Medical Legal

Identification of Aphysiologic Sway Patterns Using Computerized Dynamic Posturography

Joel A. Goebel, MD, FACS, Associate Professor
Washington University School of Medicine
St. Louis, Missouri

The complete evaluation of the dizzy or balance-disordered patient involves a thorough history, physical examination of eye movement, posture and gait, as well as laboratory quantification of visual, vestibular and proprioceptive contributions to ocular and balance control. In most cases of dizziness and/or imbalance the historical severity of the problem correlates with the physical and laboratory findings. Occasionally, however, particularly in cases of head trauma, the mechanism and severity of injury are out of proportion to the physical or laboratory findings of posture and gait control. Distinguishing between normal, true physiological abnormalities, and exaggerated symptoms of balance dysfunction is an important element of the diagnostic work-up of dizziness and balance disorders, whether or not medical-legal issues are involved.

The clinical utility of Computerized Dynamic Posturography (CDP) in the management of a variety of balance disorders is well established. Certain patterns have emerged which correlate with the historical, exam and laboratory findings to aid the clinician in the formulation of a successful treatment plan. In certain cases, however, the mechanism of injury and the symptoms and signs of instability due not fit together. For these instances, CDP has been proposed as a useful tool to document excessive or voluntary sway patterns as an indication of non-organic (aphysiologic) etiology.

Numerous clinical studies have demonstrated the utility of CDP for detection of aphysiologic sway. Excessive sway on condition one of the Sensory Organization Test (SOT) is rarely seen in the ambulatory patient population and serves as an excellent marker for non-organic sway. Moreover, since exaggerating patients usually have no knowledge of the test sequence (easier tests first) or normative performance (SOT 5, 6 norms lower than SOT 1, 2), they frequently exhibit excessive rhythmic sway on the earlier conditions while exceeding the norms on the more difficult trials. They also may not know that lateral sway is unusual during SOT conditions where the platform moves only in the anterior-posterior plane. During the Motor Control Test (MCT), normal responses to small platform translations are minimal in normal subjects and most patients, but are excessive and non-congruent in malingering subjects. Finally, the addition of sway frequency to amplitude data using discriminate analysis enhanced both sensitivity and specificity.

The clinician, however, must exercise caution when assigning motive to such patterns in CDP. In addition to malingering for secondary gain, excessive voluntary sway can be seen (usually to a limited extent) in anxious patients or patients with real pathology who are eager to “show” their deficits on platform testing. Nevertheless, armed with the results of the studies reviewed, the practitioner can approach this vexing situation with objective evidence to support his clinical suspicions.