spinal shock

spinal shock The term ‘spinal shock’ refers to the fact that transection of the spinal cord produces an initially complete but temporary absence of spinal reflexes in body parts whose innervation arises from levels of the spinal cord below the level of transection. In current clinical usage, the state of spinal shock would never be considered in isolation, but together with the absence of voluntary movement in, and the loss of sensation from, corresponding regions of the body, it forms the basis of the clinical diagnosis of a functionally-complete transection of the spinal cord. This distinction between current and past usage is not without academic interest, as, historically and conceptually, spinal shock could not be understood until the ‘spinal reflex’ itself was fully defined and its nature investigated by experiment. Thus, although the phenomena of spinal shock were first described and investigated by the physician Whytt in 1750, its naming as such, by the physician and physiologist Marshall Hall, did not occur until a hundred years later. That naming was the outcome of animal experiments in which he clearly defined spinal reflex for the first time and later their counterpart cranial reflexes, such as the blink reflex. Whytt had recognized that mechanical stimulation of the foot in a decapitated frog resulted in withdrawal of the hindlimb. He termed such movements ‘vital motions’ and recognized that they depended on the spinal cord. However, in accord with lingering ideas of ‘vitalism’ (for which ‘soul’ corresponds to the contemporary usage of ‘consciousness’), he deemed the spinal cord to contain a sentient principle or ‘soul’. Hall, who can be regarded as the first professional physiologist, and wedded therefore to functional explanations, conceived a distinct class of ‘involuntary motions’, the spinal reflexes, that depended on ingoing influences to the spinal cord on the spinal cord itself, and on outgoing influences to the muscles (and glands); such ‘reflected’ actions were purposive in nature, but not dependent on sensory experience and hence not involving consciousness, which he attributed to the brain alone. Indeed, these and related experiments and philosophical enquiry all contributed to the then current debate as to whether movements induced by touching a part of the body (e.g. the tail) which, together with the spinal cord, had been surgically isolated from the rest of the body, were the result of a sensory experience, i.e. whether they concerned the ‘soul’. In contrast, Hall believed reflex action to be a manifestation of sensitivity to the stimulus but without sensibility; for him the ‘soul’ could not be so divided between brain and spinal cord.

As evident from the early experiments on the frog, spinal shock can be very transitory, lasting only for a few minutes, but it is of increasing duration according to cerebral dominance, lasting weeks in monkeys and still longer in apes and humans. In man the effect of injury of the spinal cord depends on whether it is completely or incompletely divided and on the level of the spinal cord that is affected. For example, with transection at the third cervical level or above, functions depending on the cranial nerves, such as swallowing and facial movements, persist, but all breathing movements cease and life sup-port by artificial ventilation is necessary. Speech remains possible, so long as a source of air pressure is provided below the vocal cords, to energize their oscillation when they are brought together (through activity of the still intact cranial nerves serving the larynx) as the patient attempts to speak. With transection a little lower, below the fourth cervical level, speech and also breathing are now independent because the brain stem motor control of the diaphragm remains mainly intact, via the phrenic nerve, whose motor neurons leave the cord mainly above this level; however all active expiratory-dependent activities, such as coughing, straining in defecation, and vocal power, remain absent, because the motor innervation of the relevant muscles lies below the transection. Even one segmental level can make a remarkable difference in the person's dependence on others or independence. A lesion at the seventh thoracic segment would leave the person independent for much of his personal needs, but standing unassisted and walking would be impossible, as would normal control of defecation and micturition. The ‘tendon jerk’ is important to the assessment of spinal transection, because normally it would be present in a range of muscles in the arms and legs. This allows the level of transection to be identified, along with other features, based on knowledge of the spinal segmental motor nerve supply to the individual muscles. Furthermore, extensive anatomical and physiological research has clearly established that the reflex pathway of the tendon jerk is monosynaptic. This means that when the muscle receptors are briefly stretched by the tap, the nerve impulses in the afferent pathway travel directly to the motor neurons and excite them reflexly to cause the normally visible muscle twitch. Thus the complete absence of this particular class of spinal reflex activity, in the initial phase of spinal shock that follows spinal transection, indicates how strongly in man motor neuron excitability is dependent on impulses descending from the brain stem and above.

However, not all the pathways are necessarily excitatory. The spinal neural circuitry is itself extremely complex, and some descending pathways may equally normally inhibit ‘inhibitory’ interneurons whose activity is then ‘released’ by the loss of the descending inhibitory control, causing the motor neurons to be inhibited. It is not surprising, therefore, that the basis of spinal shock remains an enigma; its unravelling would undoubtedly contribute to future attempts to restore — prosthetically, or biologically by cell transplantation, for example — useful function to spinal man. But until more research is done, any such interventions when first undertaken would be unlikely to be introduced at the time of ‘spinal shock’, because at that stage the final clinical outcome would remain uncertain if not unknown.

With regard to the reflexes, spinal shock is not permanent and spinal reflex activity is restored; this is a gradual process starting some weeks following the lesion. It is not simply as before but has a distinct bias in which increasingly the flexor muscles are readily thrown into reflex contraction by cutaneous stimulation or muscle stretch, the process commonly being first seen in the big toe (Babinski's sign) and ankle, and later in the knee and hip. Still later, reflexes return in the extensor muscles. Another aspect of this functional recovery is the enlargement of the receptive field of the cutaneous reflexes so that they can be elicited by minimal stimulation from a progressively wider area of skin. Spinal reflexes involving micturition and defecation are also affected during spinal shock. In particular the bladder is completely without its normal ‘tone’ and the immediate loss of the ‘voiding’ reflex, whose reflex pathway normally involves the brain stem, can result in overfilling of the bladder, with urination only by overflow if not managed clinically. Eventually, as spinal shock diminishes, a wholly spinal reflex emerges to create an ‘automatic bladder,’ which the patient can learn to empty by manual stimulation in the groin.

The study of spinal shock indicates the extraordinary capacity of the nervous system to reorganize after a lesion, and raises many important questions and theoretical concepts about the way the central nervous system functions. The International Spinal Research Trust (now usually known as ‘Spinal Research’) funds the majority of research in spinal cord injury and for further information readers are referred to this Trust.

L. S. Illis

Bibliography

Manuel, D. E. (1980). Marshall Hall, F. R. S. (1700–1857): a conspectus of his life and work. The Royal Society Notes and Records, 35, 135–65.
International Spinal Research Trust, Station Road, Bramley, Surrey, GU5 0AZ, UK. e-mail: isrt@spinal-research.org

See also breathing; central nervous system; paralysis; reflexes; spinal cord.