upward. When cool air is used to irrigate the ear, endolymph in the horizontal semicircular canal becomes heavier and flows downward. This flow of endolymph results in the patient feeling as if his/her head moving. When a patient experiences this spinning sensation, or vertigo, the eyes reflexively move in a specific pattern called nystagmus. It is normal to experience vertigo and nystagmus during caloric testing. The intensity of the nystagmus is measured to determine the strength of the vestibular response at each ear. A unilateral vestibular weakness is revealed when one ear’s response is weaker than expected. A bilateral vestibular weakness is revealed when the sum of responses from both ears is weaker than expected.
vHIT assesses the integrity of the vestibulo-ocular reflex (VOR) relative to each semicircular canal. For more information regarding the VOR and its relationship with the semicircular canals please refer to the vestibular anatomy section. During vHIT, the patient wears goggles that are used to measure eye movements. The provider instructs patient to keep eyes focused on a visual target and then moves the patient’s head in short, quick motions. The measurement of eye movement is then compared to head movement. In a patient with normal VOR function, eye movement should be equal but opposite of head movement, allowing the patient’s eyes to stay focused on the visual target despite head movement. In a patient with impaired VOR function, patient’s eyes will lose focus on visual target when head is moved. vHIT may reveal impaired VOR function related to the side of weakness.
Unilateral and Bilateral Vestibular Weakness Management:
Unilateral vestibular weakness may resolve itself as the brain completes the compensation process. The more the patient moves, the more vestibular input to the brain, resulting in faster compensation. The compensation process is inhibited in cases of bilateral vestibular weakness because there is no “normal” input for the brain to reference. Both unilateral and bilateral vestibular weaknesses can be furthered addressed via vestibular rehabilitation therapy as prescribed by a physical therapist. Refer to this link for more information regarding vestibular rehabilitation therapy: http://www.fyzicalbalance.com/our-approach/howdoesitwork