Bilateral Vestibulopathy

Bilateral impairment or loss of peripheral vestibular function, producing unsteadiness and oscillopsia rather than vertigo, since there is no asymmetry between the two sides to generate a sense of spinning. This absence of vertigo is precisely why the condition is missed, often for years, while patients are investigated for cerebellar and neurological disease.

Functional consequence

Loss of the vestibulo ocular reflex means that images are not stabilised on the retina during head movement, producing oscillopsia. Loss of vestibulospinal input means balance depends entirely on vision and proprioception, so patients are severely destabilised in darkness and on uneven or compliant surfaces where both of those channels are degraded.

Causes

• Ototoxicity, principally aminoglycosides, which accounts for around 15 to 20 percent. Gentamicin is the classical agent and can produce complete bilateral vestibular loss with entirely preserved hearing at conventional doses and normal serum levels, since vestibulotoxicity is not dose predictable.
• Idiopathic in around 20 to 50 percent.
• Bilateral Ménière disease.
• Meningitis, particularly bacterial.
• Autoimmune inner ear disease, including Cogan syndrome with interstitial keratitis, and systemic vasculitis.
• Neurodegenerative disease, notably cerebellar ataxia with neuropathy and vestibular areflexia syndrome, which combines cerebellar ataxia, sensory neuropathy and bilateral vestibulopathy and is commonly caused by biallelic RFC1 repeat expansions.
• Bilateral vestibular schwannomas in neurofibromatosis type 2, and after bilateral tumour surgery.
• Superficial siderosis, producing hearing loss, ataxia and vestibular loss with characteristic magnetic resonance findings.
• Sequential bilateral vestibular neuritis, which is uncommon.
• Congenital, in association with syndromic hearing loss including Usher type 1 and CHARGE syndrome, presenting as delayed independent walking.

• Unsteadiness of gait, markedly worse in the dark, on uneven ground, and when visual or proprioceptive cues are removed. Patients describe being unable to walk at night without a light and being unsteady in the shower with the eyes closed.
• Oscillopsia: the visual scene appears to bounce or jump during walking, running or driving on a rough surface. Patients report being unable to read road signs while walking or to recognise faces of people passing them.
• Absence of vertigo, unless the loss developed asymmetrically.
• Falls, and in the elderly this may be the only presentation.
• Cognitive complaints, including spatial memory and navigation difficulty, since vestibular input contributes to hippocampal spatial processing.
• Anxiety and avoidance of activity, with progressive deconditioning.
• History of aminoglycoside exposure, meningitis, or the systemic conditions. Ask specifically about intravenous gentamicin, which patients rarely volunteer.

Examination

• Bilaterally positive head impulse test, with catch up saccades on turning to both sides. This is the key bedside finding.
• Positive dynamic visual acuity: a drop of three or more lines on the Snellen chart when the head is oscillated at 2 Hz compared with the static acuity.
• Positive Romberg test, dramatically worse with eyes closed, and inability to perform tandem gait.
• Wide based unsteady gait, worse with eyes closed.
• No spontaneous nystagmus, which is expected since the loss is symmetrical.
• Examine for associated neurological signs: cerebellar signs, peripheral neuropathy with absent ankle reflexes and impaired vibration sense, and ocular abnormalities.

The diagnostic criteria require chronic vestibular syndrome with unsteadiness worse in darkness or on uneven ground, or oscillopsia during walking or head movement, plus objectively reduced bilateral vestibular function on testing.

Vestibular testing, which is required for diagnosis

• Video head impulse testing with horizontal vestibulo ocular reflex gain below 0.6 bilaterally.
• Caloric testing with a reduced response bilaterally, conventionally a sum of bithermal maximal peak slow phase velocity below 6 degrees per second on each side.
• Rotational chair testing with a horizontal gain below 0.1 at 0.1 Hz and a phase lead above 68 degrees. Rotational testing is the most sensitive method for detecting bilateral loss and can identify partial loss missed by other tests.
• At least one of these objective tests must be abnormal to establish the diagnosis.

