Understanding Bilateral Vestibular Loss
Bilateral vestibular loss, often abbreviated as BVL, refers to absent or severely reduced vestibular function on both sides. Patients commonly report oscillopsia, meaning that the visual world appears to bounce or shift during head movement. They may also experience unsteadiness, particularly when walking in the dark or on uneven ground, because they cannot rely on vestibular input to stabilise their gaze and posture. The difficulty with balance often becomes more apparent during activities that simultaneously remove vision and destabilise the support surface.
Before testing begins, a careful history is important. The clinician will ask about the duration of symptoms, possible triggers, prior exposure to ototoxic medications, episodes of meningitis, or a history of autoimmune disease. However, diagnosis cannot rest on history alone. Objective testing is required to confirm that the deficit is bilateral, to estimate its severity, and to distinguish it from other balance disorders.
The Role of Vestibular Testing
Vestibular testing provides a structured way to measure the function of the semicircular canals and the otolith organs. In bilateral vestibular loss, the main goal is to determine whether responses are reduced or absent on both sides. A single abnormal test result is not enough for a confident diagnosis. Ideally, the clinician will use a combination of tests that assess different vestibular reflexes and different frequency ranges of head motion.
Caloric Testing
Caloric testing is one of the oldest and most widely available tests of vestibular function. The clinician slowly infuses warm or cool water, or air, into the external ear canal. This creates a temperature gradient that stimulates the horizontal semicircular canal on that side. The resulting nystagmus is measured, often with video goggles. In a healthy person, warm irrigation causes nystagmus toward the irrigated ear and cool irrigation causes nystagmus away from it.
In bilateral vestibular loss, both ears show markedly reduced or absent responses to warm and cool irrigation. Caloric testing is particularly useful for detecting severe and longstanding losses because it stimulates the lateral semicircular canal at a very low frequency. It also compares the left and right ears separately, which can reveal asymmetry when only one side is affected. However, caloric testing has limitations. It evaluates only the horizontal canal at low frequencies, and it can be unpleasant for the patient. Care must also be taken in patients with eardrum perforation, in whom water irrigation is contraindicated.
Video Head Impulse Testing
Video head impulse testing, known as vHIT, assesses the high-frequency angular vestibulo-ocular reflex. The patient wears lightweight video goggles that contain a camera and a gyroscope. The clinician asks the patient to fixate on a stationary target and then applies brief, rapid, passive head turns to the left and right. Any corrective saccade after the head movement indicates that the vestibular system failed to keep the eyes on the target.
The key measurement from vHIT is the gain of the vestibulo-ocular reflex, which is roughly the ratio of eye velocity to head velocity. A normal gain is close to one. In bilateral vestibular loss, gains for both leftward and rightward head impulses are abnormally low, and catch-up saccades are typically present in both directions. vHIT is well tolerated, quick to perform, and can be repeated easily in a clinical setting. It tests each horizontal canal individually, and newer equipment permits vertical canal assessment as well. This makes vHIT an essential tool in the modern workup for bilateral vestibular loss.
Rotatory Chair Testing
Rotatory chair testing provides another method for evaluating the horizontal vestibulo-ocular reflex. The patient sits in a chair that rotates within a controlled environment. The chair may be moved with a sinusoidal pattern at various frequencies or delivered as a single velocity step. During rotation, eye movements are recorded, usually with video goggles or electro-oculographic electrodes.
In a healthy person, rotation of the chair stimulates the horizontal canals on both sides in a complementary way. One side is excited while the other is inhibited. The resulting eye movement is a slow-phase nystagmus that drives the eyes in the opposite direction to the head. In bilateral vestibular loss, the response to rotation is greatly reduced. There is less nystagmus, lower gain across multiple frequencies, and a shorter time constant after a velocity step. Rotatory testing is valuable because it stimulates both ears simultaneously and does not depend on an intact eardrum. However, it usually cannot determine which ear is worse, because the response represents a summed signal from both sides.
