Visual system: Our eyes give us depth perception and process movement in our central and peripheral vision and can serve as a very powerful stabilizer when we have issues in the other systems. Examples of deficits in this system are cataracts, macular degeneration, exotropia or esotropia, and convergence disorders.
Vestibular system: Our inner ears serve as two gyroscopes which let us process movements of the head in angular and linear directions. There is redundancy in the system as the two ears should report the same information to the brain. When the vestibular function is different between the two ears, this can result in dizziness and vertigo. Examples of deficits in this system are viral infections in the inner ear, benign positional vertigo ("loose crystals in the ear"), Meniere's disease, and inner ear trauma.
Proprioceptive system: The sensory nerves in our legs and feet send signals up to our brain to let us know the type of ground we are standing on and where our center of gravity is. These send signals to our spinal and extensor muscles to help us maintain an upright posture and our center of gravity.
All three of these systems send information to our cerebellum, which processes all the information and lets us know where we are in space and how we are moving. Examples of deficits in this system are peripheral neuropathies (due to diabetes, degenerative neuromuscular diseases, herniated spinal disks) and edema in the legs and feet.
The cerebellum is the part of our brain that is responsible for balance. It processes all the information from these three systems to let us know where we are in space. Diseases in the cerebellum such as tumors, strokes, concussions, and migraines can also generate dizziness. Problems in any one system noted above can cause imbalance, however, with time and/or physical therapy, the other two functioning systems can give us enough information to resolve our balance issues. When more than one system is involved, the likelihood of regaining normal balance diminishes with the increasing severity of dysfunction in the systems.
A perilymphatic fistula is a leak of the fluid of the inner ear (perilymph) from the oval or round windows in the middle ear. These leaks occur with either a pressure event (e.g., straining to go to the bathroom or lifting weights, inability to equalize pressure on a plane, when scuba diving) or with trauma to the ear (e.g., slap to the ear, fracture of the temporal bone). The leaks occur at the ligament surrounding the stapes bone in the oval window or through a rupture of the round window membrane. Such leaks are often difficult to assess as normal middle ear fluids can accumulate in these areas while operating and can mimic perilymph leaking from the inner ear. The fluid volumes of a perilymphatic leak are microscopic and would not result in filling the middle ear with fluid. The symptoms of a perilymphatic fistula are a sensorineural hearing loss (i.e., "nerve" hearing loss) and vertigo. The onset of the symptoms are temporally related to the insult to the ear.
Videonystagmography is comprised of multiple tests to assess balance function. Ocular tests assess the movement of the eyes; abnormalities can signify central pathology (in the brain). Positional testing assesses whether certain head positions or changes in head position induce movements in the eyes called nystagmus, which is a specific jerking of the eyes. This can determine if there is pathology in the inner ear, and rarely in the central nervous system. Caloric testing uses warm and cold water or warm and cold air injected into the ear canal to thermally stimulate the fluid in the inner ear. Such testing can determine if there is a discrepancy in function between the two ears.