Long-lasting involuntary motor activity after spinal cord injury

Abstract

Study design: The study design used is prospective cohort study.

Objectives: This study was designed to neurophysiologically characterize spinal motor activity during recovery from spinal cord injury (SCI).

Setting: University of Louisville, Louisville, Kentucky, USA.

Methods: Twenty-five consecutive acute SCI admissions were recruited for this study. The American Spinal Injury Association Impairment Scale (AIS) was used to categorize injury level and severity at onset. Surface EMG recording was carried out initially between the day of admission and 17 days post-onset (6.0 ± 4.3, mean ± s.d. days). Follow-up recordings were performed for up to 9 months after injury. Initial AIS distribution was 7 AIS-A; 3 AIS-B; 2 AIS-C; 13 AIS-D.

Results: Twelve subjects (48%) showed long-duration involuntary motor-unit activation during relaxation. This activity was seen on initial examination in nine and on follow-up by 3 months post-injury in three others. It was seen in muscles innervated from the injury zone in 11 and caudal to the lesion in 9 subjects. This activity was independent of the presence or absence of tendon reflexes and the ability to volitionally suppress plantar stimulation elicited reflex withdrawal.

Conclusion: The form of involuntary activity described here is the likely result of the altered balance of excitation and inhibition reaching spinal motor neurons because of the loss of inhibitory interneurons or their reduced activation by damaged supraspinal drive and the synaptic reorganization that follows SCI. As such, this activity may be useful for monitoring the effects of neuroprotective and restorative intervention strategies in persons with SCI.

Figure 1

Measurement of surface EMG signals from the initial recording from a subject with a motor-incomplete C4 injury carried out 3 days post onset (Left), comparing activity observed in relaxation, measurement of noise level, activity of interest, and volitional activation of the Right Vastus Lateralis (RVL) and Soleus (RSOL) muscles. Note that the activity of interest during relaxation develops very little amplitude above noise when only a few motor units are firing (RVL) and is of much lower amplitude than volitional activation.

Figure 2

Surface EMG activity from the final 30 seconds of the 5-minute relaxation period from the Right and Left upper trapezius (UT), Biceps Brachii (BB), Triceps Brachii (TB), Wrist Extensor (WE), Wrist Flexor (WF), Adductor Policis Brevis (APB), Adductor Digiti Quinti (ADQ), Rectus Abdominus (RA), Vastus Lateralis (VL), Adductor (ADD), Hamstring (H), Ttibialis Anterior (TA), Soleus (SOL), Extensor Digitorum Brevis (EDB), Adductor Hallicus (AH) muscles. Shown are three serial recordings from a person with a C4 AIS category C spinal cord injury. Note the repeating single motor unit firing in the RSOL recorded 6 days after injury and the increasing number of muscles showing increasing motor unit activity 47 days and 6 months after onset.

Figure 3

Results for 9 subjects in which long-lasting activation was observed during the initial recording: prevalence of activation from right and left sides in 9 subjects (top); group mean amplitudes ± SD for each muscle recorded (bottom).

Figure 4

Number of muscles showing long-duration activation during relaxation in serial recordings from 7 subjects. Note the increase in number of muscles active with time.