Neurologist and dizziness specialist Kevin A. Kerber of the University of Michigan at Ann Arbor offers the following explanation:
In general, the most common causes of dizziness are activities everyone experiences, at least as children, namely running around in circles or riding carnival attractions that spin, loop or twist. These movements cause an asymmetry in the signals that stem from the vestibular system--a sensory system situated on each side of the head in the inner ear compartments--and that are processed in the brain. This alteration leads to the sensation known as dizziness during, and even for some time after, the provoking event.
But dizziness can also occur as an unprovoked and severe episodic or even constant occurrence--an understandable source of distress for the person experiencing it. In fact, dizziness is one of the most common reasons for a visit to a doctor's office. The first step in determining the cause of dizziness is to clarify exactly what the individual is experiencing. The most common types of dizziness are vertigo, light-headedness and imbalance.
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Vertigo refers to the sensation of being in a spinning environment. At rest, continuous and balanced signals from the peripheral vestibular system keep the eyes stationary via connections in the brain. When the head moves, a physiological imbalance in the signals leads to small movements of the eyes that keep vision optimal. When a sudden abnormality in the balance of the signals occurs, the result is a pattern of eye movements referred to as nystagmus. When the eyes move in this fashion, the world is perceived to be rotating even though the person remains still. Benign paroxysmal positional vertigo (BPPV) is a common variety of vertigo that is caused by aberrant stimulation of the vestibular system by small, displaced particles in the inner ear fluid chambers. Though particle displacement can be caused by head trauma, oftentimes we do not know what causes it. When the head is turned in certain directions, the particles shift, triggering nystagmus and thus the symptom of vertigo. Other common causes of vertigo are too much fluid in the inner ear chambers, commonly known as Meniere's disease, or a viral infection of the vestibular system on one side. Vertigo can also occur when a brain condition, such as a tumor, stroke or multiple sclerosis, infringes on the vestibular pathways.
Light-headedness is a very different type of dizziness that does not involve the apparent movement of the environment. Instead, patients with this type of dizziness generally feel as though they are floating or as though they may faint. Common causes of light-headedness include decreased blood flow to the brain, a medication side effect, or extreme anxiety.
Some patients describe dizziness not as a sensation in their head, but rather as a feeling of being off-balance when walking. These individuals often have impairment of sensory inputs to the brain or impairment of the brain's ability to interpret these inputs, the most common being nerves in the feet, the peripheral vestibular system and the visual system. Disorders of the cerebellum, the brain region where the sensory signals are processed, can also lead to imbalance.
I recommend a visit to a physician if dizziness occurs for an unknown reason. Oftentimes, a correctable cause, typically BPPV or a side effect of a medicine, can be quickly identified. Other, more serious causes should be ruled out as well.