Canalith
Canalith
You turn onto your left side and the ceiling slides. Ten, twenty, thirty seconds of the room moving without you, then it stops as suddenly as it started. You lie very still. Nothing hurts. Nothing else is wrong. And the next night it happens again, on the same side.
People describe this to a doctor in almost identical words, which is a clue in itself. The trigger is a movement, not an event. It is short. It is repeatable. And it is fussy about position in a way that no general feeling of unwellness ever is. That specificity is what makes positional vertigo one of the more tractable problems in the dizziness family, and it is worth understanding what is actually moving inside your head when the room appears to.
Behind each eardrum, past the small bones, sits a structure the size of a fingernail that does two unrelated jobs. One is hearing. The other is telling your brain which way is up and how your head is moving.
The balance half is built from two kinds of sensor. The first is a set of three fluid-filled loops, the semicircular canals, set at roughly right angles to one another, one for each plane in which a head can turn. When you nod, or shake your head, or tip it towards a shoulder, the fluid in the relevant loop lags behind the bone for a moment. That lag bends a small gelatinous flap at the base of the loop, and bending the flap is what the nerve reports: we are rotating, this fast, in this plane.
The second is a pair of patches called the otolith organs, which sense gravity and straight-line movement rather than rotation. They manage this by being weighted. Sitting on top of the sensing cells is a layer of tiny calcium carbonate crystals, so the patch is heavy and gravity tugs on it, and the tug tells your brain which way down is. These crystals are the reason the whole system works, and the reason it fails.
The crystals are shed and replaced throughout life. Usually the loose ones are reabsorbed and nothing happens. Occasionally, some of them drift out of the patch where they belong and fall into one of the semicircular canals, where they very much do not belong. A canal is a rotation sensor. It is calibrated on the assumption that only fluid moves inside it.
Now consider what happens when you roll over. The canal turns with your skull, and gravity pulls the loose crystals down through the fluid to the new lowest point. Dragging debris through the canal drags fluid with it, and that fluid deflects the sensing flap — the same deflection that means rotating. Your ear reports a strong rotation that is not happening.
Meanwhile the ear on the other side, which has no debris in it, reports the truth: you tipped over, briefly, and stopped. Your eyes agree with the second ear. So does the pressure on your skin. Your brain receives one loud, confident, wrong signal and several quiet, correct ones, and for a few seconds it cannot reconcile them. It resolves the conflict the only way it knows: by moving your eyes, as if to track a world that is turning. The spinning you see is your own eyes being driven by a false rotation signal. This eye movement, called nystagmus, is also what a clinician looks for during a positional test, because its direction and timing say a great deal about which canal is involved.
The crystals fall until they reach the lowest point available, and then they stay there. The fluid settles. The flap returns to neutral. The false signal ends — which is why an episode of this kind is usually measured in seconds rather than hours, and why lying perfectly still ends it faster than anything else you can do.
It also explains the short delay many people notice: a second or two of nothing after the movement, and then the room goes. That is the time it takes for the debris to start moving and for the fluid to catch up.
And it explains the pattern of triggers. Rolling over in bed. Lying down. Sitting up. Tipping your head back to rinse shampoo out, or to look at a high shelf, or into the dentist's chair. Bending to load a washing machine. What all of these share is that they change the orientation of your head relative to gravity. Walking briskly across a room does not, and generally does not trigger anything.
Two words get used interchangeably in conversation and should not be. Dizziness is a broad word covering lightheadedness, wooliness, feeling faint, and being unsteady on your feet. Vertigo is the specific illusion of movement — you or the room turning, tilting or sliding. Telling a clinician which one you mean, in those terms, narrows the field immediately.
Within vertigo, duration and trigger separate the common causes more than severity does. Seconds, triggered by a change of head position, is the classic positional pattern. Minutes to hours, often with headache, light sensitivity or a history of migraine, points elsewhere. Hours with hearing changes and a full or roaring ear points elsewhere again. Days of constant vertigo that is present even when you hold still is a different picture entirely. None of these are things to sort out yourself, but knowing that the axis is how long and what set it off — not how bad — will change what you notice and what you write down.
The details that matter to a diagnosis are exactly the details an episode destroys. In the middle of a spin, nobody is checking the clock. Afterwards you remember that it was awful and roughly when it happened, and by the appointment six weeks later you have three episodes blurred into one impression.
A diary fixes a small number of facts at the time: the moment it started, roughly how long it lasted, which movement set it off, which side you had turned towards, and whether anything came with it. Over a few weeks that turns into a shape — one side or both, mornings or evenings, clustered or evenly spread — and that shape is far more useful to a clinician than any single description. Our next post covers what to record and why, field by field.
If a repositioning procedure has already been discussed with you, you may also want to read how Epley, Semont and Brandt-Daroff differ in principle — a conceptual explainer, not instructions to follow at home.
This article explains a mechanism; it is not medical advice and it is not a diagnosis. Several conditions cause vertigo and they cannot be distinguished from a description on a website. See a clinician about new or recurring vertigo, and treat it as urgent — emergency care, the same day — if vertigo comes on suddenly together with any of: double vision, weakness or numbness on one side, trouble speaking or swallowing, a severe or unusual headache, difficulty walking or a new inability to stand, or new hearing loss in one ear.
Canalith is a vertigo and dizziness diary for iPhone and Apple Watch: log an episode with one drag of a dial, have the barometric pressure and weather recorded with it, follow the exercise your clinician prescribed, and export a one-page report for your appointment. It records and guides; it does not diagnose or treat. The app is not on the App Store yet — join the waitlist on the home page.