Dizziness is a vague term than different people use to indicate a variety of experiences including lightheadedness, unsteady gait and vertigo.
Vertigo is a more precise term, which indicates a sense of false movement – if feels like the world is moving (using spinning).
To understand what causes vertigo, you first have to understand the vestibular system, and the vestibular ocular reflex (VOR).
The vestibular system is made up of 2 systems of semicircular canals, within the inner ears (or labyrinths), designed to detect head position and movement:
Each semicircular canal works like a spirit level to determine head position in it’s plane:
The VOR is a connection between the vestibular system and the muscles that move the eyes, which allows us to seamlessly compensate for every little head movement.
Life without a normally functioning VOR would be something like this:
For the sake of simplicity, we’re just going to talk about the horizontal canals.
There is tonic input to the brain from each side at rest.
When the head moves to the right, input from the right side increases, input from the left side decreases, this creates a mismatch between the two sides, and brain knows the head is moving:
The VOR then provides a reflex corrective eye movement to the left, so that the eyes keep looking in the same direction to maintain fixation:
Now, let’s say there’s a medical problem like a viral infection or a tiny stroke, that injures the L sided labyrinth, we call this labyrinthitis or acute labyrinthine failure. This reduces tonic input from the damaged L side, and will lead to the same mismatch as if the head were turning to the right side:
This, in turn, will:
1. Fool the brain into thinking the head is turning to the right side, creating an illusion of movement, “vertigo” .
2. Lead to unnecessary “corrective” eye movements to the L side, which (in the alert patient) will be followed by a voluntary eye movement back to the R to maintain fixation. This combination of a slow drift of the eyes to the underactive side, and a rapid eye movement back to the middle is know as nystagmus.
Nystagmus from L labyrinthine failure – slow eye drift to L, then rapid eye movement back to R side.
We can reproduce this phenomenon with the cold caloric – the cold water causes temporary vestibular hypofunction, vertigo and nystagmus:
Ultimately, the brain will “reset” and the vertigo and nystagmus improve. During this phase, stable visual input (which says the world is stationary) overcomes the abnormal vestibular input (which says the world is moving). You are probably aware of this phenomenon from sea and car sickness – You are less likely to get sick if you are on deck (looking at the horizon) or in the front seat of a car (looking out the window).