Extent and pattern of lower motor neuron damage following cervical spinal cord injury
Franz S, Eck U, Wolf M, Wilder-Smith E, Weber M, Rupp R, Weidner N
Heidelberg University Hospital, Heidelberg, , Germany
The integrity of lower motoneuron function is essential for innovative therapeutic approaches like the application of electrical stimulation (ES) with the objective of restoring grasping function and pharmacological strategies for promoting axonal regeneration in the spinal cord [1, 2]. Extensive lower motoneuron damage (LMND) could heavily challenge such therapeutic considerations, raising the essential requirement of its precise assessment [3, 4]. Previous investigations already reported a widespread LMND, not only in the vicinity, but also remote from the injury site . Thus, the intention of this study was to determine the potential cranio-caudal extent and the pattern of LMND at the cervical level in tetraplegic patients by needle electromyography (EMG).
12 acute individuals with traumatic and ischemic cervical spinal cord injury (SCI) have been included in this still ongoing descriptive observational pilot study. For detecting LMND, needle EMG was performed on one upper extremity per patient 27 to 77 days post-injury (mean=51±15.8). Previously, preexisting peripheral nerve injury was excluded by evaluation of sensory nerve conduction studies. Aside from one exception, the chosen muscles were in accordance with the upper extremity key muscles of the International Standards for Neurological Classification of Spinal Cord Injury (ISNCSCI). Due to better accessibility for EMG, the abductor pollicis brevis muscle was chosen for evaluation of C8 nerve roots. The degree of LMND was classified based on three different findings: (1) the severity of abnormal spontaneous activity, (2) if present, discharge rates of motor unit potentials (higher or lower than 20 Hz), and (3) density of innervation (interference pattern: no discharges, single motor unit discharges, reduced or normal innervation density). According to the relative manifestations of these criteria, muscles were allocated to the respective subgroups “no LMND”, “moderate LMND” and “severe LMND”.
11 patients (2 x ASIA Impairment Scale (AIS) A, 1 x AIS B, 5 x AIS C and 3 x AIS D) had a neurological level of injury (NLI) above C5. 1 patient had a NLI at C5 (AIS D). Of 60 examined muscles, 34 showed signs of moderate and 2 severe LMND (overall 60 %). Most commonly affected was the extensor carpi radialis muscle (C6) with proof of LMND in 75 % of the cases (n=9). Severe LMND could only be seen in the segments C8 and Th1 of 1 patient (NLI C4; AIS C). AIS A and B patients displayed the most widespread LMND. In these patients (n=3) on average 4.3 out of 5 myotomes were affected with an upper extremity motor score (UEMS) of 7.6 out of 50. In all other patients (n=9) LMND could be observed in 2.4 segments with an UEMS of 16.6.
Our results confirm a widespread yet discontinuous at and below-level LMND pattern following ischemic/traumatic cervical SCI. Moreover, our data indicate that the spatial extent of LMND generally depends on the severity of injury. Aside from the spatial extent, LMND within each segment appears mostly moderate. In these cases, LMND might not rule out the chance of providing a patient with a neuroprosthesis, which requires at least partial integrity of the relevant lower motoneuron pool.
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