Additional assessment

• Pure tone audiometry, since concurrent hearing loss points to specific causes and influences rehabilitation.
• Vestibular evoked myogenic potentials to assess otolith function.
• Magnetic resonance imaging of the brain and internal auditory meati to exclude bilateral schwannomas, superficial siderosis with its characteristic hypointense rim on gradient echo sequences, and cerebellar atrophy.
• Neurological assessment with nerve conduction studies for peripheral neuropathy, particularly where cerebellar ataxia with neuropathy and vestibular areflexia syndrome is suspected. Genetic testing for RFC1 repeat expansion where the triad is present.
• Autoimmune screen: erythrocyte sedimentation rate, antinuclear antibody, antineutrophil cytoplasmic antibody, and syphilis serology.
• Detailed drug history with specific enquiry about aminoglycosides, including intravenous courses during hospital admissions for other reasons.
• Full blood count, renal function, glucose, vitamin B12 and thyroid function.

There is no treatment that restores lost vestibular function. Management centres on rehabilitation using sensory substitution, prevention of further loss, and falls prevention.

Treat identifiable causes

• Stop and permanently avoid all ototoxic drugs, and flag the record clearly.
• Immunosuppression for autoimmune disease: prednisolone 1 mg/kg daily initially with steroid sparing agents under rheumatology supervision, which may arrest progression in Cogan syndrome and vasculitis.
• Treat the underlying systemic or neurological condition.
• Identify the cause of superficial siderosis, which is usually a chronic source of subarachnoid bleeding, and treat it surgically where identified.

Vestibular rehabilitation, adapted to the absence of function

The strategy differs fundamentally from unilateral loss. There is no residual function to adapt, so the goal is substitution rather than adaptation.

• Substitution training using vision and proprioception, teaching patients to use saccadic and pursuit strategies and pre programmed compensatory eye movements to reduce oscillopsia.
• Gaze stabilisation exercises are still valuable in partial loss, where residual function can be enhanced.
• Balance training progressing through conditions of reducing sensory input, always within safe limits.
• Gait training with an emphasis on visual scanning strategies and on shortening stride and widening base on uneven terrain.
• Aerobic conditioning, since deconditioning compounds the disability.
• Programmes must be prolonged, typically 6 months or more, and outcomes are more modest than in unilateral loss.

Practical adaptations and safety, which matter more here than in any other vestibular condition

• Night lighting throughout the home, particularly on stairs and the route to the bathroom, since darkness removes the visual channel on which the patient now depends entirely.
• A walking aid, which provides both support and additional proprioceptive input through the hand.
• Remove home hazards: loose rugs, trailing cables, poor lighting, and low contrast steps.
• Advise against swimming alone and against swimming underwater, where disorientation is potentially fatal because visual and proprioceptive cues are removed simultaneously.
• Advise against working at height, on ladders and with dangerous machinery.
• Driving assessment, since head movement while driving produces oscillopsia. Many patients can drive safely with adaptations, but this requires formal assessment.
• Review all medications for those that impair balance, particularly sedatives, antihypertensives and anticholinergics.
• Falls prevention: strength and balance training, vitamin D supplementation where deficient, bone density assessment and osteoporosis treatment, and a formal falls service referral.

Emerging interventions

• Vestibular implants, which deliver motion encoded electrical stimulation to the ampullary nerves, have shown restoration of a partial vestibulo ocular reflex and functional improvement in early trials and are available in a small number of specialist centres. They remain investigational.
• Sensory substitution devices delivering head position information through vibrotactile or tongue based stimulation have shown benefit in trials but are not widely available.

Psychological support and follow up

• Anxiety and depression are common and disabling. Screen for them and treat.
• The condition is frequently misattributed for years before diagnosis, and patients often derive substantial benefit simply from having an explanation.
• Regular review with objective vestibular testing to detect progression, annual falls risk assessment, and audiological review where hearing is also affected.
• Genetic counselling where a hereditary cause is identified.