14Microvascular Decompression of the Cochleovestibular Nerve

FThe neurovascular conflict hypothesis

Just as a loop of artery pressing on the trigeminal nerve can cause trigeminal neuralgia, and on the facial nerve can cause hemifacial spasm, a vessel resting against the cochleovestibular (eighth) nerve at its root-entry zone has been proposed to cause auditory and vestibular symptoms [1991]. The idea is that pulsatile contact at the transition between central and peripheral myelin produces ectopic, ephaptic discharges that the brain hears as sound.

The usual culprit is a loop of the anterior inferior cerebellar artery (AICA). The hypothesis is attractive because it offers a discrete, surgically correctable target — but most vascular loops touching the eighth nerve on imaging are entirely asymptomatic, which is the central difficulty in selecting patients [2017].

TThe typewriter-tinnitus phenotype

The one subtype with a reasonably coherent story is ‘typewriter’ tinnitus: brief, staccato, machine-gun or Morse-code-like bursts, often intermittent, that patients describe as clicking or popping rather than a steady tone. Levine characterised it as a carbamazepine-responsive syndrome attributed to vascular compression of the eighth nerve [2006].

The diagnostic clue with real predictive value is the dramatic response to a sodium-channel blocker such as carbamazepine or oxcarbazepine — the same drug class that quiets trigeminal neuralgia. Auditory brainstem response can show abnormalities consistent with a conduction problem in the nerve and may carry prognostic information [2023]. A carbamazepine-responsive typewriter phenotype with a matching vascular loop is the closest thing to a sound indication.

CPatient selection: who (rarely) qualifies

Microvascular decompression is justifiable only when several criteria converge: a typewriter/paroxysmal phenotype, demonstrable medical response (so that the drug both treats and confirms the diagnosis), high-resolution imaging (3D-CISS/FIESTA, MR angiography) showing a vessel in genuine contact with the symptomatic nerve, and exclusion of the far more common subjective and vascular-venous causes [2006].

Patients with steady-tone subjective tinnitus, no medication response, or merely an incidental vascular loop should not be offered surgery; the loop is probably a bystander [2017]. Because carbamazepine often controls the symptom medically, many appropriate candidates never need an operation at all — surgery is reserved for those who respond to the drug but cannot tolerate it long-term [2015].

TThe operation and what it can achieve

The procedure is a retrosigmoid craniotomy that exposes the cerebellopontine angle. The offending vessel is identified and gently mobilised off the eighth-nerve root-entry zone, and a non-absorbable cushion (typically Teflon felt) is interposed to prevent re-contact. Intraoperative brainstem auditory evoked potential monitoring is used to protect hearing [2010].

In carefully selected, predominantly typewriter/paroxysmal cases, case reports and small series describe complete or near-complete relief [2015]. De Ridder and colleagues reported significant improvement in a selected vascular-compression cohort [2010]. Crucially, these outcomes do not generalise to ordinary subjective tinnitus.

CThe risks that limit it

MVD is real intracranial surgery on a nerve that carries hearing and balance. The principal hazards are sensorineural hearing loss and vestibular disturbance from manipulating the eighth nerve, plus the generic risks of posterior-fossa surgery: cerebrospinal-fluid leak, infection, facial-nerve injury and, rarely, more serious neurological events [2010].

A systematic review found the overall tinnitus literature for MVD to be low-quality, heterogeneous and dominated by small uncontrolled series, with results far better in the paroxysmal subtype than in continuous tinnitus [2017]. The honest summary is that MVD can help a very narrow phenotype but should never be offered to unselected tinnitus patients, in whom the risk of worsening hearing outweighs an uncertain chance of benefit.