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Page last modified: September 11, 2004Please read our disclaimer
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Page last modified: September 11, 2004
Moving platform posturography (MVP), or posturography for short, is a method of quantifying balance (although the definition of balance can be tricky). It is most applicable in situations where balance needs to be followed quantitatively, to determine whether a disorder is getting better or worse, or the response to treatment. There are several commercial vendors of posturography equipment. The following link has pictures of several types of equipment.
For the most useful variant of MVP, "sensory testing", there are 6 subtests, which normally are progressively more difficult. Three subtests are "sway referenced", meaning that pressure is used to control the pitch angle of the platform with the goal of keeping ankle angle constant. This is not possible, but it does distort ankle angle input.
Sensory test scores ordinarily decline with age (Wolfson et al, 1992), reflecting the usual decline in balance with age. Repeated testing over 10 days may show a learning effect with better scores (Peterson et al, 2003).
Posturography is insensitive to vestibular disorders, and normal posturography should not be considered indicative of normal vestibular function (Di Fabio, 1995). The author has had instances in which there is a severe disturbance of caloric testing and rotatory chair testing, accompanied by a normal MVP. MVP is therefore not an adequate test for vestibular disturbance, by itself.
MVP may add value to a vestibular battery, when combined with other tests of vestibular function. Stewart et al (1999) suggested that audiometry combined with posturography was a cost-effective method of documenting a vestibular disorder. Obviously, we disagree that it is sufficient.
Allum and others recently concluded that diagnosis of bilateral vestibular loss using posturography is best achieved using measure of trunk control following pure toe-up rotational perturbations under eyes-closed conditions (Allum et al, 2001). This is not a paradigm that is routinely available.
Posturography with the head held in different angles on the neck has been used in an attempt to diagnose cervical vertigo. Static posturography does not appear to be useful. Dynamic posturography, incorporating sway refencing, may be more sensitive (Alund et al, 1991).
Posturography is also very useful in medical legal situations where malingering is a possibility (see below).
Click here to see an example of a posturography output screen (courtesy of Neurocom, Inc). The main vendor of posturography equipment used in clinical context is Neurocom incorporated. Other vendors include Micromedical Technology, Metitur, and Vestibular Technologies (Tampa, maker of the "Balance Trak 500") and several makers of research balance equipment (e.g. AMTI, and Kistler).
As of July, 2002, there were 173 papers with the word "posturography" to be found indexed on Pubmed. This is substantial but not a big enough research effort to answer most of the questions posed below.
Several studies have suggested that posturography is a cost-effective and/or method of evaluating dizziness (Stewart, 1999; El-Kashian, 1998, Yardley et al, 1998). Our view is that the usefulness of posturography is a function of the type of patients that are being screened. Posturography might be of considerable utility in separating out patients with psychiatric disorders from vestibular or CNS disorders. We are less enthusiastic about the utility of posturography in a population with known inner ear disease, such as Meneire's or chronic ear disorders.
Use of a foam support surface provides a more challenging balance environment than the Equitest ankle-sway referenceing system, and Allum has suggested that it may offer a good alternative.(Allum et al. 2002)
Perilymph fistulas are a rare cause of imbalance. Although a fistula test procedure, involving aural pressure and sway, has been available using posturography, very little has emerged in the literature regarding success and failure. Ben-David and others (1997) suggested that posturography was effective in diagnosing Tullio's phenomenon (sound sensitivity, associated with fistula). This use appears reasonable to us, although it is puzzling why more has not been published as of this date (2002).
Again, little has been published to date. Sargent and others (1997) found that MVP was abnormal in bilateral loss. This is hardly suprising as bilateral loss usually causes substantial and significant effects on balance. The author has encountered patients with near complete bilateral vestibular loss, who performed normally on MVP. Thus, MVP is not 100% sensitive to bilateral loss. Baloh et al (1998) did not find MVP useful in separating patients with bilateral vestibular loss from cerebellar disease. At this writing (2002), more work is needed to calibrate MVP results to the degree of bilateral loss, and also in using MVP to differentiate among various other causes of imbalance.
