Intraoperative Motor and Sensory Monitoring of the Cauda Equina

Functional variability of sacral roots in bladder control

Object

Sacral roots are involved in sensory, autonomic, and motor innervation of the lower limbs and perineum. Theoretically, it can be assumed that the S-3 root level innervates the bladder; however, clinical practice shows that this distribution can vary. Few researchers have studied this variability.

Methods

The authors conducted a retrospective study involving 40 patients who underwent surgery requiring an electrophysiological exploration of the sacral roots. They performed stimulations for the monitoring of muscular (3 Hz, 1 V) and bladder responses under cystomanometry (30 Hz, 10 V).

Results

Although the S-3 roots were involved in bladder innervation in all cases, they were exclusively involved (i.e., the only nerve roots involved) in only 8 of 40 cases. In the remaining 32 cases, other sacral nerve roots were involved. The most common association was S-3+S-4 (12 cases), followed by S-2+S-3 (6 cases), S-2+S-3+S-4 (5 cases), and S-3+S-4+S-5 (2 cases). Stimulation of S-2 could sometimes induce bladder contraction (15 cases, 40%); however, the amplitude was often low. S-4 nerve roots were involved in 24 of 40 cases (60%) in the bladder motor function, whereas S-5 roots were only involved 7 times (17%). Occasionally, we noticed a horizontal asymmetry in the response, with a predominant response from the right side in 6 of 7 cases, always with a major S-3 response.

Conclusions

This is the first study showing a significant horizontal and vertical variability in the functional distribution of sacral roots in bladder innervation. These results show the variability of cauda equina syndromes and their forensic implications. These data should help with the monitoring of sacral roots and the performance of several tasks during surgery, including neurostimulation and neuromodulation.

Compound muscle action potentials and spontaneous electromyography can be used to identify and protect the femoral nerve during resection of large retroperitoneal tumors

BACKGROUND

Resection of large retroperitoneal neoplasms may injure the femoral nerve, thereby causing a permanent neurological deficit. We used electrical neurophysiological monitoring to identify, map, and preserve the femoral nerve during surgical resection to reduce the risk of neurological deficit.

METHODS

Seven patients with retroperitoneal neoplasms underwent eight resections. Compound muscle action potentials (CMAPs) were recorded from needle electrodes placed in the iliacus, quadriceps, and sartorius muscles. Spontaneous electromyography (EMG) was continuously monitored from the same muscle groups. A handheld monopolar stimulator was used to elicit evoked EMG responses to identify and map the course of the femoral nerve. A stimulating strength of 10 mA was used to map the nerve. The stimulation threshold was tested after neoplasm resection to predict postoperative femoral nerve function.

RESULTS

Electrical stimulation with CMAP recording and a stimulating strength of 10 mA successfully localized the femoral nerve in six cases. Monitoring with a stimulating threshold between 0.6 and 1.6 mA predicted postoperative femoral nerve preservation after tumor resection in four of the six cases.

CONCLUSION

Neurophysiological monitoring using CMAP and spontaneous EMG can protect the femoral nerve during resection of large retroperitoneal neoplasms.

Resection of myxopapillary ependymomas in children

OBJECT

Currently, the optimal treatment of children harboring myxopapillary ependymomas of the spinal cord remains somewhat debatable. The authors present a retrospective study in which they evaluated the records of patients in whom resection of these lesions had been performed.

METHODS

Fourteen pediatric patients who had undergone resection of a spinal cord tumor between September 1982 and July 2004 were identified from the database as having histologically classified myxopapillary ependymomas. There were 10 boys and four girls ranging in age from 7 to 18 years (mean age 12.6 years); 71% of the patients were boys. The clinical presentation of the tumor's course was slow and indolent, and the patients had a mean symptom duration of 19.6 months. Twelve patients, who underwent a total of 16 operations, were available for long-term follow-up review. Thirteen gross-total resections and three subtotal resections were performed. There were no deaths due to surgery. Postoperatively, patients initially remained at their preoperative level of function or improved. Patients who had undergone previous surgery and radiotherapy were treated more conservatively than patients who were undergoing surgery for the first time. Four children experienced significant complications following treatment.

CONCLUSIONS

As the authors demonstrate in this study, excellent outcomes may be obtained with the use of aggressive surgical techniques with the goal being that of gross-total resection. Despite the best of resections, however, the risk of recurrence remains. Therefore, periodic neuroimaging surveillance of the neuraxis and close clinical follow up are warranted throughout the patient's life. The role for adjunctive chemo- and radiotherapy remains to be defined in the management of myxopapillary ependymomas.

Pathophysiology of tethered cord syndrome: correlation with symptomatology

Tethered cord syndrome (TCS) is a stretch-induced functional disorder of the spinal cord. The mechanical cause of TCS is an inelastic structure anchoring the caudal end of the spinal cord that prevents cephalad movement of the lumbosacral cord. Stretching of the spinal cord occurs in patients either when the spinal column grows faster than the spinal cord or when the spinal cord undergoes forcible flexion and extension. Research in patients and experimental animals suggests that there is a link between the clinical dysfunctions that characterize TCS and putative pathophysiological changes that accompany this syndrome. Among these changes are depression of electrophysiological activity and shifts in the reduction/oxidation ratio of cytochrome oxidase. The latter suggests that there is impairment of oxidative metabolism. These putative pathophysiological changes in TCS occur mainly within the lumbosacral cord under excessive tension. The authors discuss the pathophysiology of TCS and examine related symptoms.

Intraoperative monitoring for tethered cord surgery: an update

OBJECT

Intraoperative neurophysiological recording techniques have found increasing use in neurosurgical practice. The development of new recording techniques feasible while the patient receives a general anesthetic have improved their practical use in a similar way to the use of digital recording, documentation, and video technology. This review intends to provide an update on the techniques used and their validity.

METHODS

Two principal methods are used for intraoperative neurophysiological testing during tethered cord release. Mapping identifies functional neural structures, namely nerve roots, and monitoring provides continuous information on the functional integrity of motor and sensory pathways as well as reflex circuitry. Mapping is performed mostly by using direct electrical stimulation of a structure within the surgical field and recording at a distant site, usually a muscle. Sensory mapping can also be performed with peripheral stimulation and recording within the surgical site. Monitoring of the motor system is achieved with motor evoked potentials. These are evoked by transcranial electrical stimulation and recorded from limb muscles and the external anal sphincter. The presence or absence of muscle responses are the parameters monitored. Sensory potentials evoked by tibial or pudendal nerve stimulation and recorded from the dorsal columns via an epidurally inserted electrode and/or from the scalp as cortical responses are used to access the integrity of sensory pathways. Amplitudes and latencies of these responses are then interpreted. The bulbocavernosus reflex, with stimulation of the pudendal nerve and recording of muscle responses in the external anal sphincter, is used for continuous monitoring of the reflex circuitry. Presence or absence of this response is the pertinent parameter that is monitored.

CONCLUSIONS

Intraoperative neurophysiology provides a wide and reliable set of techniques for intraoperative identification of neural structures and continuous monitoring of their functional integrity.