POSTURAL EVOKED RESPONSE (PER)
Description
PER Report
Functional Implications
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Description
The Postural Evoked Response test (PER) analyzes response of the gastrocnemius and tibialis muscles that comprise a freely standing patient's automatic reaction to an unexpected external balance perturbation. The three components of the PER are commonly referred to as the short latency (SL), medium latency (ML), and long latency (LL) responses. In patients with prolonged Motor Control Test (MCT) response latencies, the latency information provided by the PER further differentiates between peripheral and central causes. The PER is also indicated in patients with suspected CNS deficits affecting peripheral and/or central sensorimotor pathways innervating the lower extremities.
The PER analyzes the automatic response by quantifying the contractile activities of the principle extensor and flexor muscles of the ankle joint elicited by rapid toes-up (or toes-down) rotations of the support surface. The contractile profiles are recorded from the surface EMG electrodes placed on the medial gastrocnemius and the tibialis anterior bilaterally. Twenty randomly timed support surface rotations provide the stimulus.
The raw EMG is sampled at 1000hz, full-wave rectified and averaged. During the first phase of analysis, the operator views the raw EMG signals of individual muscles for each trial and is given the option of selectively removing individual trials from the group of twenty when appropriate. The operator then visually inspects the averaged waveforms for each muscle and manually identifies and labels response onset and termination times. Based on the data points selected, the system calculates the latencies, peak amplitudes, areas under the curves, and durations for three automatic response components including, the short latency (stretch reflex), the medium latency (segmental) response, and the long latency (central) response.
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PER Comprehensive Report
- Latency quantifies in milliseconds the time between the stimulus onset and the activation of the muscle. Separate latencies are displayed for the short (SL), medium (ML), and long (LL) components of the automatic response.
- Amplitude quantifies the magnitude in microvolts of the average EMG signal at the marker point identified by the operator. Separate amplitude values are calculated for each marker.
- Peak quantifies the maximum amplitude in microvolts of the EMG signal between the response onset and termination markers.
- IEMG quantifies the area under the EMG signal in units of microvolt-seconds between the response onset and termination markers. Separate IEMG scores are displayed for each of the three response components.
- The four averaged EMG signals are displayed graphically, along with arrows identifying the response onset and termination points selected by the operator.
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Functional Implications
The PER protocol can provide site specific information for differentiating lesions between spinal cord, brainstem, areas of the cerebellum, internal capsule, and sensory motor cortex. The PER has also been shown to differentiate pathological processes such as demyelinating disorders from Parkinson's Disease. Specific differentiation of this degree has not been ascribed to the Motor Control Test (MCT).
It is important to highlight a major difference in the information provided by the MCT and PER studies. While both tests measure latencies, the two latency measures are based on different response components. MCT latencies are based on the forces associated with the body's coordinated response to induced forward or backward sway, including passive viscoelastic forces and active contractile forces generated by all participating muscles. The PER protocol, in contrast, quantifies the onset times and amplitudes of individual muscle contractions. Thus, MCT latencies may be delayed and the PER latencies can be within normal limits. One cause for such a difference is an antalgic body response secondary to lower back or joint pain, resulting in a slowed onset of force development. Although uncommon, the reverse can be true if the contractions of the ankle muscles are delayed while normal responses in the more proximal knee and thigh muscles produce force responses within normal limits.
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