There has been considerable evidence in the literature that posturography is helpful in detecting symptom exaggeration (see below). At this writing, we think that this indication for posturography needs to be pursued. In particular, how accurately can exaggeration be differentiated from imbalance due the large variety of organic balance disorders ? Several studies (Cevette, Gianoli, Goebel, Krempl) suggest that MVP is useful in detection of malingering. Uimonen (1995) found that static posturography fared no better than clinical observation. We conclude from data available to date that MVP is useful in this context.
One would not think that MVP would be very useful in episodic disorders such as migraine that are largely characterized by headache. The literature here is scanty and mixed. Cass (1997) reported that MVP was useful in identifying patients who could benefit from vestibular rehabilitation. Dimitri et al, in a careful study, found that MVP does not differentiate Migraine associated vertigo from Meniere's disease.
Only a handful of papers have been published regarding movement disorders such as Parkinsonism and PSP. More work is needed in this area.
We do not think that MVP is a reasonable procedure in persons with multiple sclerosis, as reported by Williams (1997), as MS is a multifocal disorder.
In spite of a considerable research outflow, we do not think that posturography is a reasonable primary method of diagnosing peripheral neuropathy, as there are many more direct methods of doing this. It might be a useful method of partitioning out how much imbalance is due to sensory disturbances to the feet, among other sensory impairments.
Most MVP research to date has been done on vestibular disorders. It appears clear right now that MVP is moderately sensitive to vestibular disorders. The author has encountered patients with complete unilateral loss of vestibular function, who had normal results on MVP. Thus MVP is not 100% sensitive. MVP results depend on the significance of the unilateral lesion, the age of the person in which it has appeared, and their motivation to compensate. MVP does not appear to be very specific -- MVP measures imbalance, which is found in a variety of sensory and central medical disorders.
At the present writing, we think MVP is useful in certain clinical situations. As we get more experience with MVP, it seems likely that it's indications will expand. Much more research is needed on MVP.
MVP is not uniformly covered by health insurance in the United States. Medicare covers it in some areas but not in others. In part this is related to the relative recency that MVP has been developed. In part, however, the lack of coverage may be related to inadaquete knowledge about situations in which MVP is justified.
In our opinion, MVP should be covered by health insurance. MVP is FDA approved and has a CPT code. It has one well documented diagnostic indication - detection of malingering (Cevette et al. 1995; Goebel et al. 1997; Krempl et al. 1998; Morgan et al. 2002). MVP data can be pivotal in medicolegal situations as it can strongly suggest that individuals are feigning imbalance.
Nevertheless, the use of MVP in diagnostic contexts outside that of malingering is not well established. El-Kashlan and others found MVP to be more sensitive than physical examination in distinguishing patients with vestibular disturbances from normals (El-Kashlan et al. 1998). Stewart and others suggested that MVP is as cost-effective as audiometry and ENG testing for the evaluation of vertigo (Stewart et al. 1999). However, in the authors opinion, this conclusion differs from that of most specialists that evaluate vertigo. Dimitri and associates (Dimitri et al. 2001) found MVP of no utility in distinguishing migraine associated vertigo from Meniere's disease. Baloh and associates (Baloh et al. 1998) found MVP unable to distinguish between cerebellar and bilateral vestibular patients, a task that most clinicians would find quite simple.To summarize, there is presently only a small amount of data about broader diagnostic usefulness of MVP, and it is conflicted.
Another potential indication for MVP is to guide physical therapy or to document the outcome of physical therapy. While several authors suggests that it is useful in this context, (Mirka et al. 1990; Shepard 1996; El-Kashlan et al. 1998), others find it of no use in documenting functional status (O'Neill et al. 1998). Outcome studies in which therapy guided by MVP is compared to therapy without MVP are presently lacking.
We thank Neurocom Inc, for use of figures of their equipment to illustrate this page..
General References: