Chapter 4
Medical Conditions Causing Dizziness
Glenn Johnson, MD

Dartmouth Hitchcock Medical Center
Lebanon, NH

Dizziness can be caused or aggravated by a large variety of medical conditions and medications. General levels of strength, physical conditioning and activity levels will affect the degree of dizziness that is experienced, whatever the underlying cause. Even when the onset of dizziness can be clearly attributed to a very specific cause such as an acoustic neuroma the intensity, duration and disability caused by the sensation of dizziness can be modified by medical conditions. Unless recognized and managed, these medical issues can prevent the success of what otherwise would be an appropriate treatment program. In an attempt to simplify this complex area, it’s helpful to review some basic concepts of the physiology of postural stability.

Our sense of balance (how stable we are in a variety of situations) is a very unique sense. As opposed to the senses of sight, hearing, touch, smell and taste, balance requires an integration of information from multiple sensory inputs: vestibular input from the inner ear, perception of movement perceived from vision and sensation of movement in our joints from proprioception (see Chapter 1). There’s no such thing as a universal sense of balance that is true and accurate in all conditions. Each of the three senses that we (or more accurately our brains) use to give us a perception of where our body is in space and how fast we’re moving is in relation to something else.

The inner ear relies on a change in head movement. The fluids of the semicircular canals obey the laws of physics and tend to stay put when the head is suddenly moved. This causes a deflection of the nerve endings in those semicircular canals which the brain uses to calculate speed and direction of head movement. A change in movement or acceleration of the head is needed to cause a movement of the skull in relation to the fluid in the semicircular canals. If the head and semicircular canal fluids are moving at the same speed (there is no acceleration), the inner ear perceives no movement. A simple example of this is traveling in your car at a steady rate of 65 miles per hour. Obviously, you’re moving at a very fast speed, but the ear cannot perceive this since there’s no acceleration.

Vision uses a different perspective for its relativity. We rely on seeing our head move in relation to objects that we can see to judge direction and speed of movement. When objects are close by, large and motionless, this sense is wonderfully accurate. If objects are far away and thus harder to see, this sense does not work as accurately. If all the objects around us are moving (such as when walking in a crowded shopping mall) our brain would need to calculate how fast we’re moving in relation to many moving objects. This is too complicated to work well and in this situation vision is less helpful. . .

All these senses (and the brain) that need to blend the information into a single perception of movement and postural stability are affected by a variety of medical conditions. The brain is a crucial element in the sense of balance. It needs a certain critical amount of information from these three senses to feel confident in our stability. When these senses don't give enough information, the brain can not tell accurately how fast we are moving or where our body is in relation to our environment. We perceive this lack of accurate information as . . .