Page last modified:
March 13, 2008
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Timothy C. Hain, MD Page last modified:
March 13, 2008
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Equitest dynamic posturography machine sold by Neurocom Inc. This device is the one we use in our clinic in Chicago. |
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.
It is a neat and "easy" test. No wires, gel or cameras are needed. You just step on the plate, put on a safety harness, and try to stand up. A full "Equitest" takes only about 20 minutes. A technician can be trained to do an Equitest in about 20 minutes.
The best equipment is generally thought to be the "dynamic posturography" systems, which incorporate servomotors into their design. An example of this is shown above -- actually the "Equitest" apparatus from Neurocom. A similar but less expensive machine is the "Smart-Balance Master". We will be mainly talking about the full "Equitest" type apparatus here. If it doesn't move the floor and visual surround, we are not talking about it's utility -- it should be safe to assume though that it is less than the full system.
Static posturography systems are much cheaper, but of course, have less general utility. Neurocom also sells one of these, called a "Basic Balance master". These can also be easily found from other vendors. We find generally reports generated by these devices of very little utility. We would just as soon have reported out a timed-tandem Romberg.
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).
There are norms for the "motor tests" of MVP, but we will not dwell on them as their utility is unclear.
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. Sataloff and others (2005) recently suggested that MVP adds value because it is abnormal in situations where ENG is normal. This is not necessarily a reason to use MVP, however. If we flipped a coin, and called heads abnormal, we could also make the same observation.
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 referencing, 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).
Although one might think that the CDP machine is "foolproof", because you essentially just strap the patient in and operate a computer console, practically mistakes are easily made. Most of these problems arise from overly cautious operators (and perhaps patients).
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.
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| Posturography sensory test profile suggestive of malingering. Scores on test 1 are unusually low. Scores on test 6, which is more difficult than test 5, are better than test 5. The "Cevette" algorithm applied to this profile suggested that it is an "aphysiologic" pattern. |
Here is another example of an "aphysiologic" pattern.
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.
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| Posturography in person with MS and spastic gait. |
We do not think that CDP is generally a reasonable procedure in persons with multiple sclerosis, as reported by Williams (1997), as MS is a multifocal disorder. We have tested occasional persons however. The picture above shows the result in one person, who had a spastic gait. Balance was poor. The "Cevette" analysis suggested an "aphysiologic" score. This is appropriate as this person was neither normal nor did he have a vestibular disorder.
These patients have a central ataxia, due to cerebellar damage. As posturography is mainly tuned to vestibular problems, one would think that it might show an "aphysiologic" pattern. This is sometimes the case, as illustrated here.
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 (such as measuring sensation). It might be a useful method of partitioning out how much imbalance is due to sensory disturbances to the feet, among other sensory impairments.
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| Posturography is generally normal in persons with orthostatic hypotension. |
Posturography should be normal in persons with adequate blood pressure to stand upright during the test. An example of this is shown above.
Most MVP research to date has been done on vestibular disorders. It appears clear right now that MVP is moderately sensitive to vestibular disorders. On the other hand, 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 dynamic 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.
As dynamic MVP is not overwhelmingly useful (but does have a definite utility in malingering), it also follows that stripped-down versions of MVP that omit the servo-motor or the visual surround are rather marginal in utility.
MVP is not uniformly covered by health insurance in the United States. Medicare covers it in some areas but not in others. In Illinois, rather peculiarly, Blue Cross Blue Shield doesn't cover posturography. 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. There may also be some reluctance to cover posturography because of abuse -- some practices use a device vaguely similar to MVP, and then bill for the full test. In our opinion, this is fraud.
By MVP, we mean dynamic moving platform posturography. Static systems, such as for the "Basic Balance-master", can't do what dynamic systems (e.g. the "smart balance-master") can do by definition, and thus their utility is even less than what has been documented for the dynamic systems.
The vendor (Neurocom) has confused the situation by using very similar names for these limited products as for their more capable products. For example, the "Balance-master" is a static system, the "Basic Balance-master" is a static system, while the "Smart Balance-master" is a dynamic system. There are also a large number of variant posturography systems -- in our view, it is unclear whether they have much if any clinical utility.
Nevertheless, In our opinion, dynamic MVP (namely Equitest and very close relatives, that explicitly include motorized visual surround and platform movement, such as "Smart Balance-Master" ) 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). Dynamic MVP data can be pivotal in medicolegal situations as it can strongly suggest that individuals are feigning imbalance.
On the other side of the issue, 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 would like to see more work done here.
We thank Neurocom Inc, for use of figures of their equipment to illustrate this page..
General References:| © Copyright May 11, 2008 , Timothy C. Hain, M.D. All rights reserved. Last saved on May 11, 2008 |
