Bilateral vestibular impairment is a major handicap like deafness or blindness, and typically presents with gait unsteadiness and oscillopsia, with vertigo being less prominent.  Vestibulotoxic antibiotics and autoimmune disease are important causes and the diagnosis relies on the use of simple bedside tests including the head impulse test, dynamic visual acuity and a Romberg test on foam rubber.

There are held to be a number of different subtypes:

(1) Recurrent vertigo and bilateral vestibular loss: this clinical subtype is characterized by episodes of vertigo, followed by the development of symptoms of bilateral vestibular function loss.

(2) Rapidly or slowly progressive forms of bilateral vestibular loss: patients may experience a sudden onset or rapid progression in the course of bilateral vestibular loss, or the opposite situation, where clinical symptoms develop gradually, mostly without episodes of vertigo.

(3) With neurological deficits: the clinical symptoms of bilateral vestibular loss are combined with neurological deficits, such as peripheral polyneuropathy and/or cerebellar ataxia, with disabling gait ataxia, abnormal eye movements and cerebellar atrophy on imaging.

(4) A primary otological group with hearing affected; cerebellar dysfunction and peripheral neuropathy are rarely detected. 

Criteria³

From: Strupp M, Kim JS, Murofushi T, Straumann D, Jen JC, Rosengren SM, Della Santina CC, Kingma H. Bilateral vestibulopathy: Diagnostic criteria Consensus document of the Classification Committee of the Bárány Society. J Vestib Res. 2017;27(4):177-189. 

Symptoms

Core symptoms are³:

• Imbalance (91 and 86%)
• Chronic dizziness (58 and 62%)
• Oscillopsia (50 and 70%),
• Recurrent vertigo (33 and 67%)

The core symptoms of bilateral vestibulopathy (BV) are motion-dependent postural vertigo with unsteady gait and impaired balance in virtually all patients, particularly in darkness and on uneven ground, representing vestibulospinal impairment⁴.  Patients are typically symptom free when sitting or lying.  Bilateral vestibular loss may present with or without vertigo and hearing loss: as with acute vestibular neuropathy, the cochlea may be spared. Vertigo and nystagmus depend on an imbalance between the vestibular receptor inputs, and if the process is symmetrical, such an imbalance will not develop. Rotational vertigo may be a common feature early on, but equally, may abate as the lesions become chronic¹.

Although acute (typically unilateral) vestibular dysfunction receives the most attention, slowly decreasing or relatively stable but permanent function loss is more frequent (eg, aging) and, despite central compensation and sensory substitution, leads to a wide range of other complaints due to the impaired ability of the normally extremely sensitive labyrinthine sensors to detect head motion and head orientation relative to gravity.  Commonly, vision is used for sensory substitution, and in bilateral vestibular failure, during visual motion stimulation patients have enhanced activity in the visual cortex bilaterally, including the motion sensitive areas MT/V5, representing the substitution of the missing vestibular input by the visual system².  Significant decreases in gray matter in the mid- and posterior hippocampal regions associated with functional changes with delayed spatial learning performance and higher spatial anxiety are found³.

These symptoms are described in an article called, "Living Without a Balance Mechanism", published in the New England Journal of Medicine in 1954 and written by a physician named J.C. who himself lost labyrinthine function as a toxic side effect of an antibiotic.  Initially, J.C. could not read without the words on the page bouncing up and down about once per second due to the transmission of the pulse of his heart to his head. Without a labyrinthine sense, his brain could not compensate even for the small head bobs produced by the pumping of the heart. At first, J.C. had to hold his head still between his hands when he tried to read from a book. Crawford J. Living without a Balancing Mechanism. Br J Ophthalmol. 1964;48(7):357-360. doi:10.1136/bjo.48.7.357

Due to the redundant sensorimotor control of posture, the visual system can basically substitute for any defective regulation of postural control in light.  In addition, the somatosensory system also contributes to the maintenance of balance, above all via the muscle spindle afferents and the mechanoreceptors of the skin.  If the contribution of the visual system is reduced (whether in darkness or due to visual disorders), gait imbalance increases and patients will develop a tendency to fall. This problem will be further intensified if the patient walks in the dark over uneven or springy ground, or has a sensory polyneuropathy.

In about half of patients with BV there is associated oscillopsia and blurred vision when walking or moving the head (due to impairment of the VOR with involuntary retinal slip⁵). The patient is unable to make compensatory eye movements for head and body movements and, thus, is unable to maintain a visual target on the fovea⁶. Oscillopsia is also reported during trivial tasks such as reading or watching television.  Conversely, when head movements are slow, the smooth-pursuit system is able to sufficiently stabilize the gaze in space, and no illusory movement or blurriness occurs.  
Examining a patient with oscillopsia, it is important to ask whether the oscillopsia is present with or without head movements. Oscillopsia that is present during head movements usually occurs in patients with bilateral peripheral lesions of the vestibular end-organs or nerves, or in patients with central vestibular dysfunction as a result of lesions in the brainstem or cerebellum.
Oscillopsia that is present independent of whether the head is stationary or moving is commonly seen in nystagmus (vestibular (both central and peripheral), with eccentric gaze (Gaze-evoked, centripetal, rebound, dissociated, and Bruns’ nystagmus), acquired pendular) and saccadic dyskinesia (intrusions and oscillations).

Prognosis

Unfortunately 80% of patients with the condition do not improve and the prognosis is therefore poor.