Dynamic Visual Acuity Testing
Dynamic visual acuity, or DVA, tests the functional consequence of vestibular loss. Visual acuity is first measured while the head is still. It is then measured while the patient actively moves the head, typically in the horizontal plane at a set speed. A healthy vestibulo-ocular reflex keeps images stable on the retina, so acuity changes little during head movement. In bilateral vestibular loss, the vestibulo-ocular reflex is insufficient, and the patient is not able to compensate with smooth pursuit or saccades. The result is a meaningful drop in visual acuity during head motion.
DVA testing can be performed as a formal computerised test or with a simple wall chart and a procedure called the dynamic illegible E test. It is quick, low cost, and directly linked to the patient's complaint of oscillopsia. While an abnormal DVA result alone does not prove that the loss is bilateral, it supports the diagnosis and helps grade the functional impact on daily life.
Vestibular Evoked Myogenic Potentials
Vestibular evoked myogenic potentials, or VEMPs, assess otolith function rather than canal function. Cervical VEMPs, recorded from the sternocleidomastoid muscles, reflect function of the saccule and the inferior vestibular nerve. Ocular VEMPs, recorded from the extraocular muscles, reflect function of the utricle and the superior vestibular nerve. These tests are useful because bilateral vestibular loss does not always affect all five vestibular end organs equally. Some patients have preserved otolith responses despite absent canal responses, while others show global loss.
In cervical VEMP testing, a loud click or tone burst is delivered to one ear while the patient contracts the neck muscle on that side. In bilateral loss, the ipsilateral inhibition response is absent or markedly reduced on both sides. Ocular VEMPs produce a small excitatory potential in the contralateral eye muscles, and these are also reduced or absent bilaterally in many patients. VEMPs provide complementary information about the extent of loss and help determine whether the condition is limited to the semicircular canals or involves the entire vestibular end organ system.
Posturography and Gait Assessment
Computerised dynamic posturography measures the contribution of vision, vestibular input, and somatosensation to standing balance. Platforms and visual surrounds are moved in ways that remove accurate visual or ankle support information. In bilateral vestibular loss, posturography typically shows normal stability when the patient stands on a fixed platform with eyes open, but marked sway or falls when both vision and foot support cues are removed. This pattern, sometimes described as vestibular hypofunction during sensory organisation testing, reflects the brain's reliance on the remaining intact modalities.
Additional functional assessment includes observation of gait, ability to stand on foam, and the timed up-and-go test. None of these measures is diagnostic by itself, but they document the disability and provide a baseline for rehabilitation.
Interpreting the Test Results
The diagnosis of bilateral vestibular loss rests on a convergent picture of history and objective findings. The most commonly used diagnostic criteria require bilaterally absent or markedly reduced responses on caloric testing, rotatory chair testing, or vHIT. No single numerical cut-off applies perfectly to every patient, so the clinician considers the gain values, presence of corrective saccades, response symmetry, and the functional DVA result together.
One practical issue is that the severity of loss exists along a continuum. A patient may have a severe but incomplete loss, with low vHIT gains and reduced caloric responses that are not entirely absent. Diagnosing this partial bilateral loss is more challenging, and repeat testing over time may be needed to demonstrate stability or progression. Audiological assessment should accompany vestibular testing because many causes of bilateral vestibular loss also affect hearing.
Choosing the Right Test Battery
No single test identifies every case of bilateral vestibular loss. A comprehensive battery usually combines bedside examination with instrumented testing. The clinical head impulse test is a simple bedside screen, but it can miss mild loss when a patient uses covert saccades. Formal vHIT is much more sensitive. Caloric testing provides information about very low frequency function and side-specific asymmetry, while rotatory chair offers a broader frequency view and can be performed even when the ear canal is not suitable for calories. VEMPs add otolith information, and DVA or posturography reveal the functional handicap.
The exact test battery depends on the equipment available at the clinic and the patient's ability to cooperate. Some patients cannot tolerate water calories because of tympanic membrane perforation. Others cannot complete posturography because of severe imbalance. In every case, the clinician should choose tests that answer the specific question at hand and explain the purpose of each step in clear terms.