CI Atlas · Preoperative Imaging · Module 09

9Ossification & fibrosis — the patency question

Meningitis can do something uniquely cruel to the cochlea: it fills the very channel the electrode must enter with first fibrous tissue and then new bone, and it does so on a clock that races the surgeon. The earliest, fibrous stage of this labyrinthitis ossificans is invisible to CT — there is no bone yet to see — but it shows on MRI as a quiet loss of the bright fluid signal, which is why the two modalities are read together and why a normal CT can falsely reassure. Where the bone has spread, a patency grade translates directly into a surgical manoeuvre, from a normal cochleostomy through a drill-out to a split array. The right response to ossification is almost never exclusion; it is to implant early, choose the better ear and the right array, and not to wait. This module is about reading patency.

Imaging note

Representative CT and MRI images for this chapter are being added soon. The interactive figures here are original schematic teaching diagrams; to respect copyright we do not reproduce third-party radiographs.

TLabyrinthitis ossificans

Labyrinthitis ossificans is pathological ossification of the membranous labyrinth after a meningogenic, tympanogenic, haematogenic or post-traumatic insult. Post-meningitic LO usually begins in the scala tympani of the basal turn near the round window (where the cochlear aqueduct enters) and spreads apically, through acute → fibrous → ossific stages — neo-ossification begins ~8 days to weeks, becomes CT-detectable by ~2 months, and can continue up to ~30 years.

CMRI sees it before CT

For patency, MRI beats CT: a loss of the bright T2 fluid signal (± contrast enhancement acutely) detects the early fibrous stage before CT shows any bone — a loss of lateral-SCC fluid signal is an early warning. CT misses early obstruction in 15–50% of candidates; T2 MRI for patency is ~94% sensitive.[1987][1990]

Ossification passes through acute → fibrous → ossific stages. The fibrous stage obliterates the lumen before any bone forms, so the bright T2 fluid signal fades on MRI while CT still looks normal — CT misses early obstruction in 15–50% of candidates. MRI for patency is ~94% sensitive vs CT's variable 53–90%, and a loss of lateral-SCC fluid signal is an early warning. Because ossification progresses (and can do so up to ~30 years post-insult), the finding drives early implantation and immunisation. Schematic.

CPatency grades & surgical stages

The Balkany–Dreisbachpatency grades — C0 normal, C1 indistinct basal endosteum, C2 definite basal narrowing, C3 bony obliteration — and the surgical stages (I round window → II inferior basal turn drill-out → IIIa 180–360° scala-vestibuli or compressed/split array → IIIb >360° total drill-out or ABI) translate how far the bone has spread into what the surgeon does.[1988]

Labyrinthitis ossificans usually begins in the scala tympani of the basal turn near the round window (where the cochlear aqueduct enters) and spreads apically. The Balkany–Dreisbach grades (C0–C3) and surgical stages (I–IIIb) translate how far the bone has spread into what the surgeon must do — from a normal cochleostomy, through drill-out and scala-vestibuli or compressed/split arrays, to a total drill-out or ABI. Because significant spiral-ganglion neurons survive even with bony occlusion, ossification drives early implantation and array choice — not exclusion. Schematic.

CNot exclusion — early implantation

Because significant spiral-ganglion neurons surviveeven with bony occlusion (Module 8's neural reserve), ossification drives early bilateral implantation, ear selection (the least-ossified scala tympani) and array choice (compressed, split or double arrays) — not exclusion.[1984] Because LO progresses, a post-meningitic candidate is implanted promptly, and pneumococcal immunisation ≥2 weeks before surgery is stressed.

Case 12.9 · CT looks normal after meningitis

Six weeks after meningitis, a candidate's CT shows a normal-looking cochlea, but the team suspects early obstruction.

What is the best next step?

Self-assessment — Module 82 questions

Question 1 · Trainee

Why does MRI beat CT for early post-meningitic obstruction?

Question 2 · Clinician

What is the right response to cochlear ossification?

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