Could the subject test and computerized posturography reveal a brain stem leison that could cause vestibular abnormality? If not, what tests would be needed?
Thanks
Answer ---------------------
I am unaware of the reliability of dynamic posturography testing for localizing a balance disorder to a brain stem lesion. It should be noted however, that the brain stem consists of various nerve tracts and nuclei of nerves that are all involved with spatial orientation, coordination, and balance. It can be assumed that a brain stem lesion therefore could cause abnormal findings on various balance tests including posturography. The important issue however, is whether posturography is useful by itself in localizing a balance disorder to a brain stem lesion.
Dynamic posturography is a new diagnostic measure used in the assessment of balance disorders. It is new enough such that it has not really been established as a routine and conventional means of testing for balance disorders, however it has become a much more common component in the testing battery used by ear specialists when evaluating a patient who complains of dizziness.
Dynamic posturography is primarily used to measure vestibular (inner ear) function. Information concerning an individuals spatial orientation, and therefore balance, comes from numerous and somewhat redundant sources including vision, structures in the inner ear, as well as from nerve endings located in various areas of the body limbs collectively labeled the somatosensory system. Tests designed to measure the function of a single component of the entire balance system must take into account or cancel out the contributions of the other components. Vision's contribution to equilibrium can be cancelled easily by blindfolding. Dynamic posturography cancels out the somatosensory component, thus allowing the measurement of the inner ear's contribution to balance and equilibrium relatively independent of other parts of the balance system. One disadvantage however is that dynamic posturography cannot distinguish between a right or left inner ear problem.
Preliminary research has suggested that dynamic posturography, when combined with the more standard and commonly used balance test known as electronystagmography, increases the sensitivity with which an inner ear disorder affecting balance can be detected.
As an aside, brain stem lesions often cause a relatively consistent pattern of neurologic symptoms including dizziness. However, balance disturbance as an isolated symptom and sign is unlikely. There are usually other neurologic findings beyond those that involve balance in brain stem lesions. Therefore one needs to be cautious about focusing on the inner ear as a possible source of dizziness if other neurologic symptoms or findings are present. It may indeed be the balance disturbance that bothers the patient the most, but a careful physical examination would likely uncover other neurologic findings which would lead the physician to suspect a central nervous system problem as a more likely cause as opposed to an inner ear abnormality. In this situation it would likely be more prudent, if supported by medical history and physical examination findings, to proceed directly to an MRI scan of the brain stem.
Cerbello-pontine angle lesions, the most common being acoustic neuromas, are tumors at the base of the brain which may impinge on either the brain stem or the inner ear nerve. Acoustic neuromas are benign tumors that originate in the sheath surrounding the inner ear nerve. Hearing abnormalities are more likely to be a symptom of such lesions as opposed to balance disorders, however, balance can certainly be affected. As in brain stem lesions, balance tests, including posturography, may show abnormalities with acoustic neuromas, but it is unlikely that the abnormalities would point to the cerebello-pontine angle as the location of the problem. Furthermore, posturography would be unable to identify the side of the lesion. A more commonly used test to screen for acoustic neuromas is known as a brain-stem evoked auditory response (BAER, or ABR). This test essentially measures the speed with which an electrical impulse generated by sound travels from the inner ear to the brain and compares the right side to the left. Abnormalities on this test frequently justify proceeding to an MRI scan.
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Steve Dankle, MD
Otolaryngology-Head and Neck Surgery
Milwaukee, Wis
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