Causes

Half of patients are idiopathic.  The commonest causes include the following:
Conditions which affect BOTH cerebellar and vestibular systems are in bold.

1. Vestibulotoxic antibiotics (especially gentamicin); furosemide, cisplatin, aspirin, alcohol, vitamine-B12 deficiency, folate deficiency, hypothyroidism.
2. Autoimmune ear diseases: Cogan's syndrome, Susac (Susac, Sarcoïdosis, Wegener’s, Sjögren, colitis, celiac disease, polyarteritis nodosa, antiphospholipid syndrome)
3. Meniere's disease, otosclerosis, bilateral labyrinthitis,
4. Meningitis, cerebellitis
5. Multisystem atrophy
6. Cerebellar Ataxia, Neuropathy,and Vestibular Areflexia Syndrome (CANVAS), which consists of bilateral vestibulopathy (dysfunction of all semicircular canals) in combination with cerebellar ataxia.  There are associated oculomotor disturbances (gaze evoked, and a sensory axonal polyneuropathy. The lesion must damage the vestibular nuclei (affecting the VOR) as well as the cerebellum, giving rise to a smooth pursuit deficit⁷.
7. Neurofibromatosis type 2
8. Hereditary ataxias  (such as Friedreich's ataxia and spinocerebellar ataxia SCA 3 and 6)
9. Episodic ataxia type 2
10. Superficial siderosis
11. Amiodarone

Examination
This serves chiefly to identify loss of vestibulo-ocular and vestibulospinal functions.

1. Head Impulse Test: catch-up saccades appear with head impulses to either side in bilateral vesibulopathy due to reduced or absent function of the vestibulo-ocular reflex (VOR): bilateral pathological HIT
2. Smooth pursuit should be normal, unless there is associated cerebellar dysfunction 
3. Rebound and downbeat nystagmus may be found
4. Saccades may be abnormal
5. In CANVAS, there is an impaired visual VOR, with inability to stabilize gaze at slow frequencies (below 1 Hz) which indicates double pathology involving both the vestibular as well as the cerebellar pathways⁷. VOR suppression is normal, since the VOR is effectively absent: there is therefore an unusual dissociation of normal VOR suppression with abnormal smooth pursuit
6. Dynamic visual acuity: a decline of more than two lines is considered abnormal, but severely affected patients may show a decline of five or more lines.
7. Loss of vestibulospinal function: unsteadiness of gait, with broad stance, and patients may function better when running than with walking.  Increased body sway during the Romberg test, which becomes more obvious during tandem standing and walking. Proprioceptive loss is a risk factor for falling and is best identified by carrying out the Romberg test on foam³.

 

Differential Diagnosis³

Central lesions or the combination of central and peripheral lesions with selective canal involvement can mimic BV.

• Acute floccular lesions may lead to isolated bilateral horizontal SCC impairment,
• Llesions of the vestibular nuclei can affect horizontal and posterior SCC function only.
• In Wernicke encephalopathy, a specific pattern of both horizontal SCC involvement is found, most likely related to enhanced vulnerability of the medial vestibular nuclei neurons because of thiamine deficiency.

Treatment

1. Explanation of the function of the VOR and the consequences of abnormalities of the VOR.
2. Balance training: daily, permanent and lifelong (noting that during balance training, patients will tend to feel worse, that being the purpose of the balance training).
3. Vestibular exercises include those that elicit the VOR (i.e., head turning in the horizontal plane for 1 min while staring at a finger held at eye level, and repeated in the vertical plane). The exercises were performed first sitting and then standing, four to five times per day. Bedridden subjects may be encouraged to begin exercises lying down⁸.

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(vv)Balance Movie.mp4(tt)

 

From: Kingma H.  Clinical Relevant Knowledge of the Vestibular System. 2016. From: https://www.youtube.com/watch?v=yB850dIIGIY

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Normal compensatory eye movements with slow head movements, indicating the importance of the visual system with slow head movements.

(vv)Halmagyi.mp4(tt)

Halmagyi GM. Clinical Examination of the Vestibular System. J Vestib Res. Teaching Course, 29th Bárány Society Meeting, Lecture 2, June 5, 2016, Seoul, Korea. From: https://www.youtube.com/watch?v=ehR7SOlBBow

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Bilateral vestibulopathy secondary to gentamycin toxicity.

(vv)Halmagyi2.mp4(tt)

From: Halmagyi GM. Clinical Examination of the Vestibular System. J Vestib Res. Teaching Course, 29th Bárány Society Meeting, Lecture 2, June 5, 2016, Seoul, Korea. From: https://www.youtube.com/watch?v=ehR7SOlBBow

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Patients are characterized by complete absence of compensatory eye movemens, either visual or vestibular. The test carried out is the VOR.

(vv)CANVAS.mp4(tt)

From: Halmagyi GM. Clinical Examination of the Vestibular System. J Vestib Res. Teaching Course, 29th Bárány Society Meeting, Lecture 2, June 5, 2016, Seoul, Korea. From: https://www.youtube.com/watch?v=ehR7SOlBBow

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(vv)CANVASZee.mp4(tt)

From: Zee DS. The cerebellum for the neuro-ophthalmologist: A video tutorial of some signs and syndromes to recognize  NANOS 42nd Annual Meeting 2016. Retrieved from: https://collections.lib.utah.edu/ark:/87278/s6nw2r5g

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