Incomplete spinal cord injury: Neuronal mechanisms of motor recovery and hyperreflexia

Objective: To understand neuronal mechanisms of motor recovery and hyperref lexia after incomplete spinal cord injury (SCI), and their role in rehabili tation. Design: Reviewed and compared clinical, neurophysiologic, and neuropatholog ic data from human SCI patients with behavioral, neurophysiologic, and neur oanatomic data from animals to postulate underlying neuronal mechanisms. Outcome: A postulation that two neuronal mechanisms-receptor up-regulation and synapse growth-act sequentially, to explain the gradual appearance of m otor recovery after incomplete SCI, These same mechanisms may also act in s pinal reflex pathways to mediate hyperreflexia cauda to SCI. Results: After incomplete SCI, walking ability and hyperreflexia often deve lop. Initially, cord neurons are hyperpolarized and less excitable because of loss of normal descending facilitation; this is spinal shock. Then, grad ually, voluntary movement recovers and hyperreflexia develops. Early (hours to days), these changes develop simultaneously, suggesting a common postsy naptic mechanism-likely an increase in postsynaptic receptor excitability, possibly receptor up-regulation. Late (weeks to months), recovery and refle x changes occur at a slow rate, are no longer simultaneous, and are long-la sting, which suggests a presynaptic mechanism, such as local synapse growth in spared descending pathways and in reflex pathways. This presumed synaps e growth is seemingly enhanced by active use of the growing pathway. Also, developing hyperreflexia appears to limit motor recovery. Conclusions: These observations suggest that rehabilitation for incomplete SCI should (1) increase activity in spared descending motor pathways, (2) i nitially use reflex facilitation or central nervous system stimulants to as sist spared descending inputs in depolarizing cord neurons, and (3) later m inimize reflex input, when spared descending inputs can depolarize cord neu rons without reflex facilitation. Better understanding of neuronal mechanis ms that underlie motor recovery after incomplete SCI promises better outcom es from rehabilitation. (C) 1999 by the American Congress of Rehabilitation Medicine and the American Academy of Physical Medicine and Rehabilitation.