Electrodiagnostic investigation of motor neuron and spinal reflex arch (H-reflex) in spinal cord injury

Neuromuscular consequences of spinal cord injury: New mechanistic insights and clinical considerations

The spinal cord facilitates communication between the brain and the body, containing intrinsic systems that work with lower motor neurons (LMNs) to manage movement. Spinal cord injuries (SCIs) can lead to partial paralysis and dysfunctions in muscles below the injury. While traditionally this paralysis has been attributed to disruptions in the corticospinal tract, a growing body of work demonstrates LMN damage is a factor. Motor units, comprising the LMN and the muscle fibers with which they connect, are essential for voluntary movement. Our understanding of their changes post-SCI is still emerging, but the health of motor units is vital, especially when considering innovative SCI treatments like nerve transfer surgery. This review seeks to collate current literature on how SCI impact motor units and explore neuromuscular clinical implications and treatment avenues. SCI reduced motor unit number estimates, and surviving motor units had impaired signal transmission at the neuromuscular junction, force-generating capacity, and excitability, which have the potential to recover chronically, yet the underlaying mechanisms are unclear. Furthermore, electrodiagnostic evaluations can aid in assessing the health lower and upper motor neurons, identify suitable targets for nerve transfer surgeries, and detect patients with time sensitive injuries. Lastly, many electrodiagnostic abnormalities occur in both chronic and acute SCI, yet factors contributing to these abnormalities are unknown. Future studies are required to determine how motor units adapt following SCI and the clinical implications of these adaptations.

Some Problems of the Lower Extremity in Patients with Spinal Cord Injuries

Spinal cord injury (SCI) is devastating, leaving patients wholly or partly paralyzed. Health care providers who care for SCI patients during the acute or chronic phases are faced with different phenomena in the lower extremities of these subjects. In this article, the authors review the relevant changes associated with SCI. Preventive measures of these medical complications are directed according to the specific cause. Early comprehensive rehabilitation carried out by a specialized team prevents complications while enhancing functional gains.

An Examination of the Motor Unit Number Index (MUNIX) in muscles paralyzed by spinal cord injury

The objective of this study was to assess whether there is evidence of motor unit loss in muscles paralyzed by spinal cord injury (SCI), using a measurement called motor unit number index (MUNIX). The MUNIX technique was applied in SCI (n=12) and neurologically intact (n=12) subjects. The maximum M waves and voluntary surface electromyography (EMG) signals at different muscle contraction levels were recorded from the first dorsal interosseous (FDI) muscle in each subject. The MUNIX values were estimated using a mathematical model describing the relation between the surface EMG signal and the ideal motor unit number count derived from the M wave and surface EMG measurements. We recorded a significant decrease in both maximum M wave amplitude and in estimated MUNIX values in paralyzed FDI muscles, as compared with neurologically intact muscles. Across all subjects, the maximum M wave amplitude was 8.3 ± 4.4 mV for the paralyzed muscles and 14.4 ± 2.0 mV for the neurologically intact muscles (p<0.0001). These measurements, when combined with voluntary EMG recordings, resulted in a mean MUNIX value of 112 ± 71 for the paralyzed muscles, much lower than the mean MUNIX value of 228 ± 49 for the neurologically intact muscles (p<0.00001). A motor unit size index was also calculated, using the maximum M wave recording and the MUNIX values. We found that paralyzed muscles showed a mean motor unit size index value of 80.7 ± 17.7 ìV, significantly higher than the mean value of 64.9 ± 10.1 ìV obtained from neurologically intact muscles (p<0.001). The MUNIX method used in this study offers several practical benefits compared with the traditional motor unit number estimation technique because it is noninvasive, induces minimal discomfort due to electrical nerve stimulation, and can be performed quickly. The findings from this study help understand the complicated determinants of SCI induced muscle weakness and provide further evidence of motoneuron degeneration after a spinal injury.

Reorganization of reflex responses mediated by different afferent sensory fibers after spinal cord transection

Adult rats were submitted to a complete spinal cord transection at T9 level to address peripheral and spinal reflex changes in the caudal lumbar segments. Compound muscle and nerve action potentials decreased in amplitude and increased their duration between 14 and 30 days but recovered to near to normal values thereafter. The H wave amplitude increased during follow-up, resulting in significantly higher H/M ratio in tibialis anterior (223%), gastrocnemius (160%), and plantar (304%) muscles with respect to preoperatory values (P < 0.01). Sixty minutes after spinal cord transection, component C1 (conveyed by Aalphabeta afferents) disappeared in the crossed but not in the ipsilateral withdrawal reflex. Components C2 (Adelta) and C3 (C afferents) were abolished on both. C1 and C3 reappeared for both reflexes in all injured animals, while C2 reappeared in a few cases. C1 ipsilateral component became highly facilitated (209% of presurgery values, P < 0.01), whereas C3 (82%) and C2 (24%) recovered partially. Crossed reflex component C1 attained in all animals similar to normal values (85%) but with longer duration. C3 increased with time although it remained significantly lower than the original (67%) whereas C2 reappeared in only 2/8 animals. In conclusion, spinal cord injury induces a transient disability of caudal spinal cord segments that progressively reverts along time. Ipsilateral reflex components mediated by thick Aalphabeta fibers (H reflex and C1) but not those mediated by thin fibers (C2 and C3) remained present after injury showing long-lasting facilitation whereas contralateral reflex components were abolished after injury and showed limited recovery.

Long-term continuously infused intrathecal baclofen for spastic-dystonic hypertonia in traumatic brain injury: 1-year experience

OBJECTIVE

To determine if the long-term use of continuously infused intrathecal baclofen (ITB) over a 1-year period will control spastic-dystonic hypertonia in patients with traumatic brain injury (TBI).

SETTING

Tertiary care outpatient and inpatient rehabilitation center directly attached to a university hospital.

SUBJECTS

Persons with TBI and intractable spasticity and dystonia for more than 6 months' duration recruited in a consecutive manner.

DESIGN

TBI patients were admitted to the study after screening via a bolus injection of either intrathecal normal saline or 50 microg of baclofen. Data for Ashworth rigidity scores, spasm scores, and deep tendon reflex scores were collected for both the upper extremities (UE) and lower extremities (LE). Patients whose LE Ashworth scores decreased an average of 2 points were then offered implantation of a computer-controlled pump for continuous ITB. Changes over time were assessed statistically via Friedman's analysis for ordinal data and ANOVA for linear data. Differences between set points in time were also assessed via Wilcoxon signed rank.

DATA SET

Seventeen patients (average age 29+/-11 yrs) with spasticity and/or dystonia treated over 1 year via a computer-controllable intrathecal delivery system for the delivery of ITB.

RESULTS

After 1 year of continuous ITB treatment the average LE Ashworth score decreased from 3.5+/-1.3 (SD) to 1.7+/-0.9 (p < .0001), spasm score from 1.8+/-1.3 to 0.2+/-0.5 (p< .0001), and reflex score from 2.5+/-1.1 to 0.1+/-0.3 (p < .0001). The average UE Ashworth score decreased from 2.9+/-1.5 to 1.6+/-1.0 (p < .0001), spasm score from 1.2+/-1.5 to 0.2+/-0.6 (p < 0.0001), and reflex score from 2.2+/-0.5 to 1.0+/-0.8 (p < .0001). The average ITB dose required to attain these effects at 1 year was 302 microg continuously infused per day.

CONCLUSION

Continuous intrathecal infusion of baclofen is capable of maintaining a reduction in spasticity and dystonia in both the upper and lower extremities of TBI patients.