6One Chamber for Everything: The Common Cavity

FWhat a common cavity is

A common cavity is a single ovoid chamber in which the cochlea and vestibule are confluent, with no internal partition and no interscalar septum. It reflects arrest around the fourth-to-fifth gestational week, at the otocyst stage, before the cochlear and vestibular components differentiate. Radiographically it is an empty ovoid space, classically longer horizontally than vertically, with no modiolus. It accounts for a sizeable minority of cochlear malformations and hearing is usually, but not always, poor.[1987][2010]

TWhere the neural tissue lives

Because there is no modiolus, neural elements are distributed unpredictably around the wall of the cavity rather than in an organised central core. An electrode aimed at a central modiolus would find nothing; stimulation must reach receptor/neural tissue scattered along the periphery. This favours an array that lies against the cavity wall - a straight array hugging the wall, or a ring/custom array designed to contact the perimeter. The cochlear nerve may be present, hypoplastic, or absent, and MRI nerve assessment remains essential before committing to implantation.[1997][2010][2017]

CThe surgical problem: gusher, facial nerve, and access

Opening a common cavity carries a high risk of perilymph/CSF gusher because the cavity often communicates widely with the internal auditory canal. The facial nerve frequently follows an aberrant course over or near the cavity, so the standard facial-recess approach can endanger it. McElveen and colleagues described a transmastoid labyrinthotomy that approaches the cavity directly and laterally, circumventing the facial recess and an aberrant nerve. Gusher is managed by a small, snug cochleostomy/labyrinthotomy tightly sealed with connective tissue, and the array is positioned to contact the wall along its length.[1997][2005][2010]

COutcomes and what a common cavity is not

Outcomes are variable but real: many common-cavity children gain useful auditory benefit, though results are generally more modest than in normally formed cochleae. It is distinguished from cystic cochleovestibular malformation, in which the cochlea and vestibule remain identifiable as separate cystic structures rather than a single undivided cavity. It is distinguished from cochlear aplasia with a dilated vestibule, where the cochlea is absent but the vestibule is a separate structure rather than confluent with a cochlear component. Accurate categorisation matters because it changes both implantability and the array and approach chosen.[2017][2012][2005]