Vestibular Schwannoma (CPA)
- A vestibular schwannoma is a benign tumor growing off the vestibulocochlear nerve (CN VIII), the cable that carries hearing and balance.
- It loves the cerebellopontine angle (CPA) — the little wedge of space between the brainstem, the cerebellum, and the petrous bone — and it slips into the internal auditory canal (IAC), the bony tunnel that carries that nerve.
- The classic clue is asymmetric sensorineural hearing loss, and the test that finds it is a thin-slice, contrast-enhanced MRI of the IACs.
- The cinnamon-roll giveaway: a tumor centered on the IAC that widens the canal and forms an "ice cream cone" — the scoop in the CPA, the cone in the canal.
- The main lookalike is a meningioma; the presence of a dural tail, the angle the mass makes against the bone, and where the mass is centered usually settle the argument.
Somewhere behind your ear is a tiny bony tunnel, and through it runs the nerve that lets you hear and keeps you from tipping over when you stand up. A vestibular schwannoma is a slow, polite, benign tumor that decides to set up camp on that nerve and slowly fill the tunnel. It doesn't invade, it doesn't metastasize — it just grows, like a houseguest who never invades your fridge but also never, ever leaves.
What it actually is (and the name nobody uses anymore)
The tumor grows from Schwann cells — the insulation wrapped around the nerve, like the rubber sleeve on a phone charger. Specifically it usually sprouts from the vestibular part of the eighth cranial nerve, which is why "vestibular schwannoma" is the accurate name. You'll still hear people say acoustic neuroma, but that's a double misnomer (it's rarely acoustic, and it's not a neuroma), so the field quietly retired it. Use the joke if you want, but write the right term in the report.
It's the most common mass in the cerebellopontine angle — that triangular cul-de-sac of cerebrospinal fluid (CSF) tucked between the pons, the cerebellum, and the back of the petrous temporal bone. Most of the CPA's traffic is just CN VII and VIII heading into the IAC, so most CPA masses are tumors that grow off those nerves or the dura nearby.
Why the patient shows up
The hallmark is asymmetric sensorineural hearing loss — one ear quietly fading while the other works fine. There may be tinnitus (ringing) or unsteadiness too. The balance nerve is the one being squeezed, but the brain compensates for slow balance loss so smoothly that hearing usually steals the spotlight.
Any adult with one-sided sensorineural hearing loss earns a hard look at the IACs. The single most useful thing you can do is compare the two sides — symmetry is your friend, and a nerve that looks thicker than its twin is the whole game.
How to find it on MRI
This is an MRI story. The workhorse is a high-resolution, heavily T2-weighted sequence through the IACs (the kind that makes CSF bright white), so the nerves show up as dark threads swimming in a bright pool. A schwannoma is a filling defect in that pool — the spot where the bright CSF goes missing because something solid is sitting there.
Then you give gadolinium, the MRI contrast agent, and the tumor lights up. Vestibular schwannomas enhance avidly and usually fairly uniformly, though bigger ones can develop cystic, non-enhancing pockets. If you only remember one shape, remember the ice cream cone: the rounded scoop sits in the CPA cistern, and the cone tapers into the widened internal auditory canal.
If MRI is off the table — say, a non-compatible implant — a thin-section temporal bone CT can show a widened porus acusticus (the bony mouth of the IAC), which hints the canal is being stretched from inside. But CT can't see the tumor's soft tissue the way MRI can. (For the bony anatomy of this region, the temporal bone page is the place to go.)
The lookalike that trips everyone up
The big differential is a meningioma — a tumor of the dural lining that can also park in the CPA. Both enhance brightly, so you have to read the body language.
| Feature | Vestibular schwannoma | Meningioma |
|---|---|---|
| Center of mass | Centered on the IAC, extends into the canal | Centered on the dura next to the canal; eccentric to it |
| Effect on the canal | Widens / flares the porus acusticus | Usually leaves the canal alone |
| Angle with the bone | Acute angle, "ice cream cone" | Broad base, obtuse angles against the bone |
| Dural tail | Absent | Often a "tail" of enhancing dura |
A dural tail and a broad, obtuse base against the petrous bone point to meningioma, not schwannoma. And don't forget the third CPA classic: an epidermoid, which looks like CSF on routine sequences but does NOT suppress on diffusion-weighted imaging — it stays bright when true CSF goes dark.
Two-sided tumors mean something more
If you see vestibular schwannomas on both sides, stop and think about neurofibromatosis type 2 (NF2) — a genetic condition where these tumors are essentially the defining feature. Bilateral disease in a younger patient isn't a coincidence; it's a diagnosis, and it changes the whole conversation about screening and family.
The takeaway
When an adult loses hearing in one ear and the audiogram says the problem is in the nerve, the picture you want is a contrast-enhanced MRI of the IACs — and the thing you're hunting is an enhancing ice cream cone widening the bony canal. Find the cone, check it isn't a meningioma's broad-based scoop, peek at the other side for NF2, and you've told the whole story. For a broader tour of how these enhancing brain and CPA masses sort out, swing by meningioma and the approach to brain MRI.